DNA Structure and Replication COMPLEMENTARY BASE PAIRING Adenine

DNA Structure and Replication

COMPLEMENTARY BASE PAIRING Adenine and thymine - H H N Adenine N HC C N N CH 3 O H C N Thymine C C C N C H N O TWO hydrogen bonds H H

COMPLEMENTARY BASE PAIRING Cytosine and Guanine - H N Guanine C C C HC N N N H O H Cytosine CH CH N C C N H N O H THREE hydrogen bonds

Models for DNA replication double-stranded DNA original double strand conserved CONSERVATIVE replication + all-new double strand formed

Models for DNA replication double-stranded DNA DISPERSIVE replication new strands are mixture of original and new DNA +

Models for DNA replication double-stranded DNA one original strand conserved SEMI-CONSERVATIVE replication one new strand formed + one new strand formed

Very basic theory DNA replication template strand complementary base pairing between free nucleotides and template strands

Very basic theory DNA replication backbone forms between paired nucleotides

DNA replication – strand separation Double stranded DNA replication fork helix unwinds and H bonds between bases break helicase enzyme

DNA replication – primer attachment and elongation enzyme not shown replication fork DNA nucleotides pair with template strand extend primer RNA nucleotides complementary to DNA bases form primer unpaired bases

DNA replication – chain extension is in 5’ to 3’ direction 5’ 3 P HOCH 2 Sugar 2 P nucleotide triphosphate BASE existing DNA strand P 1 P BASE Sugar Newly-formed phosphodiester bond OH P BASE new nucleotide added at 3’ end, therefore chain extends in 5’ to 3’ direction Sugar 3’ OH + PPi (pyrophosphate)

DNA replication – chain extension on two strands 5’ 3’ 3’ 5’ doublestranded DNA with antiparallel strands 5’ to 3’ extension possible chain cannot extend 3’ to 5’

DNA replication – chain extension on two strands 5’ 3’ Leading strand: continuous replication 5’ to 3’ RNA primer forms 3’ 5’ 3’ Short Okazaki fragment forms 5’ to 3’ RNA primer forms 5’ 3’ 3’ 3’ 5’ RNA primer forms Lagging strand: discontinuous replication by Okazaki fragments from multiple RNA primers Short Okazaki fragment forms 5’ to 3’ 3’ 5’