Unit 5: DNA Replication, Transcription and Translation

1. DNA Replication in Prokaryotes

DNA replication is the process by which a cell makes an identical copy of its DNA. In prokaryotes, this process is essential for binary fission and genetic continuity.

Mechanism and Types

• Semi-conservative: Each new DNA molecule consists of one original parent strand and one newly synthesized daughter strand.
• Bidirectional: Replication proceeds in both directions from a single origin of replication (oriC).
• Enzymatic Action:
  • Helicase: Unwinds the double helix.
  • DNA Polymerase III: The main enzyme that adds nucleotides in the 5' to 3' direction.
  • Primase: Synthesizes short RNA primers required for DNA polymerase to start.
  • Ligase: Seals gaps between Okazaki fragments on the lagging strand.

2. Mechanism of Transcription in Prokaryotes

Transcription is the synthesis of RNA from a DNA template. In prokaryotes, this occurs in the cytoplasm.

Steps in Transcription

1. Initiation: RNA polymerase binds to a specific DNA sequence called the promoter. The sigma factor helps in recognizing the promoter.
2. Elongation: RNA polymerase moves along the DNA template strand, synthesizing a complementary RNA strand in the 5' to 3' direction.
3. Termination: Transcription stops when the enzyme reaches a terminator sequence on the DNA. This can be Rho-dependent or Rho-independent.

3. Genetic Code and Translation in Prokaryotes

Translation is the process where the genetic information in mRNA is decoded to build a protein.

The Genetic Code

The Genetic Code is a set of rules used by living cells to translate information encoded within genetic material into proteins.

• Triplet: Each "word" (codon) consists of three nucleotides.
• Degenerate: Multiple codons can code for the same amino acid.
• Universal: The same code is used by almost all organisms.

Mechanism of Translation

• Initiation: The small ribosomal subunit binds to mRNA at the start codon (AUG).
• Elongation: tRNAs bring amino acids to the ribosome, where peptide bonds are formed between them.
• Termination: The process ends when a stop codon (UAA, UAG, UGA) enters the ribosome, releasing the newly formed polypeptide.

4. Exam Focus Enhancements