Topic 3 (DNA Replication and Telomeres)

Conservative replication

2 original strands stay together in one molecule (double helix)

Dispersive replication

Each strand gets broken down into pieces, copied, and then reassembled

Semiconservative replication

Each strand serves as a template, is copied, and each strand stays intact but 2 strands do not come back together.

Messel and Stahl

Equilibrium density gradient centrifugation

14N

COMMON FORM

15N

RARE, HEAVY FORM

Initation

DnaA protein binds to oriC

Unwinding

Helicase breaks hydrogen bonds and unzips 5’ to 3’ direction.

SSBs protect DNA and form tetramers

DNA gyrase

• Topoisomerase (type 2)

  1. Relieves torsion by cutting DNA

  2. Removes twist

  3. Reseals broken DNA ends

Elongation

DNA pol adds nucleotides to the growing DNA strand in a 5′ to 3′ direction, utilizing the parental DNA as a template. Accurate duplication

Primase

• an RNA pol

• Synthesizes short RNA primers

• Does not need 3’-OH group to start

• Complexes with helicase at replication fork

DNA pol 3

• Large multiprotein complex

• Synthesizes DNA strand 5’ to 3’

• Attaches new nucleotides to 3’-OH sugar phosphate

Leading strand

• DNA synthesized towards the fork

• One continuous DNA strand

Lagging strand

• DNA synthesized away from fork

DNA pol 1

• Large multiprotein complex

• Synthesizes DNA strand 5’ to 3’

• Has 3’ to 5’ exonuclease activity

• Removes and replaces primers w/ DNA nucleotides

• Low processivity

DNA ligase

Fuses breaks in DNA strand that cannot be fused by DNA pol 1.

• fuses okazaki fragments together

Termination

DNA replication ends when 2 replication forks meet

Ter

• specific termination sequence

• STOPS DNA replication

Theta Replication

• Produces 2 circular DNA molecules

• Unwinding of double stranded DNA and fork proceeds around circle.

Rolling Circle Replication

• Initiated by break of one nucleotide strands

• Synthesis begins at 3’ end of broken strand.

• Inner strand is template

• Cleavage releases linear and circular DNA

Origin recognition complex

• Loads helicase onto dsDNA

End replication problem

Each time cell replicates ends get shorter and chromosomes shrink with each cell division.

Shelterin

• Multiprotein complex

• Binds to telomeres and prevents DNA repair mechanisms from recognizing telomere ends as a double stranded break.

Telomerase

• Ribonucleoprotein

• Enzyme that elongates the ends of eukaryotic chromosomes.

Werner syndrome

• Mutation in WRN gene (type of helicase necessary for telomere replication)

• Premature aging