Class 12 Biology Chapter 5 Molecular Basis of Inheritance

Key Concept: Genome annotation, ESTs

b) To identify all genes expressed as RNA
[Solution Description]
Expressed Sequence Tags (ESTs) are short sub-sequences of cDNA (complementary DNA) derived from mRNA. They represent expressed genes and are used to identify coding regions in the genome. In the rice genome project, ESTs helped map which parts of the genome are actively transcribed into RNA, thus distinguishing functional genes from non-coding regions. This was crucial for genome annotation and understanding gene function.

Key Concept: IRGSP collaboration, sequencing methodology

c) Involvement of multiple countries sharing responsibilities
[Solution Description]
The IRGSP involved collaboration from 11 nations, including Japan, Korea, the United States, China, and India. This international collaboration allowed pooling of resources, expertise, and technologies, making it feasible to sequence a complex genome like rice efficiently. The shotgun approach and use of PAC/BAC clones were enhanced by this distributed effort, ensuring comprehensive coverage and accuracy in sequencing.

Key Concept: Rice genome size, sequencing progress

c) 420-450 Mb
[Solution Description]
To find the estimated genome size, we can use the proportion of the sequenced part to the total genome. Given that 3.5% corresponds to 15 Mb, we set up the equation:
3.5% of Total Genome = 15 Mb
Let Total Genome be $G$. Then:
$0.035 \times G = 15 \text{ Mb}$
Solving for $G$:
$G = \frac{15 \text{ Mb}}{0.035} \approx 428.57 \text{ Mb}$
This closely aligns with the syllabus's mention of the rice genome being estimated between 400-430 Mb.

Key Concept: Sequencing status, Chromosome assignment

c) 15 Mb
[Solution Description]
To calculate the sequenced portion:
1. Total genome size = 430 Mb.
2. Sequenced percentage = 3.5%.
3. Calculation: 430 × 0.035 = 15.05 Mb ≈ 15 Mb.

Key Concept: Gene estimation, Functional genomics

c) 5,632
[Solution Description]
To find the number of stress tolerance genes:
1. Total genes = 37,544.
2. Percentage for stress tolerance = 15%.
3. Calculation: 37,544 × 0.15 = 5,631.6 ≈ 5,632 genes.

Key Concept: Sequencing, BAC/PAC clones, Repetitive DNA

c) 600
[Solution Description]
To determine the number of subclones needed for 8x coverage:
1. Calculate total base pairs to be sequenced: Coverage × BAC size = 8 × 150,000 bp = 1,200,000 bp.
2. Each subclone is 2 kb (2,000 bp), so divide total base pairs by subclone size: 1,200,000 ÷ 2,000 = 600 subclones.

Key Concept: Estimated Genes and Applications, Functional Genomics

d) 37,544
[Solution Description]
From the syllabus, it is estimated that the rice genome may contain 37,544 genes. This information is explicitly mentioned in the "Estimated Genes and Applications" section.
Step 1: Locate the "Estimated Genes and Applications" section in the syllabus.
Step 2: Note the number of genes stated (37,544).
Step 3: Match this number with the provided options.

Key Concept: Sequencing Approach, Functional Genomics

b) 1 to 3 kb
[Solution Description]
The shotgun approach involves breaking the DNA into small fragments for sequencing. From the syllabus, the Rice Genome Research Programme used a shotgun library with insert sizes of 1 to 3 kb.
Step 1: Refer to the "Sequencing Approach" section, which states the insert sizes are 1 to 3 kb.
Step 2: Identify the option that matches this range.

Key Concept: International Collaboration, Sequencing Approach

a) Germany
[Solution Description]
The IRGSP included 11 nations: Japan, Korea, China, India, Taiwan, France, Thailand, Canada, Brazil, and the United Kingdom. To identify the country not part of this collaboration, we can refer to Table 7 in the syllabus, which lists all participating centers and their respective countries.
Step 1: List all countries from the IRGSP collaboration: Japan, Korea, China, India, Taiwan, France, Thailand, Canada, Brazil, and the United Kingdom.
Step 2: Compare with the options. If an option is not in the listed countries, it is the correct answer.

Key Concept: Rice Genome Project - Implications

c) Improvement in crop yield, disease resistance, and stress tolerance
[Solution Description]
The rice genome project has contributed significantly to agricultural biotechnology by providing tools for data analysis and technology transfer. It also helps improve crop yield, disease resistance, and stress tolerance in rice and other cereals, as mentioned in the syllabus.

Key Concept: Rice Genome Project - Sequencing Method

c) Shotgun sequencing of PAC/BAC clones
[Solution Description]
The syllabus states that shotgun sequencing of PAC or BAC clones is the primary method used in the Rice Genome Research Programme. Fragments are subcloned and sequenced to provide coverage of the total sequence.

Key Concept: Rice Genome Project - Collaboration

d) 11 nations
[Solution Description]
The Rice Genome Research Programme involves 11 nations, as mentioned in the syllabus. These include Japan, Korea, the United States, China, India, Taiwan, France, Thailand, Canada, and the United Kingdom.

Key Concept: Functional Genomics

c) 37,544
[Solution Description]
The IRGSP estimates that the rice genome contains approximately 37,544 genes, which are crucial for functional genomics studies aimed at improving crop traits.

Key Concept: Genome Size and Repetitive DNA

b) 50%
[Solution Description]
According to the provided syllabus, repetitive DNA constitutes at least 50% of the rice genome, highlighting its impact on genome organization.

Key Concept: Sequencing Methodology

b) Shotgun sequencing
[Solution Description]
The Rice Genome Research Programme (Japan) employed a shotgun sequencing approach, where PAC/BAC clones were broken into smaller fragments (1–3 kb) and sequenced randomly. These sequences were then reassembled using software like PHRED and PH.

Key Concept: Functional Genomics

c) To analyze gene functions for breeding improved varieties
[Solution Description]
Functional genomics helps in understanding gene functions and interactions, enabling breeders to develop improved rice varieties with traits like stress tolerance and disease resistance more efficiently than traditional methods. The syllabus emphasizes this significance.

Key Concept: Sequencing Methodology

b) Shotgun sequencing
[Solution Description]
The Rice Genome Research Programme (Japan) utilized the shotgun sequencing approach, which involves breaking DNA into small fragments (1-3 kb), sequencing them, and reassembling the entire sequence later. This is explicitly stated in the syllabus.

Key Concept: Genome Size

c) 400 to 430 Mb
[Solution Description]
The rice genome is one of the smallest among major cereal crops and is estimated to be between 400 to 430 megabases (Mb). This information is directly mentioned in the syllabus.

