Transcription, RNA processing and translation
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- From gene to protein
- Transcription
- A past-paper example
- A worked example
Read the transcript of the teaching part
Last time
Before we start: last time, in DNA, RNA, ATP and the genetic code, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
From gene to protein
The D N A never leaves the nucleus, yet its instructions end up building proteins out in the cytoplasm. A messenger makes the trip. First the gene is copied into messenger R N A, which is transcription. Then the message is read at a ribosome to build a polypeptide, which is translation. Both steps come up in every paper.
Transcription
Transcription happens in the nucleus. D N A helicase is not needed. R N A polymerase unwinds a short length of the helix and breaks the hydrogen bonds between the two strands. R N A polymerase binds to the template strand, also called the transcribed strand. Free R N A nucleotides pair with the exposed bases: A with U, T with A, C with G. Polymerase joins the nucleotides by phosphodiester bonds, in the five prime to three prime direction. The finished strand is pre messenger R N A. The mRNA sequence matches the non template, or coding, strand, except that uracil replaces thymine.
A past-paper example
October November 2024, Paper 13, question 23. Which strand of D N A does R N A polymerase bind to? Lagging, leading, non transcribed or template? R N A polymerase binds to the template strand, the one that is transcribed. Lagging and leading strands are words for replication. The answer is D.
A worked example
A short template strand reads three prime T A C, G G A, T T C, five prime. Pair each base with its R N A partner. The mRNA reads five prime A U G, C C U, A A G, three prime. A U G is the start codon. Three codons code for three amino acids: methionine, proline, lysine.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Transcription and RNA processing · Translation and tRNA.
The whole chapter: Nucleic Acids and Protein Synthesis, Nucleic Acids and Protein Synthesis.
Chapter 6: Nucleic acids and protein synthesis
- DNA, RNA, ATP and the genetic code 6 min
- DNA replication and gene mutations 5 min
- Transcription, RNA processing and translation 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.