Cell Division, Genetics, Molecular Biology

1. Cell Division, Genetics, Molecular Biology 20.1b DNA Replication

2. DNA vs. RNA • DNA: deoxyribonucleic acid (double stranded) • RNA: ribonucleic acid (single stranded) • Both found in most bacterial and eukaryotic cells • RNA molecule can assume different structures- results in different types of RNA with each having a particular function (some important in DNA replication) • 3 key differences: • Sugar component of RNA is ribose not deoxyribose • RNA does not have nucleotide thymine (T). It is replaced with nucleotide uracil (U) • RNA single stranded

4. Genes and the Genome • Genes are functional subunits of DNA • Direct production of one or more polypeptides (protein molecules) • Genome of an organism: sum of all DNA carried in each cell of the organism- includes non-coding regions as well • Genes not spaced regularly along chromosomesex) Chromosome 4: 200 000 000 base pairs long, 800 genes Chromosome 19: 55 000 000 base pairs long, 1500 genes • No relationship between number of genes and size of genomeex) human genome: 3 billion bp’s, 20 000 – 25 000 genes amoeba: 650 billion bp’s, fewer than 7000 genes

5. DNA Replication • Occurs during S phase of interphase in mitosis • DNA must copy itself and be equally divided between daughter cells- must be exact copy of parent- human cell replicates in a few hours, error rate of one per one billion nucleotide pairs

6. DNA Replication • Semiconservative replication: separating two parent strands and using them to synthesize two new strands • Hydrogen bonds break, DNA helix unzips • Each single strand acts as a template to build the complementary strand • Errors then repaired, result is TWO identical DNA molecules- one for each daughter cell

7. Initiation & Separation • Replication starts at a specific nucleotide sequence- replication origin- can have many replication origins simultaneously • DNA helicase bind to DNA at replication origin- unwinds segment of helix by breaking hydrogen bonds- proteins bind to separated strands to prevent reformation • Opening of DNA creates Y-shaped replication fork • Separated strands now template strands with exposed unpaired bases- one strand runs in 3’ to 5’ direction, other in 5’ to 3’ direction (in relation to replication fork)

8. Replication occurs in both directions and bubbles growuntil they meet.

9. DNA Replication

10. Building Complementary Strands • Synthesis begins of two new DNA strands on template strands- complementary base pairing • DNA polymerase III: adds free nucleotides one at a time that are complementary to the template- elongation • RNA primer: short piece of RNA attached to template strand- gives DNA polymerase III a starting point • Nucleotides added in only ONE DIRECTION- 5’ to 3’ • Leading strand: synthesized continuously in 5’ to 3’ direction TOWARD replication fork- free 5’ end of nucleotides bind to free 3’ hydroxyl end on template

11. Building Complementary Strands • Lagging strand: synthesized away from replication fork, in short fragments later joined together- Okazaki fragments • Synthesized in 5’ to 3’ direction as well- able to do so since it runs in opposite direction of leading strand • RNA primers needed in multiple locations- recall: lagging strand synthesized in fragments- then primers cut out and replaced with DNA nucleotides by DNA polymerase I • Nicks left in between fragments- DNA ligase links sugar-phosphate backbone of fragments

12. Building the Lagging Strand

13. Review DNA Replication fork

14. DNA Repair • DNA polymerase III and I used as checkers throughout synthesis of complementary strands • Mistake occurs?? DNA polymerases backtrack!- cut out incorrect nucleotide, continue adding correctly • Prevents mistake from being copied in future replications

15. Termination • Replication fork progresses throughout helix- only short region of DNA unravelled in single stranded form at a given time • Newly formed strands completed: rewind automatically into helix structure • Replication proceeds until new strands complete, DNA separates from one another • TERMINATION.