How is the information in a gene used to build a protein, and how can one gene give several proteins?
Gene expression through transcription and translation, including the structure and roles of mRNA, tRNA and rRNA, RNA splicing of the primary transcript, and how alternative splicing and post-translational modification produce protein diversity.
An SQA Higher Human Biology answer on gene expression, covering transcription by RNA polymerase, the splicing of introns from the primary transcript, translation at the ribosome using mRNA and tRNA, the role of the genetic code and codons, and how alternative splicing produces protein diversity.
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What this dot point is asking
The SQA wants you to describe transcription of a gene into a primary transcript, explain RNA splicing, describe translation at the ribosome using mRNA and tRNA, and explain how alternative splicing lets one gene produce several proteins.
Transcription
RNA polymerase moves along the DNA, separating the two strands and adding complementary RNA nucleotides to one template strand. RNA differs from DNA in three ways: it has the sugar ribose, it is normally single-stranded, and it uses the base uracil (U) in place of thymine, so uracil pairs with adenine. The product is the primary transcript, a full-length mRNA copy of the gene that still contains both coding and non-coding regions.
RNA splicing
Splicing is important because it allows the cell to control which exons end up in the final mRNA. By joining exons in different combinations (alternative splicing), a single gene can produce several different mature mRNA molecules and therefore several different proteins. This greatly increases the number of proteins a genome can make without needing more genes.
Translation
The mature mRNA carries the genetic message as a series of codons, each a triplet of three bases that specifies one amino acid. Three RNA molecules cooperate in translation:
- mRNA carries the coded sequence of codons from the gene.
- tRNA (transfer RNA) carries a specific amino acid and has an anticodon that pairs with a complementary codon on the mRNA.
- rRNA (ribosomal RNA) forms part of the ribosome, the structure on which translation takes place.
At the ribosome, the mRNA is read codon by codon. Each tRNA anticodon pairs with the matching mRNA codon, delivering its amino acid; the amino acids are joined by peptide bonds to build a growing polypeptide. A start codon begins the chain and a stop codon ends it, after which the tRNA is released and reused.
After translation, the polypeptide may be folded and modified (for example by cutting or by adding carbohydrate groups) to become a functional protein. Together with alternative splicing, this post-translational modification is another way the body makes many proteins from a limited number of genes.
Examples in context
Example 1. Antibody variety from splicing. Cells of the immune system use alternative splicing of antibody genes to help produce a vast range of antibody proteins. By joining exon segments in different combinations, relatively few genes give rise to antibodies able to recognise many different antigens.
Example 2. Insulin production. A pancreatic beta cell transcribes the insulin gene, splices the primary transcript and translates the mature mRNA into a polypeptide, which is then cut and folded into functional insulin. This shows the full pathway from gene to active protein, including post-translational modification.
Try this
Q1. State the base that replaces thymine in RNA. [1 mark]
- Cue. Uracil.
Q2. Explain why splicing exons in different combinations increases the number of proteins a cell can make. [1 mark]
- Cue. Each combination gives a different mature mRNA, so one gene can be translated into several different polypeptides.
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 the splicing of the primary transcript in a eukaryotic cell.Show worked answer →
A 4-mark answer needs transcription followed by splicing.
In transcription, the enzyme RNA polymerase moves along the DNA, separating the strands and adding complementary RNA nucleotides to a template strand. In RNA, the base uracil pairs with adenine in place of thymine. This produces a primary transcript, a molecule of mRNA that is a complete copy of the gene including introns and exons.
The primary transcript is then spliced. The non-coding regions, the introns, are removed, and the coding regions, the exons, are joined together to form the mature mRNA transcript, which leaves the nucleus.
Award (1) RNA polymerase separating strands, (2) complementary RNA nucleotides with uracil for thymine, (3) introns removed, and (4) exons joined to give mature mRNA.
SQA Higher 20213 marksExplain how alternative RNA splicing allows one gene to code for more than one protein, and describe the role of tRNA in translation.Show worked answer →
This is a 3-mark application and recall question.
Alternative splicing means the exons of one primary transcript can be joined in different combinations, with different exons kept or removed. Each combination gives a different mature mRNA, so a single gene can give rise to several different polypeptides and therefore several proteins.
In translation, each tRNA carries a specific amino acid and has an anticodon. The anticodon pairs with a complementary codon on the mRNA at the ribosome, so tRNA brings amino acids in the correct order set by the mRNA.
Markers reward (1) exons joined in different combinations, (2) giving different mRNA and different proteins, and (3) tRNA carrying amino acids and anticodon-codon pairing.
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