Overview

The eukaryal genome is located in the cell's nucleus, packaged in structurally complex chromosomes. Through the cell cycle, chromosomes condense and decondense, undergoing dramatic changes in thickness and length. With few exceptions, eukaryal chromosomes consist of thread-like chromatin, composed of particles of histones wrapped by DNA, called nucleosomes. Nearly all eukaryal chromosomes are linear and possess a pair of telomeres. In most species, each chromosome has a single centromere, which functions in chromosome separation and movement.

Condensed versus Uncondensed Nuclear Chromosomes

In eukarya, chromosomes reside in the nucleus, a large spherical organelle bounded by a double-membraned envelope. In most cases chromosomes are visible by light microscopy only during mitosis, the brief nuclear division phase. Mitotic chromosomes are short and thick (d ~12 µm, “packed to travel”) and take up colorful stains used to detect their presence. They consist of twin copies called sister chromatids and are attached to spindle fibers (microtubules that move chromosomes). Genes are not transcribed into RNA during mitosis. Mitotic chromosomes are useful for the study of chromosome structure, even though they bear little resemblance to “chromosomes at work,” and even though mitosis takes up a small fraction of the life span of a typical cell (Figure 5.1).

Chromosomes perform most of their genetic functions during interphase, the cell's lengthy, nondividing period.