DNA, and in some cases RNA, is the primary source of heritable information

1. DNA, and in some cases RNA, is the primary source of heritable information Genetic information is transmitted from one generation to the next through DNA or RNA

2. Structure of DNA • Building Blocks • Nucleotide • Deoxyribose sugar • Phosphate group • Nitrogenous bases • Uprights / Backbone • Alternating DNA and Phosphate • Rungs / Core • Nitrogenous bases • A-T and G-C • A and G – purines (double ring) • T, C and U – pyrimidines (single ring)

3. Structure of DNA • Double helix • 2 polynucleotides bonded together by hydrogen bonding between nitrogen bases • Antiparallel • one strand runs 5’ to 3’ • one strand runs 3’ to 5’

4. DNA replication • Replication is a semiconservative process • Each strand serves as the template for a new strand • Ends with two identical strands of DNA; each having one old and one new strand • http://youtu.be/teV62zrm2P0

5. DNA replication Helicase unwinds DNA forming a Y-shaped region where new DNA strands are produced - replication fork/bubbles. There can be 100's or even 1,000's of replications forks/bubbles in a eukaryotic chromosome. Single-stranded binding proteins bind to and stabilize the single-stranded DNA so that it can be used as a template (see above). Topoisomerases correct "overwinding" ahead of the replication fork by breaking, swiveling, and rejoining DNA strands. Primaseadds a short RNA primer (5 - 10 nucleotides long) to add an initial starting point for the new DNA strand. DNA polymerase III can then add complementary DNA nucleotides to the 3' end of the RNA primer (at a rate of about 50/sec in humans.)

6. DNA Replication Each new nucleotide that is added to the growing DNA strand is a nucleoside triphosphate (see right). As each nucleoside triophosphate monomer is added it loses two phosphate groups - hydrolysis of 2 phosphates is exergonic & powers the polymerizeration of monomers. One strand is continuously replicated. This strand is referred to as the leading strand and it is the strand that is replicated in a 5' to 3' direction. The lagging strand on the other hand has discontinuous replication. The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined by DNA ligase (see right). When the replication fork opens a RNA primer is needed to change the change the direction of replication so that replication can run 5' to 3'. When the entire replication is complete DNA polymerase I removes RNA primers and replaces them with DNA. DNA polymerases also proofread newly made DNA replacing any incorrect nucleotides.

7. Replication of Telomeres • Tips of linear chromosomes are called telomeres • DNA polymerase works in a 5’ to 3’ direction • Tips of lagging strand stops short of the end during replication • Telomerase is an enzyme that extends the lagging strand • Telomerase prevents the premature shortening of telomeres / chromosomes • Length of telomeres has been linked to cell death (apoptosis) • http://youtu.be/AJNoTmWsE0s

8. Enzymes of Replication that you should know… • Helicases • enzymes that untwist the double helix at the replication forks • Topoisomerases • corrects "overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands • RNA Primase • synthesizes an RNA primer at 5' end of leading strand and of each Okazaki fragment of lagging strand • DNA Polymerase • using parental DNA as a template synthesizes new DNA strand by covalently adding nucleotides to the 3' end of a pre-existing DNA strand or RNA primer; removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides • DNA Ligase • joins Okazaki Fragments