How is the information in a gene used to make a protein?
Gene expression through transcription and translation, the role of mRNA, tRNA and ribosomes, RNA splicing of the primary transcript, and how one gene can give rise to different proteins.
An SQA Higher Biology answer on gene expression, covering transcription and translation, the structure of RNA, the roles of mRNA, tRNA and ribosomes, RNA splicing of the primary transcript, and how alternative splicing produces different proteins.
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What this key area is asking
The SQA wants you to explain how genes are expressed through transcription and translation, describe the structure and roles of mRNA, tRNA and ribosomes, explain how the primary transcript is processed by RNA splicing, and explain how one gene can give rise to more than one protein.
RNA and how it differs from DNA
These differences matter for gene expression: because RNA is single-stranded and short-lived, it can carry a copy of a gene out to the cytoplasm and be made and broken down quickly, without putting the master copy of DNA in the nucleus at risk.
Transcription
RNA polymerase moves along the gene, unwinding the DNA and adding complementary RNA nucleotides to build the primary transcript. It pairs each DNA base with its RNA partner, so DNA adenine pairs with RNA uracil, DNA thymine with adenine, and the G-C pairs as usual. This first copy contains both exons (coding regions) and introns (non-coding regions).
RNA splicing
Because the exons of one gene can be spliced together in different orders or combinations (alternative RNA splicing), a single gene can give rise to different mature transcripts and therefore different proteins. This is one reason humans have only about 20,000 genes yet make a far larger number of proteins.
Translation
The mature mRNA leaves the nucleus and binds to a ribosome. The order of codons in the mRNA is read three bases at a time, and this determines the order of amino acids in the polypeptide. Each codon (a triplet of mRNA bases) is read in turn:
- tRNA molecules carry specific amino acids; each has an anticodon of three bases that pairs with a complementary codon on the mRNA.
- The ribosome joins the amino acids together with peptide bonds, forming the polypeptide.
- Translation continues until a stop codon is reached and the polypeptide is released. A start codon marks where translation begins.
The polypeptide then folds into a specific three-dimensional shape, and may be modified, to form the functional protein. The shape determines the protein's function, which is why even a small change to the amino acid sequence can stop a protein working.
Examples in context
Example 1. Antibody diversity from alternative splicing. The cells of the immune system make a huge variety of antibody proteins, partly through alternative RNA splicing of the same genes. By joining exons in different combinations, one gene region can produce both a membrane-bound antibody and a secreted antibody, showing how splicing expands the range of proteins from a limited number of genes.
Example 2. Sickle-cell anaemia and the protein link. In sickle-cell anaemia, a single base change in the gene for haemoglobin changes one codon, so one amino acid in the polypeptide is altered. This small change in the amino acid sequence alters the shape of the haemoglobin protein, making red blood cells sickle-shaped. The example shows the direct chain from the mRNA codon sequence, through translation, to the final protein and the organism's characteristics.
Try this
Q1. State where transcription and translation each occur in a eukaryotic cell. [2 marks]
- Cue. Transcription in the nucleus; translation at a ribosome in the cytoplasm.
Q2. Explain how one gene can code for more than one protein. [2 marks]
- Cue. The exons of the primary transcript can be spliced together in different combinations, giving different mature transcripts.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe the process of transcription and explain how RNA splicing produces a mature mRNA transcript.Show worked answer →
A 4-mark answer needs the transcription steps plus the role of splicing.
In transcription, RNA polymerase moves along the gene in the nucleus, unwinding the DNA and adding complementary RNA nucleotides to a DNA template strand to build a primary transcript. The primary transcript contains both exons (coding regions) and introns (non-coding regions).
In RNA splicing, the introns are removed and the exons are joined together to form the mature mRNA transcript that will be translated.
Markers reward (1) RNA polymerase makes a complementary copy from a template, (2) the primary transcript contains exons and introns, (3) introns are removed and (4) exons are joined to give the mature transcript.
SQA Higher 20213 marksA length of mature mRNA contains 360 bases. Determine the maximum number of amino acids in the polypeptide it codes for, and explain your reasoning using the genetic code.Show worked answer →
This is a triplet code calculation.
Step 1. Each amino acid is coded by a codon, which is a triplet of three mRNA bases.
Step 2. Divide the number of bases by three: codons.
Step 3. One of these codons will be a stop codon, which does not code for an amino acid, so the maximum number of amino acids is .
Markers reward the triplet rule, the division giving 120 codons, and the adjustment for the stop codon.
SQA Higher 20182 marksExplain how one gene can give rise to more than one protein.Show worked answer →
A 2-mark explain answer needs alternative splicing.
The primary transcript of a single gene contains several exons. During RNA splicing these exons can be joined together in different combinations, producing different mature mRNA transcripts from the same gene.
Because each different mature transcript is translated into a different sequence of amino acids, one gene can give rise to several different proteins.
Markers reward (1) exons spliced in different combinations and (2) different mature transcripts give different proteins.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)