Chapter Eight DnA

Watson and Crick, circa 1953

James Watson (an American biologist) and Francis Circk (an Englsih physicist) made remarkable strides in the study of DNA. Building on the progress made by Friedrich Miescher and Erwin Chargaff, Watson and Crick were the first to propose the structure of DNA. In the early 1950s, they proposed their groundbreaking theory that DNA was a double-stranded helix held together by hydrogen bonds. They also concluded that DNA was either right or left handed (referrring to the direction which the helix wrapped around the sugar-phosphates). They were assisted in their research by Rosalind Franklin, but she unfortunately passed away before Watson and Crick were given the Nobel Prize.

Chargaff's Rule, devised by Austrian scientist Erwin Chargaff, states that purines and pyrimidines bond to each other in a 1:1 ratio. Furthermore, Adenine will always bond to Thymine and Guanine will always bond to Cytocine.

The Bond between Adenine and Thymine

Base pairs (Adenine to Thymine and Guanine to Cytocine) will alwyas bond in a 1:1 ratio. They are held together by the bonding of hydrogen ions.

Chomosomes are structures composed of many interwoven strnds of DNA. They are found in the nucleus if every eukaryotic cell and are clumped together in prokaryotic cells. They possess both "p" and "q" arms held together in a center point known as the centromere. They form the basis of genes.

A karyotype is a profile which provides the number, shape, and organization of the chromosomes in a given nucleus of an organizsm or species.

To replicate, the DNA double-helix first "unzips" by allowing enzymes known as helicases to break the hydrogen bonds which hold the strands of base pairs together. After being divide into a leading strand and lagging strand (forming a Y-shape), an RNA molecule called a primer bonds to the end of both the leading and lagging strand. DNA polymerase now binds to the strand and gradually goes along it, adding complementary base pairs to complete the strand. Meanwhile, RNA primer binds chunks of DNA to the lagging strand. Once all the base pairs are completed, the primer is stripped away and an enzyme called ligase helps complete the bond between the base pairs. The two strands now now wind up into two different helixes.

DNA can be damaged by natural mutation, radiation, heat, and also through mechanical seperation.

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