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Unformatted text preview: MOLECULAR GENETICS 4.2: DNA Structure • DNA, deoxyribonucleic acid is the hereditary substance of all living organisms • DNA has 3 main components: a deoxyribose sugar (a 5 C sugar), a phosphate group that is negatively charged and a nitrogenous base (these 3 components form a nucleotide) • The source of variation in DNA is found in the nitrogenous bases • There are 4 such bases: Adenine (A) and Guanine (G) both are purines (composed of double rings) and Cytosine (C ) and Thymine (T) both are pyrimidines (composed of single rings) • A and T form 2 hydrogen bonds, G and C form 3 hydrogen bonds • DNA is composed of many nucleotides by phosphodiester bonds and is therefore, a polymer • The proportion of adenine is equal to the proportion of thymine in DNA and the proportion of guanine is equal to that of cytosine • The total amount of purines in DNA equals the total amount of pyrimidines in an organism’s genome • A deoxyribose sugar has a hydroxyl (OH) group on carbon 3 (3’) while it has a hydrogen atom on carbon 2 (2’) • The nitrogenous base is attached to the 1’ carbon of the sugar by a glycosyl bond • The phosphate group is attached to the 5’ carbon by an ester bond • Rosalind Franklin used X-ray diffraction for the analysis of the structure of DNA and the structure indicated a double helix from the image of X-ray • With her analysis, Watson and Crick discovered DNA had a helix shape about 2 nm (nanometers) in diameter • Using this, Watson and Crick built their famous model of the double-helix structure of DNA • DNA consists of two anti- parallel strands of nucleotides • Anti- parallel: parallel but running in opposite directions; the 5’ end of one strand of DNA aligns with the 3’ end of the other strand in a double helix • The bases of one strand are paired with the bases of the other strand, facing inward toward each other • The nitrogenous base pairs are arranged above each other, perpendicular to the axis of the molecule • The purine is always bonded to a pyrimidine: A (purine) is always paired with T (pyrimidine) in one strand and G (purine) is always paired with T (pyrimidine) in the other strand • This type of pairing is called complementary base pairing • If you know the sequence of one of strand, you know the sequence of the other because of the complementary base pairing • DNA has a constant diameter of 2 nm so if you bond 2 purines together, the DNA molecule would be wider at some points • Conversely, if you bonded 2 pyrimidines together, the molecule would be less than 2 nm wide in some places • The nitrogenous bases are held together by hydrogen bonds which can only occur in A-T and G-C arrangements • An individual hydrogen bond itself is weak but many hydrogen bonds in a molecule make it really strong, which explains the high stability of DNA • Thymine could not bond with guanine, even though these bases constitute the desired diameter of DNA but because hydrogen bonds cannot form between them •...
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This note was uploaded on 02/05/2011 for the course PHYS 1010 taught by Professor Tomkirchner during the Spring '11 term at York University.
- Spring '11