DNA REPLICATION, TRANSCRIPTION AND TRANSLATION Part-IV

 

DNA REPLICATION, TRANSCRIPTION AND TRANSLATION Part-IV

Introduction

DNA replication is an extraordinarily accurate process. The human genome contains approximately 3.2 billion base pairs, yet almost every daughter cell receives an almost identical copy of DNA after each cell division.

This remarkable accuracy is achieved because cells possess:

• High-fidelity DNA polymerases
• Proofreading mechanisms
• DNA repair systems
• Cell-cycle checkpoints

Without these protective mechanisms, mutations would accumulate rapidly, causing genetic diseases, developmental abnormalities, ageing, and cancer.

How Accurate Is DNA Replication?

·       DNA polymerase alone makes approximately: 1 error in 10⁵ nucleotides

·       After proofreading: 1 error in 10⁷ nucleotides

·       After DNA repair: Approximately 1 error in 10⁹–10¹⁰ nucleotides

This makes DNA replication one of the most accurate biological processes.

Why Is High Fidelity Necessary?

Accurate DNA replication ensures:

• Genetic stability
• Correct protein synthesis
• Normal growth
• Proper embryonic development
• Prevention of inherited disorders
• Prevention of cancer

Sources of DNA Damage

DNA is continuously damaged by both internal and external factors.

Endogenous (Internal)

• Reactive oxygen species (ROS)
• Replication errors
• Spontaneous base loss (depurination)
• Spontaneous deamination

Exogenous (External)

• Ultraviolet (UV) radiation
• X-rays
• Gamma rays
• Chemical mutagens
• Environmental toxins
• Tobacco smoke
• Certain viruses

Proofreading

Definition

Proofreading is the immediate correction of wrongly inserted nucleotides during DNA replication.

Enzyme Responsible- DNA polymerase

Many replicative DNA polymerases possess: 3' → 5' exonuclease activity

Mechanism

Step 1

Wrong nucleotide inserted.

↓

Step 2

DNA polymerase detects mismatch.

↓

Step 3

Incorrect nucleotide removed.

↓

Step 4

Correct nucleotide inserted.

↓

Replication continues.

Importance of Proofreading

• Prevents mutations
• Maintains genome stability
• Increases replication fidelity
• Ensures correct inheritance

DNA repair

Even after proofreading, some mistakes remain. Therefore, cells possess specialized repair systems.

Types of DNA Repair

Major DNA repair mechanisms include:

1. Mismatch Repair (MMR)
2. Base Excision Repair (BER)
3. Nucleotide Excision Repair (NER)
4. Double-Strand Break Repair (DSBR)

1. Mismatch Repair (MMR)

Function

Corrects mismatched bases that escape proofreading.

Example- A paired with C, instead of- A paired with T

Steps

Mismatch detected

↓

Incorrect nucleotide removed

↓

Gap filled by DNA polymerase

↓

DNA ligase seals strand

Importance

Prevents permanent mutations after replication.

2. Base Excision Repair (BER)

Repairs

Small damaged bases such as:

• Oxidized bases
• Deaminated bases
• Alkylated bases

Steps

Damaged base recognized

↓

DNA glycosylase removes damaged base

↓

AP endonuclease cuts DNA backbone

↓

DNA polymerase inserts correct nucleotide

↓

DNA ligase seals nick

3. Nucleotide Excision Repair (NER)

Repairs- Large DNA lesions. Especially: UV-induced thymine dimers.

Mechanism

Damaged DNA segment removed

↓

DNA polymerase synthesizes replacement DNA

↓

DNA ligase joins strand

Clinical Importance

Defect causes: Xeroderma pigmentosum

Patients show:

• Extreme UV sensitivity
• Early skin cancers
• Defective nucleotide excision repair

4. Double-Strand Break Repair

Double-strand DNA breaks are among the most dangerous forms of DNA damage. Cells repair them mainly by:

Homologous Recombination (HR)

• Uses the sister chromatid as a template.
• Highly accurate.
• Predominant in S and G₂ phases.

Non-Homologous End Joining (NHEJ)

• Directly joins broken DNA ends.
• Faster but more error-prone.

Cell-Cycle Checkpoints

Cells do not divide until DNA replication is complete and DNA damage is repaired. Important checkpoints include:

G₁/S Checkpoint

• Verifies DNA integrity before replication.

Intra-S Checkpoint

• Monitors ongoing DNA replication.
• Slows replication if damage is detected.

G₂/M Checkpoint

• Ensures DNA replication is complete.
• Prevents entry into mitosis if DNA remains damaged.