Key Concept: Genome Sequencing Progress

c) 3.5%
[Solution Description]
The syllabus states that as of the current progress, about 3.5% of the rice genome has been completed (15 of 430 Mb). This figure is clearly mentioned in the syllabus.

Key Concept: Collaborative Effort

d) 11
[Solution Description]
According to the syllabus, the Rice Genome Research Programme involves 11 nations, including Japan, Korea, the United States, China, and India. This detail is explicitly stated in the syllabus.

Key Concept: Rice Genome Project Overview

c) 430 Mb
[Solution Description]
The rice genome size mentioned in the syllabus is approximately 430 Mb. This information is directly provided in the syllabus under the Rice Genome Project: Conclusion section.

Key Concept: Chromosome Assignment

c) Indian Rice Genome Programme (University of Delhi)
[Solution Description]
According to the table, the Indian Rice Genome Programme (University of Delhi) and the University of Wisconsin were assigned to sequence chromosome 11.

Key Concept: Rice Genome Size

c) 400-430 Mb
[Solution Description]
The rice genome is reported to have an estimated size range of 400 to 430 Mb (megabases), making it the smallest among major cereal crop genomes.

Key Concept: Sequencing Approach

b) Shotgun sequencing
[Solution Description]
The Rice Genome Research Programme (Japan) used a shotgun sequencing approach to sequence PAC or BAC clones. In this method, DNA is fragmented into small pieces, cloned, and sequenced at random to achieve coverage.

Key Concept: Sequencing Technique

b) Shotgun sequencing
[Solution Description]
The Rice Genome Research Programme (Japan) utilized a shotgun approach to sequence PAC or BAC clones. This method involves shearing these clones into smaller fragments, subcloning them, and sequencing them randomly before reassembling the entire sequence.

Key Concept: International Rice Genome Sequencing Project

b) Russia
[Solution Description]
The IRGSP collaboration includes Japan, Korea, China, India, Taiwan, France, Thailand, Canada, the United Kingdom, Brazil, and the United States. Russia was not part of this project.

Key Concept: Rice Genome Size

c) 400 to 430 Mb
[Solution Description]
The rice genome is the smallest among major cereal crops, with an estimated size ranging between 400 to 430 megabases (Mb).

Key Concept: False Positives and Negatives, Cultural Implications

c) False positives in DNA testing
[Solution Description]
The incorrect identification due to sample contamination raises the ethical concern of false positives in DNA fingerprinting, which can lead to wrongful convictions. This undermines justice and highlights the need for stringent accuracy measures in DNA testing.

Key Concept: Discrimination, Data Security

c) Genetic discrimination
[Solution Description]
The situation highlights genetic discrimination, where an individual faces adverse treatment based on their genetic information. While data security might be a concern if the information was obtained improperly, the primary ethical issue here is the discriminatory use of genetic data.

Key Concept: Privacy Concerns, Consent

c) Breach of informed consent
[Solution Description]
The scenario describes a violation of informed consent, where participants are not fully aware of how their genetic information will be used or the risks involved, such as data breaches. Privacy concerns are also implicated as unauthorized access to DNA data can lead to misuse.

Key Concept: PCR, Genetic monitoring

d) Progressive reduction in heterozygous loci
[Solution Description]
Reduced heterozygosity in PCR results indicates loss of genetic variation, a critical concern for small, reintroduced populations facing increased extinction risk.
Step-by-step reasoning:
1. PCR amplifies specific VNTR regions.
2. Multiple bands per individual show heterozygosity.
3. Fewer bands suggest homozygosity.
4. Conservation aims to maintain genetic diversity.

Key Concept: Southern blotting, Population structure

c) The groups represent distinct genetic subpopulations
[Solution Description]
Different Southern blot banding patterns indicate genetic differentiation between the groups, likely due to lack of gene flow. This has implications for conservation strategies.
Step-by-step reasoning:
1. Southern blot shows DNA fragment lengths after restriction digestion.
2. Different patterns = different VNTR distributions.
3. Geographic isolation prevents genetic mixing.

Key Concept: VNTR, Conservation genetics

a) Low genetic diversity due to inbreeding
[Solution Description]
The high percentage (40%) of individuals sharing identical VNTR patterns suggests reduced genetic diversity due to factors like inbreeding or small population size. This is problematic for conservation as it reduces adaptability and increases disease vulnerability.
Step-by-step reasoning:
1. VNTR patterns are unique in genetically diverse populations.
2. High similarity indicates limited allele variation.
3. Conservation genetics uses such data to assess genetic health.

Key Concept: Hybridisation, VNTR Probes, Inheritance

c) Mutations or recombination events
[Solution Description]
Non-matching bands can arise due to mutations or recombination events during meiosis, leading to new VNTR patterns not present in either parent. However, most bands are inherited, so the majority should match one of the parents.

Key Concept: PCR, Restriction Digestion, Autoradiography

c) PCR amplification
[Solution Description]
Polymerase Chain Reaction (PCR) is used to amplify the DNA extracted from the single cell, making millions of copies. This ensures there is enough DNA for subsequent steps like restriction digestion and autoradiography, even when starting with a minimal sample.

Key Concept: VNTR, Southern Blotting, Paternity Testing

d) 5
[Solution Description]
In DNA fingerprinting, a child inherits half of their VNTRs from each parent. If the child has 8 bands and 3 match exclusively with the mother, the remaining 5 must be inherited from the father. Therefore, at least 5 bands should match with the potential father's DNA fingerprint to confirm paternity.

Key Concept: Restriction digestion, VNTR

b) Due to differences in the number and position of restriction sites
[Solution Description] Restriction endonucleases cut DNA at specific recognition sites. Due to polymorphisms in VNTR regions, the number and location of these sites differ between individuals, leading to varying fragment lengths. This variation forms the basis of DNA fingerprinting. The correct answer involves understanding genetic variability in restriction sites.

Key Concept: PCR, Southern blotting

d) Polymerase chain reaction
[Solution Description] Polymerase Chain Reaction (PCR) is used to amplify tiny amounts of DNA, making it suitable for degraded or limited samples. It creates multiple copies of the target DNA sequence, enabling further steps like restriction digestion. Other options describe unrelated techniques. The correct answer requires knowledge of PCR's role in DNA amplification.

Key Concept: VNTR, PCR, Gel electrophoresis

c) To identify and visualize specific DNA sequences
[Solution Description] After gel electrophoresis, DNA fragments are separated by size. However, these fragments need to be visualized and compared. VNTR probes are used for hybridization because they bind specifically to the repetitive sequences unique to an individual. This allows detection via autoradiography, revealing a distinct band pattern. The correct answer involves understanding that VNTR probes help identify specific DNA sequences for comparison.

