Chromosome- Definition, Structure, Function and Classification

“An essential element of life- chromosomes are located in the nucleus of a cell on which the entire genomic DNA of an organism is arranged.” 

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By coiling and supercoiling with each other DNA- deoxyribose nucleic acid forms threads of the chromosome. 

We all know about DNA right? If not, read our previous article on DNA: DNA story: structure and function of DNA. 

Let me give you a  brief overview of nucleic acid, 

DNA- a type of nucleic acid is present in live-cell, in a membrane-bounded structure called a nucleus. 

It is a basic unit of inheritance and was discovered by Watson and Crick in 1953. As we said above it is located on chromosomes, all the DNA of us is known as a genome- tailored with coding DNA and non-coding DNAs. 

Chromosomes are basic inheritance unit varies from species to species. In the present article, I will explain to you the structure and function of the chromosome along with other information related to it. 

What is a chromosome? 

Chromosome= chroma (colour) + some (body); 

Chromosomes are a complex network of proteins and DNA helps DNA to organize during cell division and protects it. 

In 1842, Karl Wilhelm von Nageli, a swiss botanist discovered a structure later known as chromosomes. 

In 1882, Wilhelm Hofmeister denoted the term as “chromosomes”. Later on in the year, 1883 Wilhem roux speculated chromosomes as a carrier of genetic information.

Unlike the prokaryotic DNA, the DNA of eukaryotes is arranged on chromosomes having centromere, telomere, p arm and q arm. 

A prokaryotic chromosome is either made up of DNA or RNA (RNA in the case of some viruses)- but is not located in the nucleus. 

Different species have a different number of chromosomes based on their genome size. The number of chromosomes in different organisms are Given below,

Organism	Number of chromosomes (2n)
Human	46
Potato	48
RIce	24
Monkey	42
Mouse	40

Interesting information: 

Adders- tongue (Ophioglossum) has the highest number of chromosomes – 1260 while jack-jumper ant (Myrmecia pilosula) has the lowest number of chromosomes- 2.  

Definition of chromosome: 

“DNA of eukaryotes such as plants and animals is arranged as a tightly packed thread-like structure known as a chromosome.” 

Or 

“A complex network of DNA and protein- coiled around each other and helps to fit DNA inside the nucleus is known as a chromosome.” 

Human chromosomes: 

46 chromosomes are present in each human cell, except germ cells. All chromosomes are present in a pair thus a total of 46 chromosomes are present in 23 pairs called a diploid number of chromosomes. 

All the somatic cells have a diploid number of chromosomes while egg and sperm-germ cells have a haploid number of chromosomes. 

 All the nuclear- DNA of a cell is arranged on these 23 pairs of chromosomes, however, some linear or circular DNA is also present in the cytoplasm. 

X and Y chromosome: 

The 22 pairs of chromosomes are the same in all-male and females, however, one pair of sex chromosomes decide their gender. 

A pair of the X chromosome is present in females while a single X along with a short Y chromosome is present in males. Hence XX in females and XY in males are denoted as sex chromosomes. 

Interestingly, a mechanism called X-chromosome inactivation helps to inactivate one X chromosome for regulating gene expression, especially in females.

The SRY- a sex-determining region of Y, responsible for maleness and helps in the development of male phenotypes although it is solely not decided the fate of the embryo. 

The presence or absence of the X chromosome decides whether the developing embryo becomes female or male. 

Structure of the chromosome: 

How DNA is arranged on the chromosome is a complex process. Let’s start with the DNA itself. DNA is a double-stranded molecule and helical in shape.

Imagine a rope and arrangement of threads in a rope. This spiral arrangement creates tension on the remaining strand of DNA. Now the DNA wraps on each other and creates a supercoiled structure.

It binds with Histone protein molecules (H1, H2A, H2B, H3 and H4) and forms a nucleosome.

It looks like a solenoid that has a beads-on-string-like structure and, later turned into long chromatin. Finally, the chromatid formed from chromatin is attached to the centromere and formulates the chromosome.

Learn more about how chromatin is formed: Inside the chromatin.

Centromere and arms are the main components of a chromosome: 

Centromere: 

First of all, the centromere is not the center of the chromosome, remember that, based on the location of the centromere, chromosomes are categorized into different categories. 

