IB Biology HLClassifying Living DiversityPaper 1 & 2~11 min read
Comparing Genome Sizes
A human genome runs to about 3000 million base pairs. A small Japanese woodland plant carries around 150 000 million. If genome size measured how sophisticated an organism is, that plant would be running the planet.
📚 What you need to know
Sequencing lets genome-wide comparisons be made between individuals and between species.
Viruses and bacteria have very small genomes; prokaryotes generally have smaller genomes than eukaryotes; plant genome sizes vary widely.
Genome sizes can be looked up in databases such as the Animal Genome Size Database and the Plant DNA C-Values Database.
Databases give sizes as a C-value: the haploid nuclear DNA content of an organism.
C-values are given in picograms (pg), where 1 pg = 10–12 g, or in megabases (Mb), where 1 Mb = 106 bases.
1 pg = 978 Mb.
Genome size is not clearly related to complexity. Similar species can have very different genome sizes.
Reasons include: the genome holds all an organism’s DNA, plants can be polyploid, and our idea of ‘complexity’ may be wrong.
How much the numbers differ
Organism
Common name or description
Genome size (million base pairs)
Enterobacteria phage T2
A virus that infects E. coli
0.17
Escherichia coli
E. coli bacteria
5
Drosophila melanogaster
Fruit fly
140
Homo sapiens
Human
3000
Paris japonica
Japanese canopy plant
150 000
Those numbers span nearly a million-fold range, which is impossible to draw on a normal scale. Plotting them on a scale where each step is ten times the last one makes the pattern visible.
Watch the scale carefully. Each bar looks only a little longer than the one above, but every tick along the bottom is a ten-fold jump.
C-values and units
Anyone can look up a genome size in a database. Sizes are given as a C-value:
C-value
The haploid nuclear DNA content of an organism
Two sets of units are used, and questions often ask you to move between them.
Unit
What it measures
The relationship
Picogram (pg)
The mass of DNA
1 pg = 10–12 g
Megabase (Mb)
The number of bases
1 Mb = 106 bases
Converting
Mass to bases
1 pg = 978 Mb
Notice that a C-value is the haploid content. A human body cell is diploid, so it holds about twice the C-value. If a question gives you a C-value and asks about a body cell, that factor of two is usually where the marks are.
Size against complexity
You would expect bigger, more complicated organisms to have bigger genomes. Plenty of examples refuse to cooperate:
Organism
Genome size (Mb)
The problem
Human
3100
The baseline we would expect to be near the top
Hagfish
4200
A jawless fish, with a larger genome than a human
Common wheat
17 000
Over five times the human genome
Red wild einkorn wheat
5000
A closely related wheat with roughly a third of the DNA
Three things explain most of these surprises.
The genome contains all of an organism’s DNA, not just the parts that code for proteins. A large genome may simply hold a great deal of non-coding DNA.
Plants can be polyploid — their cells contain many complete sets of chromosomes rather than two. That multiplies the genome size without adding new kinds of gene.
Our view of complexity may not be correct. We tend to equate complexity with brain function, but there are other ways for an organism to be complex.
Polyploidy explains the wheat pair. Common wheat and red wild einkorn wheat are close relatives, and the difference in genome size is not about one being more advanced. Common wheat carries several full sets of chromosomes, so it has several copies of much the same information.
Worked examples
WE 1
Convert a C-value
A database gives the C-value of an organism as 3.5 pg. Calculate its genome size in megabases. (2 marks)
Step 1: the conversion factor1 pg = 978 MbStep 2: multiply
3.5 × 978 = 3423
3423 Mb, or about 3400 Mbkeep the unit — a bare number scores nothing in a conversion question
WE 2
Compare two genome sizes
Using the table above, calculate how many times larger the Paris japonica genome is than the human genome. (2 marks)
Step 1: set up the ratio
150 000 Mb ÷ 3000 Mb
Step 2: calculate
= 5050 times larger“how many times larger” means divide, and the answer has no unit
WE 3
Explain an unexpected result
A student is surprised that a hagfish has a larger genome than a human. Suggest two reasons for this. (3 marks)
Reason 1: what a genome contains
A genome contains all of an organism’s DNA, including non-coding sequences, so a larger genome does not mean more genes.
Reason 2: the assumption behind the surprise
Genome size is not related to complexity, and our idea of what makes an organism complex may itself be wrong.
Adding weight
Other examples show the same thing, such as common wheat having a far larger genome than either species.
Genome size measures DNA, not sophisticationmention polyploidy if the organism in the question is a plant
💡 Exam tips
Learn the conversion 1 pg = 978 Mb. It is the one number that gets tested.
Define a C-value as the haploid nuclear DNA content.
State the general pattern: viruses and bacteria small, prokaryotes smaller than eukaryotes, plants highly variable.
Never say a larger genome means a more advanced organism.
Use polyploidy when explaining very large plant genomes.
Read log-scale axes carefully — each tick is a ten-fold jump, not a fixed addition.
⚠ Common mistakes
Assuming genome size tracks complexity. Hagfish and wheat both break that idea.
Treating a C-value as diploid. It is the haploid content.
Mixing up pg and Mb. One is a mass, the other a number of bases.
Reading a log axis as if it were linear. The bars are compressed on purpose.
Saying a bigger genome must have more genes. Much of it is non-coding.
Forgetting units in a conversion answer.
Up next: Uses of Genome Sequencing — what all this sequence data is actually for, from evolutionary trees to choosing a patient’s drug.
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