DNA and the gene: Synthesis and repair
What is gene DNA is compromised of 4 different nucleotides. Gene is made of DNA. But many
made of biologist thought they were made of protein
Hershey Chase T2 virus infects the bacterium Escherichia coli (E.coli)
experiment ◦ virus capsid injects its gene into the cell. Capsid does not enter(virus’ protein coat).
◦ production of new virus particles directed by the virus gene
◦ End of injection: New generation of virus particles burst from the cell.
◦ Does DNA or protein enter? DNA
Experiment Hershey Chase—viral gene consist of DNA or protein.
◦ label virus: virus grown in the presence of P ( P is only found in DNA and not in
protein) and some virus grown in the presence of S(present in protein only amd not
in DNA).
◦ infects bacteria: Allowed each labelled viruses to infect one culture of E.coli.
◦ Agitate them in kitchen blender to separate empty viral capsids from bacterial cells in
each culture.
◦ centrufuge solution of bacterial cells from each culture to force cells into pellet.Only
radioactive DNA was found in the pellet while radioactive protein was in the solution.
Hence, Gene consist of DNA.
Secondary
structure of The primary structure of each strand of DNA has
DNA
◦ A backbone made up of the sugar and phosphate groups of deoxyribonucleotides
◦ A series of nitrogen-containing bases that project from the backbone
DNA has directionality
◦ The 3ʹ end has an exposed hydroxyl group attached to the 3ʹ carbon of deoxyribose
◦ The 5ʹ end has an exposed phosphate group attached to a 5ʹ carbon
Purine: Adenine and Guanine
Pyrimidine: Thymine, cytosine and uracil (RNA)
Purine+ pyrimidine= A-T or A-U(in RNA) and G-C
Watson and crick propsed that the two DNA strand runs in antiparallel fashion.
◦ The antiparallel strands twist to form a double helix.
◦ the structure is stabilized by complementary base pairs i,e hydrogen bonds between A
and T , C and G.
Three hypotheses for how parental strands and daughter strands act during replication:
1.Semiconservative replication
2.Conservative replication
3.Dispersive replication
Semi conservative Parental strands separate, and each is a template for
replication a new strand. Each daughter has one old and one
new strand
, Conservative The parental molecule serves as a template for an entirely
replication new molecule. One daughter has both old strands; the
other has both new strands
Dispersive
replication
The parent molecule is cut into sections. Each daughter has
old and new DNA interspersed
Meselson and
In the Meselson and Stahl experiment, E. coli were first grown in a medium with heavy
stahl experiment nitrogen (15N), then transferred to a medium with normal nitrogen (14N). After allowing
the bacteria to divide once, they centrifuged the DNA to separate it by density. The DNA
showed an intermediate density, indicating each strand had one 15N and one 14N (hybrid).
After a second round of division in 14N, they found two bands: one intermediate and one
lighter. This confirmed that DNA replication is semi-conservative, with each new DNA
molecule containing one old and one new strand.
DNA polymerase e monomers
The enzyme that catalyze DNAare
synthesis. It adds deoxyribonucleotide to the 3’ end
of DNA strand.Hence, DNA strand synthesis is in 5’—. 3’ direction.
model for DNA The monomers are deoxyribonucleoside triphosphates (dNTPs)•
synthesis ◦ Have high potential energy because of their three closely
packed phosphate groups
◦ They have enough potential energy to make the
formation of phosphodiester bonds exergonic
Parental strand 3’— 5’ and daughter strand 5’— 3’.
DNA synthesis involves the condensation rxn that forms
phophodiester bond between 3’ end of the DNA strand and 5’ carbon
of the incoming deoxyribonucleoside triphosphate monomer.
Where does DNA replication begins at a specific sequence called the origin of replication, where a
replication start? replication bubble forms. In bacteria, there is one replication bubble, while in eukaryotes, there
are many on each chromosome. Each bubble has two replication forks because DNA is
synthesized in both directions, so the bubble grows outward from both sides.
