Two types of cell:
- prokaryotic (e.g. bacteria) - DNA molecules smaller, form a circle, not associated with protein molecules, do not have chromosomes.
- eukaryotic cells - DNA molecules larger, form a line (linear), occur in association with proteins to form chromosomes.
- Only visible as distinct structures when cell is dividing, widely dispersed throughout cell the rest of the time.
- Appear as two threads, joined at single point (centromere).
- Each thread = chromatid.
- DNA in chromosomes held in position by proteins, length of DNA coiled and folded:
- DNA is a double helix.
- Helix is wound around proteins to fix it in position.
- DNA-protein complex coiled.
- Coil is looped and further coiled, then packed into the chromosome.
Chromosome contains just a single molecule of DNA that is very long - lots of DNA condensed into single chromosome.
Single DNA molecule has lots of genes along its length, each gene occupies a specific position along the DNA molecule.
Number of chromosomes same for individuals of a species but different for…