Slide 1
DNA
replication is the process of replicating a chromosome - this occurs prior to
cell division and is the process that creates the sister chromatids.
Slide
2
Recall from our discussions of mitosis and meiosis that before a cell
divides it must make a copy of each chromosome. In the image on this
slide we see on the left the unreplicated chromosome and on the right the
replicated chromosome comprised of two identical sister chromatids. DNA
replication is the process that creates the identical sister chromatids.
Slide
3
In order to understand the process of DNA replication, we must understand
the structure of the DNA molecule. As mentioned in the previous
tutorial, a DNA molecule is double stranded molecule that is held togther by
hydrogen bonds between nitrogenous bases of the two strands.
Slide 4
The
hydrogen bonds between the two nitrogenous bases are specific and only form
between an adenine and thymine and a guanine and cytosine - these are the DNA
base pairing rules.
There is another rule to the way the two DNA strands pair. Each strand of DNA has a 5' end and a 3' end. The 5' and 3' refer to the numbering of the carbons in the sugar molecule of the nucleotides. The top nucleotide on the left of this image shows the numbering of the carbons. On the strand on the left, the 5' carbon is oriented toward the 5' end of the molecule. The 3' end of the molecule is oriented toward the 3' sugar. The strand on the right has the opposite orientation - the 3' end is toward the top of the molecule and the 5' end is toward the bottom. Whenever nucleic acids base pair together - the 3' end of one strand must be aligned with the 5' end of the other. This rule will be very important when we discuss both DNA replication and protein synthesis.
Slide 5
The process of DNA
replication takes place within the nucleus (the DNA in our cells is mainly in
the nucleus). In order for a cell to build a new molecule of DNA it
needs some raw materials.
First, there needs to be a template or original molecule of DNA. During DNA replication, both strands of DNA are copied.
There also needs to be a supply of nucleotides to build the new DNA.
Finally, there needs to be enzymes to actually do the job of building the molecule. Enzymes are proteins that perform jobs within cells. The enzymes that do the job of replicating DNA are helicase, primase, DNA polymerase, and Ligase. We can tell by the names that these are all enzymes because they all end in the suffix "ase". Most enzymes are named this way.
DNA polymerase is the enzyme that actually builds the DNA polymer. It has one very important limitation - it can only add nucleotides to the 3' end of the newly synthesized DNA strand. We will see why this is important shortly.
Slide 6
This image shows most of
the details about DNA replication. Note that this image shows the
overall direction of replication - this means that parental DNA molecule is
unwinding from right to left.
Before the DNA can be replicated - the two strands must unwind and separate from each other. It is the job of helicase to unwind the parental strands and proteins known as single-strand binding proteins anchor the two strands so they cannot reattach to each other.
Slide
7
To begin replication, the process must be primed. This means that
an existing piece of nucleic acid must be placed along the original DNA to
start the process of replication. The enzyme primase is responsible for
priming the process. It places a small RNA primer along the original
DNA. Each strand is being copies so each strand requires primers.
The reason the process requires a primer is because DNA polymerase cannot start producing new DNA from scratch. DNA polymerase can only add to an existing piece of nucleic acid.
Slide 8
DNA polymerase adds
nucleotides to the 3' end of the newly placed primer and continues to add
nucleotides until the entire molecule has been replicated. Each strand
is replicated so DNA polymerase is working along each strand. Because
the two DNA strands are antiparallel DNA polymerase cannot add continously
along both strands. Along the top strand - referred to as the leading
strand, the newly synthesized DNA is oriented in the 5' to 3' direction.
So DNA polymerase can happily go along adding nucleotides in the
direction of replication in a continous fashion.
Along the bottom strand - known as the lagging strand, DNA polymerase has to work opposite to the overall direction of replication. This is because DNA polymerase can only add nucleotides to the 3' end of the new DNA. The result is that along the lagging strand DNA gets produced in discontinous fragments known as Okazaki fragments. Each fragment requires its own primer so primase has to work multiple times along the lagging strand.
Slide 9
The
final enzyme to work is ligase. Its job is to connect the Okazaki
fragments together into one seamless piece of DNA. First the RNA primers
are removed and then the fragments are sealed together. The result is
that at the end of DNA replication - an entire chromosome has been copied to
create an identical sister chromatid.
Slide 10
The next
three slides are practice problems. Work them through and then access
the next Voice Thread to see the solutions.