“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.” 


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 present in live-cell, in a membrane-bounded structure called nucleus. 

It is a basic unit of inheritance and 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 non-coding DNAs. 

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

Content of the article:

  • What is a chromosome?
  • Definition of chromosome
    • Human chromosomes
    • X and Y chromosome 
    • Structure of chromosomes
    • Diagram of human chromosomes  
    • Function of chromosomes 
  • Chromosome abnormalities 
  • Chromosome analysis 
    • Karyotyping 
    • Chromosome microarray 
  • Conclusion
Graphical representation of how DNA arranged on a chromosome.

Graphical representation of how DNA arranged on a chromosome.

What is a chromosome? 

Chromosome= chroma (colour) + some (body)

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

In 1842, Karl Wilhelm von Nageli, a swiss botanist discovered a structure which 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 are arranged on chromosomes having, centromere, telomere, p arm and q arm. 

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

Different species have a different number of chromosome based on their genome size.

The number of chromosomes in the different organism: 

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 (Mymecia 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 are arranged on this 23 pair 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 sex. 

A pair of the X chromosome is present in females while a single X along with a short Y chromosome is present in males. 

Thus XX in females and XY in the male are denoted as sex chromosomes. 

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

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

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

The location of genes on a chromosome.

The location of genes on a chromosome.

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 which has the beads-on-string like structure and, later turned into long chromatin. Finally, the chromatid attached with the centromere and creates the chromosome.

Centromere and arms are the main components of a chromosome: 

The graphical representation of the structure of chromosome- centromere, arms and telomeres.

Centromere: 

First of all, the centromere is not a centre of the chromosome, based on the location of the centromere, chromosomes are categorised into different categories. 

Actually, a constricted, narrow and somewhat rounded region of a chromosome which 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 with 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 it. 

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

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

On the other hand, the loosely packed region is known as 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 which protects gene-rich region- and are made up of the repetitive DNA sequence, the non-coding DNA sequences on the telomeres are categorised in microsatellite and minisatellite. 

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

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

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

After each round of replication, the length of telomeres reduces, once it disappears cell 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 not happened, it can cause cancer. 

Read more on telomere and ageing: role of telomere in ageing. 

Classification of chromosomes: 

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

Based on the location of the centromere, the 23 pairs of human chromosome 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 human. 

Metacentric chromosomes: 

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

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

Submetacentric chromosomes: 

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

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

Acrocentric chromosomes: 

In the acrocentric chromosomes, the centromere 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 little larger than the telocentric chromosomes. 

Telocentric chromosomes: 

In this type of chromosomes, 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. 

Classification of chromosomes based on the location of centromere.

Classification of chromosomes based on the location of centromere.

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, that 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 offsprings. 

Furthermore, sex chromosomes decide the sex of the embryo, we had discussed it 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

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

Read more: What is genome?

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 chromosome 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 occurs. This condition results in genetic abnormality called down syndrome- a type of mental disorder. 

Another example of trisomy is trisomy 18 called Edwards syndrome. 

Monosomy: once chromosome lacks from the pair, the condition is called monosomy. 

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

Uniparental disomy: as we know that one chromosome from father and one chromosome from 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 chromosome 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 human. 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 use to encounter numerical and lager 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 since long for 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 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 used 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 on it, the complementary DNA sequence will bind with it and 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, arrange 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, 

Image explanation (A) results of conventional karyotyping, (B) results of Chromosome microarray.

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

Conclusion:

A chromosome is a complex network of DNA and protein. It is very important that each and every chromosome inherited properly. Karyotyping and microarray like tools are required for chromosomal analysis.