IB Biology SLTopic 3 — Classifying Living DiversityPaper 1 & 2Core idea~10 min read
Genomes
The genome is not “all your genes”. It is all your DNA — genes, the stretches between them, and the DNA hidden inside your mitochondria. That distinction is the difference between a mark and no mark.
📚 What you need to know
The genome is all of the genetic information in an organism.
It includes genes that code for proteins and non-coding DNA.
In eukaryotes it includes mitochondrial DNA and chloroplast DNA.
In prokaryotes it includes plasmid DNA.
DNA sequencing works out the base sequence, allowing genome-wide comparisons.
Humans share about 99.9% of their DNA with each other and about 99% with chimpanzees.
All humans have the same coding genes; the differences are which alleles we carry.
A single nucleotide polymorphism (SNP) is a difference of one base between individuals.
Sequence data is stored in online databases such as GenBank and NCBI, which anyone can search.
What counts as part of the genome
The genome refers to the DNA present in every cell of an organism, not to one cell’s worth of it.
Test yourself with a quick question: is the DNA in a plant’s chloroplasts part of that plant’s genome? Yes. Is a bacterium’s plasmid part of its genome? Also yes. If it is DNA and the organism carries it, it is in.
How similar are genomes?
Advances in DNA sequencing mean the entire base sequence of a genome can now be worked out. Once you can do that for many individuals and many species, you can compare them directly.
The comparisons show a striking amount of similarity:
Any two humans share about 99.9% of their DNA.
Humans and chimpanzees share about 99%.
In fact, all humans carry the same coding genes. What differs is which alleles — which versions of those genes — each person has. New alleles arise through mutation.
SNPs: one letter at a time
Where a difference between two people’s DNA comes down to a single base, it is called a single nucleotide polymorphism, or SNP (said “snip”).
Most SNPs change nothing about the protein produced. A few do, which is why scientists use them to estimate ancestry and disease risk.
Why SNPs are so useful. Because we are 99.9% identical, the places where we differ are rare and therefore informative. A pattern of SNPs acts like a fingerprint that can be matched to populations, families or particular health risks.
Comparing genomes between species
Eukaryotic genomes differ from each other in two ways.
Size. Some organisms simply have genes that others lack. A plant needs genes for the enzymes of photosynthesis; you do not.
Base sequence. Even for a gene both species have, the exact sequence differs.
All this data is stored in public online databases such as GenBank and NCBI. Scientists worldwide upload their sequences, and anyone can retrieve them for analysis.
🧩 Using a sequence database
Choose a gene that is present in all the organisms you want to compare.
Choose the species to compare it across.
Choose the data type — DNA base sequence or amino acid sequence.
Run the comparison and read off the percentage similarity.
Interpret it: more similar sequences mean more closely related organisms.
Worked examples
WORKED EXAMPLE
The human genome contains about 3000 million base pairs. Two people share 99.9% of their DNA. Calculate roughly how many base pairs differ between them.
Step 1: find the percentage that differs100 − 99.9 = 0.1%Step 2: take 0.1% of the genome3000 × 0.001 = 3 million base pairsAbout 3 000 000 base pairsa tiny percentage of a huge number is still a very big number
WORKED EXAMPLE
A student writes: “The genome is all of the genes in an organism.” Correct this definition and give two things the student has left out. [3]
Step 1: the correct definition
The genome is all of the genetic information in an organism — all its DNA.
Omission 1
Non-coding DNA, which does not code for proteins.
Omission 2
Organelle DNA in mitochondria and chloroplasts (and plasmid DNA in prokaryotes).
All the DNA, not just the genes
WORKED EXAMPLE
Explain why two humans differ from each other even though they carry the same coding genes. [2]
Step 1: what is shared
All humans have the same set of coding genes.
Step 2: what differs
They carry different alleles — different versions of those genes — which arise through mutation.
Same genes, different alleles“same genes, different alleles” is the phrase to have ready
💡 Exam tip
Define the genome as all the genetic information, then add that it includes non-coding DNA.
Remember the extras: mitochondrial and chloroplast DNA in eukaryotes, plasmids in prokaryotes.
Learn the two percentages: 99.9% human to human, 99% human to chimpanzee.
Define a SNP precisely: a difference in a single nucleotide between individuals.
Say same genes, different alleles when explaining human variation.
Name a database if asked — GenBank or NCBI will do.
⚠ Common mix-up
Defining the genome as “all the genes”. Non-coding DNA is included too.
Leaving out organelle DNA. Mitochondrial DNA is part of your genome.
Confusing gene and allele. A gene is the instruction; an allele is a version of it.
Saying every SNP changes the protein. Most have no effect at all.
Thinking 99.9% similar means nearly identical. It still leaves millions of differences.
Mixing up genome and karyotype. One is the DNA content; the other is the chromosome appearance.
Up next: Comparing Genome Sizes (Skills) — how genome sizes are measured and compared, and why the biggest genome does not belong to the cleverest organism.
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