An Overview of Human Chromosomes

A chromosome is found in the nucleus of the human cell, the building block of the human body. It contains a linear thread of DNA (deoxyribonucleic acid) and associated proteins. A chromosome carries the genes and plays a major role in transmitting hereditary information. Human beings have 23 pairs of chromosomes (22 pairs of autosomes and two sex chromosomes, XX and XY). Each pair has unique characteristics.

A chromosomal DNA molecule contains a centromere and a telomere. Centromere is necessary to attach the DNA to the mitotic spindle. Telomere is located at each end of the linear chromosome. During cell division, the chromosome is compactly coiled. This facilitates movement with minimum entanglement. 

Each chromosome duplicates itself before cell division takes place (during S phase of the cell cycle). Chromosomes are too tenuous to be seen under a microscope, but the duplicated chromosomes can be stained appropriately and viewed using a microscope. The duplicated chromosomes are called dyads. The centromere divides the chromosome into two arms (sections). The short arm is known as the “p arm” and the long arm is known as the “q arm”. 

The karyotype (the complete set of chromosomes) of the human male contains 22 pairs of autosomes, one X chromosome and one Y chromosome. The karyotype of the human female contains 22 pairs of autosomes and a pair of X chromosomes. The 22 pairs of chromosomes are same in both males and females. 

Chromosome 1 is the largest human chromosome. It contains more than 3000 genes. These genes perform various functions in the human body. Chromosome 22 was the first human chromosome to be fully sequenced. 

Any variation in the number of chromosomes usually causes harmful health problems. It may happen during the formation of reproductive cells or during early fetal development. It may also occur after the child is born. 

People affected by Down syndrome have 3 copies of chromosome 21 in each cell. The presence of an extra chromosome is known as trisomy. Turner syndrome is a medical condition caused by monosomy. Women affected by this medical condition have only one copy of the X chromosome. Mosaic Turner syndrome is a medical condition characterized by the absence of X chromosome in some cells. 

Chromosomes continue to fascinate scientists and extensive research is being conducted in this area at various labs all over the world. The subject is very interesting.