Biology · Ch 5 — Molecular Basis of Inheritance
Transcription
Transcription
Transcription is the first step in gene expression — the process by which a gene’s DNA sequence is copied into RNA. In molecular terms, it is the synthesis of RNA from a DNA template. This RNA copy then carries the genetic message to the ribosome for protein synthesis (translation). The key idea is that only one of the two DNA strands serves as the template for RNA synthesis, and the RNA produced is complementary to that template strand.
The DNA template and the coding strand
A gene is a stretch of double-stranded DNA. During transcription, only one of the two strands — called the template strand — is read by the enzyme RNA polymerase. The other strand, which is not transcribed, is called the coding strand (or sense strand). The coding strand has the same sequence as the RNA transcript, except that thymine (T) in DNA is replaced by uracil (U) in RNA.
The RNA transcript is identical in sequence to the coding strand (with U instead of T) and complementary to the template strand.
The transcription unit
A transcription unit in DNA consists of three regions:
- Promoter — the binding site for RNA polymerase, located upstream of the gene. It defines the start point and direction of transcription.
- Structural gene — the actual DNA sequence that gets transcribed into RNA.
- Terminator — the sequence that signals the end of transcription.
RNA polymerase and the process
In bacteria, a single RNA polymerase enzyme carries out transcription. In eukaryotes, there are three different RNA polymerases: RNA polymerase I (for rRNA), RNA polymerase II (for mRNA), and RNA polymerase III (for tRNA and other small RNAs).
Transcription occurs in three steps:
- Initiation — RNA polymerase binds to the promoter region. In bacteria, a sigma factor helps the polymerase recognise the promoter. The DNA unwinds locally to expose the template strand.
- Elongation — RNA polymerase moves along the template strand in the 3′ → 5′ direction, adding complementary RNA nucleotides (A, U, G, C) in the 5′ → 3′ direction. The growing RNA strand peels away from the DNA, and the DNA helix re-forms behind the polymerase.
- Termination — When RNA polymerase reaches the terminator sequence, it detaches from the DNA, and the newly synthesised RNA is released.
Differences between prokaryotic and eukaryotic transcription
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Location | Cytoplasm (no nucleus) | Nucleus |
| RNA polymerase | One type | Three types (I, II, III) |
| Promoter recognition | Sigma factor helps | Multiple transcription factors required |
| Processing of RNA | None — mRNA is directly usable | Pre-mRNA is processed (capping, tailing, splicing) |
| Coupling with translation | Transcription and translation can occur simultaneously | Transcription and translation are separated in space and time |
Post-transcriptional modifications in eukaryotes
The primary transcript (pre-mRNA) in eukaryotes is not ready for translation. It undergoes three major modifications:
- Capping — a modified guanine nucleotide is added to the 5′ end of the transcript. This protects the RNA from degradation and helps ribosome binding.
- Tailing — a string of adenine nucleotides (poly-A tail) is added to the 3′ end. This also protects the RNA and aids in export from the nucleus. …