DNA Replication and Repair: Human Biology Study Notes

DNA Replication

Overview of DNA Functions

DNA is essential for life, serving as the blueprint for all cellular activities. Its three main functions are replication, transcription, and translation. Replication ensures that each new cell receives an identical copy of DNA during cell division.

• Replication: The process of making a copy of DNA prior to cell division, specifically during the S-phase of Interphase.

• Transcription: Producing a working copy (RNA) of a single gene that can leave the nucleus.

• Translation: The production of proteins required by specific genes.

• Example: Before mitosis, a cell duplicates its DNA so both daughter cells inherit identical genetic material.

Principles of DNA Replication

DNA replication is a semi-conservative process, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. This ensures genetic consistency across cell generations.

• Template Mechanism: The original DNA acts as a template for the formation of new strands.

• Semi-Conservative Replication: Each daughter DNA molecule contains half the original DNA and half new DNA.

• Timing: Occurs during S-phase before cell division.

Steps of DNA Replication

DNA replication involves several coordinated steps, each facilitated by specific enzymes.

1. Unwinding: The enzyme helicase unwinds and "unzips" the DNA helix at multiple locations, breaking hydrogen bonds between nucleotide pairs. Single-stranded binding proteins stabilize the separated strands.

2. Formation of Replication Bubbles: Multiple helicase enzymes create several replication bubbles along the DNA, allowing replication to occur simultaneously at different sites.

3. Complimentary Pairing: Primase inserts an RNA primer to mark the starting point. DNA polymerase binds at the primer and adds new complementary nucleotides to the template strand. Base pairing rules:

   • New A pairs with old T

   • New T pairs with old A

   • New C pairs with old G

   • New G pairs with old C

4. Strand Synthesis: Leading strand (3') is synthesized continuously. Lagging strand (5') is synthesized in short segments called Okazaki fragments due to its opposite orientation.

5. Finishing: Ligase repairs the sugar-phosphate backbone and connects gaps in Okazaki fragments. The DNA recoils into its double 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	Inserts RNA primer
DNA Polymerase	Adds new nucleotides to template strand
Ligase	Repairs backbone, joins Okazaki fragments

Replication Directionality

DNA polymerase can only add nucleotides in the 5' to 3' direction. This results in continuous synthesis on the leading strand and discontinuous synthesis (Okazaki fragments) on the lagging strand.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized in short fragments away from the replication fork.

• Example: Okazaki fragments are joined by ligase to form a complete strand.

Completion of Replication

At the end of S-phase, two identical daughter DNA molecules are formed, each containing half original and half new DNA. The process typically takes 7–8 hours.

• Result: Two identical DNA molecules ready for cell division.

• Importance: Ensures genetic consistency in daughter cells.

DNA Mutation and Repair

Mutations During Replication

Replication of billions of base pairs in a short time can lead to errors or mutations. Mutations may arise from biological, chemical, or physical factors.

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

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

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

• Example: UV radiation can cause thymine dimers, leading to mutations.

Mutation Checking and Repair

Cells check for mutations during the G2 phase. If errors are not repaired, they can affect cell function, be life-threatening, or contribute to genetic diversity.

• Mutation Checking: Occurs in G2 phase before mitosis.

• Consequences: Unrepaired mutations may prevent DNA replication or be passed to new cells.

• Genetic Diversity: Some mutations are beneficial and contribute to evolution.

DNA Repair Mechanisms

The body uses specialized enzymes to repair DNA mistakes. These enzymes are most active between replication and mitosis.

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

• "CUT" enzymes: Remove mismatched base pairs.

• "REPLACE" enzymes: Insert appropriate nucleotides.

• Efficiency: Repair is efficient unless damage is severe.

• Example: DNA polymerase has proofreading ability to correct errors during replication.

Summary Table: DNA Repair Enzymes

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

Key Terms and Definitions

• DNA Replication: The process of copying DNA before cell division.

• Semi-Conservative Replication: Each new DNA molecule contains one original and one new strand.

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

• Mutation: A change in the DNA sequence.

• DNA Repair: Cellular mechanisms to correct DNA errors.

Relevant Equations

Base pairing rules can be summarized as:

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

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

Directionality of DNA synthesis:

$\text{DNA Polymerase: 5' \rightarrow 3'}$

Number of base pairs replicated:

$\text{Total base pairs replicated} \approx 3 \text{ billion}$

Time for replication:

$\text{Replication time} \approx 7-8 \text{ hours}$

Additional info:

• DNA replication is fundamental to cell reproduction and differentiation, linking to topics such as genetics, inheritance, and cancer.

• Errors in replication and repair mechanisms are central to understanding genetic diseases and cancer development.