Molecular Basis of Inheritance – CBSE NCERT Study Resources

We studied that DNA replication is semi-conservative, where each new DNA molecule has one parental and one newly synthesized strand. This was proven by Meselson and Stahl using E. coli and nitrogen isotopes.

• Meselson-Stahl experiment used ¹⁵N (heavy) and ¹⁴N (light) isotopes to label DNA.
• After one generation, DNA had intermediate density, confirming semi-conservative replication.

This mechanism prevents mutations by using the parental strand as a template, ensuring accuracy. Errors are minimized by proofreading by DNA polymerase.

Understanding this process aids in genetic engineering and cancer research, where DNA replication errors play a key role.

The lac operon, studied in our textbook, regulates lactose metabolism in E. coli. It consists of structural genes (lacZ, lacY, lacA) and regulatory sequences.

• In the absence of lactose, the lac repressor binds to the operator, blocking transcription.
• When lactose is present, it acts as an inducer, inactivating the repressor and allowing enzyme synthesis.

This model shows how prokaryotes efficiently regulate metabolism. It also explains how gene expression responds to environmental changes.

Studying operons helps in designing synthetic biology systems, like insulin production using recombinant DNA technology.

Transcription is the synthesis of RNA from DNA. Our textbook shows that prokaryotes and eukaryotes differ significantly in this process.

• Initiation: Prokaryotes use sigma factors, while eukaryotes require transcription factors (TFs) and RNA polymerase II.
• Elongation: Eukaryotic mRNA undergoes splicing to remove introns, unlike prokaryotic mRNA.
• Termination: Prokaryotes use rho-dependent or independent mechanisms, while eukaryotes rely on polyadenylation signals.

These differences reflect the complexity of eukaryotic gene regulation, allowing for alternative splicing and post-transcriptional modifications.

Understanding these mechanisms aids in developing gene therapies targeting specific transcriptional errors.

The central dogma states that genetic information flows from DNA → RNA → protein. However, retroviruses like HIV reverse this flow.

• Retroviruses use reverse transcriptase to convert RNA into DNA, which integrates into the host genome.
• This challenges the unidirectional flow proposed by Crick.

Exceptions like retroviruses show that biological systems are more flexible than originally thought. This has implications for viral treatments.

Studying reverse transcription helps in designing antiretroviral drugs and understanding viral evolution.

The HGP, completed in 2003, mapped the entire human genome. Our textbook highlights its role in identifying genes linked to diseases.

• HGP revealed ~20,500 genes, enabling the study of mutations causing disorders like cystic fibrosis.
• It also discovered SNPs (single nucleotide polymorphisms) associated with diseases like diabetes.

This project revolutionized personalized medicine by enabling gene therapy and early diagnosis of genetic conditions.

Ongoing research uses HGP data to develop CRISPR-based treatments and predict disease risks.

The central dogma states genetic information flows from DNA → RNA → Protein. It was proposed by Francis Crick in 1958.

• In retroviruses like HIV, reverse transcriptase converts RNA to DNA, violating the dogma.
• Prions replicate without nucleic acids, another exception.

These exceptions show genetic information flow is more complex. However, the dogma remains foundational for most organisms.

Studying exceptions helps develop antiviral drugs targeting reverse transcriptase.

Transcription is the synthesis of RNA from DNA. Prokaryotes (e.g., E. coli) and eukaryotes (e.g., humans) differ significantly.

• Location: Prokaryotes lack nuclei; transcription occurs in cytoplasm. Eukaryotes transcribe in nuclei.
• Processing: Eukaryotic pre-mRNA undergoes splicing; prokaryotic mRNA does not.
• Initiation: Prokaryotes use sigma factors; eukaryotes use transcription factors (TFs).

These differences reflect cellular complexity. Eukaryotic processing allows alternative splicing, increasing protein diversity.

Understanding transcription aids gene therapy development.

The lac operon in E. coli controls lactose metabolism. It consists of promoter, operator, and structural genes (lacZ, lacY, lacA).

• Without lactose, the repressor binds the operator, blocking transcription.
• Lactose acts as an inducer, inactivating the repressor and allowing transcription.