Actually, a constricted, narrow and somewhat rounded region of a chromosome holds chromatids known as the centromere. 

During the process of cell division, and when replication is in progress, the centromere plays an important function to align chromosomes properly to replicate. 

It provides an attachment point to complete the replication properly. Notably, important genes are not present in the centromeric region of the chromosome. 

Arms: 

The “p arm” and “q arm” of the chromosome are attached to the centromeres. Although, based on the location of the centromere, the length of the arms varies. 

For example, the p and q arm of the metacentric chromosome 1 has almost the same length while the p arm of acrocentric chromosome 21 is very short. The arms are, we can say the main body of the chromosomes, having all the essential genes on them. 

Arms are the complex network of protein and DNA where genes are located. 

The densely packed area of arms is called the heterochromatin region- a gene-less region, rich in non-coding DNA. 

On the other hand, the loosely packed region is known as the euchromatin region- a gene-rich region. The tip of each arm is protected by the structure called telomeres. 

“Higher the length of arms, more genes it contains.”

Telomeres: 

Telomeres are the end of chromosomes that protects gene-rich region- and are made up of repetitive DNA sequence, the non-coding DNA sequences on the telomeres are categorized in microsatellite and minisatellite. 

The repetitive sequences are highly packed and thus do not encodes any protein however, it protects other genes from the cell’s own mistake called “end replication problem.” 

The end replication problem occurs when DNA polymerase starts incorporating the wrong nucleotides, especially at the end of the replication. Protein-coding genes are located on arms and thus replicate properly. 

But once the polymerase reaches to end, the replication stops or incorporates the wrong bases henceforth after each round of replication some sequences from the telomeres can not be replicated and are lost. 

After each round of replication, the length of telomeres reduces, once it disappears cells will die and consequently the organism. Therefore it is believed that the length of the telomere is directly proportional to the age of the organism. 

Contrary to this, if telomere shortening does not happen, it can cause cancer. 

Classification of chromosomes: 

As we discussed above, the centromere of chromosomes plays an important role in its characterization.

Based on the location of the centromere, the 23 pairs of human chromosomes can be divided into 7 categories. 

Group	Size	Position of centromere	Number of chromosomes
Group A	Large	Metacentric	1, 2 and 3
Group B	Large	Submetacentric	4 and 5
Group C	Medium	Submetacentric	6, 7, 8, 9, 10, 11,12 and X
Group D	Medium	Acrocentric	13, 14 and 15
Group E	Relatively short	Submetacentric	16, 17 and 18
Group F	Short	Meta or submeta	19 and 20
Group G	Short	Acrocentric	21, 22 and Y

*telocentric chromosomes is another category but not present in humans. 

Metacentric chromosomes: 

In metacentric chromosomes, a centromere is located exactly or nearly exactly in the center of both p and q arms. 

Due to this p and q arms are equal in length. 

Submetacentric chromosomes: 

In this type of chromosome, a centromere is shifted a little towards one of the two arms. 

Thus the lengths of the p and q arm are almost similar but not equal. 

Acrocentric chromosomes: 

In the acrocentric chromosomes, the centromere is located very close to the p arm and therefore the p arm is very shorter than the q arm. 

The q arm is longer than the p arm but the p arm is a little larger than the telocentric chromosomes. 

Telocentric chromosomes: 

In this type of chromosome, the centromere is located very close to the p arm and thus the p arm is barely visible or absent. However, the q arm is still long enough to distinguish. 

Notably, humans do not have telocentric chromosomes while all the chromosomes of the house mouse are telocentric. 

The function of chromosomes: 

Chromosomes facilitate proper cell division and replication. 

The main function of the chromosome is to fit the DNA inside the nucleus. As we all know, our DNA is too long, if we unwind all the DNA of a cell, it is up to 2 meters in length. 

Hence it is very important to fit it inside the nucleus which is facilitated by chromosomes. By interacting with proteins DNA forms a coiled structure- chromosome. 

Chromosomes also help in inheriting genes or DNA from parents to their offspring. 

Furthermore, sex chromosomes decide the sex of the embryo, as we had discussed above. 

The process of sex determination and sex differentiation is governed by genes located on autosomes and sex chromosomes. 

