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4.8.9Additional lessons

Genes and Polypeptides: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 76 of 101

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Finding Genes and Loci

Genome databases
  • 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
  • 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
  • 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
  • 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.

Part 2 of 5Comparing Sequences

Comparing Sequences
Alignment tools
  • Alignment tools Programs such as BLAST line up two base or amino acid sequences and mark where they match and differ.
Percent identity
  • Percent identity The percentage of positions that are identical in an alignment is used as a measure of similarity.
Evolutionary meaning
  • 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
  • Conserved sequences Regions that are the same across many species are important for function, so mutations there are usually harmful.
Protein comparison
  • Protein comparison Amino acid sequences change more slowly than base sequences because of the degenerate genetic code.
StepAction
1Search the gene name in the database
2Record locus, chromosome and product
3Copy two sequences into an alignment tool
4Calculate 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?

  1. Percent identity = identical positions ÷ total positions × 100.
  2. 19 ÷ 20 × 100 = 95%.
  3. 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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