4.2.1Additional lessons
Transcription in Protein Synthesis — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 30 of 101
Part 1 of 5Two Stages of Gene Expression
Protein synthesis needs the gene (DNA) to be read in two linked steps.
Transcription
- Transcription The base sequence of one gene is copied into a single-stranded messenger RNA molecule,
- and in eukaryotes this takes place inside the nucleus.
Translation
- Translation A ribosome reads the mRNA codons in the cytoplasm and joins amino acids into a polypeptide in the order the codons specify.
mRNA
- mRNA This is a mobile working copy of one gene, so the permanent DNA never has to leave the safety of the nucleus.
RNA polymerase
- RNA polymerase This enzyme builds the mRNA strand by joining RNA nucleotides that pair with the exposed DNA template.
| Feature | DNA | mRNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Double stranded | Single stranded |
| Length | Whole gene plus control regions | Copy of one gene only |
Part 2 of 5Stages of Making mRNA
1
RNA polymerase attaches to the start of a gene and the two DNA strands separate, breaking hydrogen bonds between base pairs.
2
One strand, the template strand, is read while free RNA nucleotides line up opposite it by complementary base pairing.
3
Pairing rules are A with U, T with A, C with G and G with C, so the mRNA carries a sequence complementary to the template.
4
RNA polymerase links neighbouring nucleotides with phosphodiester bonds and the mRNA grows by adding to its 3' end.
5
At the end of the gene the mRNA is released, the DNA strands rejoin, and the mRNA moves out through a nuclear pore.
Part 3 of 5Why Copy the Gene?
Why Copy the Gene?
Protection
- Protection Keeping the original DNA in the nucleus lowers the risk of damage from enzymes and chemical reactions in the cytoplasm.
Amplification
- Amplification Many mRNA copies can be made from one gene, so a cell can make large amounts of the same protein quickly.
Regulation
- Regulation A cell can control which proteins it makes by switching particular genes on or off at the transcription stage.
Short life
- Short life mRNA is broken down after use, so protein production can be stopped when the product is no longer needed.
Part 4 of 5Worked Example
Question
- A template strand reads 3' TACGGATTC 5'.
- Write the mRNA sequence produced.
- Pair each template base with its RNA partner: T gives A, A gives U, C gives G, G gives C.
- T-A-C-G-G-A-T-T-C becomes A-U-G-C-C-U-A-A-G.
- The mRNA is made 5' to 3', which matches the order read from the template 3' to 5'.
Answer 5' AUGCCUAAG 3'
Part 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Transcription Synthesis of an RNA molecule using one strand of DNA as a template.
- Translation Assembly of a polypeptide at a ribosome according to the codon sequence of mRNA.
- Template strand The DNA strand that RNA polymerase reads to make the complementary RNA.
- RNA polymerase Enzyme that links RNA nucleotides together during transcription.
- Complementary base pairing Specific pairing of A with T or U, and of C with G.
Common Mistakes
- Saying that DNA leaves the nucleus; only the mRNA copy does.
- Writing thymine in an mRNA sequence; RNA contains uracil instead.
- Confusing transcription with replication;
- transcription copies one gene into RNA, not the whole genome into DNA.
- Forgetting that only one DNA strand, not both, is used as the template for a given gene.
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