Size and number of genes on different human chromosomes: 

Chromosome	Number of genes
1	2000
2	1300
3	1000
4	1000
5	900
6	1000
7	900
8	700
9	800
10	700
11	1300
12	1100
13	300
14	800
15	600
16	800
17	1200
18	200
19	1500
20	500
21	200
22	500
X	800
Y	200

A total of around 21,000 to 22,000 genes are present in the human genome in which chromosome 1 has the highest number of genes (2000) while the Y chromosome has the lowest number of genes (200). 

Read more: What is a genome?

Conclusively the functions of a chromosome are,

• Make DNA fit inside the nucleus
• protect genes
• Facilitate DNA replication and transcription
• Store and transfer genetic information
• Protect DNA from damage
• Helps in regulating gene expression.

Chromosomal anomalies: 

Chromosomal abnormalities occur due to the change in either chromosome number or chromosome structure. 

Deletion, duplication, translocation, inversion and addition are some of the structural chromosomal abnormalities while trisomy, tetrasomy and monosomy are numerical chromosomal abnormalities. 

Interestingly, in numerical chromosomal abnormalities change in the total number of chromosomes results in an abnormal genetic condition. 

Trisomy: one extra chromosome is added into the genome. For example trisomy 21, in which three different 21st chromosomes occur. This condition results in a genetic abnormality called down syndrome- a type of mental disorder. 

Another example of trisomy is trisomy 18 called Edwards syndrome. 

Monosomy: One chromosome is absent from the pair, the condition is called monosomy. 

For example, monosomy of X often known as XO or turner syndrome is a type of complex genetic disorder. 

Uniparental disomy: as we know that one chromosome from the father and one chromosome from the mother comes in offspring. But in uniparental disomy, both chromosomes come from the same parents. Although the total number of chromosomes remains unchanged, it causes genetic abnormality. 

For example Prader-Willi syndrome. 

We have covered a dedicated article on structural chromosomal abnormalities hence we are not discussing it here. 

Anyway, 

One of the classical examples, of structural chromosomal abnormality, is the translocation between chromosomes 9 and 22 which is called as Philadelphia chromosome. We also have an entire article on the Philadelphia chromosome, read it here: Philadelphia Chromosome, BCR-ABL1 Gene Fusion And Chronic Myeloid Leukemia.

Chromosome analysis: 

Alike DNA mutations, chromosomal abnormalities are also common in humans. Thus for identifying chromosomal abnormality, we need techniques for chromosomal analysis. 

In recent days two of the best chromosomal analysis methods- karyotyping and chromosomal microarray are used routinely. 

Karyotyping: 

Karyotyping- a cytogenetic method is often known as PBLC- peripheral blood leukocyte culture is employed to encounter numerical and larger structural chromosomal abnormalities. 

In this method, the metaphase cells are harvested for getting metaphase chromosomes and stained using the Giemsa stains. 

A traditional, conventional and widely used method karyotyping is used for a long for the evaluation of chromosomal abnormalities. 

It is also used for the analysis of chromosomes in other species as well. 

The major limitation of karyotyping is, it can not identify minor deletions or duplications or other structural abnormalities because of the lower resolution banding. 

We have covered an amazing article on karyotyping, its protocol and the different components used in it. 

Read it here: A Karyotyping Protocol For Peripheral Blood Lymphocyte Culture.

Chromosome microarray: 

Chromosome microarray is a DNA hybridization-based method practiced for detecting many structural chromosomal aberrations in a single assay. 

In the chromosome microarray often known as “whole chromosome microarray,” thousands of different probes are immobilized on the solid glass surface. Once we apply our DNA sample to it, the complementary DNA sequence will bind with it and be detected in the microarray detector. 

Though it is called chromosome microarray we are using total genomic DNA, instead of metaphase chromosomes. 

The microarray detector detects abnormality if any, and arranges it accordingly on each chromosome. 

The main advantage of the present technology is that it is robust and automated as compared with conventional karyotyping. Besides this, we can screen thousands of different copy number variations at once. 

The results of karyotyping and chromosome microarray are shown in the figure below, 

Read our article on microarray: Genome-On-A-Chip: DNA Microarray.

Conclusion:

Histon protein and DNA are the constituents of a chromosome. Every chromosome should have been inherited correctly, otherwise, it may cause structural, developmental or reproductive defects. Techniques like karyotyping, FISH or chromosome microarray are commonly practiced to encounter chromosomal anomalies.