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A principal difference between prokaryotic and eukaryotic
DNA replication is
a) completely different proteins/enzymes in eukaryotes
b) multiple origins in eukaryotes
c) no requirement for topoisomerase activity in prokaryotes
d) the absence of a nucleus in prokaryotes
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Answer Posted / ravi p
Replication in Prokaryotes
Initiation: DNA replication starts at oriC with three
repeats of a 13 bp sequence and four repeats of a 9 bp
sequence. A single complex of four to five DnaA protein
molecules binds to the four 9 bp repeats and successively
denatures DNA in region of three 13 bp repeats. DnaC
protein then loads DnaB protein onto unwound region. Two
ring shaped hexamers of DnaB, one loaded onto each DNA
strand, act as helicases, unwinding DNA bidirectionally and
creating two potential replication forks.
Dna Replication in Eukaryotes
Eukaryotic DNA replication proceeds in same way as that of
prokaryotic DNA replication except for following aspects:
1. Eukaryotic DNA has multiple “origin of replication”
sites, so several replication forks create many bubbles
along DNA length. These replication forks are formed at
autonomously replicating sequences (ARS) that contain
degenerate 11-bp sequences known as origin replication
element (ORE). ORE is located adjacent to an 80-bp AT rich
sequence that is easy to unwind.
2. DNA polymerases α and δ are main DNA replication enzymes
in eukaryotic cell. DNA polymerase α has 5’---> 3’
polymerase activity and synthesize primer on lagging strand
which are then extended by multisubunit DNA polymerase. DNA
polymerase δ has a 3’ à5’ proofreading exonuclease activity
and carry out both leading and lagging strand synthesis in
a complex comparable to dimeric bacterial DNA polymerase
III.
DNA polymerase ε removes primers of Okazaki fragments on
lagging strand. DNA polymerase γ is responsible for
replication of mt DNA.
3. Telomeres, structures at ends of linear eukaryotic
chromosomes, consist of many tandem copies of a short
oligonucleotide sequence with TxGy in one strand and CyAx
in complementary strand, where x and y are typically in
range of 1 to 4. Telomerase contains RNA that act as a
template for synthesis of TxGy strand of telomere.
Protein component of telomerase act as a cellular reverse
transcriptase for RNA-dependent DNA synthesis. After
extension of TxGy strand by telomerase, complementary CyAx
strand is synthesized by cellular DNA polymerases, starting
with an RNA primer.
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