8876 / 2027

Lesson 4 of 6 / DNA and gene expression

Copy a gene into RNA

Which DNA strand is the template?

I can describe transcription and relate template, coding and RNA sequences.

  • I can describe transcription, pre-mRNA processing and translation.Syllabus

    Syllabus 8876, Core 2(c). Describe transcription, pre-mRNA processing and translation.

About 5 min

Make a guess. You are not marked.

Is mRNA identical to the DNA template strand it is copied from?

  1. It is identical, base for base.
  2. It is complementary to the template.
  3. It is identical except U replaces T.
Show the answer

It is complementary to the template.

mRNA matches the other strand, the coding strand, except that U replaces T.

The key idea

RNA polymerase uses one DNA strand as a template, making a complementary RNA strand 5' to 3'. The DNA sequence remains in the chromosome.

Explore the idea

Pair RNA with the template

DNA template3' - TAC GGA ACT - 5'Predict the RNA first

Work along the displayed template 3' to 5'. Pair T with A, A with U, and G with C to make the 5'-to-3' RNA.

Explanation

Transcription makes an RNA copy from a particular DNA region. RNA polymerase binds to the promoter. The promoter is a DNA sequence at the start of a gene where RNA polymerase attaches. RNA polymerase unwinds the DNA and breaks the hydrogen bonds between complementary bases, so the two strands separate locally. One DNA strand acts as the template. Only a short stretch is open at a time.

The template is read 3' to 5' while RNA is synthesised 5' to 3'. Free RNA nucleotides pair with template bases by complementary base pairing: A pairs with U in RNA, T with A, G with C and C with G. RNA polymerase joins adjacent RNA nucleotides with phosphodiester bonds as the chain extends.

The non-template, or coding, DNA strand has the same base order as the RNA when both are written 5' to 3', except that DNA has T where RNA has U. This is a comparison of sequences, not a claim that the coding strand is the one being copied by complementary pairing.

Transcription stops when RNA polymerase reaches the terminator. The terminator is a DNA sequence at the end of a gene that signals the end of transcription. The RNA transcript is released and the DNA strands rejoin. For a eukaryotic protein-coding gene, the initial transcript is pre-mRNA that must be processed. Transcription does not join amino acids or convert the DNA molecule into RNA material.

Step by step
  1. 1

    Find the template direction

    RNA polymerase reads the template 3' to 5'.

  2. 2

    Apply RNA complementarity

    Use U opposite template A, with the other complementary pairs.

  3. 3

    Keep the stages separate

    The product is RNA, which can later be processed and translated.

Worked example

Transcribe a short template

A template segment is 3'-TAC GGA ACT-5'. Write the corresponding RNA.

One way to explain it

5'-AUG CCU UGA-3'. The RNA is complementary and antiparallel to the template, and contains U rather than T.

Why this answer works
  • Start at the template's 3' end as displayed.
  • Pair each base correctly.
  • Label the RNA 5' to 3'.

Watch out for this

A student says: "mRNA is identical to the DNA template strand." What is wrong with this?

Show the answer

mRNA is complementary to the template. It resembles the coding strand when both are written 5' to 3', with U replacing T.

You may meet this in school papers

Not required by the 2027 syllabus, but school papers still ask it. Learn it after the main idea.

The lac operon in bacteria

Older H1 and H2 papers ask about the lac operon. In E. coli, the genes for lactose use sit together under one promoter and an operator. When no lactose is present, a repressor protein binds the operator and blocks RNA polymerase. When lactose is present, a form of it binds the repressor and changes its shape, so it leaves the operator and the genes are transcribed. The 2027 H1 syllabus does not require operons; it is a clear example of a gene being switched on and off at transcription.

Check your understanding

Which RNA matches the template 3'-TGA-5'?

  1. 3'-ACU-5'.
  2. 5'-ACU-3'.
  3. 5'-TGA-3'.
Show the answer

5'-ACU-3'.

The RNA is complementary and antiparallel.

Exam-style questions

Check your understanding

A DNA template strand reads 3' TAC GCA 5'. Which mRNA is transcribed?

  1. 3' AUG CGU 5'
  2. 5' ATG CGT 3'
  3. 5' UAC GCA 3'
  4. 5' AUG CGU 3'
Show the answer

5' AUG CGU 3'

mRNA is complementary to the template: T to A, A to U, C to G, G to C, running 5' to 3'.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which DNA strand is the template?

You can return to this lesson any time.