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4.2.1Additional lessons

Transcription in Protein Synthesis — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 102 of 142

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Part 1 of 5Stages of Transcription

4.2.1 Transcription in Protein Synthesis

Transcription is the first stage of gene expression, making an RNA copy of a gene.

1
In the nucleus, the DNA double helix unwinds at the gene and the hydrogen bonds between base pairs break.
2
One strand, the template strand, is exposed and free RNA nucleotides pair with its bases by hydrogen bonds.
3
RNA polymerase joins the RNA nucleotides by covalent bonds, building the sugar-phosphate backbone of the mRNA.
4
When the gene has been copied, the mRNA separates from the DNA and the DNA double helix re-forms.
5
The mRNA leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm.
DNA templateATGC
mRNA baseUACG

Part 2 of 5Key Features

Several terms describe the molecules and strands involved and are often confused.

Template strand
  • The DNA strand that is read by RNA polymerase and is complementary to the mRNA that is made.
Coding strand
  • The other strand, whose base sequence matches the mRNA apart from thymine being replaced by uracil.
RNA polymerase
  • The enzyme that links RNA nucleotides together; DNA polymerase, in contrast, is used in DNA replication.
mRNA
  • A single-stranded molecule that carries the genetic message from the nucleus to the ribosome.

Part 3 of 5Transcription and Gene Expression

Transcription and Gene Expression
Selective expression
  • Cells transcribe only the genes they need, so not all of the roughly 20,000 protein-coding genes are active in every cell.
Switching genes
  • Controlling whether a gene is transcribed is a key way to switch it on or off.
DNA stability
  • Hydrogen bonds and strong phosphodiester bonds make DNA stable, so it remains a reliable template over many cell divisions.
Why mRNA
  • DNA is too large to pass through nuclear pores, so a smaller mRNA copy carries the message out.

Part 4 of 5Worked Example

Question

  • The template strand of a gene reads TAC GGA AGA CTT GGG.
  • Write the mRNA sequence.

  1. Pair each template baseT with A, A with U, C with G, G with C.
  2. TAC becomes AUG and GGA becomes CCU.
  3. AGA becomes UCU, CTT becomes GAA and GGG becomes CCC.

Answer The mRNA is AUG CCU UCU GAA CCC.

Part 5 of 5Exam Checklist & Tips

4.2.1 – Key Points for the Exam

  • Transcription The synthesis of mRNA using a DNA template strand.
  • RNA polymerase The enzyme that builds an mRNA strand from RNA nucleotides.
  • Template strand The DNA strand that is transcribed.
  • mRNA Messenger RNA, carrying the code for a polypeptide to the ribosome.
  • Gene expression The use of a gene's information to make a product such as a protein.

Common Mistakes

  • Mixing up RNA polymerase with DNA polymerase.
  • Writing thymine in mRNA; RNA contains uracil, which pairs with adenine.
  • Saying the mRNA copies the coding strand base by base; it is complementary to the template strand.
  • Placing transcription in the cytoplasm of eukaryotes; it takes place in the nucleus.

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