Unit 5: DNA Replication, Transcription and Translation
Table of Contents
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
- Initiation: RNA polymerase binds to a specific DNA sequence called the promoter. The sigma factor helps in recognizing the promoter.
- Elongation: RNA polymerase moves along the DNA template strand, synthesizing a complementary RNA strand in the 5' to 3' direction.
- 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
Exam Tips
- Directionality: Always remember that both DNA and RNA are synthesized in the 5' to 3' direction.
- Enzymes: Be ready to name the specific DNA polymerases involved in prokaryotic replication (Pol I, II, and III).
- Start and Stop: Memorize the start codon (AUG) and the three stop codons (UAA, UAG, UGA) for translation questions.
Common Mistakes
- Confusing Transcription with Translation. Remember: Transcribe (DNA to RNA) comes before Translate (RNA to Protein).
- Forgetting the Primer. DNA polymerase cannot start from scratch; it always needs a 3'-OH group provided by an RNA primer.
Frequently Asked Questions
- Explain the semi-conservative mode of DNA replication.
- What is the role of RNA polymerase in prokaryotic transcription?
- Discuss the salient features of the genetic code.