A eukaryotic chromosome is therefore a long, linear piece of DNA (a very long piece - the average chromosome length in humans is about 130 million base pairs of DNA) wrapped around histones to form nucleosomes.

Chromosomes are not uniform, however. During interphase, most of each chromosome would exist as euchromatin (in the 10 nm fiber configuration), but some regions remain as heterochromatin (in the tightly wrapped 30 nm configuration) even during interphase. Since heterochromatin is genetically inactive, any genes found in these heterochromatic region will not function. Most of the heterochromatic regions do not contain genes, however. They tend to be repetitive DNA, or other functional regions of the chromosome such as centromeres and telomeres. (One example of heterochromatin that contains inactive genes is the inactive X chromosome (Barr body) of female mammals. For a discussion of this, see the section on dosage compensation.)

Centromeres

The centromere is the region of each eukaryotic chromosome that attaches to spindle fibers during mitosis and meiosis. Proper centromere function is therefore very important for cell division and reproduction. Centromeres have been best studied in yeast, and all yeast centromeres have been shown to be remarkably similar to each other. Each centromere consists of three regions. The two flanking regions (regions I and III) are short, very highly conserved sequences (this means that virtually all of the centromeres have the same sequence, indicating that the particular sequence is important). Of these two regions, region III seems to be the most critical, and may be essential for binding to the spindle fibers. The central region is not highly conserved, but it is very rich in adenine and thymine - over 90% of the bases in this region are A or T!

Telomeres

Telomeres are the ends of the chromosomes. They consist of many tandem (head to tail) repeats of short (generally hexamer) sequences. In humans the telomeric sequence is TTAGGG. This sequence is repeated over 50 times at the end of each chromosome. These sequences are added to the ends of the chromosomes by an enzyme called telomerase. The reason telomeres exist is to provide stability to the chromosomal ends. As we'll see in the module on DNA replication, addition of telomeric sequences prevent the shortening of chromosomes that would otherwise occur during replication.

Chromosome Assembly: Summary of Key Points

• In eukaryotes, DNA is normally complexed with histone proteins to form chromatin. Histones H2A, H2B, H3, and H4 form a histone octamer, around which DNA wraps, to form a nucleosome core. The nucleosome core plus 60 bp of linker DNA and one molecule of histone H1 forms a nucleosome, which is the basic unit of chromatin structure.
• This chromatin (known as the 10 nm fiber) can be more highly organized and condensed, into a 30 nm fiber, which can in turn be folded further to form the highly condensed metaphase chromosomes. The 10 nm fiber corresponds to euchromatin, while the 30 nm fiber corresponds to heterochromatin.
• Chromosomes are long linear pieces of DNA that associate with histones, forming chromatin. Some regions of each chromosome exist as euchromatin during interphase, whereas other regions remain as heterochromatin all the time. The latter regions include repetitive DNA, centromeres, and telomeres.
• Yeast centromeres have conserved structures - they are very similar on every chromosome.
• Telomeres are the ends of chromosomes, and consist of numerous repeats of a six base sequence.