Robs_EX2_Guide08

Robs_EX2_Guide08 - Life Sciences 1a, 2008 Robs Study Guide...

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1 Life Sciences 1a, 2008 Rob’s Study Guide of Key Points for Exam 2 Nucleic acids and the chemistry of replicating information 1. The biological role of DNA Nucleic acids carry the information that specifies the amino acid sequences of proteins The informational capacity of nucleic acids depends on their nucleotide sequence 2. The double helix structure of DNA Deoxyribonucleic acid (DNA) exists as a linear polymer of nucleotide subunits, each consisting of a nitrogenous base, deoxyribose sugar, and a phosphate group DNA contains four nitrogenous bases that define the sequence used to encode genetic information: the purines, adenine and guanine; the pyrimidines, cytosine and thymine The DNA polymer adopts a double-stranded helical structure – the so-called double helix The bases are precisely arranged in pairs to form optimal hydrogen bonds The individual nucleotide strands of the double helix have a chemical directionality with distinct 5’- and 3’-ends The DNA strands in the double helix are oriented antiparallel to each other Complementary base-pairing allows for error correction during DNA polymerization Purine bases form specific pairings with pyrimidine bases thanks to optimally aligned hydrogen bonds (A to T, and G to C) The hydrogen bonds formed between base pairs are particularly strong because of their optimal alignment of electron donors and acceptors Significant driving forces for the formation of the DNA double helix include induced dipole interactions between stacked bases, and the hydrophobic effect 3. Chemical interactions involving DNA At physiological pH, the phosphate groups in DNA are deprotonated, and therefore carry a negative charge The bases of DNA point towards the center of the double helix, while the sugar-phosphates form a negatively-charged backbone The negatively-charged phosphates protect the DNA backbone from hydrolysis by repelling water molecules The negatively-charged DNA backbone can form electrostatic interactions with positively- charged amino acid side chains in proteins The DNA double helix has a major groove and a minor groove The patterns of hydrogen bond donors and acceptors exposed at the sides of the bases allow for protein binding interactions in the major and minor grooves 4. The molecular basis of DNA replication DNA replication requires the polymerization (synthesis) of an identical copy of the entire cellular genome DNA replication is an essential preparative step for cell division DNA is replicated semi-conservatively such that each daughter cell receives one strand of parental DNA base-paired with a newly synthesized strand of DNA
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2 Deoxyribonucleotide triphosphates add to the 3’-end of a growing DNA strand with the release of
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Robs_EX2_Guide08 - Life Sciences 1a, 2008 Robs Study Guide...

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