What are Chromosomes? :Its function, and Structure

What are Chromosomes?

Inside the nucleus, Chromosomes are the thread like structure of Nucleic acids (DNA and RNA) and proteins (Histones and non-histones) present in most living cells in the form of genes (unit of heredity)  which contains all the genetic information. The chromosomes were first described in 1875 by Strausberger and the term chromosomes was first used in 1888 by Waldeyer. Chromo means color Soma means body. They are given this name because they have a marked affinity for the basic dyes. The number of chromosomes in the nucleus is directly proportional to the quantity of DNA inside a cell.

  • Each chromosome is made up of two identical strands known as chromatids which are joined at the centromere with the terminal regions known as a telomere.
  • Circular double stranded DNA molecule which is almost protein free is present in Prokaryotic chromosomes.
  • Linear double stranded DNA molecule complicated throughout their length with proteins is present in the Eukaryotic chromosome.

Functions of chromosomes

  1. Chromosomes have two basic functions the first one is constant transmission and the second one is the appropriate expression of genetic information.
  2. During cell division, Chromosomes allow the DNA to be copied accurately.
  3. Gene regulation, protein synthesis, and cellular replication are also carried by the chromosomes.

Morphological structure of chromosomes

During cell division, the following structures can be seen by staining with the help of a light microscope.


Two identical halves of a chromosome are called chromatids. During mitosis and meiosis, chromatids are produced from the chromatin fibers. Chromatids consist of DNA and proteins and when the DNA is wrapped around these proteins it is called Nucleosomes. Chromatids help the cell to replicate and thus play role in cell division. A pair of sister chromatids is called a dyad. Each chromosome consists of two sister chromatids which are joined with each other with the help of cohesins.


At the ends of linear eukaryotic chromosomes, telomeres are specialized DNA Protein complexes. The two ends of the chromosomes are called telomeres. They cap and provide protection at the ends of the chromosomes.


It is a protein structure that provides the points of attachment where spindle fibers attach to pull the sister chromatids apart during the cell division and present at the center of the chromosome called a centromere. Chromosomes can only bend at the site of primary constriction during the anaphase.

Primary constriction

The chromatids of the chromosomes are attached to each other by primary constriction. In simple words, the centromere of the chromosome is called primary constriction.

Secondary constriction

In addition to the primary constriction or centromere, the constricted region present at any point on the chromosome is called secondary constriction. It may be present in the short arm or long arm near the centromere. The main purpose of this constriction region is that it is used in the identification of chromosomes from a set. During anaphase in a cell, there can be zero, one, two, three, or four secondary restriction sites. They are also called Nucleolar organizers because they represent the sites of nucleolus formation.


It is the region where the two sister chromatids are held together. It provides the site for the attachment of the spindle fibers during cell division.

Types of chromosomes

On the bases of the location of the centromere, a chromosome is divided into four types


If the centromere is present in the middle of the chromosome dividing the chromosomes in two arms of roughly equal length it is called Metacentric.

Sub metacentric

If the centromere is present near the middle of chromosomes and thus has a long arm or a short arm it is called Sub metacentric. A human’s second number chromosome is metacentric.


If the centromere is located near the end of the chromosome it is called Acrocentric. Five pairs of autosomal chromosomes are acrocentric (13,14,15,21 and 22). The Y chromosome is also acrocentric.


If the chromosome is located at one end it is called Telocentric.

