4.8.9Additional lessons
Genes and Polypeptides: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 76 of 101
Part 1 of 5Finding Genes and Loci
Genome databases
- After the human genome was sequenced, public databases recorded the exact position and sequence of each gene for anyone to search.
Locus notation
- A locus is written as chromosome number, arm and band, for example 7q31.2, where p is the short arm and q is the long arm.
Gene to protein
- Entries link a gene to its mRNA and to the amino acid sequence of the polypeptide that it codes for.
Disease genes
- CFTR,
- which codes for a chloride channel and causes cystic fibrosis when mutated, lies on chromosome 7;
- the HBB gene for the β-globin chain of haemoglobin lies on chromosome 11.
- which codes for a chloride channel and causes cystic fibrosis when mutated, lies on chromosome 7;
Part 2 of 5Comparing Sequences
Comparing Sequences
Alignment tools
- Programs such as BLAST line up two base or amino acid sequences and mark where they match and differ.
Percent identity
- The percentage of positions that are identical in an alignment is used as a measure of similarity.
Evolutionary meaning
- The more similar the sequences of a gene in two species, the more recently they are likely to share a common ancestor.
Conserved sequences
- Regions that are the same across many species are important for function, so mutations there are usually harmful.
Protein comparison
- Amino acid sequences change more slowly than base sequences because of the degenerate genetic code.
| Step | Action |
|---|---|
| 1 | Search the gene name in the database |
| 2 | Record locus, chromosome and product |
| 3 | Copy two sequences into an alignment tool |
| 4 | Calculate the percent identity |
Part 3 of 5Using Databases Critically
1
Use a recognised database run by a research institute so that the data are curated and reliable.
2
Search by gene name, then check the species to be sure you are looking at the human gene and not a similar gene from another organism.
3
Record the locus, the length of the gene and the name and function of the polypeptide product.
4
Run an alignment against the same gene from another species and note the percent identity.
5
Use the result to support an argument, remembering that genes close together on one chromosome are likely to be inherited together.
Part 4 of 5Worked Example
Question
- Two aligned segments of the same gene from human and from chimpanzee are each 20 bases long and match at 19 positions.
- What is the percent identity, and what does this suggest?
- Percent identity = identical positions ÷ total positions × 100.
- 19 ÷ 20 × 100 = 95%.
- A high value suggests a recent common ancestor, since few mutations have accumulated since the two lineages separated.
Answer Percent identity is 95%, which indicates the species are closely related.
Part 5 of 5Exam Checklist & Tips
4.8.9 – Key Points for the Exam
- Locus The position of a gene on a chromosome.
- Database An organised collection of biological data that can be searched online.
- Percent identity The proportion of positions that are identical in an alignment of two sequences.
- Polypeptide A chain of amino acids produced by translation of an mRNA.
Common Mistakes
- Confusing the p arm (short) with the q arm (long) of a chromosome.
- Treating high similarity of a short sequence as proof of relatedness without considering the length of the sequence.
- Assuming that every gene codes for a protein; some genes code for functional RNA.
- Using unreliable websites as sources of sequence data.
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