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Lecture

DNA as Genetic Material Notes

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Department
Biological Sciences
Course
BIOSC 0160
Professor
All Professors
Semester
Fall

Description
2/6 Chapter 14: DNA as Genetic Material Deoxyribonucleic Acid • Structure o The Double Helix  Consists of two strands  Each has a backbone of sugar and phosphate  Each sugar has a nitrogenous base attached to it • A, adenine (purine) • C, cytosine (pyrimidine) • G, guanine (purine) • T, thymine (pyrimidine)  Complementarity and Antiparallel Strands • Two strands are held together by hydrogen bonds between bases • Bases pair in a way that is most energetically favorable (purine with pyrimidine) o Adenine bonds with thymine o Guanine bonds with cytosine • Antiparallel o Refers to the strand directionality o Each strand has two ends  5’ end with free phosphate  3’ end with free hydroxyl on ribose o Phosphodiester bond forms between phosphate and hydroxyl o Always synthesized 5’  3’ 2/6 o Strands are parallel but run in opposite directions • Replication o Copying DNA o Uses enzymes called DNA polymerases o Begins at the origin of replication  There are several along the length of the DNA strand  At each one, strands separate and yield two replication forks  One bubble, to replication forks moving in opposite directions 5’ Replication Site 3’ 3’ 5’ Replication Site 5’ Replication Replication 3’ 3’ Bubble Forks 5’ • Replication cont. o Complementarity of strands is key to accurate replication  Semiconservative model • Each original strand acts as a template for the new strand • Meselson-Stahl Experiment o Hypotheses:  Semiconservative: One new strand, one old  Conservative: Parental DNA, offspring DNA  Dispersive: Segments of each are interspersed 2/6 o Leading and Lagging Strands  Nucleic acids are always synthesized 3’  5’  Thus, template is always read 3’  5’  Requires RNA primers, which are inserted to temporarily create the double strand construct  Leading Strand • Grows continuously because replication leads (is in front of) new DNA strand 2/6 • Direction of synthesis is the same as direction of replication fork  Lagging Strand • Grows in short pieces (Okazaki fragments) because the fork lags behind (is in back of) new DNA strand o Direction of synthesis is opposite the direction of the replication fork = RNA Primer 5’ 3’ 3’ 5’ Leading
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