What is gene DNA is compromised of 4 different nucleotides. Gene is made of DNA. But many
made of biologist thought they were made of protein
Hershey Chase T2 virus infects the bacterium Escherichia coli (E.coli)
experiment ◦ virus capsid injects its gene into the cell. Capsid does not enter(virus’ protein coat).
◦ production of new virus particles directed by the virus gene
◦ End of injection: New generation of virus particles burst from the cell.
◦ Does DNA or protein enter? DNA
Experiment Hershey Chase—viral gene consist of DNA or protein.
◦ label virus: virus grown in the presence of P ( P is only found in DNA and not in
protein) and some virus grown in the presence of S(present in protein only amd not
in DNA).
◦ infects bacteria: Allowed each labelled viruses to infect one culture of E.coli.
◦ Agitate them in kitchen blender to separate empty viral capsids from bacterial cells in
each culture.
◦ centrufuge solution of bacterial cells from each culture to force cells into pellet.Only
radioactive DNA was found in the pellet while radioactive protein was in the solution.
Hence, Gene consist of DNA.
Secondary
structure of The primary structure of each strand of DNA has
DNA
◦ A backbone made up of the sugar and phosphate groups of deoxyribonucleotides
◦ A series of nitrogen-containing bases that project from the backbone
DNA has directionality
◦ The 3ʹ end has an exposed hydroxyl group attached to the 3ʹ carbon of deoxyribose
◦ The 5ʹ end has an exposed phosphate group attached to a 5ʹ carbon
Purine: Adenine and Guanine
Pyrimidine: Thymine, cytosine and uracil (RNA)
Purine+ pyrimidine= A-T or A-U(in RNA) and G-C
Watson and crick propsed that the two DNA strand runs in antiparallel fashion.
◦ The antiparallel strands twist to form a double helix.
◦ the structure is stabilized by complementary base pairs i,e hydrogen bonds between A
and T , C and G.
Three hypotheses for how parental strands and daughter strands act during replication:
1.Semiconservative replication
2.Conservative replication
3.Dispersive replication
Semi conservative Parental strands separate, and each is a template for
replication a new strand. Each daughter has one old and one
new strand
, Conservative The parental molecule serves as a template for an entirely
replication new molecule. One daughter has both old strands; the
other has both new strands
Dispersive
replication
The parent molecule is cut into sections. Each daughter has
old and new DNA interspersed
Meselson and
In the Meselson and Stahl experiment, E. coli were first grown in a medium with heavy
stahl experiment nitrogen (15N), then transferred to a medium with normal nitrogen (14N). After allowing
the bacteria to divide once, they centrifuged the DNA to separate it by density. The DNA
showed an intermediate density, indicating each strand had one 15N and one 14N (hybrid).
After a second round of division in 14N, they found two bands: one intermediate and one
lighter. This confirmed that DNA replication is semi-conservative, with each new DNA
molecule containing one old and one new strand.
DNA polymerase e monomers
The enzyme that catalyze DNAare
synthesis. It adds deoxyribonucleotide to the 3’ end
of DNA strand.Hence, DNA strand synthesis is in 5’—. 3’ direction.
model for DNA The monomers are deoxyribonucleoside triphosphates (dNTPs)•
synthesis ◦ Have high potential energy because of their three closely
packed phosphate groups
◦ They have enough potential energy to make the
formation of phosphodiester bonds exergonic
Parental strand 3’— 5’ and daughter strand 5’— 3’.
DNA synthesis involves the condensation rxn that forms
phophodiester bond between 3’ end of the DNA strand and 5’ carbon
of the incoming deoxyribonucleoside triphosphate monomer.
Where does DNA replication begins at a specific sequence called the origin of replication, where a
replication start? replication bubble forms. In bacteria, there is one replication bubble, while in eukaryotes, there
are many on each chromosome. Each bubble has two replication forks because DNA is
synthesized in both directions, so the bubble grows outward from both sides.
