A chromosome, by definition is a threadlike strand of DNA in a cell nucleus that carries genes, the units of heredity, in a linear order. Human beings have 22 chromosome pairs and a pair of sex chromosomes. Apart from genes, the chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-bound proteins which control functions of the DNA.
Interestingly, the word 'chromosome' originated from the Greek word, chrome meaning color. Chromosomes got their name owing to their property of being stained by dyes. The structure and nature of chromosomes varies across different kinds of organisms.
Human chromosomes have always been a subject of interest for researchers working in genetics. The wide range of factors that human chromosomes determine, the abnormalities they are responsible for, and their complex nature have always invited the interest of many.
Interesting Facts about Human Chromosomes
Human cells have 23 pairs of nuclear chromosomes. A chromosome is made up of a DNA molecule that contains genes. A chromosomal DNA molecule contains three nucleotide sequences, which are required for replication. On staining the chromosomes, the banded structure of the mitotic chromosomes becomes apparent. Each band contains numerous DNA nucleotide pairs.
Human beings are sexually reproducing species and have diploid somatic cells having two sets of chromosomes. One set is inherited from the mother and the other from the father. As against the body cells, reproductive cells have a single set of chromosomes. The crossover between chromosomes results in the creation of new ones. The newly created chromosomes are not inherited from any single parent. This is why not all of us exhibit traits that are directly derived from one of our parents.
There are 24 distinct human chromosomes out of which 22 are autosomal chromosomes and the remaining two are sex-determining chromosomes. The autosomal human chromosomes are numbered from 1 to 22 in the decreasing order of their size. An individual has two sets of the 22 chromosomes, X chromosome from the mother, and an X or a Y chromosome from the father.
An abnormality in the content of the chromosomes of a cell can cause certain genetic disorders in human beings. Chromosomal abnormalities in human beings are often responsible for the appearance of genetic disorders in their children. Those with chromosomal abnormalities are often only the carriers of the disorder, while their children actually exhibit the disorders.
An extra copy of chromosome 21 is responsible for the genetic disorder known as Down's syndrome. The trisomy of chromosome 18 results in Edwards syndrome that may cause death in infancy.
The deletion of a part of the fifth chromosome leads to a genetic disorder known as 'cri du chat', meaning 'cry of a cat'. Those affected by this disorder show a cat-like crying in childhood and mostly, are mentally retarded.
Disorders caused by sex chromosomes include the Turner syndrome wherein female sexual characteristics are present but underdeveloped, the Triple-X syndrome in girls and the XXY syndrome in boys, both causing dyslexia in the affected individuals.
Chromosomes were first discovered in plants. Van Beneden's monograph on the fertilized eggs of a roundworm, led the research further. Later in time, August Weismann proclaimed that the germ line was different from the soma and he discovered that the cell nucleus housed the hereditary material. He also proposed that fertilization results in a new combination of chromosomes.
These discoveries were cornerstones in the field of genetics. Researchers have achieved a sufficient amount of knowledge of human chromosomes and genes but there is still a lot to be discovered.