BIOL 114
Biology Dept
Kenyon College
Chapter 1D.  Cell Division
Fall SectionSpring Section 1Spring Section 2

Cell division in Bacteria; in Eukaryotes

For reproduction, all cells need to copy their chromosomes exactly and pass on an identical copy to each daughter cell. (Of course rare "mistakes" occur -- cells actually have evolved to favor rare mistakes. Why?)

Two different mechanisms do this in cells:

Bacterial cell fission, in which the circular chromosome is replicated.

Eukaryotic cell cycle, including Mitosis, in which multiple linear chromosomes are separated and passed on.

Bacterial Cell Division

Replication of the chromosome starts at the origin attached to the cell wall, near the midpoint of the cell. Replication occurs bidirectionally around the chromosome, as the cell elongates. New replicating forks can start before the first cell division takes place; this phenomenon allows an extremely rapid rate of reproduction.

Image 69091JWA from Dennis Kunkel Microscopy, Inc.

DNA starts replication at the origin, and two DNA polymerase complexes proceed in opposite directions.  Each complex (called a replisome) contains two whole DNA polymerases (WHY?).
So at the origin, that's four DNA polymerases in all.

Before the DNA circle has completed replication a second round of replication can begin.  Thus two "generations" of DNA replication occur simultaeously.

Can this ever happen in eukaryotes?  Why or why not? 

Eukaryotic Cell Division

In eukaryotes, DNA replication actually occurs in S phase of interphase. 
Stages of interphase include: 
  • G1, growth
  • S, semiconservative synthesis of DNA
  • G2, preparation for mitosis. 
Mitosis only separates the newly replicated chromosomes; DNA replication does not occur during mitosis.

The big problem with eukaryotes is that they have to replicate linear chromosomes with special ends called telomeres.   To do this, they need to use a special enzyme called telomerase, actually related to the reverse transcriptase of HIV virus.  Telomerase activity may play a crucial role in human aging; if the chromosome ends fail to replicate properly, the chromosomes gradually lose parts of their end sequence.

For more information on telomerase, go here.
Telomerase gene can extend life of human cells, perhaps preventing aging!  (Or will it cause virulent cancer?)

Eukaryotic Cell Division--Mitosis Click on picture for stages:
 
 

 week01.htm
Trent University, Biology Department

Mitosis -- The Movie

Meiosis:
Division to produce sex cells

T. A. Brown, Genomes, BIOS

What happens to chromosome copy number (ploidy) during DNA replication followed by mitosis or meiosis?
Mitosis: 2N -> 4N -> 2N
Meiosis: 2N -> 4N -> 2N -> 1N

Problems:

(1) Bacteria can divide TWICE in the time it takes to complete replication of
their entire circle of DNA.  (This is one reason kids get sick so fast after
eating E.coli-contaminated hamburger.)
How is this possible?  Can animal cells do the same thing?  Why or why not?

(2) Suppose that in a field of cells in tissue culture, about five percent of cells show the condensed chromosomes of mitosis. If the duration of mitosis is five minutes, what is the overall generation time of the cells?

 Solutions
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