Biology 1202B Lecture Notes - Lecture 12: Semiconservative Replication, Phosphodiester Bond, Polynucleotide

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Published on 19 Apr 2013
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Western University
Department
Biology
Course
Biology 1202B
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Bio 1
Biology Exam Review
Lecture 11: Chapter 12DNA Structure and Organization
DNA structure
Watson and Crick won Nobel Prize for discovery
Franklin played role because she helped discover it through X-Ray Diffraction
died because of things she worked with and her notes ended up with Watson. X-
shaped pattern of spots was correctly interpreted by Franklin to indicate that
DNA has a helical structure similar to spiral staircase
Nucleotides
DNA contains nucleotides joined to form a polynucleotide chain. In a
polynucleotide chain, the deoxyribose sugars are linked by phosphate groups in
an alternating sugar-phosphate-sugar-phosphate pattern, forming a sugar-
phosphate backbone
Each phosphate group is a ―bridge‖ between the 3’ carbon of one sugar and the
5’ carbon on the next sugar; the entire linkage, including the bridging phosphate
group is called a phosphodiester bond
At one end, a phosphate group is bound to the 5’ carbon of a deoxyribose sugar
(5’ end), whereas at the other end, a hydroxyl group is bonded to the 3’ carbon
of a deoxyribose sugar (3’ end)
Each of the 2 sugar-phosphate backbones of the double helix qualifies as a
moleculedouble helix is technically composed of 2 poly-nucleotide molecules
held together by hydrogen bonds
Full turn takes 3.4nm therefore there are 10 base pairs in a full turn
Semiconservative replication
1. Complementary base pairing in the DNA double helix: A pairs with T; G with C
2. The 2 chains unwind and separate
3. Each ―old‖ strand is a template for the addition of bases according to the base-
pairing rules
4. The result is 2 DNA helices that are exact copies of the parental DNA molecule
with one ―old‖ strand and one ―new‖ strand
DNA replication
DNA polymerase catalyzes assembly of template strand
Can add a nucleotide only to the 3’ end of an existing nucleotide chain
As DNA is assembled, a 3’ –OH group is always exposed at its ―newest‖ end.
Oldest‖ end of the new chain has an exposed 5’ phosphate
DNA polymerase assemble nucleotide chains 5’ 3’ direction
Read in the 3’ 5’ direction for new synthesis
Key molecular events
Bio 2
1. The 2 strands of the DNA molecule unwind for replication to occur
2. DNA polymerase can add nucleotides only to an existing chain
3. The overall direction of new synthesis is in the 5’ 3’ direction, which is a
direction antiparallel to that of the template strand
4. Nucleotides enter into a newly synthesized chain according to the A-T and G-
C complementary base-pairing rules
Lecture 12: Chapter 12DNA Replication
Semi-conservative: double helix opens,
each strand is a template, after
replication, each strand has a new and an
old
Conservative: double helix opens,
parental stands pair back up, there is a
new double strand and an old double
strand
Dispersive: splits into double stranded
segments, on which new double stranded
segments form. Some new parts, some
old parts
Meselson and Stahl showed that DNA replication is semiconservative…
Grew bacteria for several generations in 15N (heavy) mediumincorporated into
nitrogenous bases of DNA
Transferred bacteria to 14N (normal/light) medium and allowed it to grow and
divide for several generations
Mixed with CsCL and centrifugedthings with highest density go to bottom
Results…
Supports the semi
conservative model
Bio 3
DNA replication
Unwinding of DNA occurs at specific
sequence in bacterial chromosome called
origin of replication (ori)
Proteins bind to ori and promote binding
of DNA helicase, which unwinds the DNA
strands and produces a replication fork,
which consists of the 2 unwound template
strands transitioning to double helical
DNA
SSBs keep the strands from binding back
together
Troposomerase prevents twisting ahead of the replication fork as the DNA
unwinds
DNA polymerases cannot add nucleotides when there is no existing strand, so a
primer made of RNA is there instead and gets synthesized by primase
DNA polymerase III adds DNA nucleotides to the RNA primer
DNA unwinds more, leading strand is
continuously made, and lagging strand is
discontinuously made
Primer will form at fork (Okasaki
fragment) and DNA polymerase will synthesize
until it reaches the previously formed Okasaki
fragment
DNA polymerase I removes primer of
okasaki fragment using exonuclease activity
Replaced with DNA through
polymerizing activity. A nick (break in sugar
phosphate backbone) remains
DNA ligase seals the nick between the 2 lagging
strand fragments
Helicase continues to unwind and the same steps are
repeated
Biodirectional synthesis of DNAreplication goes in opposite
directions and occurs on many areas of the eukaryotic
chromosome for fast replication. Eventually all of the forks
will meet up and 2 strands are there
Replication is activated only once during the S (synthesis) phase on interphase of
eukaryotic cell cycle (so no portion of the DNA is replicated more than once)
Removal of the RNA primer leaves the 5’ end of the newly synthesized DNA
strand with a gap

Document Summary

Watson and crick won nobel prize for discovery. Franklin played role because she helped discover it through x-ray diffraction died because of things she worked with and her notes ended up with watson. X- shaped pattern of spots was correctly interpreted by franklin to indicate that. Dna has a helical structure similar to spiral staircase. Dna contains nucleotides joined to form a polynucleotide chain. In a polynucleotide chain, the deoxyribose sugars are linked by phosphate groups in an alternating sugar-phosphate-sugar-phosphate pattern, forming a sugar- phosphate backbone. Each phosphate group is a bridge between the 3" carbon of one sugar and the. 5" carbon on the next sugar; the entire linkage, including the bridging phosphate group is called a phosphodiester bond. At one end, a phosphate group is bound to the 5" carbon of a deoxyribose sugar (5" end), whereas at the other end, a hydroxyl group is bonded to the 3" carbon of a deoxyribose sugar (3" end)