Gene expression converts information stored in DNA into functional products. In protein-coding genes, transcription produces an RNA copy from a DNA template, and translation uses the codons in messenger RNA (mRNA) to build a polypeptide.
1. Transcription: DNA to RNA
RNA polymerase binds at or near a promoter with the help of organism-specific factors, opens a short region of DNA and synthesizes RNA complementary to the template strand. RNA is synthesized 5′ to 3′. The RNA sequence resembles the coding DNA strand except that RNA uses uracil (U) in place of thymine (T).
Stages of transcription
- Initiation: RNA polymerase and required factors recognize the promoter and begin RNA synthesis.
- Elongation: RNA polymerase moves along the DNA template and extends the RNA chain.
- Termination: transcription ends through mechanisms that differ among organisms and genes.
In eukaryotes, many primary mRNA transcripts undergo processing that can include 5′ capping, splicing and 3′ polyadenylation before export and translation. Prokaryotic mRNA generally does not undergo this same combination of processing steps, and transcription and translation can be coupled.
2. Translation: mRNA to polypeptide
Translation occurs on ribosomes. The ribosome reads mRNA in three-nucleotide units called codons. Transfer RNA (tRNA) molecules deliver amino acids and use their anticodons to pair with compatible mRNA codons. The ribosome catalyzes peptide-bond formation as the chain grows.
Stages of translation
- Initiation: the small ribosomal subunit, mRNA, initiator tRNA and other factors assemble; the large subunit joins.
- Elongation: aminoacyl-tRNAs enter, codon recognition occurs, peptide bonds form and the ribosome translocates along the mRNA.
- Termination: a stop codon is recognized by release factors, allowing the completed polypeptide to be released.
The standard genetic code includes AUG as a common start codon and UAA, UAG and UGA as stop codons. AUG also encodes methionine; the initiating amino acid and details can vary in some biological contexts.
3. Transcription vs translation
| Feature | Transcription | Translation |
|---|---|---|
| Information flow | DNA to RNA | mRNA to polypeptide |
| Main machinery | RNA polymerase and transcription factors | Ribosome, tRNA and translation factors |
| Template/readout | DNA template strand | mRNA codons |
| Building blocks | Ribonucleotides | Amino acids |
| Product | RNA | Polypeptide/protein precursor |
| Direction | RNA made 5′ to 3′ | mRNA read 5′ to 3′; polypeptide grows from N-terminus to C-terminus |
| Location in eukaryotes | Usually nucleus for nuclear genes | Cytoplasm, on free or ER-associated ribosomes |
| Location in prokaryotes | Cytoplasm/nucleoid region | Cytoplasm; can occur while transcription is ongoing |
4. Important exam distinctions
- A promoter is a DNA sequence involved in transcription initiation; it is not a translation start codon.
- A codon is on mRNA; the complementary anticodon is on tRNA.
- RNA polymerase makes RNA. Ribosomes synthesize polypeptides.
- Not every RNA is translated: rRNA, tRNA and many regulatory RNAs function as RNA.
- Transcription and translation are spatially separated for nuclear genes in eukaryotes, but can be coupled in prokaryotes.
5. Practice: 10 MCQs with explanations
Q1. Which enzyme synthesizes RNA during transcription?
Answer: RNA polymerase. Why: It uses a DNA template to polymerize ribonucleotides into RNA.
Q2. What is the usual direction of RNA synthesis?
Answer: 5′ to 3′. Why: New ribonucleotides are added to the growing RNA’s 3′ end.
Q3. What is the main role of a promoter?
Answer: It helps direct transcription initiation. Why: RNA polymerase and associated factors recognize promoter DNA.
Q4. Which RNA carries codons for protein synthesis?
Answer: mRNA. Why: Its codon sequence specifies the order of amino acids according to the genetic code.
Q5. Where is the anticodon found?
Answer: On tRNA. Why: The anticodon pairs with a compatible codon on mRNA.
Q6. What is the primary site of translation?
Answer: Ribosomes. Why: Ribosomes coordinate codon reading and peptide-bond formation.
Q7. Which is a common start codon?
Answer: AUG. Why: In the standard code it specifies methionine and commonly initiates translation.
Q8. Which set contains the standard stop codons?
Answer: UAA, UAG and UGA. Why: They signal termination rather than encoding a standard amino acid in the standard genetic code.
Q9. What is one key difference between prokaryotic and eukaryotic gene expression?
Answer: Transcription and translation can be coupled in prokaryotes. Why: Prokaryotes lack a membrane-enclosed nucleus; eukaryotic nuclear transcripts are made and processed in the nucleus before translation.
Q10. Which statement is correct?
Answer: Not all RNA is translated. Why: rRNA, tRNA and regulatory RNAs have functional roles as RNA molecules.
Related revision guides: PCR vs qPCR vs RT-PCR, Gel Electrophoresis, CSIR-NET Biotechnology Revision Framework, and Study Resources.
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