Molecular Genetics
Who discovered DNA Structure?
James Watson and Francis Crick
DNA Model
Consists of two strands of nucleotides.
Each nucleotide contains
a Deoxyribose sugar
a Phosphate group
and a nitrogen base
There are 4 different nucleotides.
DNA Model
The strand of DNA represents a twisted ladder.
The backbone of the DNA is made up of the
sugar and phosphates. 🡪 vertical sides of the
ladder
The nitrogen bases are the horizontal rungs of
the ladder.
These parts twist in a clockwise direction to
make a double helix.
DNA Model
Complementary Base Pairing
Provides the rationale how the base pairs are
paired and how the two strands of the DNA
molecule are associated to one another.
The nitrogen bases of one strand are paired with
another nitrogen base from the other strand.
A purine is always paired with a pyrimidine.
Purines 🡪 adenine and guanine
Pyrimidines 🡪 thymine and cytosine
Complementary Base Pairing
The base pair rules:
Adenine always bonds with thymine. (A-T)
Guanine always binds with cytosine. (G-C)
Complementary Base Pairing
The strands are complementary to one another
based on the association of the base pairings.
The complementary base pairs are held together
by hydrogen bonds.
Between adenine and thymine there are 2
hydrogen bonds.
Between guanine and cytosine there are 3
hydrogen bonds.
Hydrogen Bonds
Antiparallel
The two strands of a DNA molecule are antiparallel to
each other.
The are arranged in the opposite directions.
At one end one strand will have the phosphate group
attached to the 5’ carbon of the deoxyribose sugar and
at the other end it will have a phosphate group attached
to the 3’ carbon and the hydroxyl group of the
deoxyribose.
The second strand will have the same characteristics
but will run in the opposite direction. 🡪3’ – 5’.
Deoxyribose Sugar
Antiparallel
DNA Replication
DNA replication is a process when DNA makes
exact copies of itself.
Occurring before cell division during the
synthesis phase of interphase.
This ensures that each daughter cell has an
identical copy of DNA.
The daughter cells will be able to carry out their
intended function.
Semiconservative Replication
Is the process that DNA goes through to
produce new identical DNA for the daughter
cells.
The parent DNA is split and used as the
template to develop the two identical copies.
First the hydrogen bonds are broken between
the nitrogen bases on the parent DNA.
This allows the double helix to unwind/unzip.
Semiconservative Replication
Once unzipped, the individual strands of the
parent DNA become the template and attract
complementary nucleotides.
The nucleotides find their complementary
nitrogen base along the template and gradually
form a complete set of DNA.
As each template completes the positioning of
nitrogen bases, two new completely identical sets
of DNA are produced.
Semiconservative Replication
The completed DNA strands are composed of a parent
strand and a new strand making a complete DNA
molecule.
The DNA coils again and is ready to be separated
during cell division.
DNA Replication Procedure
Separating the DNA Strands
DNA helicase (an enzyme)
breaks the bonds between the
nitrogen bases.
Proteins attach to the leading
strand to prevent the
nitrogen bases reforming
with their compliment.
The DNA strands separate
and create a replication fork.
DNA Replication Procedure
The separated strands
create a leading strand
and a lagging strand.
Leading strand is the
strand that is
continuously replicated
from 5’ to 3’.
The lagging strand is
replicated in fragments
from 5’ to 3’.
DNA Replication Procedure
RNA primers are
positions on the
leading and lagging
strands to identify
starting points for
replication.
Leading strand one
primer.
Lagging strands
multiple primers.
DNA Replication Procedure
The leading strand will add
nucleotides one by one.
The lagging strands adds
fragments of nucleotides
identified by the multiple
primers.
Fragments are called Okazaki
fragments.
Both replicate in the 5’ to 3’
direction.
DNA Replication Procedure
DNA polymerase III is responsible for the placement
of nucleotides and fragments.
The DNA polymerase identifies the primer (starting
point) and works 5’ to 3’ to complete the two new
DNA strands.
DNA Replication Procedure
Another enzyme DNA polymerase I follows
DNA polymerase III.
DNA polymerase I remove the RNA primers
and replaces them with DNA nucleotides.
DNA Replication Procedure
DNA Ligase
Moves along the newly formed DNA strands
and joins the fragments together. 🡪 glue
The nick left by DNA polymerase I is glued to
the next DNA nucleotide.
DNA Repair
DNA polymerase I and III also act as
proofreading enzymes.
If a mistake is found they cut it out and replace
the incorrect nucleotide with the correct
nucleotide.
The 80/20 rule, also known as the Pareto Principle, suggests that roughly 80% of effects come from 20% of causes. When applied to the structure and replication of DNA, we can focus on the key aspects that yield the most understanding:
DNA Structure:
Key Components: The DNA model consists of two strands, a sugar-phosphate backbone, and nitrogen base pairing. The crucial aspects to remember are the four nitrogen bases (adenine, thymine, guanine, cytosine) and their pairing rules (A-T and G-C).
Model: Visualize DNA as a twisted ladder (double helix). The sides are the sugar and phosphate, and the rungs are the base pairs.
DNA Replication:
Key Process: Focus on semiconservative replication where each new DNA molecule consists of one original strand and one new strand. Understanding that DNA unzips and uses each strand as a template is fundamental.
Key Enzymes: The role of DNA polymerase III in adding nucleotides and DNA ligase in joining fragments together are crucial to the replication process.
By concentrating on these essential components, you can grasp the foundational concepts of DNA structure and its replication without getting overwhelmed by every detail