Download DNA polymerase

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) He called this the transforming
principle, and thought it was a protein
responsible for this transference.
Rosalind Franklin – Used X-ray
crystallography photo of DNA. The
discovery of its structure helped
scientists understand how genetic
information was passed from parents to
children (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.
During the cellular reproductive processes, such as mitosis
and meiosis, DNA must replicate itself. Replication happens
during the “S” or synthesis phase of interphase of the cell
cycle.
Exactly how does
replication happen?
First, a portion of the DNA double helix
unwinds through the work of an enzyme called
helicase, which serves to break the hydrogen
bonds between the nitrogenous bases.
The section that
opens up in the
DNA molecule is
called a replication
fork.
As this replication fork opens
up, it exposes the 3’ and the 5’
ends of the original DNA
strand. In this diagram, that is
represented by the blue
molecule.
• An enzyme known as DNA
polymerase adds nucleotides to the
original strand’s free 3’ end, in a
continuous fashion. The new DNA
strand is therefore forming from a 5’
direction, and is referred to as the
“leading strand”.
• On the open 5’ end, the new
Assembly is discontinuous because
DNA is built in a discontinuous the exposed –OH group on the 3’
fashion, from the region nearest end is the only place where
the fork opening. This new strand nucleotides can be joined together.
is known as the “lagging strand”. DNA ligase then helps to join the
Okazaki fragments are formed, or fragments and wind the strands back up
little “chunks” of DNA.
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.
Parental DNA
DNA Template
New DNA
When the process is complete, two DNA molecules
have been formed identical to each other and to
the parent molecule
Errors during replication are rare, as each cell contains a
family of more than thirty enzymes to ensure the accurate
replication of DNA
DNA polymerase makes very few errors, and most of those
that are made are quickly corrected by DNA polymerase
which "proofreads" the nucleotides added into the new
DNA strand. If a non-complementary nucleotide is added
in error, enzymes remove the nucleotide and replace it with
the correct one. A cell’s DNA is copied with less than one
mistake in a billion nucleotides!
This is equal to a person copying 100 large (1000 page) dictionaries word
for word, and symbol for symbol, with only one error for the whole
process!