Chromosome
Diagram of a duplicated and condensed
(metaphase) eukaryotic chromosome. (1)
Chromatid - one of the two identical parts of
the chromosome after S phase. (2) Cen-
tromere - 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. Chromo-
somes also contain DNA-bound proteins,
which serve to package the DNA and control
its functions. The word chromosome 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 dif-
ferent 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,
although there are many exceptions to this
rule. Furthermore, cells may contain more
than one type of chromosome; for example,
mitochondria in most eukaryotes and chloro-
plasts
in plants have their own small
chromosomes.
In eukaryotes, nuclear chromosomes are
packaged by proteins into a condensed struc-
ture called chromatin. This allows the very
long DNA molecules to fit into the cell nucle-
us. The structure of chromosomes and chro-
matin varies through the cell cycle. Chromo-
somes are the essential unit for cellular divi-
sion and must be replicated, divided, and
passed successfully to their daughter cells so
as to ensure the genetic diversity and surviv-
al of their progeny. Chromosomes may exist
as either duplicated or unduplicated—undu-
plicated chromosomes are single
linear
strands, whereas duplicated chromosomes
(copied during synthesis phase) contain two
copies joined by a centromere. Compaction of
the duplicated chromosomes d