Chromosomes - The Carriers of Heredity

Gregor Johann Mendel published his work on inheritance of traits in 1866, but for some reason, it remained unnoticed or unrecognised till 1900. In 1900, three scientists, Hugo de Vries, Correns and von Tschermak, independently rediscovered Mendel’s work on the inheritance of traits. Walter Sutton, along with Theodor Boveri (1903), studied the parallel behaviour of Mendel’s factors (genes) and the behaviour of chromosomes at the time of meiosis. Based on these observations, the chromosomal theory of inheritance was put forth by Sutton and Boveri. This theory identifies chromosomes as the carriers of genetic material. This theory states that the chromosomes are present in pairs in somatic cells. The nucleus of gametes contains chromosomes, which carry all hereditary traits. Male and female gametes (sperm and egg) carry all the hereditary traits. 

Chromosomes are filamentous bodies present in the eukaryotic nucleus. The term chromosomes (Gr., chromo = colour, soma = body) was coined by W. Waldeyer (1888). Chromosomes are visible during cell division. They are capable of self-replication and play a vital role in the heredity, mutation, variation, and evolutionary development of eukaryotic species. Chemically, eukaryotic chromosomes are made up of DNA, histone and non-histone proteins.

Organization of chromosome

Human somatic cells have 46 chromosomes arranged into 23 pairs. These include 22 pairs of autosomes and one pair of sex chromosomes (XX for females and XY for males). 

• Function of Chromosomes: Chromosomes mainly act as carriers of heredity. 
• Number of Chromosomes: The number of chromosomes is specific and constant for a particular species; therefore, it is of great importance in the study of the phylogeny and taxonomy of the species. 

The chromosome number of some organisms has been given in the following table:

The term ploidy refers to the degree of repetition of the primary basic number of chromosomes (i.e., ‘x’) in a cell.

Euploidy: When the chromosome number in a cell is the exact multiple of the primary basic number, then it is called euploidy

1. Monoploidy (with one set of chromosomes where x n)

2. Diploidy (2n-two sets of chromosomes)

3. Polyploidy:

• Triploidy (3n-three sets of chromosomes)
• Tetraploidy (4n-four sets of chromosomes)

Aneuploidy: When the chromosome number is not the exact multiple of the haploid set, it is described as aneuploidy.

1. Hypoploidy (Fewer numbers of chromosomes than the normal diploid number)

• Monosomy (Loss of one chromosome from the diploid set (2n-1))
• Nullisomy (Loss of both chromosomes of a homologous pair (2n-2))

2. Hyperploidy (More chromosomes than the normal diploid number)

• Trisomy (Extra chromosome added to the diploid set (2n+1))
• Tetrasomy (Two extra chromosomes added to the diploid set (2n+2))

1. Chromatids: Each chromosome is made up of two identical halves known as sister chromatids, which are linked at the centromere. 
2. Centromere: There is a constricted region on each chromosome. It is called the ‘primary constriction’ or ‘centromere’. This divides the chromosome into two parts.
3. Telomeres: Chromosome ends have protective coverings that prevent disintegration and fusion with other chromosomes.
4. Chromatin: DNA is wrapped around histone proteins, resulting in a complex that condenses to form visible chromosomes during cell division. 
5. Kinetochore: During mitosis and meiosis, spindle fibres attach to the centromere, pulling chromatids apart. 
6. Arm: Each chromatid has two arms, that is, the short arm (p arm) and a long arm (q arm). 
7. Genes: Chromosomes include DNA segments that encode proteins and determine inherited characteristics. 
8. Euchromatin and heterochromatin: Euchromatin is lightly packed and transcriptionally active, whereas heterochromatin is densely packed but transcriptionally inactive. 
9. Secondary constriction: Besides primary constriction, some chromosomes have an additional one or two constrictions called secondary constriction. At secondary constriction I, the nucleolus becomes organised during interphase. A satellite body (SAT body) is attached at secondary constriction II in very few chromosomes. 

A: Parts of chromosomes B: Showing secondary constrictions and details