DNA: The Molecule of Heredity – Structure, Function, and Replication

DNA: The Molecule of Heredity

How Did Scientists Discover That Genes Are Made of DNA?

Early geneticists recognized that genes were units of heredity, but the chemical nature of genes was unclear until the mid-20th century. Key experiments established DNA as the genetic material.

• Chromosomes and Genes: By the 1920s, scientists knew genes were part of chromosomes, which are made of DNA and proteins.

• Griffith’s Transformation Experiment (1928): Showed that a "transforming principle" from dead bacteria could genetically alter living bacteria.

• Avery, MacLeod, and McCarty (1944): Demonstrated that DNA, not protein, was the "transforming principle" responsible for heredity.

• Hershey-Chase Experiment (1952): Used radioactive labeling to show that DNA, not protein, enters bacterial cells during viral infection, confirming DNA as the genetic material.

Example: The Hershey-Chase experiment used bacteriophages labeled with radioactive sulfur (protein) and phosphorus (DNA) to track which molecule entered bacteria.

What Is the Structure of DNA?

The structure of DNA reveals how it stores and transmits genetic information. DNA is a double helix composed of nucleotides.

• Nucleotides: The building blocks of DNA, each consisting of a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Four Nitrogenous Bases:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• Chargaff’s Rules: The amount of adenine equals thymine, and cytosine equals guanine in DNA.

• Double Helix Model: Watson and Crick (1953) proposed the double helix structure, with two strands running in opposite directions and held together by base pairing.

Example: The DNA double helix resembles a twisted ladder, with sugar-phosphate backbones as the sides and base pairs as the rungs.

DNA Base Pairing and Complementarity

Base pairing is essential for DNA’s structure and function. Specific pairing ensures accurate replication and transmission of genetic information.

• Complementary Base Pairs: Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G).

• Hydrogen Bonds: Two hydrogen bonds form between A and T; three between C and G.

• Antiparallel Strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

Equation:

$\text{A} = \text{T}$ $\text{C} = \text{G}$

How Does DNA Encode Genetic Information?

DNA stores genetic information in the sequence of its bases. The order of bases forms genes, which code for proteins.

• Genetic Code: The sequence of bases is read in groups of three (codons), each coding for an amino acid.

• Genome Size: Human DNA contains about 3.2 billion base pairs.

• Information Storage: The linear sequence of bases allows for vast combinations and genetic diversity.

Example: The gene for hemoglobin contains a specific sequence of bases that determines the amino acid sequence of the protein.

How Does DNA Replication Occur?

DNA replication ensures that genetic information is accurately passed to daughter cells during cell division.

• Semiconservative Replication: Each new DNA molecule consists of one old strand and one new strand.

• Steps in DNA Replication:

  1. DNA unwinds and separates into two strands.

  2. Each strand serves as a template for a new complementary strand.

  3. DNA polymerase adds nucleotides according to base pairing rules.

• Replication Fork: The area where DNA unwinds and replication occurs.

Equation:

$\text{New DNA strand} = \text{Template strand} + \text{Complementary nucleotides}$

What Are Mutations, and How Do They Occur?

Mutations are changes in the DNA sequence. They can occur spontaneously or due to environmental factors.

• Types of Mutations:

  • Substitution: One base is replaced by another.

  • Insertion/Deletion: Bases are added or removed, potentially causing frameshifts.

• Causes: Errors during replication, exposure to chemicals or radiation.

• Effects: Mutations can be neutral, harmful, or beneficial, depending on their impact on protein function.

Example: Sickle cell anemia is caused by a single base substitution in the hemoglobin gene.

DNA Repair Mechanisms

Cells have mechanisms to correct errors in DNA and maintain genetic stability.

• Proofreading: DNA polymerase checks and corrects errors during replication.

• Mismatch Repair: Enzymes detect and fix incorrectly paired bases.

• Excision Repair: Damaged sections of DNA are removed and replaced.

Example: UV light can cause thymine dimers, which are repaired by excision repair enzymes.

Summary Table: DNA Structure and Function

Feature	Description
Nucleotide	Phosphate group, deoxyribose sugar, nitrogenous base
Bases	Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
Base Pairing	A-T (2 H-bonds), C-G (3 H-bonds)
Double Helix	Two antiparallel strands, twisted ladder structure
Replication	Semiconservative, uses base pairing rules
Mutation	Change in DNA sequence; can be substitution, insertion, or deletion

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.