1. Physical Structure and Replication of DNA

2. Property 1 - it must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction Property 1 - it must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction Property 2 - it must replicate accurately so progeny cells have the same genetic makeup Property 2 - it must replicate accurately so progeny cells have the same genetic makeup Property 3 - it must be capable of some variation (mutation) to permit evolution Property 3 - it must be capable of some variation (mutation) to permit evolution Nature of the Genetic Material

3. DNA double helix 1. Sugar-phosphate backbone 2. Base-pair "rungs" of ladder 3. Nucleotides attached to S-P molecules 4. Strands antiparallel (run in opposite directions, 5'-->3') 5. Each base-pair "rung" has a purine (A or G) and pyrimidine (C or T) 6. Strands held together by hydrogen bonds between nucleotides 7. Chemical structures of nucleotides discourage "incorrect" pairing 8. G-C pair has 3 hydrogen bonds, A-T only 2-->former is stronger

4. How DNA Replicates How DNA Replicates How DNA Replicates How DNA Replicates

5. DNA Replication 1. Semiconservative = replication results in two DNA molecules each with two strands, one original and one new. 2. Sequence of events a) Helix unwinds b) Both strands replicate simultaneously, during unwinding process c) "Leading" strand replicates continuously from 3' end of existing strand, with newest end of forming strand facing into replication fork d) "Lagging" strand replicates by a series of fragments placed end-to-end, with newest ends of fragments facing away from fork; fragments later "ligated" e) During replication, 2 polymerases "proofread" for mismatched bases

6. Speed of DNA replication: 3,000 nucleotides/min in human 30,000 nucleotides/min in E.coli Speed of DNA replication: 3,000 nucleotides/min in human 30,000 nucleotides/min in E.coli Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt) Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt) Replication of DNA and Chromosomes

8. Process of DNA Replication Process of DNA Replication Hydrogen bonds between base pairs are broken and the two sides of the ladder unzip Hydrogen bonds between base pairs are broken and the two sides of the ladder unzip Semi-conservative replication: the Semi-conservative replication: the original molecule is no longer present but each new molecule will have one original strand

9. Polymerases (enzymes) link free-floating nucleotides to their matching base on the parent strand Polymerases (enzymes) link free-floating nucleotides to their matching base on the parent strand There are estimated to be 3 billion bases in a human DNA strand There are estimated to be 3 billion bases in a human DNA strand Mistakes are rare but do happen Mistakes are rare but do happen Proofreader enzymes fix most mistakes Proofreader enzymes fix most mistakes

10. DNA has a direction. It consists of two antiparallel strands with distinguishable 5 ′ and 3 ′ ends (the numbers refer to the number of the carbon in the ribose sugar). Carbon-1 → base Carbon-3 → downstream PO 4 Carbon-5 → upstream PO 4 The original strand of DNA is read in the 3’ to 5’ direction The new strand is assembled in the 5’ to 3’ direction

12. If both sides were synthesized at once, you'd need two different DNA polymerases: one for 5′ → 3′; and one for 3′ → 5′. However, experimentally, we find that all DNA polymerases synthesize the new strand 5′ → 3′ The original strand of DNA is read in the 3’ to 5’ direction, The new strand is assembled in the 5’ to 3’ direction

13. Since DNA synthesis only occurs in the 5′ and 3′, so DNA polymerases must move in antiparallel directions to synthesise the two daughter helices.

14. Because the synthesis of DNA only occurs in one direction, different processes must occur on the two strands. These two strands are termed the leading and lagging strands. The leading strand is synthesised continuously 5′→3′. However, the other, 'lagging' strand is still synthesised 5′→3′ but in discrete chunks called Okazaki fragments, from the replication fork back towards the origin.

16. Gene Recombinations The process of cutting out damaged or foreign segments of DNA and repairing them The process of cutting out damaged or foreign segments of DNA and repairing them Restriction enzymes: The scissors that cut certain segments out of the DNA strand Restriction enzymes: The scissors that cut certain segments out of the DNA strand DNA Ligase: The glue that restores the damaged segment DNA Ligase: The glue that restores the damaged segment

17. Resources From DNA to Protein From DNA to Protein From DNA to Protein From DNA to Protein DNA and Protein Synthesis in the Cell DNA and Protein Synthesis in the Cell DNA and Protein Synthesis in the Cell DNA and Protein Synthesis in the Cell Genetics Handbook Genetics Handbook Genetics Handbook Genetics Handbook Animations at Virtual Cell Biology Classroom Animations at Virtual Cell Biology Classroom Animations Say it with DNA Protein synthesis tutorial Say it with DNA Protein synthesis tutorial Say it with DNA Say it with DNA