DNA Structure and Function

DNA Structure

• DNA, the substance of inheritance, is a double helix, comprised of two outer sugar-phosphate backbones, with nitrogenous bases paired in the molecule's interior.
• The double helix model was introduced by Watson and Crick in 1953, using X-ray images captured by Rosalind Franklin and Maurice Wilkins, and insights from Erwin Chargaff's work.
• The helical structure of DNA was deduced by interpreting the X-ray diffraction pattern of DNA.

Base Pairing

• The model proposed by Watson and Crick suggests specific base pairings: Adenine (A) with Thymine (T), and Guanine (G) with Cytosine (C).
• This pairing explains Chargaff's rules (A = T; G = C) and suggests a possible copying mechanism for the genetic material.
• The base pairings, although specific, don't result in a uniform width of the double helix, due to the differences in size and shape of the bases.

DNA Replication

• The double helix structure is essential for DNA replication, as each strand acts as a template to build a new complementary strand.
• DNA polymerases add nucleotides to the free 3' end of a growing DNA strand.
• Replication happens in the 5' to 3' direction, but the two strands are antiparallel, leading to different replication mechanisms for the leading and lagging strands.

Semiconservative Replication

• The semiconservative model of DNA replication proposes that each new molecule will have one old strand and one newly made strand.
• This model was validated by the Meselson-Stahl experiment, which used heavy (N15) and light (N14) isotopes of nitrogen to trace the parental DNA strands during replication.

Replication Process

• DNA replication begins at specific sites called origins of replication, where two DNA strands are separated, creating a replication "bubble".
• The "bubble" expands as two replication forks move in opposite directions, unzipping the DNA helix.
• Enzymes involved in DNA replication include:
  • Helicases: Untwist the double helix.
  • Single-strand binding proteins: Stabilize single-stranded DNA.
  • Topoisomerase: Relieves overwinding ahead of replication forks.
  • Primase: Synthesizes an RNA primer, which provides a starting point for DNA polymerases.
  • DNA polymerases: Add nucleotides to the new DNA strand, using the parental strand as a template.
  • DNA ligase: Joins Okazaki fragments, short segments of DNA produced during discontinuous replication of the lagging strand.

DNA Repair

• DNA can be damaged by harmful physical agents or undergo spontaneous changes.
• DNA repair mechanisms, like nucleotide excision repair, use enzymes to cut out damaged stretches of DNA and replace them with new, correctly sequenced nucleotides.
• Proofreading and repairing DNA during replication ensures a low error rate.

Evolutionary Significance

• Mutations in DNA sequences can become permanent, passed on to future generations, contributing to genetic variations.
• This genetic variation serves as a source for natural selection, driving evolution.