Please enable JavaScript.
Coggle requires JavaScript to display documents.
Chapter 12A: DNA Replication (S Phase) - Coggle Diagram
Chapter 12A: DNA Replication (S Phase)
Structure of DNA: 2 strands of nucleotides help by phosphodiester bond
5' phosphate
3' hydroxyl
strands are antiparallel so there's correct hydrogen bond pairing
direction of chain growth: 5' --> 3'
"The most beautiful experiment in biology"
Label bacteria with "heavy" DNA
transfer cells to "light DNA", grow for one generation (cells start to divide)
take DNA out of bacteria, examine the density gradient (hybrid DNA - density is in between "heavy" density and "light" density)
Grow for another cell cycle in light DNA
examine density gradient: (there's now light AND hybrid DNA)
Experiment shows that DNA was replicated conservatively
DNA Synthesis is complicated because
Unwinding the DNA helix creates tension in the unwound part
DNA polymerase can only add nucleotides to an existing nucleotide
Occurs in opposite directions on each strand because DNA polymerase works 5'-->3' (Produces multiple fragments on the lagging strand)
DNA is a long molecule and it would take a long time to copy it with a single DNA polymerase (needs to be copied by multiple DNA polymerases whose products need to be joined together)
Enzymes and STEPS
TYPES
1) Preparing DNA for copying
Single-strand binding protein: binds to/stabilizes single-stranded DNA
Topoisomerase: clips 1 of the 2 DNA strands to allow supercoiled region to unwind, rejoins strands
Helicase: unwinds parental double helix
2) Copy DNA
Primase: Synthesizes an RNA primer at 5' end, provides DNA polymerase III with a nucleotide it can add to
DNA Pol III: synthesizes new DNA strand by adding nucleotides to an RNA primer or preexisting DNA strand
3) Finishing up
DNA pol I: Removes RNA nucleotides of primer at 5' end
DNA ligase: seals nucleotides together
STEPS FOR DNA REPLICATION
Step 1: Unwinding the helix
helicase unwinds DNA duplex
Topoisomerase II relives the stress of unwinding
single-stranded binding proteins stabilize single strand of DNA
Step 2: Replicating each strand
DNA synthesis starts with an RNA primer
DNA synthesis occurs in opposite directions on each strand (5-->3')
DNA synthesis is discontinuous on the LAGGING strand and continuous on the LEADING strand because both strands are synthesized by a single protein complex
DNA exonuclease (a DNA pol I) activity (replaces RNA with DNA) and DNA ligase activity (stiches together okazaki fragments) are required because DNA synthesis is discontinuous
Coordinating polymerase activities on leading and lagging strands
lagging strand loops and gets inverted
allows DNA polymerase to copy lagging strand at the same rate/way the leading strand is synthesized
Replication bubble: growth of leading and lagging strands continues on both sides of replication bubble until there's 2 identical DNAs
DNA polymerase III has proofreading ability: can detect its mistakes, remove the incorrectly paired nucleotide (exonuclease activity), and add the correct nucleotide
Eukaryotic chromosomes have many origins of replication
DNA ligases pieces together Okazaki fragments on lagging strand AND joins leading strands from different replication forks
Replicating chromosome ends requires TELOMERASE which carries its own RNA template: keeps the chromosome at a constant length through many cell cycles
important for lagging strand because at the beginning of this strand there's an RNA primer
telomerase sits at the end of the chromosome and has an RNA that's complementary to the DNA sequence
allows for continued DNA molecules synthesis
NOW, RNA primer can sit down on new strand and synthesize another Okazaki fragment