IB Biology SLTopic 3 — Classifying Living DiversityPaper 1 & 2Core idea~12 min read
Uses of Genome Sequencing
Sequencing a genome used to take years and cost a fortune. Now it is routine, and the same technique answers two very different kinds of question: how species are related, and how one particular patient should be treated.
📚 What you need to know
DNA sequencing works out the base sequence of a genome. Modern methods are automated, fast and cheap.
Newer approaches are called next-generation sequencing (NGS).
Phylogenetics classifies species by evolutionary origins, not by appearance.
Three kinds of sequence data are compared: DNA, mRNA and amino acid sequences.
The more similar the sequences, the more closely related the species.
Species separated longer have had more time to accumulate mutations.
Comparing multiple proteins or regions gives a more accurate estimate than using just one.
A useful comparison protein must be widespread and show enough variation — cytochrome c is the classic example.
Sequence data builds phylogenetic trees, and underpins personalised medicine.
Sequencing and evolutionary relationships
Phylogenetics groups organisms by shared common ancestry rather than by how similar they look. Sequence data is far better evidence for this than appearance, because looks can mislead.
The logic is simple. Mutations build up in DNA at a rough background rate. Two species that split recently have had less time to accumulate differences, so their sequences are still very alike. Two species that split long ago have drifted much further apart.
The rule that runs this whole page
more similar sequences → more recent common ancestor
Read it right to left. Every dot is a point where one ancestral population split into two, and the percentage difference in DNA grows the further left you go.
Why cytochrome c? A protein used for comparison has to be present in a wide range of organisms and show enough variation between them to be informative. Cytochrome c fits both: it is essential to aerobic respiration, so almost everything has it, and its sequence differs just enough between species to be useful.
Sequencing really comes into its own in two awkward cases. When two species look almost identical, DNA can tell them apart. And when a species is extinct, ancient DNA can still place it on the tree — something no amount of staring at fossils could do.
Sequencing and personalised medicine
The Human Genome Project sequenced the entire human genome and stored the results in databases. That opened the door to genomic medicine: using information about genes to design and choose treatments.
Use
How it works
Benefit
Targeted drug design
Knowing a disease protein’s sequence and structure allows a drug to be designed against it, e.g. an enzyme inhibitor
More effective drugs with fewer unpleasant side-effects
Genetic screening
An individual’s genome is checked for mutations linked to particular diseases
High-risk individuals identified, so preventative steps can be taken
Predicting drug response
A patient’s genotype is used to work out how well they will respond to a treatment
Treatment chosen to suit that individual
Lifestyle decisions
Genetic risk of cancers or heart disease is combined with clinical information
Informed choices about diet and lifestyle
A concrete example: certain mutations are known to raise the risk of developing breast cancer sharply. Someone who knows they carry such a mutation can choose preventative surgery, or be monitored far more closely, long before any disease appears.
Notice how far apart the two main uses are. The technique is identical; only the question being asked changes.
Worked examples
WORKED EXAMPLE
Species A and B differ in 2% of the bases of a shared gene. Species A and C differ in 9%. State which pair shares a more recent common ancestor, and explain. [3]
Step 1: compare the differencesA and B differ by 2%; A and C differ by 9%Step 2: apply the rule
More similar sequences mean fewer accumulated mutations since the two lineages split.
Step 3: conclude
A and B have had less time to accumulate differences.
A and B share the more recent common ancestor
WORKED EXAMPLE
Explain why scientists compare several proteins rather than one when working out how closely two species are related. [2]
Step 1: the problem with one protein
A single protein may have mutated unusually fast or unusually slowly, giving a misleading result.
Step 2: the fix
Using multiple proteins or multiple regions of the genome averages out that variation.
More regions compared = a more accurate estimate of relatedness
WORKED EXAMPLE
Outline two ways in which genome sequencing can benefit a patient. [4]
Benefit 1: targeted treatment
Knowing the sequence and structure of a protein involved in a disease allows drugs to be designed against it, which means fewer side-effects.
Benefit 2: risk prediction
Screening can identify mutations that raise the risk of a disease, so preventative measures or closer monitoring can start early.
Better-targeted drugs, and earlier action on risk4 marks means two points, each with an explanation – not four separate names
💡 Exam tip
The core sentence: the more similar the sequences, the more closely related the species. Write it out.
Explain why using mutation: longer apart means more accumulated changes.
Name cytochrome c and give both reasons it is chosen — widespread and variable enough.
The three data types are DNA, mRNA and amino acid sequences.
Use the word phylogenetics when asked about classification by ancestry.
For medicine questions, give a benefit and what makes it possible, not just the name.
⚠ Common mix-up
Reading a phylogenetic tree left to right. The tips are the present; the root is the past.
Saying one species evolved from another because they sit next to each other on a tree. They share an ancestor.
Classifying by appearance and calling it phylogenetics. Phylogenetics uses ancestry.
Assuming a bigger DNA difference means a bigger difference in appearance. It means more time, not more change on the outside.
Confusing genetic screening with gene editing. Screening reads; it does not change anything.
Choosing a protein found in only a few species for comparison. It has to be widespread to be useful.
Up next: Classification & Cladistics — how the tree of life is actually built from this evidence, and why groups sometimes get rearranged.
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