Genetics - Chromosomes

Prokaryotic chromosomes consist of a single, circular $DNA$ molecule that is not associated with proteins ('naked' $DNA$).

Some prokaryotes also contain plasmids, which are small, circular, extra-chromosomal $DNA$ molecules that may contain genes for antibiotic resistance.

Eukaryotic chromosomes are linear $DNA$ molecules associated with histone proteins, forming structures called nucleosomes.

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes; they are the same size and have the same centromere position.

Haploid nuclei ($n$) have one chromosome of each pair (found in gametes), whereas diploid nuclei ($2n$) have pairs of homologous chromosomes (found in somatic cells).

The number of chromosomes is a characteristic feature of members of a species; for example, $Oryza$ $sativa$ has $2n = 24$ and $Homo$ $sapiens$ has $2n = 46$.

Sex is determined by sex chromosomes: females are $XX$ and males are $XY$. All other chromosomes are called autosomes.

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length; it can be used to determine sex or diagnose chromosomal abnormalities like Trisomy $21$.

Cairns' technique used autoradiography to measure the length of $DNA$ molecules, demonstrating that the bacterium $E. coli$ has a single circular chromosome.