Download Chapter 12 The Cell Cycle

Cell division creates duplicate
offspring in unicellular
organisms (Asexual
Reproduction)
 Provides for growth,
development, and repair in
multicellular organisms
(Sexual Reproduction)
 The cell cycle extends from
the creation of a new cell by
the division of its parent cell
to its own division into two
cells

 The
process of recreating a cell necessitates
the EXACT duplication and equal division of
the DNA
 Each
diploid eukaryote has a characteristic
number of chromosomes in each somatic cell
 Reproductive cells, or gametes, have half
that number of chromosomes.
Diploid:
Haploid:
Mosquito 6
Frog 26
Corn 20
 Each
c’some is a
very long DNA
molecule with
associated
proteins. This
DNA-protein
complex is called
chromatin.
Before cell division, a
cell copies its DNA
and each c’some
densely coils and
shortens.
 Duplicated c’somes
consist of 2 identical
sister chromatids.
 A duplicated c’some
has a centromere
where the chromatids
are most closely
attached.

 The
sister chromatids separated during
mitosis (division of the nucleus)
 Then the cytoplasm divides during
cytokinesis, producing 2 separate,
genetically equivalent daughter cells.



Cell cycle (or life of the
cell) lasts for about 20
hours
There are 2 main stages
in the cycle of the
eukaryotic organism
The cell cycle consists
of the mitotic (M)
phase, which includes
mitosis and cytokinesis,
and interphase, during
with the cell grows and
duplicates its c’somes
 Usually
lasting 90% of the cell cycle
 Includes the G1 phase, the S phase, and the
G2 phase
 Gap 1 (G1) phase- growth period of the cell
 S (synthesis) phase- period when DNA
replication occurs
 Gap 2 (G2) phase- cell prepares for division
 Mitosis-
equal division of genetic information
(chromosomes) between the two new cells
(division of the nucleus)
 4 phases of M: prophase, metaphase,
anaphase, and telophase
1. Prophase- nuclear
envelope disintegrates;
nucleolus disappears;
and, the chromatin
fibers coil and fold into
visible
In cells that have
centrioles, such as
animals’ cells, the
centrioles replicate
 Filaments called
asters develop &
radiate from the
centrioles
 The asters lengthen
and become
connected to the
centromeres
 The asters are now
called spindle fibers

 Chromatids
become aligned at the center of
the cell or at its equator
 Centrioles have migrated to opposite sides of
the cell, called the poles
 The
centromeres separate and the daughter
chromosomes begin to migrate to opposite
poles
 This migration occurs as a result of a
shortening of the spindle fibers.
 Begins
when all chromosomes reach the 2
poles
 Spindle fibers begin to disappear and the
nuclear membranes begin to reappear
 Chromosomes begin to disappear as the
chromatin material disperses through the
new nuclei
 Cytokinesis
begins during telophase
 In animal cells the cytoplasm begins to pinch
inward
 In plant cells, a partition called a cell plate
begins to form and divides the cytoplasm
 One
special type of cell reproduction is for
organisms that produce offspring by sexual
reproduction
 Special reproductive cells must be produced
by each parent. These are the cells that
unite to form the new cell.
 Gamete-
special type of reproductive cell
that contains only one set of chromosomes
(sperm & egg)
 A special reproductive process that cells use
to produce gametes is called meiosis
(reduction division)
 Prophase
I- nuclear membrane disappears.
The homologous chromosomes (made up of 2
chromatids) pair up next to each other.
 At this time, genetic material can be
exchanged by a process called crossing over.
 Homologous
chromosomes move to the
equator of the cell
 Homologs
of each pair go to opposite poles
(chromatids remain attached at the
centromeres)
2
daughter cells results but the chromosome
number has not been halved which results in
a second division…Meiosis II