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Warm-up : 4/4/11
Complimentary Strand
• Do this on a separate sheet in your notebook.
• What pairs up with:
– Adenine?
– Guanine?
• For the following strands give the complimentary
(other side of the double helix). Write out both.
1) ATGCGTAAT
2) GCGTTAACT
What does DNA look like?
• DNA has..
• A sugar/phosphate
backbone
• 4 Chemical bases
Sugar
The bases
• It is composed of 4
chemical bases:
Adenosine (A), Thymine
(T), Cytosine (C) and
Guanine (G).
• A always pairs with T
• C always pairs with G
• The pairs are held together
by hydrogen bonds
(WNC’s).
What does DNA do?
• DNA is the genetic code.
• It determines our physical characteristics:
from our hair color to what we are allergic
to.
• Our DNA codes for 20 amino acids which
are the building blocks of life.
What Is DNA Replication
• DNA Replication is the
process in which the DNA
within a cell makes an
exact copy of itself.
– Why does DNA replicate?
– During which phase of the
cell cycle does DNA
replicate?
Steps of Replication
•
•
•
•
Unwind
Unzip
Insert
Proofread
DNA Replication models
The Three Possible DNA
Replication Models
• Conservative- would leave
the original strand intact
and copy it.
• Dispersive-would produce
two DNA molecule with
sections of both old and
new along each strand.
• Semiconservative –would
produce DNA molecule
with both one old strand
and one new strand.
DNA replication
• DNA replication: the DNA strand “unzips”,
the weak hydrogen bond comes undone, and
the base pairs separate
Insert
• After the DNA molecule opens up, new
nucleotides are brought in and match up to
their complimentary base. (A-T, C-G)
DNA Replication
Replication occurs during Interphase
Replication
fork
Replication
fork
Replication
bubble
Hydrogen
bond
DNA replication is the process where an entire double-stranded
DNA is copied to produce a second, identical DNA double helix.
DNA Replication
DNA
helicase
• Helicase unwinds the double helix starting at a
replication bubble.
• The two strands separate as the hydrogen bonds
between base pairs are broken.
• Two replication forks form and the DNA is unwound in
opposite directions.
DNA Replication
•Helicase has completed unwinding the DNA strand.
•Single strand Binding Proteins (SSB) keep the two
strands from re-annealing (coming back together).
DNA Replication
Leading Strand
Primase
RNA Primer
Lagging Strand
•Primase is an RNA polymerase that makes the RNA
primer.
•These primers “tell” the DNA polymerase where to start
copying the DNA.
DNA Replication
Leading Strand
5’
3’
Direction of Replication
DNA Polymerase
5’
Direction of Replication
Lagging Strand
3’
• The DNA polymerase starts at the 3’ end of the RNA primer of
the leading stand CONTINUOUSLY.
• DNA is copied in 5’ to 3’ direction.
• DNA polymerase copies the lagging strand DIS- continuously.
DNA Replication
• The dis-continuous pieces of DNA copied on the lagging
strand are known as Okazaki fragments.
DNA Replication
Another DNA Polymerase removes the RNA primers and
replaces them with DNA.
DNA Replication
ligase
Finally the gaps in the sugar phosphate backbone are
sealed by DNA ligase
There are now 2 identical double helices of DNA.
Proofread
• The nucleus has enzymes that “read”
through the molecule looking for mistakes.
• If one is found, the molecule cuts out the
incorrect nucleotide and replaces it.
• Even with this molecule, sometimes a
mistake is made, causing a change.
Something old, Something new
• The process is semi-conservative; each new
molecule has one of the old strands and one
of the new strands.
DNA Video clip
/
Remember that DNA is replicated using a
series of enzymes.
•
http://bio-rad.cnpg.com/Video/flatFiles/799
PCR= Polymerase Chain Reaction, lab
procedure that makes several copies of a
certain sequence of DNA, used to amplify a
sequence so scientists have an easier time
looking at it.
Video Animations
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/c
hapter14/animations.html#
DNA Paper Model
Must make sure you do questions 1 and 3 to
be able to do homework.
–
–
–
–
Watson/Crick- double helix structure discovery
Hershey/Chase- confirmed that DNA was genetic material
Franklin/Wilkins- X-ray crystallography of DNA structure
Pauling/Chargaff- specific proportions of bases and that DNA was
genetic material
HW: Questions 1-10
Wednesday: Finish building model in class
and create poster