2. DNA review
DNA codes for all information necessary for
life
The information is stored in the sequence of
nucleotide bases
It is a quaternary code…
The sequence is important
– Human Genome Project: Sequence of roughly
3.2 billion bases in human DNA!
3. A Perfect Copy
When a cell divides, each new cell receives a
complete set of chromosomes.
This means that each new cell has a complete
set of the DNA code.
Before a cell can divide, the DNA must be copied
so that there are two sets ready to be distributed to
the new cells.
4. DNA structure
Recall that once Watson and Crick made the
correct model, they instantly realized a
copying mechanism.
Due to the structure of DNA (specifically the
base-pairing arrangement), if the order of
nucleotides on one chain is known, the order
on the other chain is obvious.
13. DNA is copied through a process called DNA
replication.
The process, though seemingly simple, is
actually quite complex.
Energy is required.
– Comes from which molecule?
Many enzymes and proteins are required for
DNA replication.
15. In a nutshell…
To start, an enzyme breaks the hydrogen
bonds that hold base-pairs together.
This process “unzips” the DNA molecule.
Other enzymes find and correctly match free-
floating nucleotides to the two sides of the
now open DNA molecule.
This process continues until the entire
molecule has been replicated.
16.
17. Replication fork… what happens
Many different enzymes are needed to open
up the DNA helix for replication
Two strands must be separated first
Helicases do this …
Pry apart the hydrogen bonds of the two
DNA strands…
20. DNA polymerases build new strands.
Growth
Growth
Replication
fork
DNA
polymerase
New strand
Original
strand
DNA
polymerase
Nitrogenous
bases
Replication
fork
Original
strand
New strand
21. DNA Replication fork is
asymmetrical
Leading and lagging strands are different
The leading strand is continuous
The lagging strand has fragments and is
synthesized discontinuously.
– Okazaki fragments
22.
23. High fidelity requires proof
reading
DNA polymerase synthesizes in a 5’ to 3’
direction
Proof reading is in a 3’-5’ direction
Can take out mismatched bases with reverse
action
25. Helicases…
Many different helicases have been identified
Single stranded binding proteins are
important in maintaining the single stranded
nature at the replication fork & so no hair
pins can form
28. • Each eukaryotic chromosome contains one DNA double helix
• Average ~106
base pairs long
• With a replication rate of 2 kb/minute, replicating one human
chromosome would require ~35 days.
• Solution ---> DNA replication initiates at many different sites
simultaneously.
Rates are cell
specific!
29. Three main features of the DNA synthesis reaction:
1. DNA polymerase catalyzes formation of phosphodiester bond
between deoxyribose and the phosphate on the new strand.
• Energy for this reaction is derived from the release of two of the
three phosphates on the free nucleotide.
2. DNA polymerase “finds” the
correct complementary nucleotide
at each step in the
lengthening process.
• rate 50-1000 bases/second
• low error rate (~1/107
)
3. Direction of synthesis is 5’ to 3’ Image credit:
Protein Data Bank