What is a chromosome?
A chromosome is a large macromolecule which contains part of the genetic information stored in the genome. The core of the chromosome-molecule is a double stranded DNA string which is interacting non-covalently with a large number of proteins (chromatin proteins) and shorter chains of ribonucleic acid (RNA). The DNA of a chromosome within a human cell spans several centimeters and has to be compacted in order to fit into an average cells nucleus of 7 micrometer diameter. This compaction is mediated by the proteins associated with the DNA. Each chromosome must contain at least one origin of replication. While prokaryotic microorganisms usually contain a circular chromosome, the ends of linear chromosomes of eurkaryotes must be protected against degradation by special proteins. The ends of linear chromosomes are called telomeres.
In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosomes are not visible in the cell’s nucleus–not even under a microscope–when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division. Chromosome 1 (the largest human chromosome) has the most genes (2968), and the Y chromosome has the fewest (231).
