DNA Structure, Replication, and Repair

Antiparallel Strands

A critical feature of the double helix is that the two strands are antiparallel. This means they run in opposite directions: one strand is oriented in the $5' \rightarrow 3'$ direction, while its partner runs $3' \rightarrow 5'$. This orientation is vital because the enzymes responsible for copying DNA can only function in one specific direction.

DNA vs. RNA: Key Differences

While DNA is the long-term storage of genetic information, RNA plays various roles in protein synthesis and regulation.

DNA Packaging and Chromosomes

To fit inside a microscopic nucleus, long strands of DNA must undergo condensation (tightly packing). In its loose, functional state, DNA is referred to as chromatin, which allows for processes like protein synthesis. When a cell prepares to divide, the chromatin condenses into visible chromosomes.

Chromosomes can be classified by the position of their centromere (e.g., metacentric, acrocentric). At the ends of these linear chromosomes are protective caps called telomeres. With every cell division, telomeres shorten; this shortening is a key factor in cellular aging.

The Mechanism of DNA Replication

DNA replication is a semiconservative process. This means that when a new DNA molecule is formed, it consists of one "old" original strand and one newly synthesized strand.

Phase 1: Initiation

Replication begins at specific sites called the origin of replication.

1. Topoisomerase works ahead of the replication fork to relax the DNA coil, preventing the molecule from becoming over-twisted (supercoiling).
2. Helicase unwinds the double helix by breaking the hydrogen bonds between the bases, acting like a zipper to create a replication fork.
3. SSB proteins (single-strand binding proteins) attach to the separated strands to keep them from snapping back together.

Phase 2: Elongation

This is the stage where new nucleotides are added. However, a major constraint exists: DNA Polymerase can only synthesize DNA in the $5' \rightarrow 3'$ direction. This creates two different methods of synthesis on the two antiparallel strands:

• The Leading Strand: This strand is synthesized continuously toward the replication fork.
• The Lagging Strand: Because this strand runs in the opposite direction, it must be synthesized discontinuously away from the fork. This results in short segments known as Okazaki fragments.

To start this process, Primase must first lay down a short RNA primer. The primer provides the necessary $3'$-OH group that DNA polymerase needs to begin attaching nucleotides.

Phase 3: Termination

Replication concludes when the strands are complete or the enzyme reaches the end of the linear molecule. In prokaryotes, specific termination sequences signal the end, whereas in eukaryotes, the process is limited by the length of the telomeres.

Summary of Key Enzymes

Maintaining Genetic Integrity: DNA Repair

Errors during replication can lead to mutations. To prevent this, cells employ several sophisticated repair mechanisms:

1. Proofreading: As DNA polymerase adds nucleotides, it performs an immediate check. If it detects a mismatch, it can move backward, remove the incorrect nucleotide, and replace it with the correct one.
2. Mismatch Repair: Specialized enzymes recognize and fix errors that escaped the initial proofreading.
3. Excision Repair: If a larger section of DNA is damaged, nucleases "cut out" the damaged segment, and DNA polymerase and ligase work together to fill the gap and seal the backbone.