Exam III Review - Exam III Review Chpts 5 9 Chapter 5 The...

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Exam III Review Chpts. 5 – 9 Chapter 5 The polarity of a DNA strand commonly refers to the orientation of its sugar- phosphate backbone G—C base pairs are more stable than A—T base pairs o G—C base pairs are held together by three hydrogen bonds o A—T base pairs are held together by only two hydrogen bonds Complementary base pairs are formed in the DNA double helix The shapes and chemical structure of the bases allow hydrogen bonds to form efficiently only between A and T and between G and C, where atoms that are able to form hydrogen bonds can be brought close together without perturbing the double helix Two hydrogen bonds form between A and T, while three form between G and C The bases can pair in this way only if the two polynucleotide chains that contain them are antiparallel, that is, oriented in opposite polarities The two strands of the DNA double helix are held together by base pairing The nucleotides are linked together covalently by phosphodieseter bonds through the 3’hydroxyl (-OH) group of one sugar and the 5’-phosphate (PO4) of the next Thus, each polynucleotide strand has a chemical polarity; that is, its two ends are chemically different The 3’ end carriers an unlinked –OH group attached to the 3’ position on the sugar ring; the 5’ end carries a free phosphate group attached to the 5’ position on the sugar ring A mutation on the X chromosome: o Men have only the X chromosome (males are XY), and a defective gene carried on it therefore has no backup copy o Womeon have two copies of the X chromosome (females are XX),one inherited form each parent, so a defective copy of the gene on one X chromosome can generally be compensated for by a normal copy on the other chromosome o This is the case with regard to the gene that causes color blindness o However, during female development, transcription from one X chromosome is shut down because it is compacted into heterochromatin o This occurs at random in each cell to one or the other of the two X chromosomes, and therefore some cells of the woman will express the defective mutant copy of the gene, whereas others will express the normal copy o This results in a retina in which on average only every other cone cell has functional color photoreceptors, and women carrying the mutant gene on one X chromosome therefore see color images with reduced resolution o A women who is color-blind must have two defective copies of this gene, one inherited form each parent o Her father must therefore carry the mutation on his X chromosome because this is his only copy of the gene, he would be color-blind o Her mother could carry the defective gene on either or both of her X chromosomes o Her mother could therefore either be color-blind (defective genes on both X chromosomes) or have color vision but reduced resolution as described above If two strands of a DNA double helix can be separated by heating:
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