Dna - Deoxyribonucleic Acid

DNA - Deoxyribonucleic Acid

Anonymous

University of Phoenix

BIO 101

Tuesday, September 15, 2009

DNA

If man could date back to the beginning of life, not necessarily how mankind would perceive life but to the beginning of all living organisms, the starting point would begin with a speck. A speck, in which over time, has passed on a code or a message of information that allows all living things to function, grow, and reproduce. The information that has passed on from the first speck to the next living organism to eventually mankind is genetic information, chemically found in the form of a molecule; a molecule called deoxyribonucleic acid or better known as DNA. Deoxyribonucleic acid appears in the shape of a double helix, that if stretch out for simplicity, would appear as a ladder. The two longs sides of the ladder are strands of polymers called nucleotides. The two strands are linked together through bonds that consist of four types of bases: Cytosine (C), Guanine (G), Thymine (T), and Adenine (A). Each link, or step in the ladder, consists of either cytosine and guanine or thymine and adenine, commonly referred to as base pairs. It is through the sequencing of these base pairs where chemists find the genetic instructions for all living organisms. The information carried in DNA sequences is called genes, the basic unit of heredity. Man has around three billion base pairs of DNA that are organized into 46 chromosomes. In all practicality, as humans reproduce, the natural question that would arise, since man and woman each have 46 chromosomes, why do the offspring of humans not have 92 chromosomes. The answer is meiosis.

Meiosis is the basis of sexual reproduction. Without meiosis, humans would not be able to reproduce. As mentioned previously, humans have 46 chromosomes and are said to be diploid organisms because all body cells contain homologous pairs of chromosomes. Homologous chromosomes are of the same length, centromere position, and staining pattern. Within the pair, one chromosome is inherited from one's father and the other from the mother. Human females have 23 homologous pairs of chromosomes with all essentially identical in appearance however in human males, 22 appear identical with one pair that does not look alike. That one pair is known as the male's sex chromosome. In a male the sex pair of chromosomes consists of one X chromosome and one Y chromosome. In a female, both chromosomes are X chromosomes in the sex chromosome pair. The other 22 pairs in both males and females are known as autosomes.

To prevent human offspring from having 92 chromosomes, meiosis must take place. By definition, meiosis in humans is the division of a single diploid nucleus, or