What is the classification of chromosomes?

Answer 1

One classification of chromosomes is into circular and linear. Prokaryotes have circular chromosomes, but they are not geometrically circular - better to say they are in the form of a loop, so that the DNA has no end. Eukaryotic cells have linear chromosomes in the nucleus, but circular chromosomes in mitochondria and chloroplasts, probably reflecting their origins as independent prokaryotes.

Another classification is into sex chromosomes and autosomes. Sex chromosomes are responsible for sex determination in some, but not all, organisms. In humans the sex chromosomes are called X and Y, and the female is the homogametic sex (because all gametes must carry an X), while the male is the heterogametic sex (XY). In birds it is the other way round: the male is homogametic (WW) and the female heterogametic (WZ).

Another form of classification is to number the autosomes. This is done with Arabic numerals, starting with the longest chromosome. Thus the longest human chromosome is chromosome 1, the next longest is chromosome 2, and so on. Unfortunately early microscopists incorrectly ranked the two smallest, and the Human Genome Project found that chromosome 22 is actually longer than chromosome 21, but the names have stuck.