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DNA Structure, Function, and Replication

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DNA: Storage and Transmission of Genetic Information

Overview of DNA Function

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. It stores genetic information, codes for proteins, and is responsible for the transmission of traits from one generation to the next.

  • Genetic Code: DNA contains the instructions for making proteins, which perform most cellular functions.

  • Enzyme Coding: Many DNA sequences code for enzymes that catalyze biochemical reactions.

  • Inheritance: DNA is passed from parents to offspring, carrying inherited traits.

  • Mutations: Changes in DNA sequence (mutations) can lead to genetic diversity or disease.

Example: The bacterium Neurospora crassa was used to demonstrate the link between genes and enzymes.

DNA as the Storage Molecule

Evidence for DNA as Genetic Material

Experiments in the early 20th century established DNA as the molecule responsible for heredity.

  • Transformation Experiments: DNA from one bacterial strain can transform another strain, transferring genetic traits.

  • Bacteriophage Experiments: Viruses that infect bacteria (bacteriophages) inject DNA into host cells, demonstrating DNA's role in heredity.

  • Radioactive Labeling: Experiments using radioactive isotopes showed that DNA, not protein, enters bacterial cells during infection.

Example: Avery, MacLeod, and McCarty (1944) showed that DNA is the "transforming principle" in bacteria.

Structure of DNA

Nucleotides: Building Blocks of DNA

DNA is a polymer made up of repeating units called nucleotides.

  • Nucleotide Structure: Each nucleotide consists of:

    • 1. Five-carbon sugar: Deoxyribose

    • 2. Phosphate group

    • 3. Nitrogenous base

  • Nitrogenous Bases:

    • Purines: Adenine (A), Guanine (G)

    • Pyrimidines: Thymine (T), Cytosine (C)

  • Nucleosides: A base linked to a sugar (no phosphate).

  • Phosphodiester Bonds: Nucleotides are joined by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming the sugar-phosphate backbone.

  • Base Pairing: Bases attach to the sugar and pair with complementary bases on the opposite strand.

  • Polynucleotide Chains: DNA can form very long chains, often millions of nucleotides in length.

Double Helix Structure

The double helix model of DNA was proposed by Watson and Crick, based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins.

  • Antiparallel Strands: Two polynucleotide chains run in opposite directions (5' to 3' and 3' to 5').

  • Sugar-Phosphate Backbone: The backbone is on the outside, with bases on the inside.

  • Base Pairing: Hydrogen bonds link specific pairs:

    • Adenine (A) pairs with Thymine (T)

    • Guanine (G) pairs with Cytosine (C)

    Base Pairing Rule: A = T, G = C

DNA Replication

Semiconservative Replication

DNA replication is the process by which DNA makes a copy of itself during cell division. The semiconservative model, proposed by Watson and Crick and demonstrated by Meselson and Stahl, states that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

  • Replication Forks: DNA strands separate, forming replication "bubbles" with forks at each end.

  • Enzymes Involved:

    • Helicase: Unwinds the double helix.

    • Single-Strand Binding Proteins: Stabilize unwound DNA.

    • Topoisomerase: Relieves strain ahead of the replication fork.

    • Primase: Synthesizes RNA primers to initiate DNA synthesis.

    • DNA Polymerase: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

    • Ligase: Joins Okazaki fragments on the lagging strand.

  • Leading vs. Lagging Strand:

    • Leading Strand: Synthesized continuously toward the replication fork.

    • Lagging Strand: Synthesized discontinuously away from the fork as Okazaki fragments.

Strand

Synthesis Direction

Mode of Synthesis

Leading

5' to 3' (toward fork)

Continuous

Lagging

5' to 3' (away from fork)

Discontinuous (Okazaki fragments)

Summary of Replication Steps

  1. Helicase unwinds DNA at the origin of replication.

  2. Single-strand binding proteins stabilize unwound DNA.

  3. Primase synthesizes RNA primers.

  4. DNA polymerase extends the new DNA strand from the primer.

  5. Leading strand synthesized continuously; lagging strand synthesized in fragments.

  6. DNA ligase joins Okazaki fragments.

Packaging of DNA

Chromatin and Nucleosomes

In eukaryotic cells, DNA is packaged into chromosomes. Each chromosome contains a single, long DNA double helix, which is compacted by wrapping around proteins called histones to form nucleosomes.

  • Nucleosome: The basic unit of DNA packaging, consisting of DNA wrapped around a core of histone proteins.

  • Chromatin: The complex of DNA and proteins that forms chromosomes.

  • Compaction: DNA is further coiled and folded to fit inside the cell nucleus.

Example: If stretched out, the DNA in a single human cell would be about 2 meters long, but it fits into a nucleus only a few micrometers in diameter.

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