Chromosome
Diagram of a duplicated and condensed (metaphase) eukaryot-
ic chromosome. (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.
A chromosome is an organized structure of DNA and
protein that is found in cells. A chromosome is a single
piece of coiled DNA containing many genes, regulatory
elements and other nucleotide sequences. Chromosomes
also contain DNA-bound proteins, which serve to pack-
age the DNA and control its functions. The word chromo-
some comes from the Greek χρῶμα (chroma, color) and
σῶμα (soma, body) due to their property of being very
strongly stained by particular dyes. Chromosomes vary
widely between different organisms. The DNA molecule
may be circular or linear, and can be composed of 10,000
to 1,000,000,000[1] nucleotides in a long chain. Typically
eukaryotic cells (cells with nuclei) have large linear
chromosomes and prokaryotic cells
(cells without
defined nuclei) have smaller circular chromosomes, al-
though there are many exceptions to this rule. Further-
more, cells may contain more than one type of chromo-
some; for example, mitochondria in most eukaryotes
and chloroplasts
in plants have their own small
chromosomes.
In eukaryotes, nuclear chromosomes are packaged
by proteins into a condensed structure called chromatin.
This allows the very long DNA molecules to fit into the
cell nucleus. The structure of chromosomes and chro-
matin varies through the cell cycle. Chromosomes are
the essential unit for cellular division and must be rep-
licated, divided, and passed successfully to their daugh-
ter cells so as to ensure the genetic diversity and surviv-
al of their progeny. Chromosomes may exist as either
duplicated or unduplicated—unduplicated chromosomes
are single linear strands, whereas duplicated chromo-
somes (copied during synthesis phase) contain two cop-
ies joined by a centromere. Compaction of the duplic-
ated chromosom