Key Concept: Ethical issues, DNA fingerprinting applications

c) Misuse of genetic information leading to discrimination or privacy breaches
[Solution Description]
DNA fingerprinting can reveal sensitive genetic information about inherited diseases, which may lead to privacy violations or misuse. Concerns include discrimination based on genetic data, unauthorized access to personal health information, and lack of consent in genetic profiling.

Key Concept: PCR amplification, DNA fingerprinting sensitivity

c) PCR increases the amount of DNA available for analysis by making multiple copies
[Solution Description]
PCR amplifies small amounts of DNA, generating millions of copies. This increased quantity allows DNA fingerprinting to be performed even on samples containing DNA from a single cell. Without PCR, the amount of DNA might be insufficient for accurate detection via electrophoresis and autoradiography.

Key Concept: DNA fingerprinting, Southern blotting, autoradiography

c) To detect hybridized DNA fragments by visualizing the radioactive probes
[Solution Description]
After hybridization, radioactive probes are attached to specific portions of DNA fragments. The nylon membrane containing the hybridized DNA is exposed to X-ray film. The radioactivity from the probes exposes the film, revealing a unique pattern of dark and light bands. These bands are used to identify individuals based on their DNA fingerprints.

Key Concept: PCR, Probe hybridisation

c) To amplify target DNA sequences for better detection
[Solution Description]
PCR amplifies target DNA sequences, including VNTR regions, increasing their quantity. This enhances sensitivity during probe hybridisation as more copies of VNTR sequences are available for probes to bind. Higher DNA concentration improves detection via autoradiography, ensuring clear and distinguishable banding patterns.

Key Concept: Southern blotting, Autoradiography

b) No banding pattern will be observed
[Solution Description]
Southern blotting transfers DNA fragments from gel to a nylon membrane for probe hybridisation. If this step fails, DNA fragments remain on the gel and cannot hybridise with probes. This prevents detection of VNTR sequences via autoradiography, resulting in no banding pattern, thus failing the fingerprinting process.

Key Concept: Probe hybridisation, VNTR sequences

c) Ensures specific binding of probes to VNTR sequences
[Solution Description]
The probe hybridisation step involves binding of labelled DNA probes to complementary VNTR sequences. VNTRs are highly variable among individuals, making their detection essential for creating a unique fingerprint. Probes bind specifically to these regions, allowing visualization through autoradiography, which reveals distinct banding patterns unique to each individual.

Key Concept: PCR, DNA Fingerprinting Sensitivity

c) It selectively multiplies target DNA sequences for better detection.
[Solution Description]
PCR amplifies specific regions of DNA, increasing the quantity available for analysis. This allows even a single cell's DNA to generate sufficient material for electrophoresis, hybridization, and detection. Without PCR, small samples might not yield detectable bands after autoradiography.

Key Concept: Hybridization, Autoradiography

b) The VNTR probe had low radioactivity.
[Solution Description]
Faint bands in autoradiography can occur if the radioactive probes used for hybridization have low activity or if exposure time was insufficient. Since the question specifies the procedure was otherwise correct, the issue likely lies in the detection phase. Insufficient washing of unhybridized DNA could cause background noise but not faint target bands.

Key Concept: Gel Electrophoresis, Southern Blotting

d) Hybridization will fail due to lack of probe binding sites.
[Solution Description]
Before hybridization in DNA fingerprinting, DNA fragments must be denatured into single strands to allow the VNTR probes to bind. If this step is skipped, the double-stranded DNA will prevent proper binding of the probes. As a result, the autoradiography will show weak or no bands because hybridization fails.

Key Concept: Gel electrophoresis, Autoradiography

c) To visualize radioactively labeled DNA fragments
[Solution Description]
Autoradiography detects radioactive probes bound to DNA fragments after Southern blotting. Since DNA fragments themselves are not visible under normal conditions, the radioactive probes expose X-ray film to produce dark bands where DNA fragments are located.

Key Concept: DNA Fingerprinting, VNTR probes

c) VNTRs are highly polymorphic between individuals
[Solution Description]
VNTR (Variable Number Tandem Repeats) regions are highly polymorphic, meaning they vary significantly between individuals and thus provide unique patterns. Coding sequences are conserved and do not differ much among individuals, making them unsuitable for generating unique fingerprints.

Key Concept: Restriction enzymes, Gel electrophoresis

c) 6
[Solution Description]
Restriction enzymes cut DNA at specific recognition sites. The number of fragments produced is always one more than the number of recognition sites. Here, the DNA has 5 recognition sites. Therefore, the total number of fragments after digestion will be 5 + 1 = 6.

Key Concept: Autoradiography, Detection

c) A unique pattern of dark and light bands specific to an individual
[Solution Description]
Autoradiography involves exposing the nylon membrane with hybridized radioactive DNA probes to X-ray film. The radioactive probes expose the film, creating dark bands where the probes have bound to the DNA fragments. The resulting pattern of dark and light bands is unique to each individual and serves as their DNA fingerprint.

Key Concept: PCR, Sensitivity

c) To amplify DNA, ensuring sufficient quantity for analysis
[Solution Description]
Polymerase chain reaction ($PCR$) is used to amplify specific regions of DNA, making millions of copies from even a single cell's DNA. This step increases the sensitivity of DNA fingerprinting by ensuring there is enough DNA for subsequent steps like restriction digestion and electrophoresis. Without amplification, the small amount of DNA might not be sufficient for detection.

Key Concept: VNTR, Hybridization

c) To identify and bind specifically to unique VNTR sequences
[Solution Description]
The purpose of hybridization using a labeled VNTR probe is to identify and bind specifically to the unique VNTR sequences in the DNA fragments. These probes are radioactive or fluorescent and attach only to their complementary sequences. After washing away unbound DNA, the remaining hybridized fragments can be detected using autoradiography, revealing a unique band pattern for identification.

Key Concept: Restriction Enzymes, Gel Electrophoresis

d) Recognition of specific palindromic sequences
[Solution Description]
Restriction enzymes that recognize and cleave at short, specific sequences (usually palindromic) are essential for DNA fingerprinting because these sequences often vary between individuals due to SNPs or insertions/deletions, creating polymorphisms. The ability to cut at these polymorphic sites allows for the generation of unique banding patterns on gel electrophoresis.

Key Concept: Allele Frequency, Heterozygosity

c) 0.32
[Solution Description]
The expected heterozygosity ($H$) under Hardy-Weinberg equilibrium is given by $H = 1 - \sum p_i^2$, where $p_i$ is the frequency of the $i^{th}$ allele. Since there are two alleles, the heterozygosity would be $H = 2pq$. Here, $p = 0.2$ and $q = 0.8$ (since frequencies must sum to 1). Thus, $H = 2 \times 0.2 \times 0.8 = 0.32$.

