Download 12.2 DNA Replication ppt

12-2
Memory Check: Mitosis and Meiosis
When Does DNA Replicate?
Why Does DNA Replicate?
 DNA
must be replicated exactly
 All somatic cells that undergo mitosis get a
new strand of DNA and an “old” one
 This occurs in the S phase of interphase
 Cannot be accomplished without the aid of
enzymes.
1. Growth
2. Tissue Repair
3. Keeps chromosome
number constant
 Enzyme:
DNA helicase
 Action: Breaks H-bonds that link bases
together
 Result: Replication Fork (site where
separation and replication occur) made by
using additional proteins that keep the DNA
from re-winding (keep site open)
 Enzyme:
DNA polymerase
 Action: Adds new nucleotides to the exposed
bases using the base-pair rule; also
proofreads every connection at this time
(avg. 1 error per 2 billion nucleotides)
 Result: 2 new double DNA strands are
created (but still attached)
 Enzyme:
Ligase
 Action: DNA
polymerase and
proteins from
helicase are
removed,
gaps filled in
 Result: 2 new DNA
molecules twist into
their finalized
shape
 Does
this “replication fork” only occur once
and go base by base?
 What could be the potential problem if it
occurred this way?
 Too Slow! In fact, from start to finish it
would take 33 days!
 There are multiple replication forks going at
once all over DNA to speed up the process

http://www.lpscience.fatcow.com/jwanamaker/
animations/DNA%20Replication%20%20long%20.html

http://www.johnkyrk.com/DNAreplication.html

http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapt
er14/animations.html#