Chromosomes, the Cell Cycle, and Cell Division
A.
The Eukaryotic Cell Cycle
Stages in the life of an actively dividing
eukaryotic cell:
 G1
 S
 G2
 M
Stationary phase (G0): Nondividing cells
The cell cycle is controlled predominately at
two key regulatory points: between G1 and S,
and between G2 and M.
B.
Interphase and Mitosis: An Overview
 During G1: Chromosomes are uncondensed
and unreplicated
 During S: Chromosomal (DNA) replication
takes places but the two copies do not
separate. By the end of S, the chromosomes
are in an uncondensed, replicated state.
Each chromosome consists of a pair of
sister chromatids connected at a centromere
 During G2: Chromosomes are uncondensed,
replicated. The cell makes the proteins it
needs for mitosis.
 During Mitosis: The chromosomes
condense and become visible; the sister
chromatids separate and become daughter
chromosomes, the daughter chromosomes
move to opposite poles of the cell; the
daughter chromosomes decondense.
Usually after mitosis, the cell divides. By the
end of mitosis, the chromosomes have
returned to an uncondensed, unreplicated
state (G1).
C.
Stages of Mitosis
1.
Prophase
 Chromosomes condense and become
visible
 Nuclear membrane disperses
 Spindle apparatus begins to form
2.
Metaphase
 Chromosomes align on equator of the
spindle apparatus
3.
Anaphase
 Chromatids separate and become
daughter chromosomes
 Daughter chromosomes move to
opposite poles of the cell
4.
Telophase
 At each pole of the spindle, the
chromosomes decondense and form
new nuclei
 New nuclear membranes form
 Usually, cytokinesis occurs at this time.
D.
Meiosis: An Overview
 Function of meiosis: To produce gametes
 In sexually reproducing organisms
 Meiosis takes place only in germ-line cells,
found in the gonads.
 During meiosis, the number of
chromosomes is halved, so that a gamete
has half the number of chromosomes as the
original germ-line cell.
 Diploid number: The number of
chromosomes in the original germ line cell
(for example, 46 chromosomes in humans)
 Haploid number: The number of
chromosomes in the gamete (for example,
23 chromosomes in humans)
 During meiosis, homologous chromosomes
separate
 Homologous chromosomes are similar
chromosomes, each from a different parent,
in sexually reproducing species
E.
Prokaryotic Chromosomes and Division
 Prokaryotic cells (bacteria) reproduce by
simple fission: the chromosome is
replicated and then the cell divides.