Chromosomes-featured

Chromosomes – Discovery, Structure, Types & Functions of Chromosomes

Overview

The chromosome is a thread-like structure resulting from the organization of chromatin materials during cell division. Under the microscope, they appear to be made of arms (chromatids) and a centromere.

A centromere is a place where chromosome and kinetochore attach with spindle fibres during cell division.

The DNA is organized with proteins to form chromosomes. The chromosomes condense before the cell division and become thick. Now they can be seen as separate structures.

Each eukaryotic species has a characteristic number of chromosomes. A human cell has 46 chromosomes in its nucleus. But the gametes – egg and sperm have only 23 chromosomes in humans.

Chromosomes

Chromosomes are thread-like structures present in the nucleus, which brings hereditary information from one generation to another. They play an important role in cellular division, heredity, variation, repair, regeneration and mutation.

In Eukaryotic cells, the genetic product exists in the nucleus in chromosomes, which is made up of extremely organized DNA molecules with histone proteins supporting its structure. Chromosome implies ‘coloured body’, that describes its staining capability by particular dyes.

History of Discovery

Karl Nägeli in 1842, very first observed the rod-like structure present in the nucleus of the plant cell. W. Waldeyer in 1888 coined the term ‘chromosome’.

Walter Sutton and Theodor Boveri in 1902 recommended that chromosomes are the physical carrier of genes in the eukaryotic cells. The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscopic lens, he counted 24 sets, which would indicate 48 chromosomes. His mistake was copied by others and it was not until 1956 that the true number, 46, was identified by Indonesia-born cytogeneticist Joe Hin Tjio.

Chromosomes-Discovery

Chromosomes in different organisms

Chromosome number varies in different species. A nematode species includes only 2 chromosomes in a cell, whereas a protozoan species includes as much as 1600 chromosomes in the cell. Most of the plant and animal species include 8 to 50 number of chromosomes in its somatic cell. The variety of chromosomes does not show the intricacy of a species. A human cell consists of a total 23 pair of chromosomes (2n, overall, 23 × 2= 46), of which 22 are autosomes and 1 sex chromosome.

Further Reading:  Takhtajan Classification System

Karyotyping is a technique to study the structure of chromosomes present in a species. Chromosomes are separated, stained and photographed. This strategy is useful in finding out any chromosomal abnormalities.

Chromosomes-organisms

 

Structure of Chromosome

Each cell has a set of each kind of chromosome called a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome includes hundreds and thousands of genes that can specifically code for a number of proteins in the cell. Structure of a chromosome can be best seen throughout the cellular division.

Main parts of chromosomes are:

Chromatid: Each chromosome has two symmetrical structures called chromatids or sister chromatids which is visible in mitotic metaphase. Each chromatid consists of a single DNA molecule. At the anaphase of mitotic cellular division, sister chromatids separate and move to opposite poles.

Centromere and kinetochore: Sister chromatids are joined by the centromere. Spindle fibres during cell division are attached at the centromere. The number and position of the centromere differ in various chromosomes. The centromere is called primary constriction.

Centromere divides the chromosome into 2 parts, the much shorter arm is called ‘p’ arm and the longer arm is known as ‘q’ arm. The centromere includes a disc-shaped kinetochore, which has specific DNA sequence with special proteins bound to them. The kinetochore supplies the centre for polymerization of tubulin proteins and assembly of microtubules.

Secondary constriction and nucleolar organizers: Besides centromere, chromosomes have secondary constriction. Secondary constriction can be identified from centromere at anaphase due to the fact that there is flexing just at the centromere (primary constriction). Secondary constriction, which includes genes to form nucleoli are called the nucleolar organizer.

Further Reading:  Diseases due to Viruses

Telomere: Terminal part of a chromosome is referred to as a telomere. Telomeres are polar, which prevents the fusion of chromosomal sections.

Satellite: It is an extended segment that is sometimes present on a chromosome at the secondary constriction. The chromosomes with satellite are called sat-chromosome.

Chromosomes-Structure

Chromatin:

The chromosome is comprised of chromatin. Chromatin is made up of DNA, RNA and proteins. At interphase, chromosomes are visible as thin chromatin fibres present in the nucleoplasm. During cellular division, the chromatin fibres condense and chromosomes are visible with unique features. The darkly stained, condensed region of chromatin is called heterochromatin.

It contains securely loaded DNA, which is genetically non-active. The light stained; diffused area of chromatin is called euchromatin. It contains genetically active and loosely packed DNA. At prophase, the chromosomal material is visible as thin filaments referred to as chromonemata. At interphase, bead-like structures are visible, which are an accumulation of chromatin material called chromomere. Chromatin with chromomere appears like a string with beads.

Types of Chromosomes
A.Autosomes and Sex Chromosomes

Human chromosomes are of two types autosomes and sex chromosomes. Hereditary traits that are connected to the sex of the person are handed down through the sex chromosomes. The rest of the genetic info exists in the autosomes. Human beings have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one set of sex chromosomes, making an overall of 46 chromosomes in each cell.

B.On the Basis of Number of Centromeres
  • Monocentric with one centromere.
  • Dicentric with 2 centromeres
  • Polycentric with more than 2 centromeres.
  • Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes which do not make it through for long.

Diffused or non-located with indistinct centromere diffused throughout the length of the chromosome.

C.On the Basis of Area of Centromere
  • Telocentric are rod-shaped chromosomes with centromere inhabiting the terminal position so that the chromosome has just one arm.
  • Acrocentric are likewise rod-shaped chromosomes with centromere inhabiting a sub-terminal position. One arm is long and the other is short.
  • Sub-metacentric chromosomes are with centromere a little far from the mid-point so that the two arms are unequal.
  • Metacentric are V-shaped chromosomes in which centromere depends on the middle of the chromosome so that the two arms are practically equivalent.
Further Reading:  Taxonomy- Classification of Kingdom Animalia (Part 2)

Centromere

Function and Significance of Chromosomes

The number of chromosomes is constant for a particular species. Therefore, these are of great importance in the determination of the phylogeny and taxonomy of the species.

  1. Genetic Code Storage:

Chromosome includes the genetic product that is needed by the organism to develop and grow. DNA particles are made from a chain of systems called genes. Genes are those sections of the DNA which code for specific proteins required by the cell for its appropriate performance.

  1. Sex Determination:

Humans have 23 sets of chromosomes out of which one set is the sex chromosome. Females have 2 X chromosomes and males have one X and one Y chromosome. The sex of the child is identified by the chromosome passed down by the male. If X chromosome is passed out of the XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops.

Sex-Determination

  1. Control of Cellular Division:

Chromosomes check successful division of cells throughout the procedure of mitosis. The chromosomes of the parent cells ensure that the right information is passed on to the child cells needed by the cell to grow and develop correctly.

  1. Development of Proteins and Storage:

The chromosomes instruct the sequences of proteins formed in our body and also keep the order of DNA. The proteins are likewise kept in the coiled structure of the chromosomes. These proteins bound to the DNA assistance in proper packaging of the DNA.