Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Genetic information is stored in, and passed on through, DNA. Some viruses (for example influenza viruses and HIV) use RNA instead.
- Prokaryotes typically have one circular chromosome in the nucleoid region, with no nuclear envelope. It is packed by supercoiling.
- Eukaryotes typically have several linear chromosomes in the nucleus. DNA wraps around histone proteins (nucleosomes, “beads on a string”) and is folded further.
- Histones are positively charged; the DNA backbone is negatively charged. The attraction holds DNA on the histones.
- Plasmids are small, circular, extra-chromosomal DNA molecules. They are common in bacteria and also occur in some eukaryotes, such as yeasts. They often carry genes such as antibiotic resistance and can pass between cells.
- Pairing rules are the same in all known cellular life: evidence of common ancestry.
Key relationships
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Chromosome shape | typically circular | typically linear |
| Number of chromosomes | typically one | several |
| Location | nucleoid (cytoplasm) | nucleus |
| Packing | supercoiling and proteins | histones (nucleosomes) and further folding |
| Plasmids | common | in some (for example yeasts) |
| Base group | Rings | Bases | Pairs with |
|---|---|---|---|
| Purines | double | A, G | a pyrimidine |
| Pyrimidines | single | C, T (DNA), U (RNA) | a purine |
Pairs: A–T (DNA), A–U (RNA), G–C. One purine + one pyrimidine = three rings = constant helix width.
Assumptions behind the numbers
- “Purines = pyrimidines = 50%” applies only to double-stranded molecules, where every base is paired.
- Length calculations use a model value of about 0.34 nm per base pair.
Mistakes to avoid
- Saying prokaryotes have no chromosome.
- Saying all genetic material is DNA.
- Swapping purines and pyrimidines: purines are the double-ringed A and G.
- Pairing two purines (A–G) or two pyrimidines (C–T).
- Calling histones DNA; they are proteins.
- Applying the 50% purine rule to a single strand.
Quick self-check
- A double-stranded DNA molecule has 2 000 base pairs. How many purines does it contain? (4 000 nucleotides, half are purines: 2 000)
- Double-stranded DNA is 22% C. What are G, A and T? (G = 22%; purines total 50%, so A = 50 − 22 = 28% and T = 28%)
- Give two ways a bacterial chromosome differs from a human chromosome. (circular, not linear; in the cytoplasm, not a nucleus; not wound around histones; usually only one)
Next: practice questions.