Question

Read the information for transcription and then answer the question.

Protein Synthesis

Explain the process of transcription.

Answer 1

Transcription is the process by which the information in a DNA sequence is copied into an RNA molecule. It is the first step of gene expression and produces the RNA (mRNA, rRNA, tRNA or other RNAs) that will be used or processed for protein synthesis. Key features and steps:

1. Location and enzymes
- Prokaryotes: transcription occurs in the cytoplasm, carried out by a single RNA polymerase (with a sigma factor for promoter recognition).
- Eukaryotes: transcription occurs in the nucleus. Three main RNA polymerases exist; RNA polymerase II synthesizes pre-mRNA for proteins. Eukaryotic transcription requires many general transcription factors in addition to RNA polymerase II.

2. Template and directionality
- RNA polymerase reads the DNA template strand in the 3′ → 5′ direction and synthesizes RNA in the 5′ → 3′ direction by adding ribonucleotides complementary to the template.
- The non-template (coding) strand has the same sequence as the RNA transcript except T in DNA is replaced by U in RNA.

3. Stages of transcription
- Initiation:
- A promoter sequence upstream of the gene signals where transcription begins (e.g., −10 Pribnow box and −35 in bacteria; TATA box and other elements in eukaryotes).
- In bacteria the sigma factor binds the promoter and recruits RNA polymerase. In eukaryotes general transcription factors (e.g., TFIID/TBP) assemble with RNA Pol II at the promoter to form a pre-initiation complex.
- RNA polymerase melts a short stretch of DNA to expose the template strand and begins RNA synthesis at the transcription start site (+1).

- Elongation:
- RNA polymerase moves along the DNA, unwinding ahead and rewinding behind, adding ribonucleoside triphosphates (NTPs) to the 3′ end of the growing RNA chain (U pairs with A, C with G).
- The nascent RNA detaches from the DNA template as synthesis proceeds.

- Termination:
- In bacteria, termination can be intrinsic (Rho-independent: formation of a GC-rich hairpin in the RNA followed by a U-rich tract causes release) or Rho-dependent (Rho protein helps release the transcript).
- In eukaryotes, termination of Pol II is coupled to cleavage and polyadenylation: a polyadenylation signal (AAUAAA) in the pre-mRNA directs cleavage and addition of a poly(A) tail; polymerase eventually disengages.

4. RNA processing (eukaryotes only, post-transcriptional)
- Pre-mRNA is processed before export to the cytoplasm:
- 5′ capping (addition of a 7-methylguanosine cap) protects the mRNA and aids ribosome binding.
- Splicing: introns are removed and exons joined by the spliceosome (snRNPs).
- 3′ polyadenylation: addition of a poly(A) tail stabilizes the mRNA and aids export/translation.

5. Outcome and significance
- Result is an RNA molecule complementary to the DNA template. For protein-coding genes, processed mRNA is exported to the cytoplasm where it is translated into protein. Transcription regulation controls which genes are expressed and when.

Key differences to remember: RNA uses uracil (U) instead of thymine (T); transcription synthesizes RNA 5′→3′ from a DNA template read 3′→5′; in eukaryotes transcription is followed by extensive RNA processing.