Cell cycle, multiple replicones per chromosome.

Cell cycle, multiple replicones per chromosome

1. THE CELL CYCLE
«When bacteria are growing on rich
media, DNA replication occurs
nonstop throughout the cell cycle.

«However, in eukaryotes, DNA
replication is restricted to the S
phase (for synthesis).

2. «Recall that a normal eukaryotic cell
cycle consists of G, phase (immediately
following the completion of mitosis; G
for gap), S phase, G.phase (preparation
for mitosis), & M phase (mitosis).

«In rapidly dividing embryonic cells, G.&
G. are very,short.or nonexistent.

3. «In all cells, decisions to continue
on through the cell cycle occur at
two points:

»(1) entry into S phase &

«(2) entry into mitosis.

«These checkpoints help to ensure
cates once & only
cell division.

6. MULTIPLE REPLICONS
PER CHROMOSOME
«The giant DNA molecules in the largest
chromosomes of Drosophila melano-

gaster contain about 6.5 10,nucleotide
pairs.

«The rate of DNA replication in
Drosophila is about 2600 nucleotide
pairs per minute at 25C.

7. -A single replication fork would
therefore take about 17.5 days to
replicate one of these giant DNA
molecules.

«With two replication forks moving
bidirectionally from a central origin,
such a DNA molecule could be
replicated in just over 8.5 days.

8. *Indeed, the complete replication of
the DNA of the largest Drosophila
chromosome within 3.5 minutes

«would require over 7000 replication
forks distributed at equal intervals
along the molecule.

9. ‘Thus, multiple origins of
replication are required

«to allow the very large DNA
molecules in eukaryotic

chromosomes
sto replicate within the observed

10. «The first evidence for
multiple origins in eukaryotic
chromosomes

resulted from pulse-labeling
experiments with Chinese
hamster cells growing in
culture.

11. «In 1968, when Joel Huberman and
Arthur Riggs pulse-labeled cells
with H-thymidine for a few min,

* extracted the DNA, 8 performed

auto-radiographic analysis of the
labeled DNA,

«They observed tandem arrays of
exposed silver grains (Fig.31a).