Key Concept: VNTR, STR

d) Short Tandem Repeats (STRs)
[Solution Description]
Short Tandem Repeats (STRs) are widely used in forensic DNA fingerprinting because they are highly polymorphic, stable, and can be amplified using PCR. VNTRs (Variable Number Tandem Repeats) are also polymorphic but less commonly used due to their larger size and difficulty in amplification compared to STRs.

Key Concept: DNA Hybridization, Probes

d) Optimizing probe concentration and hybridization conditions.
[Solution Description]
Weak hybridization signals in DNA fingerprinting often occur due to insufficient probe binding. Increasing the stringency conditions (e.g., adjusting temperature or salt concentration) can improve specificity and signal strength. Alternatively, using a higher concentration of labeled probes or optimizing hybridization time can also enhance detection.

Key Concept: PCR, STR Analysis

d) STRs allow rapid analysis with minute DNA quantities.
[Solution Description]
STRs (Short Tandem Repeats) are preferred over RFLPs in modern DNA profiling because they can be amplified using PCR, requiring very small amounts of DNA. Unlike RFLPs, which require large DNA samples and lengthy Southern blotting procedures, STR analysis is faster, more sensitive, and suitable for degraded samples. Additionally, STRs have a higher discrimination power due to their abundance in the genome.

Key Concept: VNTR Polymorphism, Southern Blotting

d) Their length polymorphism provides unique banding patterns in gel electrophoresis.
[Solution Description]
VNTR (Variable Number Tandem Repeat) regions are highly polymorphic due to the variation in the number of repeat units among individuals. Although they are non-coding, their high variability makes them ideal for distinguishing between individuals. The differences in the number of repeats result in varying fragment lengths when digested with restriction enzymes, which can be visualized using Southern blotting.

Key Concept: PCR, Forensic Applications

b) Use shorter primer sequences
[Solution Description]
Degraded DNA has fragmented templates. Using shorter primers targets smaller intact fragments more likely to remain in degraded samples. Standard PCR uses longer primers that may fail to amplify when template DNA is broken into small pieces. Other options don't address the degradation issue as directly.

Key Concept: Southern Blotting, Hybridization

c) The VNTR probes wouldn't hybridize properly
[Solution Description]
High temperatures during hybridization can prevent proper binding of VNTR probes to target DNA sequences. Probe hybridization requires specific temperature conditions - too high prevents stable hydrogen bonding between probe and target DNA. This would lead to no/faint bands in autoradiography since unbound probes get washed away.

Key Concept: VNTR, PCR, Gel Electrophoresis

c) The unprobed VNTR region differs between samples
[Solution Description]
The question tests understanding of how VNTR probes work in DNA fingerprinting. While DNA samples may share many VNTR regions (showing identical bands), differences exist in non-probed regions. Since only probed VNTRs show up on autoradiographs, non-probed differences won't appear in the band pattern. Thus, samples can match partially but still be from different individuals.

Key Concept: Ethical Issues - Genetic Discrimination

c) They could discriminate based on genetic disease risks
[Solution Description]
Insurance companies could use genetic predispositions to deny coverage or charge higher premiums, leading to discrimination. The correct answer must reflect this potential misuse.

Key Concept: Ethical Issues - Consent

c) Individuals must be aware of how their data will be used
[Solution Description]
Informed consent ensures individuals understand how their genetic information will be used, protecting them from potential misuse. The correct option should emphasize the importance of awareness and voluntary participation.

Key Concept: Ethical Issues - Privacy Concerns

b) Unauthorized access to DNA data can result in privacy violations
[Solution Description]
A significant ethical issue in DNA fingerprinting is the violation of privacy, as DNA contains highly personal information. Unauthorized access or misuse can lead to discrimination or stigmatization. The correct answer must highlight this concern.

Key Concept: Applications in Conservation Genetics

c) By identifying individuals and assessing genetic diversity.
[Solution Description]
DNA fingerprinting helps conserve endangered species by identifying individuals, assessing genetic diversity, determining familial relationships, and detecting illegal wildlife trade through genetic matching.

Key Concept: Process of DNA Fingerprinting

c) Southern blotting.
[Solution Description]
After gel electrophoresis separates DNA fragments by size, Southern blotting is the technique used to transfer these fragments onto a nitrocellulose or nylon membrane for hybridization with labeled probes.

Key Concept: VNTR

c) Short repetitive nucleotide sequences unique to an individual.
[Solution Description]
VNTRs are short repetitive nucleotide sequences that vary in number among individuals, making them highly polymorphic markers. These repeats are used to distinguish genetic differences between individuals.

Key Concept: Restriction Digestion

c) Restriction endonuclease
[Solution Description] Restriction endonucleases are enzymes that cleave DNA at specific recognition sequences, producing fragments of varying lengths. These fragments are then separated via gel electrophoresis, allowing for the creation of a unique banding pattern used in DNA fingerprinting.

Key Concept: VNTR Comparison

d) The alleged father likely shares genetic markers with the child
[Solution Description] A child inherits half their DNA from each parent. If the child’s DNA bands match some bands from both the mother and alleged father, it indicates shared genetic markers, supporting the claim of paternity. However, complete certainty requires multiple loci analysis for accuracy.

Key Concept: DNA Extraction and Amplification

d) To synthesize new DNA strands complementary to the target sequences
[Solution Description] PCR is used to amplify (make many copies of) specific regions of DNA, especially VNTRs, which are crucial for DNA fingerprinting. This amplification ensures sufficient DNA material is available for subsequent steps like restriction digestion and gel electrophoresis. The process involves repeated cycles of denaturation, annealing, and extension.

Key Concept: Concept: Hybridization Process

c) To detect and visualize specific DNA sequences
[Solution Description]
Labeled VNTR probes bind specifically to repetitive DNA sequences unique to each individual. These probes help visualize the DNA fragments after gel electrophoresis by hybridizing to complementary sequences, enabling the identification of distinct band patterns.

Key Concept: Concept: Forensic Applications

d) Solving criminal cases such as rape or murder
[Solution Description]
DNA fingerprinting can identify individuals based on unique genetic markers. It is widely used in forensic investigations to link suspects to crime scenes by matching their DNA with evidence samples such as blood or hair. Solving criminal cases like rape or murder is a key application.

Key Concept: Concept: Restriction Digestion

d) Restriction endonuclease
[Solution Description]
Restriction endonucleases are enzymes that recognize specific DNA sequences and cleave the DNA at or near these sites. They are crucial in DNA fingerprinting for generating unique fragment patterns. The correct enzyme used is restriction endonuclease.

