2.3.4Unit 2: Cells, Development, Biodiversity and Conservation
Transcription and Translation — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 26 Unit quiz
Section 1 of 5Transcription
Transcription makes an RNA copy of a gene, and it takes place in the nucleus of a eukaryotic cell.
1
RNA polymerase binds at the start of a gene and unwinds a short section of the DNA, breaking hydrogen bonds between the strands.
2
One strand, the template strand, is used, and free RNA nucleotides pair with it by complementary base pairing, with uracil pairing with adenine.
3
RNA polymerase joins the RNA nucleotides by phosphodiester bonds, making a pre-mRNA strand in the 5′ to 3′ direction.
4
When the stop sequence is reached the enzyme detaches and the DNA strands rewind into a double helix.
5
In eukaryotes the introns are removed by splicing, and the mature mRNA leaves the nucleus through a nuclear pore.
Section 2 of 5Translation
Translation builds a polypeptide using the sequence of codons on the mRNA.
Ribosome
- A structure made of rRNA and protein with a small and a large subunit; it holds mRNA and tRNA in position.
Initiation
- The small ribosomal subunit binds to the mRNA and the first tRNA carrying methionine pairs with the AUG start codon.
tRNA
- Each tRNA carries one particular amino acid and has an anticodon that pairs with a complementary codon.
Peptide bond formation
- Two tRNAs sit in the ribosome at once, and a peptide bond forms between their amino acids by condensation.
Movement
- The ribosome moves along the mRNA by one codon at a time, so tRNAs leave and the polypeptide grows.
Termination
- When a stop codon is reached no tRNA binds, the polypeptide is released and the ribosome separates from the mRNA.
Section 3 of 5Comparing the Two Processes
Comparing the Two Processes
Location
- Transcription happens in the nucleus, whereas translation happens on ribosomes in the cytoplasm or on the rough endoplasmic reticulum.
Enzymes
- RNA polymerase carries out transcription, while peptide bond formation in translation takes place within the ribosome.
Energy
- ATP is needed to attach each amino acid to its specific tRNA molecule before translation can begin.
Product
- Transcription makes pre-mRNA from a gene, and translation produces a polypeptide with a specific primary structure.
| Feature | Transcription | Translation |
|---|---|---|
| Template | DNA template strand | mRNA |
| Product | pre-mRNA / mRNA | Polypeptide |
| Site | Nucleus | Ribosome |
| Pairing | DNA base with RNA nucleotide | Codon with anticodon |
Section 4 of 5Worked Example
Question
- An mRNA has the codons AUG GCA CUU.
- Give the tRNA anticodons that would pair with them, and state the amino acids if GCA = alanine and CUU = leucine.
- Anticodons are complementary to the codonsAUG pairs with UAC.
- GCA pairs with CGU, and CUU pairs with GAA.
- The amino acids are methionine for AUG, alanine for GCA and leucine for CUU.
Answer
- Anticodons UAC, CGU, GAA;
- the polypeptide begins Met–Ala–Leu.
Section 5 of 5Exam Checklist & Tips
2.3.4 – Key Points for the Exam
- Transcription The synthesis of mRNA by RNA polymerase using a DNA template strand.
- Translation The assembly of amino acids into a polypeptide at a ribosome, following the mRNA codons.
- RNA polymerase The enzyme that joins RNA nucleotides during transcription.
- tRNA A molecule that carries a specific amino acid and has an anticodon.
- Ribosome The site of protein synthesis.
Common Mistakes
- Saying that DNA leaves the nucleus; the mRNA copy leaves, and DNA stays in the nucleus.
- Mixing up codon (on mRNA) and anticodon (on tRNA).
- Stating that translation happens in the nucleus.
- Using DNA polymerase for transcription instead of RNA polymerase.
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