An illustration of chromosome, with its parts. (1) Chromatid. One of the two identical parts of the chromosome after S phase. (2) Centromere. The point where the two chromatids touch, and where the microtubules attach. (3) Short arm (4) Long arm.
Source: Nupedia, then en.wikipedia. http://en.wiktionary.org/wiki/File:Chromosome-upright.png
Author: Original version: Magnus Manske, this version with upright chromosome: User:Dietzel65