This model demonstrates negative regulation. It ensures enzymes are only produced when needed, conserving energy.

Operon studies help design synthetic biology systems.

The genetic code is the set of rules by which DNA/RNA sequences are translated into proteins. It is nearly universal.

• Triplet: Codons are 3-nucleotide sequences (e.g., AUG for methionine).
• Degenerate: Multiple codons code for one amino acid (e.g., UCU, UCC both code for serine).
• Non-overlapping: Each nucleotide is part of only one codon.

These features ensure accurate protein synthesis. Mutations may cause diseases like sickle-cell anemia.

Deciphering the code aids in genetic engineering.

Hershey and Chase (1952) used radioactive isotopes to prove DNA, not protein, carries genetic information.

• They labeled phage DNA with P³² and protein with S³⁵.
• Only P³² entered bacterial cells during infection, confirming DNA’s role.

This experiment resolved the debate about genetic material. It paved the way for molecular genetics.

Their work underpins modern biotechnology, like CRISPR.

Exons are coding sequences, while introns are non-coding and spliced out during mRNA processing.

• In humans, the dystrophin gene has 79 exons; mutations cause muscular dystrophy.
• Alternative splicing combines exons differently, producing multiple proteins from one gene (e.g., tropomyosin).

This mechanism allows ~20,000 genes to encode ~100,000 proteins, enhancing complexity.

Studying splicing aids in treating genetic disorders.

DNA fingerprinting identifies individuals by analyzing VNTRs (Variable Number Tandem Repeats), which are unique to each person.

• In forensics, it matches crime scene DNA to suspects (e.g., the 1986 Colin Pitchfork case).
• In paternity tests, child DNA is compared to parents’ to confirm biological relationships.

This technique is highly accurate (1 in a billion chance of error).

Advances like PCR have made fingerprinting faster and cheaper.

Transposons (jumping genes) are mobile DNA sequences that can change position within a genome.

• They cause mutations by inserting into genes (e.g., Alu elements in humans).
• In maize, Barbara McClintock showed transposons alter kernel color.

Transposons drive evolution by creating genetic diversity but can also cause diseases like hemophilia.

Gene therapy may target transposon-related disorders.

The HGP (1990-2003) mapped all human genes to understand genetic diseases and evolution.

• Found only ~20,000 genes, fewer than expected.
• Identified genes linked to diseases like BRCA1 (breast cancer).

HGP revolutionized medicine but raised ethical issues like genetic privacy.

It enables personalized medicine and CRISPR-based therapies.

The process of DNA replication in prokaryotes is semi-conservative and involves several key enzymes. Here's a step-by-step explanation:

• Initiation: Replication begins at the origin of replication (ori), where the enzyme helicase unwinds the DNA double helix, forming a replication fork. Single-stranded binding proteins (SSBs) stabilize the unwound strands.
• Elongation: The enzyme DNA polymerase III adds nucleotides to the growing DNA strand in the 5'→3' direction, using the parental strand as a template. RNA primase synthesizes a short RNA primer to initiate DNA synthesis.
• Termination: Replication ends when the replication forks meet, and DNA polymerase I replaces RNA primers with DNA. DNA ligase seals the gaps between Okazaki fragments on the lagging strand.

Differences from eukaryotic replication:
1. Prokaryotes have a single origin of replication, while eukaryotes have multiple.
2. Prokaryotic DNA polymerase III is faster than eukaryotic DNA polymerases.
3. Eukaryotic DNA is linear and requires telomerase to prevent shortening, unlike circular prokaryotic DNA.

The central dogma explains the flow of genetic information: DNA → RNA → Protein. Here's a breakdown:

• Transcription: In the nucleus, RNA polymerase synthesizes mRNA from a DNA template strand.
  1. Initiation: RNA polymerase binds to the promoter region of DNA.
  2. Elongation: Nucleotides are added complementary to the DNA strand (A-U, T-A, G-C).
  3. Termination: mRNA detaches after reaching the terminator sequence.
• Translation: Occurs in the cytoplasm at ribosomes.
  1. Initiation: mRNA binds to the ribosome; the start codon (AUG) is recognized.
  2. Elongation: tRNA brings amino acids, forming a polypeptide chain.
  3. Termination: A stop codon signals the release of the protein.

