Chromosome: Have you ever thought about how genetic information is carried out from parents to the offspring in all living organisms, including humans? DNA (Deoxyribonucleic Acid) is one of the most important molecules of life, which carries different genetic information in all living organisms, including humans.

Chromosomes are thread-like structures made up of DNA molecules and proteins. 46 chromosomes present in human cells have a total length of 200 nm, but if we unravelled chromosomes, then the total length of genetic material which chromosomes contain would measure approx 2 meters in length.

Scroll down to learn more about chromosomes and their structure and functions.

Chromosome Definition

A chromosome is the carrier of genetic information in genetic material ‘DNA’ from parents to offspring. A chromosome is an entire chain of DNA shaped like a thread located inside the cell’s nucleus. A chromosome is a combination of DNA and histone protein which gives the chromosome a stable structure. A chromosome is important in cell division, heredity, mutation, repair, and regeneration. In 1815, Strasburger first described the chromosome, and Waldeyer first used the term chromosome in 1888.

Fig: Chromosomes

Chromosome Number

A particular species always has the same number of chromosomes. It includes haploid (one set of chromosomes, n), diploid (contain two sets of chromosomes, 2n),  triploid (three sets of chromosomes, 3n), and tetraploid (four sets of chromosomes, 4n). For example, the number of chromosomes in humans, each autosome contains a diploid number, 2n = 46, and gametes have one set of chromosomes (haploid, n=23).

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Chromosome Size

The size of the chromosome depends upon the species. Some chromosomes are huge as lampbrush type. The metaphase chromosome length may vary from 0.1 to 33 micrometers and the thickness from 0.2 to 2 micrometers.

Chromosome Shape

The shape of the chromosome changes according to the stages of each cell division. In interphase, the chromosomes form a chromatin network whereas, in the metaphase, they appear short and thick.

Chromosome Types

There are different types of chromosomes depending upon different aspects.

Fig: Different Types of Chromosomes

1. Based on Number of Centromeres

Based on the quantity of centromere present in the chromosome, the chromosome is divided into the following terms:
a) Monocentric: Chromosomes consist of a single centromere.
b) Dicentric: Two centromeres present in the chromosome.

Fig: Dicentric Chromosome

c) Polycentric: More than two centromeres present in the chromosome are known as polycentric chromosomes.
d) Acentric: Chromosomes don’t have centromere, freshly broken segments of chromosomes and will fuse to form a chromosome.
e) Diffused or non located: In this type of chromosome, the centromere is diffused throughout the length of the chromosome.

2. Based on Position of Centromere

Depending upon the position of the centromere, chromosomes are again divided into four different types, they are:
a) Telocentric: These are rod-shaped chromosomes that have centromere in the end or terminal position. In this type of chromosome, ‘p’ (short arm) is missing.
b) Acrocentric: In this type, the centromere is present in a subterminal position giving rise to a tiny, short arm and another very large long arm. Chromosomes 13, 15, 21, and 22 of humans are the acrocentric chromosome.
c) Submetacentric: In this type, chromosomes look L-shaped or J-shaped. The centromere is placed submedially, so arms are unequal. Chromosomes 4 and 12 are submetacentric.
d) Metacentric: Metacentric chromosomes are v-shaped, and the centromere of the chromosome is in the center position, and the arms are almost equal. 1 and 3 chromosomes of humans are metacentric chromosomes.

Fig: Types of Chromosomes Based on the Position of the Centromere

3. Special Types of Chromosomes

Giant chromosomes are the biggest chromosomes present or discovered. Multiple replications of the chromosomal DNA without nuclear division are the main reason for this giant size. ‘Giant chromosome’ was first given by Winchester; they were also named a mega chromosome. Giant chromosomes are of two types:
a) Lampbrush Chromosome
i. Ruckert, in 1892, first described the lampbrush chromosome in shark oocytes. The Lampbrush chromosome is found in vertebrates’ immature eggs or oocytes like fish, birds, reptiles, and amphibians.
ii. DNA content in oocyte cells is very high, and the active synthesis of mRNA molecules for future egg purposes is the reason for giant size.
iii. Loops present in lampbrush chromosomes are used in chromosome mapping, also used to analyze hybridisation.
iv. Lampbrush chromosomes look like a brush which are used to clean lamps or glass.

b) Polytene Chromosome
i. Polytene chromosomes are also giant chromosomes, but they are not that large like lampbrush chromosomes.
ii. The polytene name is given due to its numerous types of strands and having large size.
iii. E.G. Balbiani first discovered the polytene chromosome in the cells of the salivary glands of Chironomus larvae in 1881.
iv. Polytene chromosomes are formed due to multiple or repeated replications of homologous chromosomal DNA without having the division of the nucleus. In this process, the newly formed sister chromatids do not get separated.

