#4.1.1
In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.
#4.1.2
In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
#4.1.3
The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.
#4.1.4
A gene is a base sequence of DNA that codes for:
- the amino acid sequence of a polypeptide
- a functional RNA (including ribosomal RNA and tRNAs).
#4.1.5
A gene occupies a fixed position, called a locus, on a particular DNA molecule.
#4.1.6
A sequence of three DNA bases, called a triplet, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate.
#4.1.7
In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, non-coding multiple repeats of base sequences between genes. Even within a gene only some sequences, called exons, code for amino acid sequences. Within the gene, these exons are separated by one or more non-coding sequences, called introns.