Regulation Of DNA Replication

Replication occurs:

• Only once during each cell cycle.
• Only during the S phase.

This prevents over-replication or incomplete replication.

DNA Replication Inhibitors

Certain drugs inhibit DNA replication and are widely used in medicine.

Drug/Class	Mechanism	Clinical Use
Quinolones (e.g., ciprofloxacin)	Inhibit bacterial DNA gyrase (Topoisomerase II)	Bacterial infections
Etoposide	Inhibits Topoisomerase II	Cancer chemotherapy
Doxorubicin	Interferes with topoisomerase and DNA	Cancer chemotherapy
Acyclovir	Inhibits viral DNA polymerase	Herpes virus infections
Zidovudine (AZT)	Inhibits reverse transcriptase	HIV infection

 

Replication Errors and Mutations

If DNA damage is not repaired:

↓

Mutation occurs

↓

Altered protein

↓

Disease or altered phenotype

Some mutations are neutral, some harmful, and a few beneficial.

DNA Replication And Cancer

Cancer develops due to the accumulation of mutations in genes controlling:

• Cell division
• DNA repair
• Apoptosis

Failure of proofreading and DNA repair greatly increases cancer risk.

Complete Flow Chart Of Dna Replication

Origin of Replication

│

▼

Helicase Unwinds DNA

│

▼

Topoisomerase Removes Supercoils

│

▼

SSB Proteins Stabilize DNA

│

▼

Primase Synthesizes RNA Primer

│

▼

DNA Polymerase Synthesizes New DNA

│

▼

Leading Strand ───────────────► Continuous

Lagging Strand ───────────────► Okazaki Fragments

│

▼

Primer Removal

│

▼

Gap Filling

│

▼

DNA Ligase Seals Nicks

│

▼

Proofreading

│

▼

DNA Repair (if required)

│

▼

Two Identical DNA Molecules

Complete Summary of DNA Replication

Feature	Description
Nature	Semiconservative
Direction	5′ → 3′ synthesis
Template read	3′ → 5′
Occurs during	S phase
Replication type	Bidirectional
Leading strand	Continuous
Lagging strand	Discontinuous
Okazaki fragments	Present only on lagging strand
Main bacterial replicative enzyme	DNA Polymerase III
Primer removal (bacteria)	DNA Polymerase I
Primer removal (eukaryotes)	RNase H and FEN1
Joining enzyme	DNA Ligase
Unwinding enzyme	Helicase
Supercoil removal	Topoisomerase
Primer synthesis	Primase
Fidelity	~1 error per 10⁹–10¹⁰ nucleotides after repair

 

Important Scientists

Scientist	Contribution
James Watson	Double-helix model
Francis Crick	Double-helix model and Central Dogma
Matthew Meselson & Franklin Stahl	Proved semiconservative replication
Arthur Kornberg	Discovered DNA Polymerase I
Reiji Okazaki	Discovered Okazaki fragments
J. Herbert Taylor	Demonstrated semiconservative replication in eukaryotes

 

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Memory Tricks for Exam

H-P-P-L Sequence

H → Helicase (opens DNA)

↓

P → Primase (makes primer)

↓

P → Polymerase (extends DNA)

↓

L → Ligase (joins fragments)

Leading vs Lagging

Leading = Long continuous

Lagging = Little pieces (Okazaki fragments)

Direction Rule

DNA Polymerase Always Adds to the 3′ End

Therefore,

DNA synthesis is always 5′ → 3′.

COMPLETE CHAPTER QUICK REVISION

• DNA replication is semiconservative.
• Occurs during the S phase.
• Begins at the origin of replication.
• Helicase unwinds DNA.
• Topoisomerase relieves supercoiling.
• SSB proteins stabilize single strands.
• Primase synthesizes RNA primers.
• DNA Polymerase III synthesizes bacterial DNA.
• DNA is synthesized only in the 5′ → 3′ direction.
• Leading strand is continuous.
• Lagging strand forms Okazaki fragments.
• DNA Polymerase I (bacteria) removes RNA primers.
• RNase H/FEN1 remove primers in eukaryotes.
• DNA Ligase seals nicks.
• Telomerase maintains chromosome ends.
• Proofreading and DNA repair maintain high replication fidelity.

Summary

• DNA replication is semiconservative, bidirectional, and template-directed.
• DNA polymerases require a primer and synthesize DNA only in the 5′ → 3′ direction.
• Leading and lagging strands arise because the two DNA strands are antiparallel.
• Okazaki fragments are synthesized on the lagging strand and joined by DNA ligase.
• High fidelity of DNA replication is achieved through complementary base pairing, proofreading, and DNA repair mechanisms.