DNA Replication, Mutation, and DNA Repair: Study Notes for Genetics

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DNA Replication

Introduction to DNA Replication

DNA replication is a fundamental process in genetics, ensuring that genetic information is accurately passed from one generation to the next. The process involves the synthesis of a new DNA strand using an existing strand as a template.

Key Enzyme: DNA polymerase catalyzes the synthesis of new DNA strands.
Directionality: New DNA is always synthesized in the 5' to 3' direction.
Replication Origin (ORI): Replication begins at specific DNA sequences called origins of replication.
Bidirectional Process: Replication proceeds in both directions from the origin, forming two replication forks.

Historical Perspective: Arthur Kornberg's Discoveries (1956)

Demonstrated DNA synthesis in cell-free extracts of bacteria.
Identified DNA polymerase as the enzyme responsible for DNA replication.
Showed that the building blocks of DNA are energy-rich deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP).
Proved that DNA polymerase requires a DNA template to determine the sequence of the new strand.
Concluded that DNA, not protein, is the direct template for its own synthesis.

Nomenclature: Nucleosides and Nucleotides

Nucleoside: A base (purine or pyrimidine) covalently linked to a 5-carbon sugar. If the sugar is ribose, it is a ribonucleoside; if deoxyribose, a deoxyribonucleoside.
Nucleotide: A nucleoside with one or more phosphate groups attached to the 5' carbon of the sugar. Nucleotides are the monomers of DNA and RNA.
Examples: dATP (deoxyadenosine triphosphate), ATP (adenosine triphosphate).

Mechanism of DNA Replication

Each DNA strand serves as a template for the synthesis of a new complementary strand.
Replication is semi-conservative: each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.
Replication starts at the origin of replication (ORI) and proceeds bidirectionally.

Leading and Lagging Strands

Leading Strand: Synthesized continuously in the 5' to 3' direction toward the replication fork.
Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments) away from the replication fork.
Each Okazaki fragment on the lagging strand requires a new RNA primer.
On the leading strand, only one RNA primer is needed at the origin.

Enzymes Involved in DNA Replication

DNA Polymerase: Synthesizes new DNA strands; has 3' to 5' exonuclease proofreading activity.
Primase: Synthesizes short RNA primers required for DNA polymerase to begin synthesis.
Helicase: Unwinds the DNA double helix at the replication fork.
Single-Stranded Binding Proteins (SSB): Stabilize unwound DNA strands.
DNA Ligase: Joins Okazaki fragments on the lagging strand by sealing nicks in the sugar-phosphate backbone.

Proofreading and Fidelity

DNA polymerase has a 3' to 5' exonuclease activity that removes incorrectly paired nucleotides, ensuring high fidelity of DNA replication.

Mutations

Definition and Impact

A mutation is an alteration in the nucleotide sequence of a gene. Mutations can affect the structure and function of proteins encoded by the gene, potentially leading to altered phenotypes or disease.

Mutations in DNA affect all proteins expressed from the mutant gene, unlike translation errors, which affect only a few proteins.

Types of Mutations

Synonymous (Silent) Mutation: A change in the nucleotide sequence that does not alter the amino acid sequence due to the degeneracy of the genetic code. No effect on protein function.
Missense Mutation: A nucleotide substitution that results in a different amino acid in the protein. May or may not affect protein function.
Nonsense Mutation: A nucleotide substitution that creates a premature stop codon, leading to a truncated, usually inactive, protein.
Frameshift Mutation: Addition or deletion of nucleotides that changes the reading frame, often resulting in a nonfunctional protein due to extensive missense and premature stop codons.

DNA Repair Mechanisms

Overview

Cells have evolved multiple repair systems to correct DNA damage and maintain genetic stability. The main repair mechanisms include base excision repair, nucleotide excision repair, and mismatch repair.

Repair System	Enzymes/Proteins	Repair System	Enzymes/Proteins
Base excision	DNA glycosylase, AP endonuclease, DNA polymerase I, DNA ligase	Mismatch repair	Dam methylase, MutS, MutL, MutH, Exonuclease, DNA helicase II, SSB protein, DNA polymerase III, DNA ligase
Nucleotide excision	UvrA, UvrB, UvrC, DNA polymerase I, DNA ligase

I. Base Excision Repair (BER)

Base excision repair corrects small, non-helix-distorting base lesions, such as those caused by deamination or alkylation.

Step 1: DNA glycosylase recognizes and removes the damaged base, creating an apurinic/apyrimidinic (AP) site.
Step 2: AP endonuclease cleaves the DNA backbone at the AP site.
Step 3: DNA polymerase I fills in the gap with the correct nucleotide.
Step 4: DNA ligase seals the nick in the sugar-phosphate backbone.
Example: Deamination of cytosine to uracil is corrected by BER.

II. Nucleotide Excision Repair (NER)

Nucleotide excision repair removes bulky, helix-distorting lesions such as pyrimidine dimers caused by UV light.

Step 1: A multienzyme complex scans DNA for distortions.
Step 2: Endonuclease cleaves the DNA on both sides of the lesion.
Step 3: DNA helicase removes the damaged fragment.
Step 4: DNA polymerase I fills in the gap.
Step 5: DNA ligase seals the remaining nick.
Example: Thymine dimers formed by UV light are repaired by NER.

III. Mismatch Repair (MMR)

Mismatch repair corrects errors introduced during DNA replication that escape proofreading by DNA polymerase.

Step 1: MutS protein recognizes and binds to the mismatched base pair.
Step 2: MutL is recruited, activating MutH, which binds to GATC sequences and cleaves the unmethylated (new) DNA strand.
Step 3: Exonuclease removes the segment containing the mismatch.
Step 4: DNA polymerase III fills in the gap.
Step 5: DNA ligase seals the nick.
In E. coli: The template strand is methylated, allowing the repair system to distinguish it from the newly synthesized strand.
In eukaryotes: The mechanism for strand discrimination is less well understood.

Summary Table: Types of Mutations

Type of Mutation	Definition	Effect on Protein
Synonymous (Silent)	Nucleotide change does not alter amino acid	No effect
Missense	Nucleotide change alters amino acid	May alter function
Nonsense	Nucleotide change creates stop codon	Truncated, usually inactive protein
Frameshift	Insertion/deletion changes reading frame	Nonfunctional protein

Key Equations and Concepts

Direction of DNA Synthesis:

Base Pairing:

Okazaki Fragments: Short DNA fragments synthesized on the lagging strand.

Example: DNA Replication in E. coli

Replication begins at a single origin (oriC) and proceeds bidirectionally.
DNA polymerase III is the main replicative enzyme.
DNA ligase joins Okazaki fragments on the lagging strand.

Additional info:

In eukaryotes, multiple origins of replication exist on each chromosome to ensure rapid and complete DNA replication.
Defects in DNA repair mechanisms can lead to genetic diseases and increased cancer risk.