16.2 DNA Replication

DNA in Prokaryotes and Eukaryotes Prokaryotes: –ring of chromosome –holds nearly all of the cell’s genetic material

DNA in Prokaryotes and Eukaryotes Eukaryotes –much larger chromosomes 1000 times more DNA than prokaryotes –Found in cell’s nucleus –Chromosomes Humans: 46 chromosomes Drosophila melanogaster: 8 chromosomes

DNA Replication Overview Replicationhttp:// Replication

DNA Replication Overview DNA splits into two strands Complementary base pairs fill in (A with T, C with G) Left with two DNA molecules –Semiconservative model One original and one new strand make up a new DNA molecule –Identical

Prokaryote DNA Replication DNA replication begins at a single point and continues to replicate whole circular strand Replication goes in both directions around the DNA (begins with replication fork)

Layout of the Eukaryote DNA Two DNA strands are antiparallel –Run in opposite directions –3’ (three prime) – 5’ (five prime) –5’ (five prime) – 3’ (three prime)

Eukaryote DNA Replication Begins in hundreds of locations along the chromosome –Origins of replication

Initiation of DNA Replication Begins when the DNA molecule “unzips” –Replication fork –Replication “bubble” Hydrogen bonds between base pairs breaks Helicase Single-strand binding proteins Topoisomerase – relieves pressure of DNA ahead of replication fork

Synthesis of a New DNA Strand Each strand serves as a template for a new strand to form Complimentary bases will attach DNA polymerase –E. coli – DNA polymerase III and DNA polymerase I –Humans – 11 different DNA polymerase molecules

Synthesis of a New DNA Strand RNA primer Nucleoside triphosphate –As each nucleotide is added to the new strand, 2 phosphates are lost Hydrolysis releases energy to drive reaction

Synthesis of a New DNA Strand Antiparallel Elongation –Remember 3’ – 5’ and 5’ – 3’ Replication in the 3’ to 5’ direction ONLY –MEANING the NEW strand of DNA will form starting with the 5’ end Leading strand (only 1 primer needed – moves toward the replication fork) Lagging strand (many primers needed – moves away from replication fork)

Important Enzymes Helicase, single-strand binding protein, topoisomerase Primase –Synthesis of RNA primer DNA polymerase III (DNA pol III) –Add new bases to DNA strand DNA polymerase I (DNA pol I) –Removes and replaces RNA primer from 5’ end DNA ligase –Links Okazaki fragments and replaces RNA primer from 3’ end

The Finished Product Each DNA molecule has one original strand and one new strand Molecules are identical

Repair of DNA DNA polymerase –Proofreads and repairs damaged/mismatched DNA Nuclease –Removes section of DNA that is damaged –DNA polymerase and DNA ligase replace missing portion

Telomeres Found at the ends of each chromosome Contain no genes Sequence that can be cut short and will not affect normal functioning TTAGGG Telomerase lengthens telomeres in gametes

16.3 A chromosome consists of a DNA molecule packed together with proteins

Chromosomes

Chromosome Structure DNA in bacteria - nucleoid Chromosomes contain both DNA and protein to form chromatin

Chromosome Structure Chromatin is DNA coiled around histones (protein) Heterochromatin – present in interphase, slightly condensed into clumps Euchromatin – uncondensed chromatin (“true chromatin”)