Chromosomes are thread-like structures made of protein and a single molecule of
DNA that serve to carry the genomic information from cell to cell. Humans have
22 pairs of numbered chromosomes (autosomes) and one pair of sex
chromosomes, for a total of 46. Typically, biologically female individuals have two
X chromosomes while those who are biologically male have one X and one Y
chromosome Chromosomes are also of different sizes. The human X chromosome
is about three times larger than the human Y chromosome, containing about 900
genes, while the Y chromosome has about 55 genes. The unique structure of
chromosomes keeps DNA tightly wound around spool-like proteins, called
histones. If all of the DNA molecules in a single human cell were unwound from
their histones and placed end-to-end, they would stretch 6 feet.
Discovery
In 1842, Karl Nageli, was the first person who had seen rod-like structures present
in the plant cells nucleus. It is generally known that chromosomes were first
discovered by Walther Flemming in 1882. Later, in 1888, von Waldeyer-Hartz coined
the term ‘chromosome', In 1902, two scientists Theodor Boveri and Walter Sutton
independently found that chromosomes are the carrier of genetic material or
genes.
Structure
Each chromosome has a constriction point called the
centromere, which divides the chromosome into two
sections, or arms. The short arm of the chromosome is
labeled the p arm. The long arm of the chromosome is
labeled the q arm. The ends of the chromosome are
called telomeres.
Chromatin
Chromatin refers to a mixture of DNA (Deoxyribonucleic acid- about 40%) and
proteins (Histones -about 60%) Many of the proteins namely, histones- package
the massive amount of DNA in a genome into a highly compact form that can fit
in the cell nucleus. DNA molecules first wrap around the histone proteins forming
beads on string structure called nucleosomes, Nucleosomes further
condense/gather to form fibrous material which is called chromatin. Chromatin
fibers can unwind for DNA replication and transcription.
● Molecular structure of DNA
Deoxyribonucleic acid is the molecule that
carries genetic information for the development
and functioning of an organism. Each strand has
a backbone made of alternating sugar and
phosphate groups. These nitrogen bases are the
rungs of the DNA ladder. The two strands are
connected by chemical bonds between the
bases: adenine bonds with thymine, and cytosine
bonds with guanine.
● Histone proteins
A histone is a protein that provides structural support for a chromosome. Each
chromosome contains a long molecule of DNA, which must fit into the cell nucleus.
To do that, the DNA wraps around complexes of histone proteins, giving the
chromosome a more compact shape.
Chromatid
Chromatid is one-half of the replicated or duplicated chromosome or one of the
two identical halves of a replicated or duplicated chromosome. A replicated
chromosome has a duplicated copy attached to itself.
Function of Chromatids
Chromatids allow cells to store two copies of their information in preparation for
cell division. This is vital to ensure that daughter cells are healthy and fully
functional, carrying a full complement of the parent cells' DNA.
Centromere
Centromere is also called the primary constriction.It is the point where sister
chromatids are attached. It is the constricted region from where chromosome's
arms originate; a short arm and a long arm . It is the link between two sister
chromatids. Centromere gives the chromosome its unique shape that helps
understand its structure and location of genes. The position of the centromere
also helps categorize chromosomes into different types.
1. Metacentric: These chromosomes have centromeres located near the
center, dividing the chromosome into two arms of approximately equal
length.
2. Submetacentric: Centromeres in submetacentric chromosomes are
slightly off-center, resulting in one arm being shorter than the other.
3. Acrocentric: In acrocentric chromosomes, the centromere is situated
quite near one end, leading to one long arm and one very short arm.
4. Telocentric: Telocentric chromosomes have their centromeres at one
end, resulting in a chromosome with a single long arm and no short arm.
However, telocentric chromosomes are rare in most organisms.