Chromosomes undergo major changes in the cell cycle

  • Especially at the S-phase when they replicated and the M-phase where the separation of the replicated chromosomes occurs and set apart into two daughter cells.
  • When DNA replication occurs at the S-phase, it produces double stranded DNA molecule that is held together at the specialized location which is known as Centromere, and these are called Sister Chromatids when sister chromatids are held together at the centromere.
  • The optimal time for viewing the chromosomes under the microscope is at the Metaphase stage of the M phase when the chromosomes are so highly condensed that the gene expression is uniformly shut down. When the GC-rich or AT-rich regions are stained with dyes that bind preferentially to these regions it allows the different chromosomes to be differentiated and thus give reproducible chromosome banding patterns.
  • The interphase is the longest phase of the cell cycle. Before moving into the mitosis during interphase cell grows and copies its DNA before moving into mitosis then during the mitosis chromosomes will first align, separate, and at last move into the new daughter cells. During interphase that separates successive M-phases, commonly chromosomes have very long extended structures. They are not visible under optical microscopy. Here the meaning of extended structure is that the genes are efficiently expressed. During the Interphase, some regions of the chromosomes are highly condensed (Heterochromatin) these regions are transcriptionally inactive and others are extended which allows gene expression (Euchromatin).
  • Sperm and egg cells are haploid means they have one copy of each chromosome, But most of the cells have two sets of chromosomes (Diploid).
  • A diploid zygote is formed when a haploid egg is fertilized by the haploid sperm. From this diploid zygote, all the other cells arise by cell division.
  • If an abnormal number of chromosomes occurs sometimes but if present in most of the cells of the body it will be lethal.
  • When the genes are deleted and incorrectly expressed by breaks in the chromosomes it leads to structural chromosomal abnormalities.
  • To transmit DNA faithfully from the mother cell to the daughter cell for the eukaryotic chromosome we required three essential and functional elements which are the following
  • Centromere ( during the cell division it ensures that the segregation of the chromosomes occurs accurately)
  • Replication origins ( which starts the replication)
  • Telomere ( it protects the internal DNA from being degraded by nucleases by providing the cap to chromosomes)

Difference between Heterochromatin and Euchromatin

Highly condensed chromosomal regionExtended chromosomal region/less condensed
Transcriptionally InactiveAllow the gene expression
Gene- poorGene-rich 
Late replicativeEarly replicated
Present only in the Eukaryotic genomePresent in both prokaryotic and Eukaryotic genome
Present at the Nucleus peripheryPresent inside of a nucleus
Usually sticky in natureNot sticky in nature
Darkly stained by G bandsLighter stained with C bands
DNA density is highLow density DNA
Makes up 97 -98% of the genomeMakes up 2-3% of the genome
Contains more DNAContains less DNA

Difference between Mitosis and Meiosis

Mitosis is the normal form of cell divisionMeiosis is specialized reductive cell division
Daughter cells diploidDaughter cells are haploid
Daughter cells are genetically identicalDaughter cells are genetically diverse
No tetrad formationTetrad formation occurs
Occurs in somatic cellsOccurs in Germ cells
Two daughter cells are formedFour daughter cells are formed
The chromosome number remains the sameChromosome numbers are reduced to half
In chromosomes, no Crossing over occursCrossing over occurs
Division of the nucleus occurs only onceDivision of the nucleus occurs twice
Occurs frequentlyOccurs less frequently
No pairing of Synapsis occursThe pairing of Synapsis takes place
In all organisms, it occurs except virusesIn Plants, animals, and fungi it occurs
Occurs in all tissuesOccurs in specialized germ cells in the testis and ovary
Per cell division one round of replicationPer two cell divisions only one round of replication
Short approximately 30 minutes in a human cellCan takes decades to complete


  • Hardin, J., Bertoni, G., & Becker, W. M. (2018). Becker’s world of the cell. Pearson.
  • Strachan, T., & Read, A. P. (2020). Human molecular genetics. CRC Press.
Mubashir Iqbal
Mubashir Iqbal

Mubashir Iqbal is a highly dedicated and motivated Microbiologist with an MPhil in Microbiology from the University of Veterinary and Animal Sciences. Currently, he is researching the efficacy of commercially available SARS Cov-2 vaccines to neutralize the omicron variant in Pakistan. He holds a Bachelor's degree in Microbiology and has experience in chemical and microbiological analysis of water samples, managing SOPs and documents according to standard ISO 17025. Additionally, he has worked as an internee in BSL 3, Institute of Microbiology, UVAS, where he gained experience in RNA extraction, sample processing, and microscopy.

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