Chromosomes: DNA (Deoxyribonucleic Acid) Is The Genetic Material of Living

Chromosomes
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Introduction
When a cell divides, one of its main jobs is to make sure that each of the
two new cells gets a full, perfect copy of genetic material. Mistakes
during copying, or unequal division of the genetic material between cells,
can lead to cells that are unhealthy or dysfunctional (and may lead to
diseases such as cancer).
But what exactly is this genetic material, and how does it behave over the
course of a cell division?
DNA and genomes

DNA (deoxyribonucleic acid) is the genetic material of living
organisms. In humans, DNA is found in almost all the cells of the body
and provides the instructions they need to grow, function, and respond
to their environment.
When a cell in the body divides, it will pass on a copy of its DNA to each
of its daughter cells. DNA is also passed on at the level of organisms, with
the DNA in sperm and egg cells combining to form a new organism that
has genetic material from both its parents.
Physically speaking, DNA is a long string of paired chemical units
(nucleotides) that come in four different types, abbreviated A, T, C, and G,
and it carries information organized into units called genes. Genes
typically provide instructions for making proteins, which give cells and
organisms their functional characteristics.
Image of a eukaryotic cell, showing the nuclear DNA (in the nucleus), the
mitochondrial DNA (in the mitochondrial matrix), and the chloroplast
DNA (in the stroma of the chloroplast).
In eukaryotes such as plants and animals, the majority of DNA is found in
the nucleus and is called nuclear DNA. Mitochondria, organelles that
harvest energy for the cell, contain their own mitochondrial DNA, and
chloroplasts, organelles that carry out photosynthesis in plant cells, also
have chloroplast DNA. The amounts of DNA found in mitochondria and
chloroplasts are much smaller than the amount found in the nucleus. In
bacteria, most of the DNA is found in a central region of the cell called
the nucleoid, which functions similarly to a nucleus but is not
surrounded by a membrane.
A cell’s set of DNA is called its genome. Since all of the cells in an
organism (with a few exceptions) contain the same DNA, you can also
say that an organism has its own genome, and since the members of a
species typically have similar genomes, you can also describe the
genome of a species. In general, when people refer to the human genome,
or any other eukaryotic genome, they mean the set of DNA found in the
nucleus. Mitochondria and chloroplasts are considered to have their own
separate genomes.
Chromatin
In a cell, DNA does not usually exist by itself, but instead associates with
specialized proteins that organize it and give it structure. In eukaryotes,
these proteins include the histones, a group of basic (positively charged)
proteins that form “bobbins” around which negatively charged DNA can
wrap. In addition to organizing DNA and making it more compact,
histones play an important role in determining which genes are active.
The complex of DNA plus histones and other structural proteins is
called chromatin.
Image of a long, double-stranded DNA polymer, which wraps around

clusters of histone proteins. The DNA wrapped around histones is further
organized into higher-order structures that give a chromosome its shape.
For most of the life of the cell, chromatin is decondensed, meaning that
it exists in long, thin strings that look like squiggles under the
microscope. In this state, the DNA can be accessed relatively easily by
cellular machinery (such as proteins that read and copy DNA), which is
important in allowing the cell to grow and function.
Decondensed may seem like an odd term for this state – why not just call
it “stringy”? – but makes more sense when you learn that chromatin can
also condense. Condensation takes place when the cell is about to
divide. When chromatin condenses, you can see that eukaryotic DNA is
not just one long string. Instead, it’s broken up into separate, linear
pieces called chromosomes. Bacteria also have chromosomes, but their
chromosomes are typically circular.
Chromosomes
Each species has its own characteristic number of chromosomes.
Humans, for instance, have 46 chromosomes in a typical body cell
(somatic cell), while dogs have 78^11start superscript, 1, end superscript.
Like many species of animals and plants, humans are diploid (2n),
meaning that most of their chromosomes come in matched sets known
as homologous pairs. The 46 chromosomes of a human cell are
organized into 23 pairs, and the two members of each pair are said to
be homologues of one another (with the slight exception of the X and Y
chromosomes; see below).
Human sperm and eggs, which have only one homologous chromosome
from each pair, are said to be haploid (1n). When a sperm and egg fuse,
their genetic material combines to form one complete, diploid set of
chromosomes. So, for each homologous pair of chromosomes in your
genome, one of the homologues comes from your mom and the other
from your dad.
Image of the karyotype of a human male, with chromosomes from the

mother and father false-colored purple and green, respectively.
Image modified from "Karyotype," by the National Institutes of Health (public domain).
The two chromosomes in a homologous pair are very similar to one

another and have the same size and shape. Most importantly, they carry
the same type of genetic information: that is, they have the same genes in
the same locations. However, they don't necessarily have the same
versions of genes. That's because you may have inherited two different
gene versions from your mom and your dad.
As a real example, let's consider a gene on chromosome 9 that
determines blood type (A, B, AB, or O)^22start superscript, 2, end
superscript. It's possible for a person to have two identical copies of this
gene, one on each homologous chromosome—for example, you may have
a double dose of the gene version for type A. On the other hand, you may
have two different gene versions on your two homologous
chromosomes, such as one for type A and one for type B (giving AB
blood).
The sex chromosomes, X and Y, determine a person's biological sex: XX
specifies female and XY specifies male. These chromosomes are not true
homologues and are an exception to the rule of the same genes in the
same places. Aside from small regions of similarity needed during
meiosis, or sex cell production, the X and Y chromosomes are different
and carry different genes. The 44 non-sex chromosomes in humans are
called autosomes.
Chromosomes and cell division
Image of a cell undergoing DNA replication (all the chromosomes in the

nucleus are copied) and chromosome condensation (all the
chromosomes become compact). In the first image, there are four
decondensed, stringy chromosomes in the nucleus of the cell. After DNA
replication, each chromosome now consists of two physically attached
sister chromatids. After chromosome condensation, the chromosomes
condense to form compact structures (still made up of two chromatids).
As a cell prepares to divide, it must make a copy of each of its
chromosomes. The two copies of a chromosome are called sister
chromatids. The sister chromatids are identical to one another and are
attached to each other by proteins called cohesins. The attachment
between sister chromatids is tightest at the centromere, a region of DNA
that is important for their separation during later stages of cell division.
As long as the sister chromatids are connected at the centromere, they
are still considered to be one chromosome. However, as soon as they are
pulled apart during cell division, each is considered a separate
chromosome.
What happens to a chromosome as a cell prepares to divide.

1. The chromosome consists of a single chromatid and is decondensed
(long and string-like).
2. The DNA is copied. The chromosome now consists of two sister
chromatids, which are connected by proteins called cohesins.
3. The chromosome condenses. It is still made up of two sister chromatids,
but they are now short and compact rather than long and stringy. They
are most tightly connected at the centromere region, which is the
inward-pinching "waist" of the chromosome.
4. The chromatids are pulled apart. Each is now considered its own
chromosome.
Why do cells put their chromosomes through this process of replication,
condensation, and separation? The short answer is: to make sure that,
during cell division, each new cell gets exactly one copy of each
chromosome.
For a more satisfying answer, check out the articles and videos on the
cell cycle and mitosis. There, you can see how the behavior of
chromosomes helps cells pass on a perfect set of DNA to each daughter
cell during division.
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Chromosomes

DNA and genomes

Image of a long, double-stranded DNA polymer, which wraps around

Image of the karyotype of a human male, with chromosomes from the

The two chromosomes in a homologous pair are very similar to one

Chromosomes and cell division

Image of a cell undergoing DNA replication (all the chromosomes in the

What happens to a chromosome as a cell prepares to divide.

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