DNA Replication: Mechanisms and Repair

DNA Replication

Overview of DNA Functions

DNA is essential for life, serving as the blueprint for all cellular activities. Its functions include:

• Replication: The process of making a copy of DNA prior to cell division, ensuring genetic continuity.

• Transcription: Producing a working copy (RNA) of a single gene that can leave the nucleus. Additional info: This is the first step in protein synthesis.

• Translation: The production of proteins as specified by the gene's instructions. Additional info: Occurs in the cytoplasm at ribosomes.

Importance of DNA Replication

DNA replication ensures that every cell in an organism contains an identical set of genetic instructions. This process occurs during the S-phase of interphase, before cell division (mitosis).

• Each daughter cell receives a complete set of DNA.

• All DNA must be duplicated, not just parts.

Semi-Conservative Replication

DNA replication is described as semi-conservative because each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

• Original DNA acts as a template.

• New strands are formed by adding complementary nucleotides.

Steps of DNA Replication

Step 1: DNA Unwinds

• Helicase: An enzyme that "unzips" the DNA helix at multiple locations, breaking hydrogen bonds between nucleotide pairs.

• Single-stranded binding proteins: Prevent the separated strands from re-forming the helix.

Step 2: Formation of Replication Bubbles

• Multiple helicase enzymes create several replication bubbles along the DNA strand.

• Replication occurs simultaneously at these bubbles, speeding up the process.

Step 3: Complementary Pairing

• Primase: Inserts an RNA primer to mark the starting point for DNA synthesis.

• DNA Polymerase: Binds at the primer and adds new nucleotides to the template strand, following base pairing rules:

  • A pairs with T

  • T pairs with A

  • C pairs with G

  • G pairs with C

• Replication proceeds until the entire strand is copied.

• Replication occurs differently on each strand:

  • Leading strand (3’): Synthesized continuously in the direction of the replication fork.

  • Lagging strand (5’): Synthesized in short segments called Okazaki fragments, opposite to the direction of the fork.

Step 4: Finishing

• Ligase: Enzyme that repairs the sugar-phosphate backbone and connects gaps between Okazaki fragments on the lagging strand.

• Each completed DNA strand recoils into its helix structure.

Summary Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds and separates DNA strands
Single-stranded binding proteins	Stabilize separated strands
Primase	Adds RNA primer
DNA Polymerase	Adds new nucleotides to template strand
Ligase	Joins Okazaki fragments, repairs backbone

Result of Replication

• Two identical daughter DNA molecules are formed.

• Each consists of half original and half new DNA.

• The process takes approximately 7–8 hours.

DNA Mutation and Repair

Mutations During Replication

Errors can occur during the rapid replication of billions of base pairs. Mutations may arise from:

• Biological exposures: e.g., Human Papilloma Virus (HPV)

• Chemical substances: e.g., pesticides, household chemicals

• Physical substances: e.g., radiation, heat

Checking for Mutations

• Occurs during the G2 phase of the cell cycle.

• If major changes are not detected and repaired, cell function may be compromised.

• Mutations can be life-threatening or prevent DNA replication.

• Some mutations contribute to genetic diversity and are not necessarily harmful.

DNA Repair Mechanisms

The body uses specialized enzymes to correct mistakes:

• "READ" enzymes: Review DNA and recognize mismatched base pairs.

• "CUT" enzymes: Remove mismatched base pairs.

• "REPLACE" enzymes: Insert the correct nucleotide(s).

• Repair enzymes are most active in the hour between replication and mitosis.

• Repair is efficient unless the damage is too severe.

Summary Table: DNA Repair Enzymes

Enzyme Type	Function
READ	Detect mismatches
CUT	Remove incorrect bases
REPLACE	Add correct bases

Key Terms and Concepts

• Replication bubble: Region where DNA is actively being unwound and replicated.

• Okazaki fragments: Short DNA segments synthesized on the lagging strand.

• Mutation: A change in the DNA sequence, which can be caused by errors or environmental factors.

• Ligase: Enzyme that joins DNA fragments together.

Example: Semi-Conservative Replication

After replication, each daughter DNA molecule contains one strand from the original DNA and one newly synthesized strand. This ensures genetic consistency across cell generations.

Relevant Equation: Base Pairing

The base pairing rules can be summarized as:

$\text{A} \leftrightarrow \text{T}$

$\text{C} \leftrightarrow \text{G}$

Where A = Adenine, T = Thymine, C = Cytosine, G = Guanine.

Additional info: DNA replication is fundamental to cell reproduction and differentiation, and errors in this process can lead to genetic diseases or contribute to evolution.