Key Concept: Southern Blotting

c) To attach to specific DNA sequences for detection
[Solution Description]
In Southern blotting, radioactive DNA probes bind to complementary sequences on the DNA fragments that have been transferred to a nylon membrane. These probes help visualize specific DNA sequences by autoradiography after exposure to X-ray film.

Key Concept: DNA Amplification

c) To create multiple copies of a specific DNA segment
[Solution Description]
PCR is used in DNA fingerprinting to amplify (make many copies of) specific regions of DNA. This step is crucial because it provides sufficient quantities of DNA for subsequent analysis, such as restriction digestion and gel electrophoresis.

Key Concept: Applications of DNA Fingerprinting

c) Differentiating between monozygotic twins
[Solution Description]
DNA fingerprinting relies on unique patterns of DNA fragments to identify individuals or relationships. However, monozygotic twins share identical DNA, making it difficult to distinguish between them using this technique. Therefore, DNA fingerprinting is least effective in differentiating between identical twins.

Key Concept: VNTR Sequences

c) They allow unique identification due to their variability among individuals
[Solution Description]
VNTR (Variable Number Tandem Repeats) are short nucleotide sequences that vary in number among individuals, making them highly polymorphic. This variability allows for unique DNA fingerprints, as no two individuals (except identical twins) share the same VNTR pattern.

Key Concept: DNA Fingerprinting Applications

c) Diagnosing viral infections in patients
[Solution Description]
DNA fingerprinting has numerous applications including identifying individuals in criminal cases, settling paternity disputes, detecting inherited diseases, monitoring bone marrow transplants, and assessing genetic diversity. However, diagnosing viral infections is not a primary application as it relies on detecting unique individual-specific VNTR patterns rather than pathogen identification.

Key Concept: Probe Hybridisation

b) To identify and bind to specific VNTR sequences in the DNA
[Solution Description]
Radioactive DNA probes are used to identify and bind to specific VNTR sequences in the fragmented DNA. The probes attach to complementary sequences, and unbound DNA is washed away. The remaining bound DNA is detected through autoradiography when exposed to X-ray film, producing a unique band pattern.

Key Concept: Banding Pattern Significance

d) It represents variations in repetitive DNA sequences
[Solution Description]
The banding pattern is unique because it reflects variations in the number of tandem repeats (VNTRs) or short tandem repeats (STRs) in an individual's DNA, except for identical twins who share the same genetic material.

Key Concept: Visualization in Gel Electrophoresis

c) By staining with ethidium bromide and viewing under UV light
[Solution Description]
The DNA fragments are visualized by staining the gel with ethidium bromide or other fluorescent dyes, which bind to the DNA. When exposed to UV light, the stained DNA emits fluorescence, revealing the banding pattern.

Key Concept: Gel Electrophoresis Principle

c) Size and charge
[Solution Description]
Gel electrophoresis separates DNA fragments based on their size and charge. The negatively charged DNA migrates toward the positive electrode, with smaller fragments moving faster and farther than larger ones.

Key Concept: VNTR Probes

c) To bind to specific repetitive DNA sequences
[Solution Description]
VNTR (Variable Number Tandem Repeat) probes are used to bind to specific repetitive sequences in the DNA fragments. This hybridization allows for the visualization of unique band patterns after autoradiography, aiding in individual identification.

Key Concept: Gel Electrophoresis

c) Size of the DNA fragments
[Solution Description]
Gel electrophoresis separates DNA fragments based on their size. Smaller fragments move faster and further through the gel matrix compared to larger fragments due to less resistance encountered.

Key Concept: Restriction Enzymes

d) Restriction endonuclease
[Solution Description]
Restriction enzymes, also known as restriction endonucleases, are enzymes that cut DNA at specific nucleotide sequences called recognition sites. These enzymes are essential in DNA fingerprinting for generating unique fragment patterns.

Key Concept: Applications of DNA Fingerprinting

d) Calculating the exact age of an individual
[Solution Description]
While DNA fingerprinting is extensively used for criminal investigations, paternity testing, disease detection, and genetic diversity studies, it cannot determine an individual's exact age as it is not based on age-related genetic markers.

Key Concept: Steps in DNA Fingerprinting

d) Southern blotting
[Solution Description]
Southern blotting is the step where DNA fragments separated by gel electrophoresis are transferred onto a nylon membrane for further hybridization with radioactive probes. This process was invented by E.M. Southern.

Key Concept: Repetitive DNA and Satellite DNA

c) Their high polymorphism makes them suitable for individual identification.
[Solution Description]
Repetitive DNA sequences show a high degree of polymorphism, meaning they vary widely among individuals, making them ideal for identification purposes. These sequences include microsatellites and minisatellites, which can be used to generate unique DNA profiles.

Key Concept: Variable Number Tandem Repeats

b) Polymerase Chain Reaction (PCR)
[Solution Description]
Polymerase Chain Reaction (PCR) is used to amplify small amounts of DNA, including polymorphic regions like Variable Number Tandem Repeats (VNTRs). The amplified fragments are then separated by gel electrophoresis to generate a unique band pattern.

Key Concept: Short Tandem Repeats

c) 2-6 base pairs
[Solution Description]
Short Tandem Repeats (STRs) are repetitive sequences of 2-6 base pairs. These are amplified using PCR and analyzed for variations in repeat numbers among individuals, forming the basis of DNA fingerprinting.

Key Concept: Single Nucleotide Polymorphism

c) A change in a single nucleotide base pair
[Solution Description]
A Single Nucleotide Polymorphism (SNP) is a variation at a single nucleotide in the DNA sequence, such as an A to T substitution. This type of polymorphism occurs commonly in the genome and is used in various genetic studies.

Key Concept: RFLP

d) To cut DNA at specific sites, producing fragments of varying lengths.
[Solution Description]
Restriction enzymes cut DNA at specific recognition sites, generating fragments of varying lengths due to polymorphisms like VNTRs or STRs. These fragments are then separated by gel electrophoresis to create a unique banding pattern for identification.

Key Concept: STRs

d) STRs are shorter (2-6 base pairs) compared to VNTRs (10-60 base pairs).
[Solution Description]
STRs are shorter repetitive sequences, typically 2-6 base pairs long, whereas VNTRs are longer, ranging from 10-60 base pairs. Both are non-coding and highly polymorphic, but their lengths distinguish them.

Key Concept: VNTRs

d) VNTRs are short repetitive sequences (2-6 base pairs) used in DNA fingerprinting.
[Solution Description]
VNTRs are repetitive DNA sequences that are 10-60 base pairs long and vary in number among individuals. They are highly polymorphic, making them suitable for DNA fingerprinting. They are non-coding sequences and are not involved in protein synthesis.

