Cell Division, Genetics, Molecular Biology

20.1b DNA ReplicationCell Division, Genetics, Molecular Biology1DNA vs. RNADNA: deoxyribonucleic acid (double stranded)RNA: ribonucleic acid (single stranded)Both found in most bacterial and eukaryotic cellsRNA 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 deoxyriboseRNA does not have nucleotide thymine (T). It is replaced with nucleotide uracil (U)RNA single stranded2

3Genes and the GenomeGenes are functional subunits of DNADirect 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 wellGenes not spaced regularly along chromosomesex) Chromosome 4: 200 000 000 base pairs long, 800 genes Chromosome 19: 55 000 000 base pairs long, 1500 genesNo relationship between number of genes and size of genomeex) human genome: 3 billion bps, 20 000 25 000 genes amoeba: 650 billion bps, fewer than 7000 genes4DNA ReplicationOccurs during S phase of interphase in mitosisDNA 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

5DNA ReplicationSemiconservative replication: separating two parent strands and using them to synthesize two new strandsHydrogen bonds break, DNA helix unzipsEach single strand acts as a template to build the complementary strandErrors then repaired, result is TWO identical DNA molecules- one for each daughter cell

6Initiation & SeparationReplication starts at a specific nucleotide sequence- replication origin- can have many replication origins simultaneouslyDNA helicase bind to DNA at replication origin- unwinds segment of helix by breaking hydrogen bonds- proteins bind to separated strands to prevent reformationOpening of DNA creates Y-shaped replication forkSeparated 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)

7

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

9Building Complementary StrandsSynthesis begins of two new DNA strands on template strands- complementary base pairingDNA polymerase III: adds free nucleotides one at a time that are complementary to the template- elongationRNA primer: short piece of RNA attached to template strand- gives DNA polymerase III a starting point Nucleotides added in only ONE DIRECTION- 5 to 3Leading strand: synthesized continuously in 5 to 3 direction TOWARD replication fork- free 5 end of nucleotides bind to free 3 hydroxyl end on template10Building Complementary StrandsLagging strand: synthesized away from replication fork, in short fragments later joined together- Okazaki fragmentsSynthesized in 5 to 3 direction as well- able to do so since it runs in opposite direction of leading strandRNA primers needed in multiple locations- recall: lagging strand synthesized in fragments- then primers cut out and replaced with DNA nucleotides by DNA polymerase INicks left in between fragments- DNA ligase links sugar-phosphate backbone of fragments

11Building the Lagging Strand

12

ReviewDNA Replication fork

13DNA RepairDNA polymerase III and I used as checkers throughout synthesis of complementary strandsMistake occurs?? DNA polymerases backtrack!- cut out incorrect nucleotide, continue adding correctlyPrevents mistake from being copied in future replications

14TerminationReplication fork progresses throughout helix- only short region of DNA unravelled in single stranded form at a given timeNewly formed strands completed: rewind automatically into helix structureReplication proceeds until new strands complete, DNA separates from one anotherTERMINATION.

15