Chromosome Structure

A chromosome typically has a centromere and one or two arms. The ultrastructure of the chromosome can show a single long DNA compacted approx 10000 fold by interactions with proteins. The morphology of a chromosome commonly changes with the stages of cell division. In mitotic metaphase morphology of chromosomes can be studied most suitably through a light microscope. The different structures of chromosomes that can be seen are described below:

a) Chromatid
i. At the time of metaphase cell division, each chromosome is divided into two equal and identical segments called chromatids.
ii. Every chromatid contains DNA that separates at the time of anaphase to develop a separate chromosome.

b) Centromere or Kinetochore or Primary Constriction
i. In the metaphase and anaphase, the chromosomes become thick and filamentous. The region in which the spindle fibers are attached during metaphase cell division is known as the centromere or kinetochore or primary constriction.
ii. Centromere allows movement of the chromosome during anaphase of cell division.
iii. Centromere divides chromosomes into two segments or arms. The short arm is known as ‘p,’ and the long arm is named ‘q.’

c) Secondary Constriction or Nucleolar Organiser
i. Secondary constriction is the constricted region present in the chromosome other than the primary construction containing the nuclear organiser.
ii. Secondary constriction is generally found on the short arm of a chromosome, away from the centromere. But in some cases, secondary constriction is located on the long arm.

d) Satellite
i. The region between secondary constriction and the nearest telomere is named a satellite. So chromosomes having secondary constriction are called satellite chromosomes or sat chromosomes.
ii. The SAT chromosomes are related to the nucleolar organiser.

e) Telomere
i. Telomeres are the end portion of each side of chromosomes.
ii. If a chromosome breaks, then broken ends of chromosomes can fuse due to a lack of telomere.

f) Chromomeres
i. Chromomeres are small beads like serially aligned small swellings present on the surface of chromosomes during interphase.

g) Chromatin
i. Chromatin is the composite DNA consisting of DNA, RNA, and proteins that form chromosomes inside the nucleus.
ii. Nuclear DNA is highly condensed and wrapped around nuclear proteins to fit inside the nucleus.
iii. Chromatin is two types depending on its staining properties during interphase.
iv. Euchromatin: It is the light-stained and partially condensed region of chromatin. This region is considered a genetically active region as it contains structural genes.
v. Heterochromatin: It is the highly stained and condensed region of chromatin. This region is considered a genetically inactive region.

Fig: Structure of Chromosome

Chromosome Organisation

1. Chromatin is mainly composed of DNA, RNA and histones, and other associated proteins.
2. The DNA is packaged in a highly organised manner in the chromosomes.
3. Histones are the most abundant protein in Chromatin. There are mainly five histone proteins in the eukaryotic chromosomes, namely H1, H2A, H2B, H3 and H4.
4. The nucleosomes are subunits of Chromatin and have a bead-like appearance.
5. Each nucleosome consists of a histone octamer and 146 base pairs (bp) of DNA.
6. Each nucleosome consists of a core particle and linker or spacer DNA.
7. The DNA wraps around the octamer core of histone proteins.
8. Spacer or linker DNA has four base pairs. One molecule of each histone H1 is connected with the linker DNA. The supercoiled nucleosome fiber is known as a solenoid.
9. According to this theory, a very long molecule of DNA (146 bp) is packed into a single unit of the nucleosome, and several nucleosome units constitute chromatin fiber.
10. The chromatin fiber of 300 Å is visible under an electron microscope at metaphase.
11. Kornberg and Thomas put forth this model of chromatin organisation in 1974. This model is universally accepted as a model of chromatin fiber organisation.

Fig: Organisation of Chromosome

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Chromosome Functions

The functions or significance of chromosomes are:

1. The most vital function of the chromosome is to pass on genetic information through DNA. DNA provides genetic information for various cellular activities. These functions are important for the growth, survival, and reproduction of organisms.
2. Particular species of organisms or the sex of that species are determined with the help of chromosomes. For specific species, the number of chromosomes is constant, the similar way humans have 23 pairs of chromosomes out of which one pair determines sex chromosomes. Females contain two X chromosomes, and males contain one X and one Y chromosome.
3. Histones and other proteins protect the chromosome from any physical or chemical forces by covering it. Thus, DNA (genetic material) gets protected from damage during cell division and ensures the proper packaging of DNA.
4. At cell division, spindle fibers attached to the centromeres contract, ensuring precise DNA distribution to the daughter cell.
5. The histone and non-histone proteins mainly help in the regulation of gene expression.
6. Chromosomes introduce variations in genes through the process of crossing over.
7. Each chromosome contains thousands of genes which are the code of multiple proteins present in the body.

Summary

A chromosome is the most important in passing genetic information from one generation to another generation. Based on the number of centromeres present in a chromosome, chromosomes again consist of five types: monocentric, dicentric, polycentric, acentric, and holocentric or diffused or non located chromosomes. Depending upon the position of the centromere, chromosomes are divided into four groups: telocentric, acrocentric, submetacentric, and metacentric.

Some other types of chromosomes are also shown, like; autosome, sex chromosomes. Giant chromosomes are polytene chromosomes and lampbrush chromosomes. In mitotic metaphase morphology of chromosomes can be studied most suitably through a light microscope. The different structures of chromosomes that can be seen are chromatid, primary constriction, secondary constriction, satellite, telomere, chromomeres and chromatin.

Frequently Asked Questions (FAQs) on Chromosome Structure and Function

Q.1. What is a satellite?
Ans: The region between secondary constriction and the nearest telomere is named a satellite.

Q.2. In humans, which chromosomes are acrocentric?
Ans: Chromosomes 13, 15, 21and 22 are acrocentric.

Q.3. Mention two giant chromosomes.
Ans: Polytene chromosome and Lampbrush chromosome are two giant chromosomes.

Q.4. Do males have a Y chromosome?
Ans: Yes, males have one Y chromosome and one X chromosome.

Q.5. What is a metacentric chromosome?
Ans: Metacentric chromosomes are v-shaped, and the centromere of the chromosome is in the center position, and the arms are almost equal.

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