DNA Replication and Nucleic Acids: Structure, Function, and Cellular Role

DNA Replication

Overview of DNA Replication

DNA replication is a fundamental process that occurs during the S phase of interphase in the cell cycle. It ensures that each somatic cell receives an identical copy of genetic material prior to cell division.

• Timing: Occurs during the S (synthesis) phase of interphase.

• Purpose: Each somatic cell makes a second copy of its DNA via DNA replication.

• Type of Molecule: DNA is a type of nucleic acid.

Nucleic Acids

Types and Structure of Nucleic Acids

Nucleic acids are polymers composed of nucleotide monomers. They are essential for storing and transmitting genetic information in cells.

• Types: Two main types in cells:

  • Deoxyribonucleic acid (DNA)

  • Ribonucleic acid (RNA)

• Nucleotide Structure: Each nucleotide contains:

  • Pentose sugar: Deoxyribose for DNA, ribose for RNA

  • Phosphate group

  • Nitrogenous base

Nucleotide Components

• Sugar: Deoxyribose (DNA), Ribose (RNA)

• Phosphate group: Provides structural support and links nucleotides together

• Nitrogenous base: Determines genetic code

Types of Nitrogenous Bases

Nitrogenous bases are classified into purines and pyrimidines, and their specific pairing is crucial for the structure and function of nucleic acids.

• DNA Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• RNA Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

• Base Pairing: In DNA, A pairs with T, and C pairs with G. In RNA, A pairs with U.

Chemical Structures of Bases

• Purines: Adenine and Guanine (double-ring structure)

• Pyrimidines: Thymine, Cytosine, and Uracil (single-ring structure)

• Example: The chemical structure of each base includes a specific arrangement of atoms that allows for hydrogen bonding between complementary bases.

DNA and Chromosomes

Chromosome Structure and Organization

Chromosomes are highly organized structures composed of DNA and proteins. They carry genetic information in the form of genes.

• Human Chromosomes: Humans have 46 chromosomes (23 pairs).

• Chromosome Composition: Each chromosome consists of two strands of DNA held together at the bases and twisted into a double helix.

• Genes: A gene is a segment of DNA within a chromosome that contains the code for making a specific protein.

Mechanisms of DNA Replication

Steps in DNA Replication

DNA replication is a semi-conservative process that ensures genetic fidelity during cell division. Several enzymes and steps are involved in this process.

• Initiation: DNA strands uncoil and "unzip" at the replication fork.

• Base Pairing: New strands are synthesized by complementary base pairing (A-T, C-G), ensuring an exact copy is made.

• Enzymes Involved:

  • Helicase: Unwinds DNA to form the replication fork.

  • Primase: Synthesizes RNA primer to mark the starting point.

  • DNA Polymerase: Adds new DNA nucleotides in the 5' to 3' direction; synthesizes Okazaki fragments on the lagging strand.

  • Ligase: Joins Okazaki fragments to create a continuous DNA strand.

  • Additional info: Other accessory proteins help stabilize the unwound DNA and ensure accuracy.

Replication Fork and Directionality

• Replication Fork: The site where DNA strands separate and new strands are synthesized.

• Directionality: DNA polymerase can only add nucleotides in the 5' to 3' direction.

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

Mutations and DNA Repair

Mutations: Causes and Effects

Mutations are changes in the DNA sequence that can occur spontaneously or due to external factors. Their effects vary from neutral to harmful or beneficial.

• Causes: Mistakes during DNA replication, chemical mutagens, physical forces (e.g., X-rays).

• Types of Effects:

  • Neutral mutations: No effect on cell function.

  • Harmful mutations: May result in cell death or cancer.

  • Beneficial mutations: Can provide evolutionary advantages.

DNA Repair Mechanisms

• Recognition and Removal: Cells have enzymes that recognize and cut out errors in DNA.

• Replacement: The damaged section is replaced with the correct sequence.

• Timing: DNA repair enzymes are most active following DNA replication and before mitosis.

• Importance: Failure to repair errors can lead to diseases such as cancer.

Summary Table: Key Components of DNA and RNA

Component	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic information storage	Protein synthesis, gene regulation

Key Equations and Concepts

• Base Pairing Rule:

$A = T$ $C = G$

• Direction of DNA Synthesis:

$\text{DNA polymerase adds nucleotides in the 5' \rightarrow 3' direction}$

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$