1. Control of gene expression in eukaryotic cells
A-Level Biology
Martin Rowland
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2. Revision from Year 1: DNA
DNA is a double-stranded molecule.
Each of the two DNA strands is a polymer of nucleotides linked by phosphodiester bonds.
The two strands are held together by hydrogen bonds between nucleotides with complementary bases (adenine– thymine and cytosine–guanine).
A gene is a sequence of nucleotides whose base sequence encodes the amino acid sequence of a polypeptide.
In eukaryotes, each gene contains non-coding sequences of bases (introns) and coding sequences of bases (exons).
In exams, students often confuse the terms:
nucleotides and bases
phosphodiester bonds and hydrogen bonds
It is worth stressing here the correct terminology to use.
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3. A DNA molecule has two antiparallel strands of nucleotides
Complementary base pairs
Phosphodiester bond between carbon 5 of the lower nucleotide and carbon-3 of the nucleotide above
DNA polymerase catalyses the addition of nucleotides to a developing polynucleotide in one direction only.
It extends DNA by forming new phosphodiester bonds at the carbon-3 end (3’ end) of the developing chain.
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4. In eukaryotic cells, each gene contains introns and exons
This diagram represent a single gene in a eukaryotic cell.
The pink regions are non-coding introns.
The yellow and purple regions are coding exons.
It might help students to remember which is which if they learn that the exons are expressed.
A single gene with four exons and four introns
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5. Revision from Year 1: DNA transcription
DNA at the gene to be transcribed unwinds as the hydrogen bonds linking the two strands break.
One of the exposed DNA strands acts as a template (often called the anti-sense strand).
RNA nucleotides already in the nucleus are attracted by base pairing to the exposed bases of the template strand (RNA nucleotides carry uracil rather than thymine).
RNA polymerase catalyses the formation of phosphodiester bonds between the RNA nucleotides to form precursor messenger RNA (pre- mRNA).
Within the nucleus, the pre-mRNA is edited to remove the introns to form mature mRNA.
Students often become confused about terms such as coding strand, template strand and antisense strand.
So long as they make it clear that only one of the DNA strands is used to provide the base sequence to be copied to RNA, they will gain marks in an exam.
The more able students might like to discuss why the strand that is copied is often referred to as ‘antisense’.
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6. Editing of pre-mRNA
The gene shown on slide 6 is shown again being transcribed to form pre-mRNA.
The pre-mRNA is edited by removing the non-coding introns.
The more able students might like to reflect on the fact that, depending on the sequence in which the exons are assembled during editing, this gene could encode more than one polypeptide chain.
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7. Control of transcription in eukaryotic cells
A-Level Biology, Year 2
Martin Rowland
Hodder & Stoughton © 2017

8. Genes ‘switched on’
Each gene is controlled by one or more promoter regions.
The promoters are close to and ‘upstream’ of the gene (before the start of the gene).
A transcription factor is a protein that binds specifically to certain DNA base sequences.
When one or more transcription factors bind(s) to the gene’s promoter:
the promoter enables RNA polymerase to attach to the template strand of the DNA near the start of the gene
the RNA polymerase moves along the template DNA strand, synthesising an mRNA molecule with a base sequence complementary to the template stand
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9. A gene is controlled by an upstream promoter
‘upstream’ of gene direction of transcription
This slide demonstrates the meaning of ‘upstream’ of a gene.
promoter DNA gene
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10. Transcription factor(s) bind to the promoter
A simplistic representation of the binding of transcription factors to a promoter.
In reality, several proteins bind to form a complex dimer.
promoter DNA gene
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11. RNA polymerase binds
Again, a simplistic representation of the attachment of RNA polymerase to the DNA.
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12. RNA polymerase moves along gene
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13. Pre-mRNA released
promoter DNA gene
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14. Genes ‘switched off’
As we have seen, in order for a gene to be transcribed:
the appropriate transcription factor(s) must be able to attach to the promoter region of that gene
RNA polymerase must be able to attach to the start of the gene’s base sequence
Both of these attachments can be prevented in one of two ways:
the DNA remains so tightly coiled around the histone proteins within the chromosome that transcription factors and RNA polymerase cannot attach to the DNA
methyl groups (CH3) added to some of the bases in the DNA prevent the attachment of transcription factors to the gene’s promoter region
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15. Acetylation of histone
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16. Methylation of cytosine
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17. Summary
A gene is transcribed when transcription factors bind to its promoter region.
This is more likely to happen if:
the histones in the region of the gene are acetylated (acetyl groups, (COCH3 added to histone tails) because this loosens the coils of the DNA allowing RNA polymerase to attach to the exposed gene
the DNA near the gene’s promoter has few methyl groups (CH3) attached to its cytosine residues
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