Review Of Replication

REVIEW OF REPLICATION

DNA as Genetic Material • established by several critical experiments – Fred Griffith (1928) – Oswald T. Avery, C. M. MacLeod, and M. J. McCarty (1944) – Alfred D. Hershey and Martha Chase (1952)

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Fred Griffith 1928--Called the change from rough to smooth by heat killed bacteria Copyright © The McGraw-Hill Companies, Inc. Permission required for3reprodu “transformation

Avery and McCloud (1944) wanted to know what in Griffith’s killed bacteria was responsible for

This suggested that DNA was responsible for Copyright © The McGraw-Hill Companies, Inc. Permission required for4reprodu transformation and there fore carrid genetic

Alfred Hershey/Martha Chase experiment (1952) ** *

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During infection phage DNA went into E. coli, phage protein Copyright © The McGraw-Hill Companies, Inc. Permission required for5reprodu

DNA Bases

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Chain growth: DNA

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DNA Structure

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DNA Structure Summary • nitrogenous bases – A, T, G, C

• pentose sugar – deoxyribose

• chain of nucleotides linked by phosphodiester bonds • usually a double helix, composed of two complementary, antiparallel strands – base pairing rules • A with T • G with C

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Some Facts •   •  – error frequency = 10-9 or 10-10 per base pair replicated – due to proofreading activity of DNA polymerases III and I

• occurs very rapidly

– 750 to 1,000 base pairs/second in procaryotes – 50-100 base pairs/second in Copyright © The McGraw-Hill Companies, Inc. Permission required for 10reprodu

Schematic of DNA Polymerase III

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TRANSCRIPTION & TRANSLATION 1st Semester 2008-2009

RNA Structure • nitrogenous bases – A, G, C, U (instead of T)

• pentose sugar – ribose

• usually consists of single strand of nucleotides linked by phosphodiester bonds – can coil back on itself • forms hairpin-shaped structures with complementary base pairing and helical organization • base pairing rules

– A with U – G with Copyright C © The McGraw-Hill Companies, Inc. Permission required for 13reprodu

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The Genetic Code • the manner in which genetic instructions for polypeptide synthesis are stored within genome • colinearity – sequence of base pairs in DNA corresponds to the amino acid sequence of polypeptide encoded

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Establishment of Genetic Code • codon – genetic code word – specifies an amino acid

• codon meanings deciphered by Marshall Nirenberg, et al. in 1960s

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Organization of the Code • code degeneracy – up to six different codons can code for a single amino acid

• sense codons – the 61 codons that specify amino acids

• stop (nonsense) codons – the three codons used as translation termination signals – do not encode amino acids

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Wobble • loose base pairing – 3rd position of codon less important than 1st or 2nd

• eliminates need for unique tRNA for each

Figure 11.19

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Gene Structure • gene – linear sequence of nucleotides with a fixed start point and end point – encodes a polypeptide, a tRNA, or an rRNA • cistron – gene that encodes a polypeptide

• reading frame – organization of codons such that they can be read to give rise to a gene product

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Importance of reading frame

Figure 11.20

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Genes That Code for tRNA and rRNA tRNA genes have promoter, leader, coding, spacer, and trailer regions

tRNA precursor

Figure 11.24a

leader, spacer, and trailer removed during maturation process

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DNA methylation • used by E. coli postreplication repair system to distinguish old DNA strands from new DNA strands – old DNA methylated; new DNA not methylated

• catalyzed by DNA methyltransferases

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