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4.2.6Genetics, Variation and Interdependence

Nucleic Acid and Amino Acid Sequence Comparison — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 14 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.2.6 Nucleic Acid and Amino Acid Sequence Comparison — AQA A Level Biology 7402 notes: Explain how comparing base sequences and amino acid sequences can be used to determine evolutionary relationships, and interpret data on sequence differences.Species descended from a common an…

  • DNA sequencing
  • DNA hybridisation
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Molecular clock

Section 1 of 9The basis of molecular comparison

Species descended from a common ancestor inherited the same genes.

The basis of molecular comparison
Over time the genes accumulate mutations,
  • so the longer ago two species diverged, the more differences exist between their base sequences,
    • and between the amino acid sequences of the proteins that those genes code for.
By counting differences, scientists can estimate how closely related species are.
This approach is called molecular phylogenetics, and it has largely replaced classification based only on physical appearance,
  • which can be misleading because of convergent evolution.

Section 2 of 9Comparing DNA

Two approaches are used to compare DNA.

DNA sequencing
DNA sequencing the order of bases in the same gene is determined for each species.
  • The sequences are aligned and the number of differences counted.
  • A smaller number of differences means a more recent common ancestor.
DNA hybridisation
DNA from two species is cut into short fragments and separated into single strands.
  • Strands from the two species are mixed, and they pair where the sequences are complementary.
  • The mixture is heated gradually, and the temperature at which the strands separate is measured.
  • The more bonds there are between complementary bases, the more heat energy is needed to break them,
    • so a higher separation temperature suggests the sequences are more similar and the species more closely related.

Section 3 of 9Comparing amino acid sequences

The sequence of amino acids in a protein is determined by the sequence of bases in the gene, so comparing proteins also gives evidence of relatedness.

1
Proteins found in many different organisms, such as cytochrome c (used in respiration) or haemoglobin, are often used.
2
For example, the cytochrome c of humans is identical to that of chimpanzees, differs by only one amino acid from that of the rhesus monkey,
  • and differs by many more from that of yeast.
3
The more differences, the more distantly related the species.

Immunological comparison is another approach. Antibodies against the albumin of one species are mixed with the serum of others, and the amount of precipitate shows how similar the proteins are.

Section 4 of 9Advantages and limitations

MethodStrengthLimitation
DNA sequencingMost precise, including non-coding DNA, which accumulates neutral mutationsRequires specialist equipment and data analysis
Amino acid sequencingReflects the functional gene productsDegenerate code means that two species with different DNA may have identical protein; fewer data than DNA
DNA hybridisationCompares whole genomesGives only an overall estimate, not exact differences

Because the genetic code is degenerate, a DNA sequence is more informative than the amino acid sequence derived from it: some mutations in the DNA are silent in the protein.

Section 5 of 9Using the data

Data are often given as a table of the number of base or amino acid differences between pairs of species, and you are asked to decide which species is the closest relative, or to construct a phylogenetic tree.

Using the data
The pair with the smallest number of differences shared the most recent ancestor.
Because the rate of mutation is roughly constant for certain sequences, the number of differences can be used as a molecular clock to estimate how long ago two species diverged.
For a valid conclusion the same gene or protein must be compared in all species, and a larger sample of sequences makes the conclusion more reliable.

This topic links to classification, the theory of evolution by natural selection and the interpretation of phylogenetic trees.

Section 6 of 9Building and reading a phylogenetic tree

The results of comparing sequences can be shown as a phylogenetic tree.

Building and reading a phylogenetic tree
Each branch point (node) represents a common ancestor, and the species that branch most recently from one another are the most closely related.
If the tree is drawn to scale, the length of each branch represents the number of changes in the sequence, or the amount of time that has passed.
To construct a simple tree from a table, find the pair of species with the fewest differences and join them first;
  • then add the species with the next lowest number of differences, and continue until all species are included.

Always check whether the conclusion is supported by enough data.

Building and reading a phylogenetic tree
A tree based on a single short gene may not show the true relationship, and different genes may give different trees,
  • so scientists compare many genes.
Remember also that rates of mutation differ between lineages, so the molecular clock is only a rough guide.
Finally, molecular evidence is combined with fossil evidence and anatomical comparisons to give the best overall picture of how organisms are related.

Section 7 of 9Exam-style questions

  1. Explain how a comparison of the amino acid sequence of cytochrome c can show how closely related two species are. [3]3 marks

  2. The cytochrome c of four species has been compared with that of humans. Differences in amino acids: chimpanzee 0, rhesus monkey 1, dog 13, yeast 44. Calculate the percentage difference in the 104 amino acid chain for the dog, and state which is the most distant relative. [3]3 marks

  3. In DNA hybridisation, hybrid DNA from species X and Y separated at 82 degrees C, and hybrid DNA from X and Z separated at 76 degrees C. Which species is more closely related to X? Explain your answer. [3]3 marks

Section 8 of 9Key terms

TermMeaning
PhylogenyThe evolutionary relationships between species.
DNA hybridisationA technique comparing DNA by the temperature at which hybrid strands separate.
Molecular clockThe use of the number of sequence differences to estimate the time since divergence.
Cytochrome cA protein of the electron transport chain used in sequence comparisons.
Convergent evolutionEvolution of similar features in unrelated organisms.
Common ancestorThe species from which two or more later species are descended.

Section 9 of 9Exam technique

1
Saying that more differences means species are more closely related.
2
Comparing the amino acid sequences and claiming this shows the whole DNA is different; the code is degenerate.
3
Forgetting that DNA hybridisation depends on hydrogen bonds between complementary bases.
4
Using sequences of different genes or proteins when comparing different species.
5
Claiming that molecular evidence proves a relationship rather than supports it.

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