Chromosomes Essay

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The genetic information encoded in DNA and carried in cells is normally packaged in molecules called chromosomes: long strands of DNA and associated proteins that carry many genes, regulatory elements, and nucleotide sequences. Merriam-Webster Online Dictionary defines chromosome as “one of the linear or sometimes circular DNAcontaining bodies of viruses, prokaryotic organisms, and the cell nucleus of eukaryotic organisms that contain most or all of the genes of the individual.” It is derived from two Greek words: chroma, meaning color, and soma, meaning body.

Chromosomes were first observed by a Swiss botanist, Karl Wilhelm von Nageli, in 1842 in plant cells. Belgian scientist Edouard Van Beneden independently observed chromosomes in Ascaris worms around the same time. Walther Flemming, a German anatomist, in 1882 discovered mitosis (the process by which a cell separates its duplicated genome into two identical halves) and described the behavior of chromosomes in animals. In 1910, an American geneticist, Thomas Hunt Morgan, proved that chromosomes are the carriers of genes by studying the common fruit fly.

Chromosomes are present in every living being, whether it is bacteria, fungi, plants, insects, animals, or humans. However, each species has different number of chromosomes (see table), but normal members of a particular species all have the same number of chromosomes. The asexually reproducing species have one set of chromosomes, which is the same in all body cells. Chromosomes in bacteria are most often circular, but sometimes they are linear as well. Most sexually reproducing species contain two haploid sets of chromosomes (chromosomes are multiples of 2, and they are therefore mostly present in pairs, though there are some animal and plant species that have more than two sets of chromosomes; for example, tobacco or wheat) and are referred to as polyploid. Some wild as well as cultivated varieties of wheat have 14 (diploid) chromosomes, whereas common pasta and bread wheat are polyploid and have 28 (tetraploid) and 42 (hexaploid) chromosomes, respectively. In humans, there are 46 chromosomes that occur as 23 pairs-22 pairs of autosomes and 1 pair of sex chromosome. Each child inherits one strand from the mother and another from the father to form a pair. Aulacantha (protozoa) has largest number of chromosomes at 1,600.

Each chromosome has two arms that differ in length and are referred to as a p (short arm) and q (long arm). The two arms are separated by a region called the centromere (see figure). The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. Chromosomes are very small and are visible only by using an optical microscope.

The abnormalities in chromosomes can occur either by change in number, size, or structure of chromosomes. The change in chromosome structure could occur due to breakage or rearrangement translocation, inversion, rings, or deletions of some of the chromosome material. This leads to disorders such as Down’s syndrome. During formation, an egg or sperm can sometimes have either an extra chromosome (24 chromosomes) or one less chromosome (22 chromosomes). When such an egg (or sperm) combines at conception with a normal sperm (or egg) with 23 chromosomes, the resulting embryo ends up with too few or too many chromosomes, i.e., 45 or 47 instead of the usual 46. These lead to disorders such as trisomy 21, XYY syndrome, and Klinefelter syndrome. Prenatal testing is available for the screening or diagnosis of the disorders of plants and animals.

Bibliography:

  1. J. Gardner, Grant R. Sutherland, and Gardner R. J. McKinlay, Chromosome Abnormalities and Genetic Counseling (Oxford University Press, 2003);
  2. F. Jorgenson, J.H. van de Sande, and C.C. Lin, Chromosoma (v.68, 1978);
  3. William Klug, Michael R. Cummings, Charlotte A. Spencer, Sarah M. Ward, Concepts of Genetics (Prentice Hall International, 2005).

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