Problem of linear templates
• Since a primer is required, how do you initiate
replication at the 5’ terminus of a DNA chain?
• How do you prevent progressive loss of DNA from
the ends after replication?
Solutions to the problem of linear templates
• Convert linear to circular DNA
• Attach a protein to 5’ end to serve as primer
• Make the ends repetitive, e.g. telomeres,
and add more DNA after replication
Telomerase adds repeats back to
Telomerase adds more
+ copies of "a’" to 3’ end of
aa strand with overhang
The segment complementary to the aaa a’a’a’a’
3’ end of template is not replicated.
a = CCCCAA,
a’ = GGGGTT a’a’a’a’
Replicated telomeres are primers for
(G T )n
(C A )
4 2 n-2 or 3
shorten the C+A rich ("bottom") strand after replication
Telomerase adds 1 nt at a time, using an
internal RNA template
Telomerase adds new copies of short repeats to the 3' end of the G+T rich ("top")
strand, using an internal RNA as the template.
telomerase 5' RNA
Add 1 nucleotide at a time
Finish synthesis of one repeat,
and shift over to synthesize the
Telomeric repeats form a primer for
synthesis of the complementary strand
After addition of more repeats, some specialized structure forms at the 3' end of
the G+T rich strand involving G-G base pairs, e.g. a hairpin or a "G quartet."
Presumably this structure serves as the primer for synthesis of the C+A rich strand.
Processing (as yet unidentified) would yield the extended telomere, with a
short overhap by the G+T rich strand.
Control of replication in bacteria
• Bacteria re-initiate replication more frequently
when grown in rich media.
– Doubling time of a bacterial culture can range
from 18 min (rich media) to 180 min (poor media).
• Time required for replication cycle is constant.
– C period
• time to replicate the chromosome; 40 min
– D period
• time between completion of DNA replication
and cell division; 20 min
– C + D = 1 hour
Multiple replication forks allow shorter
• Doubling time for a culture can vary, but
time for replication cycle is constant!
• Variation is accomplished by changing the
number of replication forks per cell.
• If doubling time of culture is < 60 min, then a
new cycle of replication must initiate before
the previous cycle is completed.
• Initiate replication at same frequency as cell
doubling, e.g. every 30 min.
E.g. every 30 min:
Cell cycle in eukarytoes
Preparation DNA synthesis
G0 G1 G2
cells 2nDNA 4nDNA
Multiple replicons per chromosome
• Many replicons per chromosome, with many
• Replicons initiate at different times of S
• Replicons containing actively transcribed
genes replicate early, those with non-
expressed genes replicate late.
Regulation at check-points
• Critical check-points in the cell cycle are
– G1 to S
– G2 to M
• Passage is regulated by environmental
signals acting on protein kinases
– e.g., if enough dNTPs, etc for synthesis are
available, then a signal activates a multi-
subunit, cyclin-dependent protein kinase.
– Increased amount of cyclin
– Correct state of phosphorylation of the kinase
More about cell cycle regulation
• BMB 460: Cell growth and differentiation
• BMB 480: Tumor viruses and oncogenes
• BMB/VSC 497A: Mechanisms of cellular