Transcript DNA polymerase

The amazing DNA
molecule is the
source of constancy
and variation among
species, and is the
foundation for the
unity and diversity of
all life on Earth.
Credit for the discovery of
DNA’s structure was given to
James Watson and Francis
Crick in 1953. They were
awarded the Nobel Prize in
1962 for this work.
DNA contains only four types
of nucleotides, which are the
building blocks of nucleic
acids.
Each nucleotide in DNA is
made of:
• a five carbon sugar Deoxyribose
• a phosphate group
• and one of four possible
nitrogen bases
The Brief History of DNA
• Discovery of the DNA double helix
Frederick Griffith – Discovers that a factor
in diseased bacteria can transform
harmless bacteria into deadly bacteria
(1928)
Rosalind Franklin - X-ray photo of DNA.
(1952)
Watson and Crick - described the
DNA molecule from Franklin’s X-ray.
(1953)
• Griffith: bacterial work; transformation:
change in genotype and phenotype due to
assimilation of external substance, which he
thought was protein, by a cell (1928)
• Avery: hereditary agent was DNA, not protein
group proved this by adding protein digesting enzymes
(1944) Avery’s
to the mix, and then DNA digesting enzymes, and observing
the reactions. Exposure to DNA digesting enzymes blocked
the transformation.
The
four
nitrogenous
bases in
DNA
are…
DNA Double Helix
5
When guanine and cytosine
bond, they form triple
hydrogen bonds
H-bonds
O
3
P
5
O
C
G
1
3
O
P
5
3
2
4
4
2
3
1
P
T
5
P
O
3
5
O
A
When thymine and
adenine bond, a
double hydrogen
bond is formed
P
3
O
5
P
There is no question of
the importance of DNA
in an organism, but in
most cases, DNA works
in conjunction with
another important
nucleic acid…RNA
The DNA molecule
serves as a template to
build the RNA molecule,
as you’ll see when we
discuss protein synthesis
next lesson. How does
the structure of these
two molecules vary?
Each nucleotide in RNA In the 2’ position, a Hydroxyl group
contains:
(Not present in the 2’ position in Deoxyribose)
• a five carbon sugar
Uracil is a
(ribose)
pyrimidine,
as you can
• a phosphate group
see from
• a nitrogenous base (all the same
ones as DNA, except the Pyrimidine the
structure
thymine is replaced with uracil.)
• Antiparallel nature: •
Sometimes called
“complementary”
sugar/phosphate backbone
runs in opposite directions
(Crick);
• one strand runs 5’ to 3’,
while the other runs 3’ to 5’;
The nucleotides connect at
the hydroxyl group of the 5
carbon sugar (at the 3’ end)
Very simply put, DNA functions in all living things as a
storehouse of information about heritable traits.
Within each chromosome lies
millions of nucleotides, and
the arrangement of their
respective nitrogenous base
pairs is the code for that
particular organism.
Remember, all life, from the simplest
prokaryote to the most complex
eukaryote, contains the same four
nitrogenous bases.
It is simply the sequence of base-pairs, and amount of
DNA that differs from organism to organism!
Think of DNA as the “keeper of the code”, or the “recipe” for the
organism that contains it. These banding patterns on chromosomes
represent “genes”. Genes are regions on chromosomes that code for
specific proteins.
While many parts of that code are important parts of the “recipe”, some
parts are simply “filler”, and are unnecessary, so far as we understand.
These unnecessary sections are called introns, and they are sequences
of DNA, within genes, which have no apparent purpose.
Within the cells of
multicellular animals
and plants almost
every gene has
introns, but they’re
fairly rare in
prokaryotes, and Exons are the working or
unicellular
expressed segments of genes.
eukaryotes.
Both terms, "introns" and "exons," were coined by Harvard biologist
Walter Gilbert in 1978
While DNA's function is to provide the code on which all
life is based, it does not accomplish this by itself. Some
RNA is fashioned from a DNA template, and is then moves
to different locations in the cell where proteins are made,
based on the information provided by the DNA. These
proteins are then used by the cells to accomplish various
tasks.
We have studied cellular reproductive
processes, such as mitosis and meiosis
already. During what stage of the cell
cycle did DNA replicate?
“S” Phase of Interphase!!!
Exactly how does this
happen?
The section that opens up, is
called a replication fork.
A portion of the DNA double helix
unwinds through the work of an
enzyme called a helicase.
• DNA polymerase only adds
nucleotides at the free 3’ end, forming
new DNA strands in the 5’ to 3’
direction only in a continuous fashion. On the other strand, assembly
is discontinuous because the
exposed –OH group is the only
DNA ligase then helps to join the
place where nucleotides can
fragments and wind the strands
be joined together.
back up into a double helix.
When DNA replication occurs, it begins somewhere in the
middle of the molecule, and not at one end. The
“replication fork” is within the bubble.
As the new molecules are built, they
progress in opposite directions from
the point of origin.
DNA Replication
• Semiconservative Model:
This model is referred to as semiconservative,
because half of every newly constructed DNA
molecule is old, and from the “parent” DNA.
1. Watson and Crick showed:
the two strands of the parental
molecule separate, and each
functions as a template for
synthesis of a new
complementary strand.
DNA Template
Parental DNA
New DNA
DNA Quiz
1. Why is replication necessary?
So both new cells will have the correct DNA
A---?
G---?
2. When does replication occur?
During interphase (S phase).
C---?
3. Use the complementary rule to
T---?
create the complementary strand:
A---?
G---?
4. What does ribose have, that
A---?
deoxyribose doesn’t?
G---?
A second hydroxyl group in the 2’ position
C---?
5. How can DNA be both the source A---?
G---?
of constancy and variety on Earth?
Because all organisms contain the exact same four T---?
nitrogenous bases, but their arrangement and amount
makes for the huge variety of life on Earth.