A-LEVEL BIOLOGY AQA NOTES
dna, genes and chromosomes
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- In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.
- In eukaryotes, the nucleus contains very long, linear DNA molecules associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
- The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.
- The genome is the full set of DNA found in an organism.
- The proteasome is the full range of proteins that can be synthesised from the genome.
- A gene is a section of DNA that code for polypeptides and functional RNA and are located at a fixed locus on a DNA molecule.
- A sequence of three DNA bases, called a codon, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate.
- In eukaryotes, sections of the nuclear DNA do not code for polypeptides (introns). Exons are sections of DNA that code for amino acid sequences.