Chromosome – Definition, Structure, Function, Examples
Chromosomes are thread-like structures present in the nucleus. They are important
because they contain the basic genetic material DNA. These are present inside
the nucleus of plants as well as animal cells. Chromosomes were first
discovered by Strasburger in 1815 and the term ‘chromosome’ was first
used by Waldeyer in 1888. Human beings have 46 chromosomes in their body.
These are arranged into 23 pairs. Let us discuss the chromosome structure
in detail.
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Table of content
1.3 Examples of
Types of Chromosomes
2 Question on
Chromosome Structure
Definition
of Chromosome
“A Chromosome looks like a thread and is coiled
material, made of proteins. Chromosomes are present in the nucleus of all the
cells and contain the basic genetic material DNA, which passes from one
generation to another”.
Structure:
A chromosome has generally 8 parts; Centromere or primary constriction
or kinetochore, chromatids, chromatin, secondary constriction, telomere,
chromomere, chromonema, and matrix.
·
Centromere or
Kinetochore: It is the primary constriction
at the center to which the chromatids or spindle fibers are attached. Its
function is to enable movement of the chromosome during the anaphase stage of
cell division.
·
Chromatid: During cell division, a chromosome is divided
into 2 identical half strands joined by a centromere. A chromatid is each half
of the chromosome joined. Each chromatid contains DNA and separates at Anaphase to form a separate chromosome.
Both chromatids are attached to each other by the centromere.
·
Chromatin: It is a complex of DNA and proteins that forms
chromosomes within the nucleus of eukaryotic cells. Nuclear DNA is highly
condensed and wrapped around nuclear proteins in order to fit inside the
nucleus. In other words, it is not present as free linear strands. The chromatin consists of DNA, RNA, and protein.
·
Secondary Constriction: It is generally present for the nucleolar
organization.
·
Telomere: Telomere is the terminal region of each side of
the chromosome. Ach chromosome has 2
·
Chromonema: It is a threadlike coiled filamentous structure
along which chromomeres are arranged. Chromonema controls the size of the
chromosome and it acts as a site of gene bearing.
·
Chromomeres: These are the bead-like structures present on
threads or chromonema. These are arranged in a row along the length of
chromonema. The number of chromosomes is constant and it is responsible for
carrying the genes during cell division to the next generation.
·
Matrix: Pellicle is the membrane surrounding each of the
chromosomes. Matrix is the jelly-like substance present inside pellicle. It is
formed of non-genetic materials.
Functions of Chromosomes
For the first time, Sutton and Bover suggested the role of chromosomes in
heredity in 1902.
1.
The most important
function of chromosomes is to carry the basic genetic material – DNA. DNA
provides genetic information for various cellular functions. These functions
are essential for growth, survival, and reproduction of the organisms.
2.
Histones and other
proteins cover the Chromosomes. These proteins protect it from chemical (e.g.,
enzymes) and physical forces. Thus, chromosomes also perform the function of
protecting the genetic material (DNA) from damage during the process of cell
division.
3.
During cell
division, spindle fibers attached to the centromeres contract and perform an
important function. The contraction of centromeres of chromosomes ensures
precise distribution of DNA (genetic material) to the daughter nuclei.
4. Chromosomes contain histone and non-histone proteins.
these proteins regulate gene action. Cellular molecules that regulate genes
work by activating or deactivating these proteins. This activation and deactivation
expand or contract the chromosome.
Examples of
Types of Chromosomes
·
Metacentric
Chromosomes: Metacentric chromosomes
have the centromere present exactly in the center. Both the sections are
metacentric chromosomes are therefore of equal length. Example: Human chromosome 1 and 3 are metacentric.
·
Submetacentric
Chromosomes: In Submetacentric
chromosomes, the centromere is not present exactly at the center. The
centromere is slightly offset from the center. Both the sections are therefore
not of equal length or are asymmetrical. Example: Human chromosomes 4 to 12 are submetacentric.
·
Acrocentric
Chromosomes: Acrocentric
chromosomes have a centromere which is highly offset from the center.
Therefore, one of the strands is very long and one very short. Example: Human chromosomes 13,15, 21, and 22 are acrocentric.
·
Telocentric
Chromosomes: In telocentric
chromosomes, the centromere is present at the very end of the chromosome.
Telocentric chromosomes are present in species such as mice. Humans do not
possess telocentric chromosomes.
Question
on Chromosome Structure
Q: How do genetic disorders occur?
Ans: When DNA sequence in chromosomes changes, genetic disorders
occur. The mutation refers to a change in the DNA sequence. The monogenic
disorder occurs when a mutation occurs in one gene. Multifactorial genetic
disorder occurs when mutation occurs in multiple genes. Human beings have 23
pairs of chromosomes. All the diseases have a genetic component and the
mutations pass from one generation to the other. Cancer, diabetes, obesity
are examples of such diseases.