Significance: This process ensures accurate protein synthesis, which is vital for cellular functions, growth, and heredity. Errors can lead to mutations or diseases.

The lac operon is a gene regulatory system in E. coli that controls lactose metabolism. It consists of:

• Structural genes: lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase).
• Regulatory elements: Promoter, operator, and repressor gene (lacI).

Regulation:
1. Absence of lactose: The repressor protein binds to the operator, blocking RNA polymerase → no transcription.
2. Presence of lactose: Lactose acts as an inducer, binding to the repressor and inactivating it. RNA polymerase transcribes the genes, producing enzymes to metabolize lactose.

Diagram (description):
1. Label the lacI gene, promoter, operator, and structural genes (lacZYA).
2. Show the repressor bound to the operator (no lactose) and detached (with lactose).
3. Indicate mRNA synthesis during active transcription.

Significance: This model demonstrates gene regulation in prokaryotes, conserving energy by producing enzymes only when needed.

Transcription is the process by which the genetic information from DNA is copied into RNA. In prokaryotes, it occurs in three main stages: initiation, elongation, and termination.

1. Initiation: The RNA polymerase enzyme binds to the promoter region on the DNA, which is a specific sequence marking the start of a gene. The promoter helps in the correct positioning of RNA polymerase. The enzyme then unwinds the DNA helix, exposing the template strand.

2. Elongation: RNA polymerase synthesizes the RNA strand in the 5' to 3' direction, using the DNA template strand. Nucleotides are added complementary to the DNA template (A-U, T-A, G-C, C-G). The DNA helix rewinds behind the enzyme.

3. Termination: Transcription stops when RNA polymerase encounters a terminator sequence. The newly formed RNA strand is released, and the RNA polymerase detaches from the DNA.

Role of RNA polymerase: It catalyzes the formation of phosphodiester bonds between nucleotides and ensures accurate copying of the genetic code.

Role of promoter: It signals where transcription begins and ensures RNA polymerase binds correctly.

Diagram: (A labeled diagram showing DNA unwinding, RNA polymerase, promoter, and the growing RNA strand should be drawn for full marks.)

The Meselson and Stahl experiment (1958) demonstrated that DNA replication follows a semi-conservative mechanism, where each new DNA molecule consists of one parental strand and one newly synthesized strand.

The experiment involved:

• Growing E. coli in a medium containing heavy nitrogen (¹⁵N) to label parental DNA.
• Transferring the bacteria to a light nitrogen (¹⁴N) medium and allowing replication.
• Analyzing DNA density using cesium chloride (CsCl) gradient centrifugation after each generation.

Results:

• Generation 0 (¹⁵N-DNA): Only heavy DNA (one band).
• Generation 1 (hybrid DNA): Intermediate-density DNA (one band), proving semi-conservative replication.
• Generation 2: Two bands (light and hybrid), confirming the mechanism.

This experiment conclusively disproved conservative and dispersive models, establishing the semi-conservative model as the universal mode of DNA replication.

The Meselson and Stahl experiment (1958) provided evidence for the semi-conservative replication of DNA. Here’s how it was conducted and interpreted:

1. Experimental Setup:
- E. coli bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, ensuring all DNA was labeled with ¹⁵N.
- The bacteria were then transferred to a medium with light nitrogen (¹⁴N) and allowed to replicate.

2. Observations:
- After one generation, the DNA formed a single band of intermediate density (hybrid ¹⁵N-¹⁴N DNA) in a cesium chloride density gradient.
- After two generations, two bands appeared: one intermediate and one light (¹⁴N-¹⁴N DNA).

3. Conclusion:
- The results matched the semi-conservative model, where each new DNA molecule consists of one parental strand (old) and one newly synthesized strand.
- This disproved the conservative (parental DNA remains intact) and dispersive (mixed fragments) models.

Significance: This experiment confirmed Watson and Crick’s hypothesis about DNA replication, a cornerstone of molecular biology.

