6.2 DNA Replication

DNA Replication

DNA replicates during the S phase of the cell cycle

the alternative models of DNA replication are conservative, semi conservative, and dispersive

the correct one for DNA is semi conservative

Conservative model

the parental strands direct synthesis of an entirely new double stranded molecule

the parental strands are fully “conserved”

Semi conservative model

the two parental strands each make a copy of itself

after one round of replication, the 2 daughter molecules each have 1 parental and 1 new strand

Meselson and Stahl performed an experiment using bacteria to confirm that this was the correct model

1. Bacteria was cultural with a heavy isotope

2. Bacteria was transferred to a medium with a light isotope

3. DNA was centrifuged and analyzed after each replication

By analyzing samples of DNA after each generation, they found that the parental strands followed the semi-conservative model

Dispersive model

the material in the two parental strands is dispersed randomly between the 2 daughter molecules

after one round of replication, the daughter molecules contain a random mix of parental and new DNA

Steps in DNA Replication

1. Proteins attach to the origin of replication and open the DNA to form a replication fork

2. Helicase unwinds the DNA strands at each replication fork

3. Primase initiates replication by adding primers to the parental DNA strand

4. DNA polymerase attaches to each primer on the parental strand and reads 3’ to 5’ direction. As it moves, it adds nucleotides to the new strand in the 5’ to 3’ direction

5. After DNAP III forms an okazaki fragment, a DNAP I replaces RNA nucleotides with DNA nucleotides

Helicase

unwinds the DNA strands at each replication fork

SSBPs

single strand binding proteins

to keep the DNA from re-bonding with itself, proteins called single strand binding proteins (SSBPs) bind to the DNA to keep it open

Topoimerase

helps prevent strain ahead of the replication fork by relaxing supercoiling

RNA Primase

enzyme that initiates replication by adding RNA primers to the parental DNA strand

the enzymes that synthesize DNA can only attach new DNA nucleotides to an existing strand of nucleotides

Primers

short segments of RNA

serve as the foundation for DNA synthesis

1 needed for leading strand, multiple needed for lagging strand

DNA Polymerase

antiparallel elongation- (DNAP III in prokaryotes) attaches to each primer on the parental strand and reads 3’ to 5’ directions(can only add nucleotides on 3’ end)

as it moves, it adds nucleotides to the new strand in the 5’ to 3’ direction(now its antiparallel to the parent strand)

the DNAP that moves towards the replication fork synthesizes the leading strand smoothly and continuously, requires only 1 primer

the DNAP on the other template strand moves away from the replication fork, synthesizing the lagging strand discontinuously short segments called okazaki fragments, requires many primers

after DNAP III forms an okazaki fragment, DNAP I replaces RNA nucleotides (from the RNA primers) with DNA nucleotides

DNA polymerase in prokaryotes

prokaryotes have 2 DNA polymerase important in replication, DNAP III and DNAP I

eukaryotes have 5 distinct DNA polymerases, but the specific names of those aren’t needed

DNA ligase

joins the okazaki fragments, forming a continuous DNA strand

acts like glue for the okazaki fragments

DNA synthesis diagram

DNA synthesis always occurs antiparallel to the template DNA and in the 5’ to 3’ direction. So, in the correct diagram, each arrow will point towards the 5’ end of its template strand (picture is an example)

Eukaryotes: Problems at the 5’ End

linear chromosomes present a problem at the 5’ end

since DNAP can only add nucleotides to a 3’ end, there is no way to finish replication on the 5’ end of a lagging strand

-over many replications this would mean that the DNA would become shorter and shorter

genes on DNA are protected from this due to telomeres

Telomeres

repeating units of short nucleotide sequences that do not code for genes

form a cap at the end of DNA to help postpone erosion

some cells have the enzyme telomerase, which adds telomeres to DNA

Proofreading and Repair

as DNAP adds nucleotides to the new DNA strand, it proofreads the bases added

if errors still occurs, mismatch repair will take place

Mismatch Repair

enzymes remove and replace the incorrectly paired nucleotide

if segments of DNA are damaged, nuclease can remove segments of nucleotides, and DNA polymerase and ligase can replace the segments