As compared to eukaryotes, nucleotide addition during DNA replication occurs almost 20 times faster in prokaryotes. DNA (deoxyribonucleotide), a.k.a. the secret code of life, is a molecule that possesses all the information that is required at every stage in the life cycle of an organism. DNA replication is the way to ensure that this information is passed down to every newly formed cell, be it a prokaryotic or eukaryotic cell.Although the basic process of DNA replication remains the same, certain differences have evolved due to the higher genomic complexity of eukaryotes. The same have been outlined below.
Location
Prokaryotes do not have nucleus and other
membrane-bound organelles, like mitochondria, endoplasmic
reticulum, and golgi bodies. The prokaryotic DNA is present as a
DNA-protein complex called nucleoid. The replication occurs in the
cytoplasm of the cell.
In case of eukaryotes, the organisms that contain
a membrane-bound nucleus, the DNA is sequestered inside the
nucleus. Hence, the nucleus is the site for DNA replication in
eukaryotes.
Stage of Cell Division
In prokaryotes, DNA replication is the first
step of cell division, which is primarily through binary fission or
budding.
In eukaryotes, cell division is a comparatively
complex process, and DNA replication occurs during the synthesis
(S) phase of the cell cycle.
Initiation
DNA replication is initiated at a specific or unique sequence
called the origin of replication, and ends at unique termination
sites. The region of DNA between these two sites is termed as a
replication unit or replicon.
Prokaryotic DNA is organized into circular
chromosomes, and some have additional circular DNA molecules called
plasmids. The prokaryotic DNA molecules contain a single origin of
replication and a single replicon. Moreover, these origin sites are
generally longer than eukaryotic origin sites.
Eukaryotic DNA is comparatively very large, and is
organized into linear chromosomes. Due to the high amount of
material to be copied, it contains multiple origins of replication
on each chromosome. DNA replication can independently initiate at
each origin and terminate at the corresponding termination sites.
Thus, each chromosome has several replicons, which enable faster
DNA replication. The human genome that comprises about 3.2 billion
base pairs gets replicated within an hour. If DNA replication was
dependent on a single replicon, it would take a month's time to
finish replicating one chromosome.
Direction of Replication
Once initiated, DNA replication assembly proceeds along the DNA
molecule, and the precise point at which replication is occurring
is termed as the replication fork. Generally, in both prokaryotes
and eukaryotes, the process of DNA replication proceeds in two
opposite directions, from the origin of replication.
However, in certain plasmids present in bacterial cells,
unidirectional DNA replication has been observed. These plasmids
replicate through the rolling circle model, wherein multiple linear
copies of the circular DNA are synthesized and then
circularized.
Enzymes
Although a similar set of enzymes are involved in prokaryotic and eukaryotic DNA replication, the latter one is more complex and varied. The initiator proteins, single-stranded DNA-binding protein (SSB), primase, DNA helicase, and DNA ligase are present in both prokaryotes and eukaryotes.
Enzymes specific to prokaryotes:
Enzyme | Activity |
DNA Polymerase I | 5' to 3' polymerase, 3' to 5' exonuclease, 5' to 3' exonuclease |
DNA Polymerase III | 5' to 3' polymerase, 3' to 5' exonuclease |
Enzymes specific to eukaryotes:
Enzyme | Activity |
DNA polymerase α | 5' to 3' polymerase |
DNA polymerase δ | 5' to 3' polymerase, 3' to 5' exonuclease |
DNA polymerase ε | 5' to 3' polymerase |
In addition, eukaryotes contain DNA polymerase γ, which is involved in mitochondrial DNA replication. Also, the topoisomerases, enzymes that regulate the winding and unwinding of DNA during the movement of replication fork, differ in their activity. Prokaryotes, generally use type II topoisomerase called DNA gyrase, that introduces a nick in both the DNA strands. On the contrary, most eukaryotes utilize type I topoisomerases, that cut a single strand of DNA, during the movement of the replication fork.
Okazaki fragments
During DNA replication, the synthesis of one strand occurs in a
continuous manner, whereas that of the other strand occurs in a
discontinuous manner through the formation of fragments. The former
strand is termed as the leading strand, the latter as the lagging
strand, and the intermediate fragments are termed as the Okazaki
fragments. The reason for such a difference is the antiparallel
nature of DNA strands, as against the unidirectional activity of
the DNA polymerase.
Prokaryotic Okazaki fragments are longer, with the
typical length observed in Escherichia coli (E. coli)
being about 1000 to 2000 nucleotides.
The length of eukaryotic Okazaki fragments ranges
between 100 and 200 nucleotides. Although comparatively shorter,
they are produced at a rate slower than that observed in
prokaryotes.
Termination
The termination of DNA replication occurs at specific
termination sites in both prokaryotes and eukaryotes.
In prokaryotes, a single termination site is
present midway between the circular chromosome. The two replication
forks meet at this site, thus, halting the replication
process.
In eukaryotes, the linear DNA molecules have
several termination sites along the chromosome, corresponding to
each origin of replication. However, the eukaryotic DNA replication
is characterized by a unique end-replication problem, wherein a
part of DNA present at the ends of the chromosome does not get
replicated. So, the lagging strand is shorter than the leading
strand. This problem is addressed in eukaryotes by the presence of
non-coding, repetitive DNA sequence called telomeres, at the ends
of chromosomes.
Compare and contrast Prokaryotic and Eukaryotic DNA replication For exam you must be able to describe...
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