Key Concept: Autoradiography

c) Hybridized radioactive probes bound to complementary DNA fragments.
[Solution Description]
In autoradiography, hybridized radioactive DNA probes bind to complementary VNTR sequences on the nylon membrane. When exposed to X-ray film, these hybridized fragments emit radiation, creating dark bands. Each band corresponds to a specific DNA fragment length, collectively forming a unique fingerprint.

Key Concept: Restriction Digestion

c) To cut DNA at specific sites, enabling fragment separation and analysis.
[Solution Description]
Restriction digestion involves cutting DNA at specific sequences using restriction endonucleases. This step breaks the DNA into fragments of varying lengths, which can then be separated via gel electrophoresis. The resulting fragment pattern is used to identify polymorphisms like VNTRs, forming the basis of unique DNA fingerprints.

Key Concept: VNTR and Polymorphism

c) They provide highly polymorphic regions used for individual identification.
[Solution Description]
VNTRs (Variable Number Tandem Repeats) are short, repetitive nucleotide sequences that show high polymorphism among individuals. These sequences serve as unique genetic markers because their repeat numbers vary widely between people. They are the basis for distinguishing individuals' DNA by generating distinct banding patterns during autoradiography.

Key Concept: ELSI

c) Ethical, Legal, and Social Issues
[Solution Description]
ELSI stands for Ethical, Legal, and Social Issues. These are concerns that arise from the use of genetic technologies like DNA fingerprinting, including privacy violations, legal implications, and societal impacts such as genetic discrimination.

Key Concept: Informed Consent

c) To uphold ethical standards by respecting individual choice
[Solution Description]
Informed consent ensures that individuals are fully aware of how their genetic information will be used and any potential risks involved before they undergo genetic testing. It respects the individual's autonomy and right to make decisions about their own body.

Key Concept: Ethical Issues

b) Privacy of genetic data
[Solution Description]
One of the major ethical concerns in DNA fingerprinting is privacy, as genetic information can reveal sensitive personal details about an individual or their family members. The misuse of such data for discrimination or other non-medical purposes is also a significant concern.

Key Concept: DNA Fingerprinting Process - Southern Blotting

c) Transferring DNA fragments to a membrane for detection
[Solution Description]
Southern blotting is a technique used to transfer separated DNA fragments from a gel electrophoresis to a membrane for hybridization with labeled probes, enabling detection of specific DNA sequences.

Key Concept: Conservation Genetics - Applications

c) Studying genetic diversity in endangered species
[Solution Description]
In conservation genetics, DNA fingerprinting helps study genetic diversity and population structure of endangered species, aiding in their management and conservation efforts.

Key Concept: DNA Fingerprinting - VNTR

b) As unique genetic markers to identify individuals
[Solution Description]
VNTRs are short, repetitive nucleotide sequences that vary in number among individuals, making them unique genetic markers. They form the basis of DNA fingerprinting because their polymorphism allows for distinguishing between individuals.

Key Concept: Paternity Testing

c) VNTR patterns are inheritable and differ between non-related individuals
[Solution Description]
DNA fingerprinting helps in resolving paternity disputes because the polymorphisms (VNTRs) are inherited from parents to children. By comparing the DNA bands of the child with those of the potential parents, parentage can be established based on shared genetic markers.

Key Concept: DNA Extraction

b) Amplification
[Solution Description]
In DNA fingerprinting, the process of making multiple copies of DNA is called amplification. This is achieved using the polymerase chain reaction (PCR), which increases the amount of DNA available for analysis.

Key Concept: VNTR in DNA Fingerprinting

c) Variable number tandem repeats (VNTR)
[Solution Description]
The nucleotide sequences used in DNA fingerprinting are called Variable Number Tandem Repeats (VNTR). These short repetitive sequences vary among individuals and help in identifying unique DNA patterns.

Key Concept: Autoradiography

d) Autoradiography
[Solution Description]
Autoradiography is the technique where radioactive DNA probes attached to specific DNA fragments are detected by exposing the membrane to X-ray film, revealing a unique band pattern.

Key Concept: PCR

d) 32
[Solution Description]
Each PCR cycle doubles the amount of DNA. Starting with one copy:
- After 1 cycle: $2^1 = 2$ copies
- After 2 cycles: $2^2 = 4$ copies
- ...
- After 5 cycles: $2^5 = 32$ copies.

Key Concept: VNTR

d) Variable Number Tandem Repeats
[Solution Description]
The short repetitive nucleotide sequences specific to a person, used for DNA fingerprinting, are known as Variable Number Tandem Repeats (VNTR). These sequences vary in length between individuals and form the basis of uniqueness in DNA profiling.

Key Concept: DNA Fingerprinting in Relationships

c) Confirming paternity disputes
[Solution Description]
DNA fingerprinting is useful in settling paternity disputes by comparing the DNA profiles of the child and alleged parents to confirm biological relationships.

Key Concept: Process of DNA Fingerprinting

c) Polymerase chain reaction
[Solution Description]
Amplification of DNA is done using Polymerase Chain Reaction (PCR), which creates numerous copies of the DNA sample for further analysis.

Key Concept: Application of DNA Fingerprinting

a) Identifying individuals involved in criminal activities
[Solution Description]
DNA fingerprinting is widely used in forensic science to identify individuals involved in crimes, such as rape cases. It helps in matching DNA samples from crime scenes with suspects or victims.

Key Concept: VNTR Probes

b) Short repetitive nucleotide sequences
[Solution Description]
VNTR (Variable Number Tandem Repeats) probes bind to short repetitive nucleotide sequences that vary in number among individuals. These repeats create a unique DNA banding pattern for each person.

Key Concept: Applications of DNA Fingerprinting

b) Determining the age of fossils
[Solution Description]
DNA fingerprinting has various applications including identification in crimes, paternity disputes, disease detection, and population studies. Monitoring bone marrow transplants is also an application, as it helps track donor cell engraftment.

Key Concept: Probe Hybridization

b) To visualize and identify specific DNA sequences
[Solution Description]
In DNA fingerprinting, radioactive DNA probes are used to bind specifically to unique DNA sequences (VNTRs). The bound probes emit radiation which exposes X-ray film, creating a band pattern unique to each individual.

Key Concept: Visualization of DNA bands

c) Staining with ethidium bromide and viewing under UV light
[Solution Description]
Separated DNA fragments are visualized by staining the gel with dyes like ethidium bromide, which binds to DNA and fluoresces under UV light, revealing distinct bands.

