6.2.46 · Nucleic Acids and Protein Synthesis
Transcription — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
6.2.4 Transcription — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how RNA polymerase makes mRNA from a DNA template, and distinguish the template strand from the coding strand.
- DNA stays in the nucleus
- Many copies
- One strand copied
- Template (antisense) strand
- Coding (sense) strand
- Codons and anticodons
Section 1 of 9Why transcription is needed
DNA stays in the nucleus
- DNA stays in the nucleus it is too valuable to leave the nucleus and too large to pass through nuclear pores.
mRNA copy
- mRNA copy transcription makes a short, disposable, single-stranded copy of one gene, and a single-stranded molecule can fit through nuclear pores.
Many copies
- Many copies many mRNA copies can be made from one gene, so much protein can be produced.
Control
- Control the amount of transcription can be regulated, so genes are switched on or off.
Section 2 of 9Template and coding strands
1One strand copied
- One strand copied only one of the two DNA strands in a gene is transcribed.
2Template (antisense) strand
- Template (antisense) strand the strand used as the template, also called the transcribed strand.
3Coding (sense) strand
- Coding (sense) strand the non-transcribed strand, which has the same base sequence as the mRNA with T in place of U.
4Codons and anticodons
- Codons and anticodons mRNA codons match the non-template strand and tRNA anticodons match the template strand, with U replacing T.
Section 3 of 9Steps of transcription
1
RNA polymerase binds to the DNA at the start of the gene, the promoter region.
2
Hydrogen bonds between base pairs break and the strands separate, exposing the template strand bases; only about ten to twenty base pairs are open at a time.
3
Free RNA nucleotides (triphosphates) pair with the exposed template bases: A with U, T with A, G with C, C with G.
4
RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds, forming the sugar–phosphate backbone of the mRNA in the 5′ to 3′ direction.
5
As the polymerase moves on, the DNA behind it re-forms its double helix and the mRNA peels away.
6
At a terminator sequence the polymerase and finished mRNA are released.
7
The mRNA leaves the nucleus through a nuclear pore.
Section 4 of 9Processing in eukaryotes
1Pre-mRNA
- Pre-mRNA the first RNA product contains exons (coding) and introns (non-coding), so it is also called the primary transcript.
2Splicing
- Splicing introns are removed and exons are joined to form mature mRNA, a process of post-transcriptional modification.
3Prokaryotes
- Prokaryotes their genes have no introns, so mRNA is used directly.
Section 5 of 9Control of transcription
Transcription factors
- Transcription factors in eukaryotes, these proteins bind the promoter before RNA polymerase can attach.
Hormone example
- Hormone example oestrogen enters a cell, binds a receptor, and the complex acts as a transcription factor that switches on specific genes.
Meaning
- Meaning a signal outside the cell changes which proteins are made by changing transcription factor or polymerase activity, not the DNA sequence.
Data
- Data more mRNA of a gene in a tissue means the gene is transcribed more actively there.
- Pancreas cells have much insulin mRNA but muscle cells almost none, though both have the gene,
- so differentiated cells express only part of their genome.
Section 6 of 9Replication and transcription compared
| Feature | DNA replication | Transcription |
|---|---|---|
| Enzyme | DNA polymerase | RNA polymerase |
| Region copied | Whole genome | One gene |
| Strands used | Both | One (template) |
| Product | Two DNA molecules | One mRNA molecule |
| Primer | Needed | Not needed |
| Proof-reading | Good | Less, but errors matter little as mRNA is short-lived and many copies are made |
Section 7 of 9More from the specification
More from the specification
Transcription in Protein Synthesis
- Transcription occurs in the eukaryotic nucleus where RNA polymerase synthesizes a single-stranded messenger RNA (mRNA) from the template DNA strand;
- base triplets on mRNA called codons code for specific amino acids.
Translation and Polypeptide Assembly
- In the second stage of protein synthesis, translation occurs in the cytoplasm where ribosomes and transfer RNA (tRNA) read mRNA codons in sequence to join amino acids with peptide bonds into a functioning polypeptide chain.
Gene Mutations and Protein Function
- A gene mutation involving nucleotide base substitution, insertion,
- or deletion changes the triplet code;
- such mutations can alter the sequence of amino acids in the primary structure, changing tertiary folding and disrupting the normal biological function of the protein.
Transcription in Protein Synthesis
- Transcription occurs in the eukaryotic nucleus where RNA polymerase synthesizes a single-stranded messenger RNA (mRNA) from the template DNA strand;
- base triplets on mRNA called codons code for specific amino acids.
Translation and Polypeptide Assembly
- In the second stage of protein synthesis, translation occurs in the cytoplasm where ribosomes and transfer RNA (tRNA) read mRNA codons in sequence to join amino acids with peptide bonds into a functioning polypeptide chain.
Gene Mutations and Protein Function
- A gene mutation involving nucleotide base substitution, insertion,
- or deletion changes the triplet code;
- such mutations can alter the sequence of amino acids in the primary structure, changing tertiary folding and disrupting the normal biological function of the protein.
Protein synthesis
- Making proteins from amino acids using the DNA code.
Polypeptide
- A chain of three or more amino acids.
Mutation
- A random change in genetic material.
Translation
- Using the mRNA code to build a protein.
Triplet
- Three consecutive DNA bases that code for one amino acid.
Insertion
- Addition of one or more nucleotides into a DNA sequence.
Complementary
- Having a matching shape.
Universal
- Describes a code that is the same in nearly all organisms.
Amino acid
- The unit from which proteins are made; the sequence decides the protein's shape and function.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| RNA polymerase | The enzyme that joins RNA nucleotides to make mRNA during transcription. |
| Template strand | The DNA strand that is copied during transcription. |
| Coding strand | The DNA strand with the same base sequence as the mRNA (T for U). |
| Pre-mRNA | The initial RNA transcript in eukaryotes, containing introns and exons. |
| Exon | A coding region of a gene that is kept in mature mRNA. |
| Intron | A non-coding region removed from pre-mRNA by splicing. |
| Promoter | A DNA sequence at the start of a gene where RNA polymerase binds. |
Section 9 of 9Exam technique
1
Saying RNA polymerase uses both DNA strands — only the template strand is copied for a given gene.
2
Saying thymine pairs with adenine in mRNA — uracil pairs with adenine on the template.
3
Confusing RNA polymerase with DNA polymerase.
4
Thinking the whole DNA molecule is transcribed — only individual genes are.
5
Writing that introns are translated — they are removed from pre-mRNA first.
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