DNA Replication

DNA Packaging and Replication

Overview of Replication

• Replication Definition: The process of copying DNA to generate more DNA.

• Location: Occurs in the nucleus of a cell.

DNA Folding

• Length of DNA: Human cells can contain up to 6 feet of DNA.

• Challenge of Packing: DNA must fit into a microscopic cell.

• Folding Mechanism: DNA is compacted into dense packets.

• Chromatin Formation: DNA is stored as chromatin, which involves DNA wrapped around proteins.

  • Proteins: These proteins are called Histones.

  • Nucleosome: A combination of DNA and one histone is referred to as a nucleosome.

  • Chromatin Structure: Multiple nucleosomes together form chromatin.

DNA Replication Enzyme Players Involved

Key Enzymes in DNA Replication

• Helicase: Unzips the DNA molecule by breaking hydrogen bonds between bases.

• Primase: Lays down RNA primers; signals where DNA polymerase should attach.

• Polymerase: The builder enzyme that pulls nucleotides from surrounding areas to attach to the parent strand.

• Ligase: Acts as glue, binding sections of nucleotides together.

First Steps of Replication

Sequence of Events

1. Helicase Action: Unzips DNA by breaking hydrogen bonds between bases.

2. Primer Formation: Primase lays down RNA primers.

3. Single Strand Binding Proteins: Prevent the DNA strands from re-annealing.

4. Polymerase Action: Builds the new DNA strand by attaching floating nucleotides to the parent strand.

5. Okazaki Fragments: Ligase binds these fragments together in the newly synthesized strand.

DNA Polymerase Functionality

• Constructs new complementary strands of DNA by attaching nucleotides to the parent strand.

• There are two strands to be built:

  • One is complementary to the leading strand.

  • Another is complementary to the lagging strand.

• Complementarity: The strands are complementary, not identical.

• Example of Complementary Strand: Given a parent strand (ACTGCGTAGT), the complementary strand would be (TGACGCATCA).

Structure of DNA

Nucleotides

A single nucleotide, the building block of DNA, is composed of three components:

• A phosphate group

• A five-carbon sugar (deoxyribose)

• A nitrogenous base

Backbone of DNA

The backbone of DNA is made up of alternating sugar (deoxyribose) and phosphate molecules. This structure provides stability and shape to the DNA molecule.

Nitrogenous Bases and Base Pairing Rules

There are four types of nitrogenous bases in DNA:

• Adenine (A)

• Thymine (T)

• Cytosine (C)

• Guanine (G)

The bases are connected through hydrogen bonds, forming base pairs, where:

• Adenine (A) pairs with Thymine (T) by two hydrogen bonds.

• Cytosine (C) pairs with Guanine (G) by three hydrogen bonds.

• Sugar Component: DNA consists of a five-carbon sugar called deoxyribose.

• Carbon Labeling: Carbons are labeled in a clockwise arrangement.

• Strands Arrangement: DNA comprises a 3' to 5' strand and a 5' to 3' strand—these strands are anti-parallel.

Backbone of DNA

• The backbone of DNA is made up of alternating sugar (deoxyribose) and phosphate molecules.

Nucleotides

• A single nucleotide, the building block of DNA, is composed of three components:

  1. A phosphate group

  2. A five-carbon sugar (deoxyribose)

  3. A nitrogenous base

Nitrogenous Bases

• There are four types of nitrogenous bases in DNA:

  1. Adenine (A)

  2. Thymine (T)

  3. Cytosine (C)

  4. Guanine (G)

Base Pairing

• The bases are connected through hydrogen bonds, forming base pairs, where:

  • Adenine (A) pairs with Thymine (T) by two hydrogen bonds.

  • Cytosine (C) pairs with Guanine (G) by three hydrogen bonds

Basic Structure

• Sugar Component: DNA consists of a five-carbon sugar.

• Carbon Labeling: Carbons are labeled in a clockwise arrangement.

• Strands Arrangement: DNA comprises a 3' to 5' strand and a 5' to 3' strand—these strands are anti-parallel.

Directionality of DNA

• Leading Strand: The strand that runs 3' to 5'; the new strand built will go in the 5' to 3' direction.

• Lagging Strand: The strand that runs 5' to 3'; the built strand will proceed in the 3' to 5' direction.

• Polymerase Activity: It only operates in the 5' to 3' direction. The newly synthesized strand will be opposite to the parent strand.

• Okazaki Fragments Description: The fragments synthesized on the lagging strand are called Okazaki fragments, which are later connected by ligase.

Result of Replication

• The result is two semi-conservative copies of DNA, with one parent strand and one new strand in each copy.

• This mechanism ensures that genetic information is accurately passed on during cell division, maintaining the integrity of the DNA across generations.