IB Biology SLTopic 3 — Classifying Living DiversityPaper 1 & 2Practical skill~10 min read
Comparing Genome Sizes: Skills
You would expect a complicated organism to need a big genome. The data says otherwise, and the reasons why are more interesting than the pattern would have been.
📚 What you need to know
Genome size varies enormously: viruses and bacteria are tiny, plants can be huge.
Prokaryotes generally have smaller genomes than eukaryotes.
Plant genome sizes vary widely, even between close relatives.
Genome size databases exist for anyone to search, e.g. the Animal Genome Size Database and the Plant DNA C-Values Database.
The C-value is the haploid nuclear DNA content of an organism.
C-values are given in mass (picograms, pg) or in bases (megabases, Mb). 1 pg = 978 Mb.
1 pg = 10−12 g and 1 Mb = 106 bases.
Genome size does not track complexity, because of non-coding DNA, polyploidy, and the fact that “complexity” is hard to define.
The range of genome sizes
The scale has to be logarithmic. On a normal axis the phage, the bacterium and the fruit fly would all sit on top of each other at zero.
Organism
What it is
Genome size (million base pairs)
Enterobacteria phage T2
A virus that infects E. coli
0.17
Escherichia coli
Bacterium
5
Drosophila melanogaster
Fruit fly
140
Homo sapiens
Human
3000
Paris japonica
Japanese canopy plant
150 000
C-values and units
Databases record genome size as a C-value: the amount of DNA in a haploid nucleus. It can be given two ways.
The conversion you need
1 pg = 978 Mb • 1 pg = 10−12 g • 1 Mb = 106 bases
Why measure DNA by mass at all? Because before sequencing existed you could weigh the DNA extracted from a nucleus long before you could count its bases. The picogram figures are a leftover from that era, and the 978 conversion is how the old and new measurements are joined up.
Why the pattern breaks
Compare these three and the expectation collapses immediately:
Human — about 3100 Mb
Hagfish — about 4200 Mb
Common wheat — about 17 000 Mb
Even close relatives can differ wildly. Common wheat has a genome of roughly 17 000 Mb while red wild einkorn wheat has around 5000 Mb — more than a threefold difference between two wheats.
🧩 Three reasons the numbers surprise you
The genome is all the DNA, not just the coding parts. A large genome can be mostly non-coding.
Polyploidy. Plant cells can carry many complete sets of chromosomes, multiplying the genome size without adding new genes.
“Complexity” is our word, not nature’s. We tend to equate it with brain function, but there are many other ways of being complex.
Worked examples
WORKED EXAMPLE
A species has a C-value of 3.5 pg. Calculate its genome size in megabases.
Step 1: recall the conversion
1 pg = 978 Mb
Step 2: multiply3.5 × 978 = 3423About 3423 Mbthat is roughly a human-sized genome
WORKED EXAMPLE
Using the table, calculate how many times larger the human genome is than the E. coli genome, and how many times larger Paris japonica is than the human genome.
Step 1: human compared with E. coli3000 ÷ 5 = 600Step 2: Paris japonica compared with human150 000 ÷ 3000 = 50600 times larger, and 50 times largera plant beats us by 50-fold – the perfect example for “size is not complexity”
WORKED EXAMPLE
A student is surprised that the hagfish genome (4200 Mb) is larger than the human genome (3100 Mb). Suggest two reasons why genome size does not reflect complexity. [2]
Reason 1
The genome includes all DNA, not only coding DNA, so a large genome may contain a great deal of non-coding sequence.
Reason 2
Our idea of “complexity” is not a scientific measure — we tend to judge it by brain function, which is only one kind of complexity.
More DNA does not mean more genes, and more genes does not mean more complexfor plants, polyploidy would be a third valid reason
💡 Exam tip
Learn the conversion 1 pg = 978 Mb. It is the only number in this section you must recall.
Define the C-value precisely: haploid nuclear DNA content. The word haploid earns the mark.
When comparing sizes, use ratios (“50 times larger”), not just differences.
If a question asks why the pattern is unexpected, give non-coding DNA and polyploidy as concrete reasons.
Watch the units: databases mix pg and Mb freely.
Prokaryote genomes are generally smaller than eukaryote ones — that part of the pattern does hold.
⚠ Common mix-up
Assuming bigger genome means more genes. Much of a large genome is non-coding.
Defining the C-value as the diploid DNA content. It is haploid.
Mixing pg and Mb in one calculation without converting.
Saying plants have big genomes because they are complex. Polyploidy is the actual reason.
Reading a log scale as if it were linear. Each tick is ten times the last, not ten more.
Confusing genome size with chromosome number. They are separate measures.
Up next: Uses of Genome Sequencing — what all this sequence data is actually for, from building family trees of species to choosing a patient’s medication.
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