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DNA Replication in Eukaryotes 
BDue to the complexity of eukaryotic genomes, DNA replication is a very difficult process that 
involves numerous proteins and enzymes. Initiation, elongation, and termination are the three main phases that 
it goes through. 
Remember that the histone proteins that bind eukaryotic DNA together to form nucleosomes? The 
proteins and enzymes needed for replication are able to access the DNA during initiation. How does the 
replication apparatus determine where to start on the DNA double helix? It turns out that replication starts at 
specific nucleotide sequences known as origins of replication. Certain proteins bind to the origin of replication 
while an enzyme called 
helicase
unwinds and opens up the DNA helix. As the DNA opens up, Y-shaped 
structures called 
replication forks
are formed (Figure 41). At the beginning of replication, two forks form, and 
as replication progresses, these forks grow longer in both directions. The eukaryotic chromosome has multiple 
replication origins, allowing for simultaneous replication from various locations throughout the genome. 
During elongation, an enzyme called DNA polymerase adds DNA nucleotides to the 3' end of the 
template. Because 
DNA polymerase
can only add new nucleotides at the end of a backbone, a 
primer
sequence, which provides this starting point, is added with complementary RNA nucleotides. Later, the primer 
is taken out, and DNA nucleotides are used in its place. The polymerase can add nucleotides in this direction 

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because the complementary DNA strand is continuously synthesized toward the replication fork. This 
continuously synthesized strand is known as the 
leading strand
. Because DNA polymerase can only synthesize 
DNA in a 5' to 3' direction, the other new strand is put together in short pieces called