Chapter 16 - Chapter 16: The Molecular Basis of Inheritance...

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Chapter 16: The Molecular Basis of Inheritance The search for the genetic material The search for the molecular basis of inheritance began in 1928 when F. Griffith found that avirulent (non-pathogenic) bacteria could be “transformed” into a virulent strain by mixing the avirulent individuals with heat-killed cell fragments from virulent strains. By the 1940’s scientists agreed that the molecule responsible for passing hereditary information was either protein or nucleic acid. The Hershey-Chase experiment of 1952 proved that DNA was responsible for the transformation observed by Griffith. Additional evidence came from E. Chargaff’s laboratory. Chargaff showed that DNA was made up of nitrogen bases (adenine, thymine, guanine and cytosine) and that these bases occurred in peculiar ratios. The amounts of the four N-bases are not all equal but are present in a characteristic ratio. The number of adenines roughly equaled the number of thymines, and the number of guanines roughly equaled the number of cytosines. And in 1953, James Watson and Francis Crick (using an X-ray diffraction photograph of the DNA molecule made by Rosalind Franklin) proposed a model for the atomic structure of DNA and its mechanism of self-replication. (a) The ribbons in the DNA double helix represent the sugar-phosphate backbones of the two DNA strands. The two strands are held together by hydrogen bonds between the N-bases, which are paired in the interior of the double helix. (b) For clarity, the two DNA strands are shown untwisted in this partial chemical structure. Notice that the strands are oriented in opposite directions (i.e., the two strands have opposite polarities). 1 of 19
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Watson and Crick also proposed a mechanism for DNA replication noting that each of the two strands serves as a template (mold) for ordering nucleotides into a new complementary strand. Prior to replication, the hydrogen bonds between complementary N-bases are broken and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain so that eventually you end up with two pairs of chains where we had only one pair before, and the sequence of the pairs of bases will have been duplicated exactly. In the late 1950’s Meselson and Stahl proved the validity of this proposed mechanism. This mechanism is known as the semi-conservative model of DNA replication. 2 of 19
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DNA Replication in Prokaryotic Cells 1. Helicase attaches to DNA’s double helix structure and causes the double-stranded DNA to unwind, forming a replication bubble with 2 Replication Forks . (The DNA double helix continues to unwind in both directions.) Enzymes called topoisomerases attach downstream of the replication forks and relieve the tension as the strands unwind. Single-strand binding proteins (SSBP) stabilize the separated strands and hold them apart while they each serve as templates for the synthesis of two new strands. 2.
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This note was uploaded on 04/07/2008 for the course BIO 101 taught by Professor Stein during the Spring '07 term at South Carolina.

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Chapter 16 - Chapter 16: The Molecular Basis of Inheritance...

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