DNA replication is the process of duplicating a cell's genome,
required at each cell division. Replication,
like all cellular activities, requires specialized proteins for carrying out the job. In the first step of replication, a special protein,
called a helicase, unwinds a portion of the parental DNA double helix. Next, a molecule of DNA polymerase – a common name for
two categories of enzymes that influence the synthesis of DNA – binds
to one strand of the DNA. DNA polymerase begins to move along the DNA strand
in the 3' to 5' direction, using the single-stranded DNA as a template.
This newly synthesized strand is called the leading strand and is necessary
for forming new nucleotides and reforming
a double helix. Because DNA synthesis can only occur in the 5' to 3' direction,
a second DNA polymerase molecule is used to bind to the other template strand
as the double helix opens. This molecule synthesizes discontinuous segments
of polynucleotides, called Okazaki fragments. Another enzyme, called DNA
ligase, is responsible for stitching these fragments together into what
is called the lagging strand.
|Before a cell can divide, it must
first duplicate its DNA. This figure provides an overview of the DNA
replication process. In the first step, a portion of the double helix
(blue) is unwound by a helicase. Next, a molecule of DNA polymerase
(green) binds to one strand of the DNA. It moves along the strand,
using it as a template for assembling a leading strand (red) of nucleotides
and reforming a double helix. Because DNA synthesis can only occur
5' to 3', a second DNA polymerase molecule (also green) is used to
bind to the other template strand as the double helix opens. This
molecule must synthesize discontinuous segments of polynucleotides
(called Okazaki Fragments). Another enzyme, DNA Ligase (yellow), then
stitches these together into the lagging strand.
The average human chromosome contains
an enormous number of nucleotide pairs that are copied at about 50 base
pairs per second. Yet, the entire replication process takes only about an
hour. This is because there are many replication origin sites on a eukaryotic
chromosome. Therefore, replication can begin at some origins earlier than
at others. As replication nears completion, "bubbles" of newly replicated
DNA meet and fuse, forming two new molecules.
With multiple replication origin sites, one might ask, how does the cell
know which DNA has already been replicated and which still awaits replication?
To date, two replication control mechanisms have been identified: one positive
and one negative. For DNA to be replicated, each replication origin site
must be bound by a set of proteins called the Origin Recognition Complex.
These remain attached to the DNA throughout the replication process. Specific
accessory proteins, called licensing factors, must also be present for initiation
of replication. Destruction of these proteins after initiation of replication
prevents further replication cycles from occurring. This is because licensing
factors are only produced when the nuclear membrane of a cell breaks down
Source: National Center for Biotechnology