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Molecular Genetics Notes

Molecular Genetics Notes - MOLECULAR GENETICS 4.2 DNA...

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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
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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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Molecular Genetics Notes - MOLECULAR GENETICS 4.2 DNA...

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