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 functions include 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" (mRNA) 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.

• Semi-conservative replication: Each daughter DNA molecule contains half of the original DNA and half new DNA.

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

• Example: After replication, both daughter cells have DNA composed of one old and one new strand.

Steps of DNA Replication

Step 1: DNA Unwinds

The enzyme helicase initiates replication by unwinding the DNA helix and separating the two strands at multiple locations. Single-stranded binding proteins stabilize the separated strands.

• Helicase: "Unzips" the DNA by breaking hydrogen bonds between nucleotide pairs.

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

Step 2: Formation of Replication Bubbles

Multiple helicase enzymes create several replication bubbles along the DNA molecule, allowing replication to occur simultaneously at different sites.

• Replication bubbles: Regions where DNA strands are separated and replication occurs.

• Multiple origins: Replication does not start at a single point but at many locations along the DNA.

Step 3: Complimentary Pairing

Replication proceeds with the help of primase and DNA polymerase enzymes. Primase inserts an RNA primer to mark the starting point, and DNA polymerase adds new nucleotides complementary to the template strand.

• Primase: Inserts an RNA primer to initiate replication.

• DNA Polymerase: Binds at the primer and adds new nucleotides (A-T, T-A, C-G, G-C).

• Leading strand: Synthesized continuously in the 3' direction.

• Lagging strand: Synthesized in short fragments (Okazaki fragments) in the 5' direction, opposite to helicase movement.

Example: New adenine (A) nucleotides pair with thymine (T) on the template strand, and vice versa.

Step 4: Finishing

The enzyme ligase completes the replication process by repairing the sugar-phosphate backbone and connecting gaps in the Okazaki fragments. The DNA strands then recoil into their helix structure.

• Ligase: Seals nicks in the backbone and joins Okazaki fragments.

• Final structure: Two identical DNA molecules, each with one old and one new strand.

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	Joins Okazaki fragments and repairs backbone

Replication Directionality and Strand Differences

• Leading strand: Synthesized continuously toward the replication fork (3' to 5' template).

• Lagging strand: Synthesized discontinuously away from the replication fork (5' to 3' template) in Okazaki fragments.

Additional info: The antiparallel nature of DNA strands requires different replication strategies for each strand.

Completion and Timing

• Duration: The entire replication process takes approximately 7–8 hours during the S-phase.

• Result: Two identical daughter DNA molecules, each composed of half original and half new DNA.

DNA Mutation and Repair

Mutations During Replication

Errors can occur during the rapid replication of billions of base pairs. Mutations may arise from biological, chemical, or physical factors.

• Biological exposures: Viruses such as Human Papilloma Virus (HPV).

• Chemical substances: Pesticides, household chemicals.

• Physical substances: Radiation, heat.

Example: Exposure to 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 consequences: May prevent DNA replication, alter cell function, 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 detect and correct DNA errors. 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 effective unless damage is too severe.

Summary Table: DNA Repair Enzymes

Enzyme Type	Function
"READ" enzymes	Detect mismatches
"CUT" enzymes	Remove incorrect bases
"REPLACE" enzymes	Insert correct bases

Key Terms and Concepts

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

• Semi-conservative: Each new DNA molecule contains one old and one new strand.

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

• 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 and Structures

• Base Pairing Rules:

  • Adenine (A) pairs with Thymine (T)

  • Cytosine (C) pairs with Guanine (G)

• DNA Structure:

  • Double helix with antiparallel strands

  • Phosphate-sugar backbone

Example DNA sequence:

• Original: ATCG

• Complement: TAGC

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