Key Concept: Gel matrix in gel electrophoresis

b) Agarose
[Solution Description]
Agarose is the most commonly used gel matrix for separating DNA fragments due to its optimal pore size for DNA migration. Polyacrylamide is also used but typically for smaller DNA fragments or proteins.

Key Concept: DNA fragment separation in gel electrophoresis

a) Length of the DNA fragment
[Solution Description]
The distance a DNA fragment travels in gel electrophoresis depends on its size. Smaller fragments move faster and farther, while larger ones move slower and remain closer to the origin. Charge is not the primary determinant since all DNA fragments are negatively charged.

Key Concept: Gel Electrophoresis

c) To separate DNA fragments by size
[Solution Description]
Gel electrophoresis separates DNA fragments based on their size. An electric current is applied, causing the negatively charged DNA to move through the gel matrix, with smaller fragments traveling farther than larger ones.

Key Concept: DNA Extraction

c) From the nuclear material in cells
[Solution Description]
The first step in DNA fingerprinting involves extracting DNA from sample cells, which could be blood, saliva, hair roots, or any biological material containing cells with nuclei.

Key Concept: Restriction Enzymes

b) Cut DNA into fragments at specific sequences
[Solution Description]
Restriction enzymes are used to cut DNA at specific sequences, producing fragments of varying lengths. These fragments are then separated using gel electrophoresis to create a unique pattern for identification.

Key Concept: Concept: Gel Electrophoresis

d) To separate DNA fragments by size
[Solution Description]
After DNA is cut into fragments using restriction endonucleases, gel electrophoresis separates these fragments based on their size. The fragments migrate through the gel at different rates depending on their length, creating distinct bands that are later visualized to form the DNA fingerprint pattern.

Key Concept: Concept: Satellite DNA

c) Satellite DNA
[Solution Description]
Satellite DNA consists of repetitive sequences that exhibit a high degree of variability (polymorphism) among individuals. These differences in satellite DNA sequences, such as microsatellites and minisatellites, allow for unique identification through DNA fingerprinting.

Key Concept: Concept: DNA Fingerprinting Technique

d) To make many copies of a specific DNA segment
[Solution Description]
Polymerase chain reaction (PCR) is used to amplify (make many copies of) a specific segment of DNA. This step is essential because DNA samples collected from crime scenes or paternity tests are often very small in quantity. By amplifying the DNA, scientists can obtain sufficient material for further analysis.

Key Concept: DNA Fingerprinting Steps

d) PCR amplification
[Solution Description]
Polymerase Chain Reaction (PCR) is used to amplify specific polymorphic regions such as STRs or VNTRs before they are separated using gel electrophoresis. This step increases the quantity of DNA for accurate analysis.

Key Concept: Short Tandem Repeats

d) To analyze repetitive DNA sequences for individual identification
[Solution Description]
STRs are repetitive sequences of 2-6 base pairs that vary in number among individuals. They are amplified using PCR and separated by gel electrophoresis to create unique banding patterns for identification.

Key Concept: Polymorphism

b) A variation in a single nucleotide in the DNA sequence
[Solution Description]
Polymorphism refers to the occurrence of different forms of DNA sequences at a specific locus. Examples include Single Nucleotide Polymorphisms (SNPs), Short Tandem Repeats (STRs), and Variable Number Tandem Repeats (VNTRs). The correct option should describe a variation in the DNA sequence.

Key Concept: Southern Blotting

c) To transfer DNA fragments to a membrane for hybridization
[Solution Description]
Southern blotting is a technique used to transfer DNA fragments from a gel to a membrane after electrophoresis. This allows for hybridization with a labeled probe to detect specific DNA sequences, which is essential for visualizing unique band patterns in DNA fingerprinting.

Key Concept: Minisatellites and Microsatellites

c) Minisatellites
[Solution Description]
Minisatellites are repetitive DNA sequences that range from 10-60 base pairs in length. These sequences are highly polymorphic and are commonly used as markers in DNA fingerprinting.

Key Concept: Polymorphism

c) Variation in the number of tandem repeats (VNTRs)
[Solution Description]
The uniqueness of DNA fingerprints arises due to variations in DNA sequences known as polymorphism. These variations occur at specific loci, such as VNTRs (Variable Number Tandem Repeats), where the number of repeats differs between individuals.

Key Concept: DNA Extraction

d) Extraction of DNA
[Solution Description]
The first step in DNA fingerprinting is the extraction of DNA from the sample cells, followed by amplification, restriction digestion, and other steps.

Key Concept: Autoradiography

c) Autoradiography
[Solution Description]
Autoradiography is the technique used to detect radioactive probes on DNA fragments by exposing them to X-ray film, revealing a pattern of dark and light bands.

Key Concept: VNTR

b) Variable Number Tandem Repeats
[Solution Description]
VNTR stands for Variable Number Tandem Repeats, which are short repetitive nucleotide sequences used in DNA fingerprinting to identify individuals.

Key Concept: Ethical Concerns, Non-Coding DNA, Genome Structure

c) Insurance denial based on genetic predispositions
[Solution Description]
The HGP raised concerns about discrimination based on genetic data, like insurance companies denying coverage to individuals with high-risk genetic markers. Ethical safeguards (ELSI program) were established to address such issues.

Key Concept: SNPs, Genetic Variation, Genome Annotation

c) Provide insights into disease susceptibility and human ancestry
[Solution Description]
SNPs are single-base differences in DNA sequences that occur at specific locations across populations. They help trace evolutionary history, identify disease-linked genes, and study genetic diversity. Over 1.4 million SNPs were mapped during the HGP.

Key Concept: Human Genome Sequencing, Expressed Sequence Tags, BAC/YAC Vectors

a) Shotgun sequencing with BAC/YAC vectors
[Solution Description]
The HGP used shotgun sequencing, where DNA was fragmented and cloned into Bacterial Artificial Chromosomes (BACs) or Yeast Artificial Chromosomes (YACs). These fragments were amplified and sequenced separately, then reassembled using overlapping regions via computational methods. This approach allowed efficient large-scale sequencing.

Key Concept: Salient Features of Human Genome

a) Less than 2%.
[Solution Description]
One of the key findings of the Human Genome Project was that less than 2% of the human genome actually codes for proteins. The rest consists of non-coding sequences, regulatory elements, and repetitive DNA.

Key Concept: Goals of HGP

c) Create genetically modified humans.
[Solution Description]
The goals of the Human Genome Project included identifying all genes in human DNA, determining the sequence of 3 billion base pairs, improving data analysis tools, transferring technologies to other sectors, addressing ethical issues, and storing information in databases. "Creating genetically modified humans" was not part of the project's objectives.

