DNA Replication
Summary of DNA’s secondary Structure
- The sugar-phosphate backbone faces the exterior, while the
nitrogenous bases face the interior
- DNA’s secondary structure consists of
- Two antiparallel strands twisted into a double helix
- By hydrogen bonding between
- The complementary base pairs
- A-T and G-C
The Sugar-Phosphate Backbone Is Directional
- The sugar-phosphate backbone of a nucleic acid is directional (has
polarity)
- One end has an unlinked 5′ carbon (5’ Free Phosphate)
- The other end has an unlinked 3′ carbon
DNA Contains Biological Information
- DNA can store and transmit biological information
- DNA carries the information required for the organism’s growth and
reproduction
- Therefore, must be able to replicate
- The language of nucleic acids is contained in the sequence of the bases
- The nucleotide sequence is written in the 5′ → 3′ direction
- The nucleic acid’s primary structure is the nucleotide sequence
Not a Xerox Machine
- There were three possible alternatives as to how the DNA could serve as
a template for the assembly of new DNA molecules (in all 3 possible
ways, the original DNA strands must separate)
- conservative replication
- the two strands of DNA completely separate to act as
templates for the assembly of two new strands
- after replicating, the original strands rejoin, preserving the
original DNA molecule and leaving a completely new one
, - semiconservative replication
- the DNA unzips and new complementary strands are
assembled using each strand as a template
- one original strand is preserved in each duplex created
- dispersive replication
- replication results in both original and new DNA dispersed
among the two daughter strands
Meselson and Stahl
- • Matthew Meselson and Franklin Stahl tested the three alternative
hypotheses for the replication of DNA (1958)
- they used radioactive isotopes of N to label DNA prior to and after
replication
- they found that DNA replication was semi-conservative
General Notes about DNA replication
- New nucleotides are added at the existing 3’ end – ONLY
- The new strand undergoes base pairing with the parent strand.
- Always using the same base pairing rules
- Always running antiparallel
Replication
- The process of DNA replication involves several families of enzymes
- Helicase
- Single-strand binding proteins (SSBPs)
- Topoisomerase
- DNA polymerases
- Primase
- DNA ligase
How Does Replication Start?
- The replication proteins bind at the origin of replication, which is
identified by a particular base sequence
- Helicase unwinds the DNA, which is held open with single strand
binding proteins.
1) As the DNA unwinds, supercoiling is prevented by
topoisomerase
, 2) Creates a Replication Fork
3) RNA primers are first laid down on each strand by the
enzyme Primase (DNA-pol a)
4) DNA nucleotides cannot be added without a RNA
nucleotide primer
- DNA polymerase III/δ incorporates complementary
nucleotides
- DNA polymerase adds nucleotides at the 3’ end,
but the new strands elongate in opposite
directions
- The leading strand elongates into the fork
- The lagging strand elongates away from the
fork
5) The gaps between the new DNA strands is closed in by DNA
ligase
6) RNase H or DNA polymerase I/ε replace RNA primers
- DNA can and must be proofread
- During DNA synthesis the newly synthesized daughter strand includes
errors.
- 3 step process
- recognition of mismatch
- excision of segment of DNA containing mismatch
- resynthesis of excised fragment
- In mammals the newly synthesized strand is preferentially nicked and
can be distinguished in this manner from the parental strand
You have a drug that stops the activity of DNA Polymerase I. Which of the
following would be affected by this drug?
A) Synthesis of a complete lagging strand
B) Synthesis of a complete leading strand
C) Synthesis of both a complete lagging strand and complete
leading strand
D) No effect, both strands would be synthesized normally
A bacteria is known to attack DNA ligase enzyme by disrupting the hydrogen
bonds responsible for protein folding
a. What level(s) of protein structure is/are affected by this bacteria
Summary of DNA’s secondary Structure
- The sugar-phosphate backbone faces the exterior, while the
nitrogenous bases face the interior
- DNA’s secondary structure consists of
- Two antiparallel strands twisted into a double helix
- By hydrogen bonding between
- The complementary base pairs
- A-T and G-C
The Sugar-Phosphate Backbone Is Directional
- The sugar-phosphate backbone of a nucleic acid is directional (has
polarity)
- One end has an unlinked 5′ carbon (5’ Free Phosphate)
- The other end has an unlinked 3′ carbon
DNA Contains Biological Information
- DNA can store and transmit biological information
- DNA carries the information required for the organism’s growth and
reproduction
- Therefore, must be able to replicate
- The language of nucleic acids is contained in the sequence of the bases
- The nucleotide sequence is written in the 5′ → 3′ direction
- The nucleic acid’s primary structure is the nucleotide sequence
Not a Xerox Machine
- There were three possible alternatives as to how the DNA could serve as
a template for the assembly of new DNA molecules (in all 3 possible
ways, the original DNA strands must separate)
- conservative replication
- the two strands of DNA completely separate to act as
templates for the assembly of two new strands
- after replicating, the original strands rejoin, preserving the
original DNA molecule and leaving a completely new one
, - semiconservative replication
- the DNA unzips and new complementary strands are
assembled using each strand as a template
- one original strand is preserved in each duplex created
- dispersive replication
- replication results in both original and new DNA dispersed
among the two daughter strands
Meselson and Stahl
- • Matthew Meselson and Franklin Stahl tested the three alternative
hypotheses for the replication of DNA (1958)
- they used radioactive isotopes of N to label DNA prior to and after
replication
- they found that DNA replication was semi-conservative
General Notes about DNA replication
- New nucleotides are added at the existing 3’ end – ONLY
- The new strand undergoes base pairing with the parent strand.
- Always using the same base pairing rules
- Always running antiparallel
Replication
- The process of DNA replication involves several families of enzymes
- Helicase
- Single-strand binding proteins (SSBPs)
- Topoisomerase
- DNA polymerases
- Primase
- DNA ligase
How Does Replication Start?
- The replication proteins bind at the origin of replication, which is
identified by a particular base sequence
- Helicase unwinds the DNA, which is held open with single strand
binding proteins.
1) As the DNA unwinds, supercoiling is prevented by
topoisomerase
, 2) Creates a Replication Fork
3) RNA primers are first laid down on each strand by the
enzyme Primase (DNA-pol a)
4) DNA nucleotides cannot be added without a RNA
nucleotide primer
- DNA polymerase III/δ incorporates complementary
nucleotides
- DNA polymerase adds nucleotides at the 3’ end,
but the new strands elongate in opposite
directions
- The leading strand elongates into the fork
- The lagging strand elongates away from the
fork
5) The gaps between the new DNA strands is closed in by DNA
ligase
6) RNase H or DNA polymerase I/ε replace RNA primers
- DNA can and must be proofread
- During DNA synthesis the newly synthesized daughter strand includes
errors.
- 3 step process
- recognition of mismatch
- excision of segment of DNA containing mismatch
- resynthesis of excised fragment
- In mammals the newly synthesized strand is preferentially nicked and
can be distinguished in this manner from the parental strand
You have a drug that stops the activity of DNA Polymerase I. Which of the
following would be affected by this drug?
A) Synthesis of a complete lagging strand
B) Synthesis of a complete leading strand
C) Synthesis of both a complete lagging strand and complete
leading strand
D) No effect, both strands would be synthesized normally
A bacteria is known to attack DNA ligase enzyme by disrupting the hydrogen
bonds responsible for protein folding
a. What level(s) of protein structure is/are affected by this bacteria