Transcription is the process by which the genetic information from DNA is copied into RNA. In prokaryotes, this process involves the enzyme RNA polymerase, which synthesizes RNA from a DNA template.

The key steps are:

• Initiation: RNA polymerase binds to the promoter region, a specific DNA sequence upstream of the gene. The sigma factor helps the enzyme recognize the promoter.
• Elongation: RNA polymerase unwinds the DNA and synthesizes RNA in the 5' to 3' direction, using one strand (template strand) as a guide.
• Termination: Transcription stops when RNA polymerase reaches the terminator region, causing the RNA transcript and enzyme to detach.

The promoter ensures accurate initiation, while the terminator signals the end of transcription. Prokaryotic transcription is efficient due to the absence of a nucleus, allowing simultaneous transcription and translation.

In prokaryotes, transcription is the process of synthesizing RNA from a DNA template. The key enzyme involved is RNA polymerase, which binds to a specific region on the DNA called the promoter. The promoter signals the start of transcription and helps RNA polymerase recognize where to begin.

Steps in prokaryotic transcription:

• Initiation: RNA polymerase binds to the promoter with the help of a sigma factor, forming a holoenzyme complex.
• Elongation: RNA polymerase unwinds the DNA and synthesizes RNA in the 5' to 3' direction, using one strand (template strand) as a guide.
• Termination: Transcription stops when RNA polymerase encounters a terminator sequence, leading to the release of the RNA transcript.

Differences in eukaryotes:

• Eukaryotes have three types of RNA polymerases (I, II, III) for different RNAs, while prokaryotes have only one.
• Eukaryotic transcription involves transcription factors and enhancers for regulation, unlike prokaryotes.
• Post-transcriptional modifications (e.g., splicing, capping, tailing) occur in eukaryotes but not in prokaryotes.

The Meselson and Stahl experiment (1958) demonstrated that DNA replication is semi-conservative, meaning each new DNA molecule consists of one parental strand and one newly synthesized strand.

Experimental Procedure:

• E. coli bacteria were grown in a medium containing heavy nitrogen (¹⁵N), making their DNA denser.
• These bacteria were then transferred to a medium with light nitrogen (¹⁴N) and allowed to replicate.
• DNA samples were extracted after each generation and analyzed using density gradient centrifugation.

Observations & Conclusions:

• First generation: DNA formed a single band of intermediate density (hybrid of ¹⁵N and ¹⁴N), ruling out conservative replication.
• Second generation: Two bands appeared—one intermediate and one light (only ¹⁴N), confirming semi-conservative replication.

Diagram: (Illustrate the centrifugation tubes showing bands for ¹⁵N, hybrid, and ¹⁴N DNA after generations.)

This experiment conclusively proved that DNA replication is semi-conservative, as predicted by Watson and Crick.

The process of DNA replication in eukaryotes is a semi-conservative mechanism where each parental DNA strand serves as a template for the synthesis of a new complementary strand. This ensures the accurate transmission of genetic information during cell division. The key steps and enzymes involved are:

• Initiation: Replication begins at specific sites called origins of replication, where the helicase enzyme unwinds the double helix, creating replication forks. Single-stranded binding proteins (SSBs) stabilize the unwound strands.
• Primer Synthesis: The enzyme primase synthesizes short RNA primers complementary to the DNA template, providing a starting point for DNA synthesis.
• Elongation: DNA polymerase III adds nucleotides to the 3' end of the RNA primer, synthesizing the new strand in the 5'→3' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as Okazaki fragments.
• Proofreading and Repair: DNA polymerase I replaces RNA primers with DNA, and DNA ligase seals the gaps between Okazaki fragments. DNA polymerase also proofreads the newly synthesized strand, correcting mismatches to maintain fidelity.

The high fidelity of DNA replication is ensured by:
1. The complementary base pairing rule (A-T, G-C), which reduces errors.
2. The proofreading activity of DNA polymerase, which removes incorrect nucleotides.
3. Mismatch repair mechanisms post-replication, which correct errors missed during synthesis.

This meticulous process ensures that genetic information is passed accurately to daughter cells, maintaining genetic stability across generations.