Key Concept: Methodologies of HGP

b) To amplify small fragments of human DNA for easier sequencing.
[Solution Description]
The primary purpose of using BAC and YAC vectors in the Human Genome Project was to clone relatively large fragments of human DNA into bacteria or yeast, allowing for amplification of these fragments. This amplification was essential for subsequent sequencing efforts, as it provided sufficient quantities of DNA for analysis.

Key Concept: Salient Features of Human Genome

c) 99.9%
[Solution Description]
The Human Genome Project revealed that almost 99.9% of nucleotide bases are identical in all humans, leaving only 0.1% variation contributing to genetic diversity.

Key Concept: Methodologies of HGP

b) Bacterial Artificial Chromosome (BAC)
[Solution Description]
In the Human Genome Project, bacterial artificial chromosomes (BAC) were used as vectors to clone DNA fragments in bacterial hosts. Yeast artificial chromosomes (YAC) were also used but for yeast hosts.

Key Concept: Goals of Human Genome Project

d) Creating transgenic organisms with enhanced capabilities
[Solution Description]
The goals of the Human Genome Project included identifying all human genes, determining DNA base sequences, improving data analysis tools, transferring technologies to industries, addressing ethical issues, and storing information in databases. However, creating transgenic organisms was not part of its goals.

Key Concept: Feedback Repression, Co-repressor

d) It enables the repressor to bind to the operator and block transcription
[Solution Description]
Feedback repression occurs when an excess metabolite (co-repressor) binds to the repressor protein, enabling it to bind to the operator region of the operon. This blocks transcription of the structural genes involved in the biosynthesis of the metabolite, conserving cellular resources.

Key Concept: Induction, Polycistronic mRNA

b) Polycistronic, encoding multiple enzymes
[Solution Description]
The lac operon consists of three structural genes ($z$, $y$, and $a$) that are transcribed as a single unit under the control of one promoter and operator. The resulting mRNA contains coding sequences for all three enzymes ($\beta$-galactosidase, permease, and transacetylase) and is termed polycistronic mRNA.

Key Concept: Operon Concept, Repressor Protein

c) It binds to the operator region to block transcription
[Solution Description]
The repressor protein, synthesized by the regulator gene ($lac I$), binds to the operator region of the lac operon when lactose is absent. This binding prevents RNA polymerase from transcribing the structural genes ($z$, $y$, and $a$) of the operon, effectively switching off the operon.

Key Concept: Operon Concept, Structural Genes

d) The structural genes are transcribed to produce tryptophan-synthesizing enzymes.
[Solution Description]
When tryptophan levels are low, the repressor protein cannot bind to the operator because tryptophan is not available to act as a co-repressor. As a result, RNA polymerase can transcribe the structural genes involved in tryptophan synthesis, leading to increased production of tryptophan-synthesizing enzymes.

Key Concept: Feedback Repression, Constitutive Genes

d) It represses the synthesis of histidine-synthesizing enzymes.
[Solution Description]
Excess histidine acts as a co-repressor in $E. coli$. It binds to the repressor protein, enabling it to attach to the operator region of the histidine operon. This prevents RNA polymerase from transcribing the structural genes involved in histidine synthesis, leading to feedback repression of these enzymes.

Key Concept: Induction, Operon Model

c) It synthesizes a repressor molecule that controls the operator.
[Solution Description]
The regulator gene in the lac operon produces a repressor protein that binds to the operator region. This binding prevents RNA polymerase from transcribing the structural genes. When lactose is present, it acts as an inducer by binding to the repressor, causing it to release from the operator and allowing transcription to proceed.

Key Concept: Constitutive Genes, Enzyme Regulation

b) Genes that are constantly expressed and not regulated by external factors.
[Solution Description]
Constitutive genes are those that are continuously expressed under normal cellular conditions because their products (e.g., enzymes like those in glycolysis) are always required for basic metabolic functions. Unlike inducible or repressible genes, their expression is not regulated by external metabolites.

Key Concept: Feedback Repression, Tryptophan Operon

b) Tryptophan acts as a co-repressor, inhibiting transcription.
[Solution Description]
The trp operon is regulated by feedback repression. When tryptophan levels are high, tryptophan acts as a co-repressor. It binds to the repressor protein, enabling it to bind to the operator region of the trp operon, thus inhibiting transcription of the structural genes involved in tryptophan synthesis.

Key Concept: Lac Operon, Induction

b) The repressor binds to the operator, blocking transcription.
[Solution Description]
The lac operon is regulated by a repressor protein produced by the regulator gene (lacI). When lactose is absent, the repressor binds to the operator region (lacO), preventing RNA polymerase from transcribing the structural genes (lacZ), (lacY), (lacA). This ensures that the enzymes for lactose metabolism are not synthesized when they are not needed.

Key Concept: Negative Feedback Repression

c) A co-repressor that enhances repression
[Solution Description]
In the $trp$ operon, tryptophan acts as a co-repressor. When present in excess, it binds to the repressor protein, enabling it to inhibit transcription of the structural genes involved in tryptophan synthesis.

Key Concept: Induction Mechanism

d) The repressor is inactivated, allowing transcription of structural genes
[Solution Description]
Lactose acts as an inducer and binds to the repressor protein, preventing it from binding to the operator. This allows RNA polymerase to transcribe the structural genes ($z$, $y$, and $a$), leading to the synthesis of $\beta$-galactosidase, permease, and transacetylase.

Key Concept: Operon Structure

c) Regulatory gene ($lac I$)
[Solution Description]
The regulatory gene ($lac I$) in the $lac$ operon encodes the repressor protein, which binds to the operator region to regulate transcription of the structural genes.

Key Concept: Operon model

d) It controls the structural gene transcription by binding to the repressor
[Solution Description]
The operator gene serves as a regulatory switch. It controls the transcription of the structural genes in the operon by interacting with the repressor protein. When the repressor binds to the operator, it blocks RNA polymerase from transcribing the structural genes. If the repressor cannot bind (e.g., due to an inducer), transcription proceeds.

Key Concept: Feedback repression

c) The metabolite binds to the repressor, which then blocks transcription
[Solution Description]
Feedback repression occurs when an excess metabolite acts as a co-repressor. It binds to the repressor protein, which then binds to the operator region of the operon, blocking RNA polymerase from transcribing the genes involved in the synthesis of that metabolite. Thus, the biosynthesis enzymes are no longer produced, conserving cellular resources.

Key Concept: Induction

c) Lactose binds to the repressor protein, preventing it from blocking transcription
[Solution Description]
Lac operon is turned on when lactose is present. The lactose acts as an inducer, binds to the repressor protein, and prevents it from binding to the operator region. This allows RNA polymerase to transcribe the structural genes coding for $\beta$-galactosidase, leading to a significant increase in enzyme production.