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Lecture 3

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Biology 1001A
Tom Haffie

September 20/2011 Biology Chapter 13- DNA Structure, Replication, and Organization (LECTURE 3/4) 13.2- DNA Structure • A DNA molecule consists of two polynucleotide chains twisted around each other into a right-handed double helix o Each nucleotide of the chains consists of the five carbon sugar deoxyribose, a phosphate group, and a nitrogenous base; adenine, thymine, cytosine or guanine. o The deoxyribose sugars are linked by phosphate groups to form an alternating sugar-phosphate backbone  Each phosphate group is a bridge between two sugars; the entire linkage, including the bridging phosphate group, is called a phosphodiester bond. o Complementary Base Pairing: Adenine+ Thymine and Guanine+ Cytosine  Purines (Adenine and Guanine) nitrogenous bases built from a pair of fused rings of carbon and nitrogen atoms  Pyrimidines (Thymine and Cytosine) built from a single carbon ring o Each full turn of the double helix involves 10 base pairs • The two strands that make up DNA are Antiparallel • X-ray diffraction suggested that DNA had helical structure Q-WHAT BONDS FORM BETWEEN COMPLEMENTARY BASE PAIRS? BETWEEN A BASE AND THE DEOXYRIBOSE SUGAR? A-HYDROGEN BONDS FORM BETWEEN THE BASE PAIRS AND GLYCOSYL BONDS FORM BETWEEN A BASE AND THE DEOXYRIBOSE SUGAR. 13.3- DNA Replication • Semiconservative Replication- two strands of a parental DNA molecule unwind and each serves as a template for the synthesis of a complimentary copy o Meselson and Stahl o Used a nonradioactive ‘heavy’ nitrogen isotope ( N)to tag the parental DNA strands 15 o Grew bacterium E. coli in a culture medium containing heavy N13sotope o Then transferred bacteria into a culture medium containing light N isotope o Then centrifuged various samples of the DNA and different densities separate • DNA Replication is catalyzed by several enzymes o
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