Download Transcript

DNA Replication
Slide 2
DNA replication is a process that all cells must go through prior to any
type of cell division. When cells replicate their DNA, they make two identical copies of
their DNA from the original copy. This process assures that the daughter cells resulting
from cell division will each have a complete copy of the DNA necessary for the cell to
survive. As we will see, DNA replication utilizes several different types of enzymes to
link free nucleotides together into new strands of DNA.
Slide 3
During DNA replication, DNA is unwound from its double helix, the two
strands are separated, and each strand serves as a template for the synthesis of a new,
complementary strand which is built one nucleotide at a time. Recall that when we use
the term ‘complementary,’ we are referring to the complementary base-pairing that
occurs between the individual strands of the double-stranded DNA molecule – A base
pairs with T, and C base pairs with G. In this way, two identical molecules of DNA are
synthesized from one original molecule, and each new double-stranded molecule contains
one parent strand and one newly synthesized strand of nucleotides. Because each new
molecule contains one of the original parent strands of DNA, the process of DNA
replication is referred to as semi-conservative replication.
Slide 4
Replication of DNA begins at a specific sequence of nucleotides in the
DNA called an origin of replication. The single, circular chromosomes of prokaryotic
organisms contain only one origin of replication. In eukaryotic organisms, however, there
are many origins of replication within each of their linear chromosomes.
Slide 5
Origins of replication serve as recognition and binding sites for a group of
enzymes that together form a replication complex. The replication complex is made up of
several enzymes, including DNA helicase, single-stranded binding proteins, RNA
primase, several types of DNA polymerase (there are at least 14 types in humans), and
DNA ligase. As DNA is replicated, it moves through this replication complex and each of
the enzymes in the complex play an important role in the synthesis of new DNA
molecules. As we describe the process of DNA replication in the following slides, note
how the names of each of the enzymes involved in this process are related to the role that
they perform – it may help you to remember them more easily.
Slide 6
When a replication complex binds to an origin of replication, one of the
initial events that occurs is the partial unwinding of the DNA double helix, and the
separation of the two strands of the DNA molecule. This is accomplished by both DNA
helicase and single-stranded binding proteins. DNA helicase unwinds the DNA double
helix and separates the strands of DNA by breaking the hydrogen bonds between the
complementary base pairs of each strand. Single-stranded binding proteins bind each
strand of DNA, and prohibit the strands from reforming hydrogen bonds.
The separation of the two strands results in what is called a replication bubble, which has
a replication fork at each end. DNA replication occurs at the replication forks, and will
proceed in both directions away from the origin of replication. Because of this, DNA
replication is often referred to as bidirectional.
Slide 7
After the strands of DNA have been separated by DNA helicase and
single-stranded binding proteins, DNA replication begins by the synthesis of short strands
of, surprisingly, RNA. These strands, called RNA primers, are complementary to the
template strands of DNA. The synthesis of RNA primers is catalyzed by the enzyme
RNA primase. RNA primers act as their name suggests, priming the synthesis of strands
of DNA, essentially like priming an engine with a small amount of gasoline. As we will
see, the enzyme that catalyzes the formation of new strands of DNA requires this primer
in order to start its work. At the end of DNA replication, however, the RNA primer is
degraded and replaced by DNA, through the activity of one type of DNA polymerase.
Slide 8
Once an RNA primer has formed, synthesis of a new strand of DNA may
begin. Remember that in the semi-conservative replication of DNA, each strand of the
original DNA molecule acts as a template for the formation of a new, complementary
strand. The synthesis of each new strand of DNA is catalyzed by DNA polymerase. DNA
polymerase lengthens the growing strand of DNA by adding nucleotides, one at a time, to
the 3’ end of the growing strand. In fact, DNA polymerase is only able to add nucleotides
in the 5’ to 3’ direction, and cannot build strands of nucleotides in the 3’ to 5’ direction.
Slide 9
As replication proceeds at a replication fork, the antiparallel nature of
DNA presents a small challenge. As the DNA is threaded through the replication
complex, one of the template strands will separate in the 3’ to 5’ direction, the other in
the 5’ to 3’ direction. From the template strand running in the 3’ to 5’ direction, synthesis
of a complementary strand will proceed smoothly and continuously, because DNA
polymerase is able to add nucleotides to the growing strand in the 5’ to 3’ direction. This
continuous strand is referred to as the leading strand. However, since DNA polymerase is
only able to add nucleotides in the 5’ to 3’ direction, synthesis of a new strand of DNA
complementary to the 5’ to 3’ template strand is discontinuous. In order to synthesize this
complementary strand, called the lagging strand, DNA polymerase synthesizes short
stretches of DNA in the 5’ to 3’ direction, then jumps ahead toward the replication fork to
synthesize another short fragment. These short fragments of DNA are called Okazaki
fragments. Note again that each fragment of the lagging strand is initially synthesized as
the leading strand of DNA is – initially an RNA primer is synthesized by RNA
polymerase, and DNA polymerase builds the strand by adding nucleotides to the primer
in the 5’ to 3’ direction.
Slide 10
As the fragments of the lagging strand are synthesized by DNA
polymerase, several other enzymes in the replication complex work together to anneal the
lagging strand into one, continuous strand of nucleotides. For example, after each
fragment is synthesized, its RNA primer is removed and replaced with DNA by another
type of DNA polymerase. The complete DNA fragments are then covalently linked
through the action of the enzyme DNA ligase.
Through the continuous synthesis of the leading strand, and the discontinuous synthesis
of the lagging strand, the entire molecule of DNA is eventually replicated so that two
exact copies of the DNA are made. These copies are eventually segregated during cell
division and distributed to daughter cells.