Cell Division
• Cell division functions in growth, repair, and
reproduction.
• Two types of cell division- mitosis and meiosis.
• Mitosis produces two genetically identical
daughter cells and conserves the chromosome
number (2n).
• Meiosis occurs in sexually reproducing
organisms and results in cells with half the
chromosome number of the parent cell (n).
Chromosomes
• A chromosome consists' of a highly
coiled and condensed strand of DNA.
• A replicated chromosome consists of
two sister chromatids, where one is
an exact copy of the other.
• The centromere is a specialized
region that holds the two chromatids
together.
• The kinetochore is a disc-shaped
protein on the centromere that
attaches the chromatid to the
mitotic spindle during cell division.
Two factors limit cell size and
promote cell division
• ratio of the volume of a cell to the surface
area
• capacity of the nucleus to control the entire
cell
The nucleus must be able to provide enough
information to produce adequate quantities to
meet the cell's needs.
Phases of the Cell Cycle
• consists of five major phases: G1, S, and G2
(which together make up interphase), mitosis,
and cytokinesis.
Interphase
• Consists of G1, S, and G2.
• The G1 is a period of intense growth and
biochemical activity.
• S is the synthesis or replication of DNA.
• G2 is the phase when the cell continues to grow
and to complete preparations for cell division.
• More than 90 percent of the life of a cell is
spent in interphase.
• During interphase chromatin is threadlike, not
condensed.
• Within the nucleus are one or more nucleoli.
• A centrosome, consisting of two centrioles,
can be seen in the cytoplasm of an animal cell.
• Plant cells lack centrosomes but have
microtubule organizing centers
Prophase
• The nuclear membrane begins
to disintegrate.
• The strands of chromosomes
begin to condense into
discrete observable structures.
• The nucleolus disappears.
• In the cytoplasm, the mitotic
spindle begins to form,
extending from one
centrosome to the other.
• Prophase is the longest phase.
Metaphase
• The chromosomes line up in a
single file located on the
equator or metaphase plate.
• Centrosomes are at opposite
poles of the cell.
• Spindle fibers run from the
centrosomes to the
kinetochores in the
centromeres.
Anaphase
• Centromeres of each
chromosome separate, as
spindle fibers pull apart the
sister chromosomes.
• This is the shortest phase of
mitosis.
Telophase
• Chromosomes cluster at
opposite ends of the cell, and
the nuclear membrane
reforms.
• The supercoiled
chromosomes begin to
unravel and to return to their
normal, pre-cell division
condition as long, threadlike
strands.
• Once two individual nucleoli
form, mitosis is complete
CYTOKINESIS
• consists of the dividing
of the cytoplasm. It
begins during anaphase.
• In animal cells, a
cleavage furrow forms
down the middle of the
cell as actin and myosin
microfilaments pinch in
the cytoplasm.
• In plant cells, a cell plate
forms during telophase.
CANCEROUS CELLS
• Normal cells grow and divide until they become too
crowded; then they stop dividing and enter Go (G zero).
• This reaction to overcrowding is called contact inhibition
or density-dependent inhibition.
• Another characteristic of normal animal cells is anchorage
dependence. To divide, a cell must be attached or anchored
to some surface, such as a Petri dish (in vitro) or an
extracellular membrane (in vivo).
• Cancer cells show neither contact inhibition nor anchorage
dependence. They divide uncontrollably and do not have to
be anchored to any membrane. That is why cancer cells can
migrate or metastasize to other regions of the body.
MEIOSIS
• Meiosis is a form of cell division that
produces gametes (sex cells, or sperm
and ova) haploid chromosome number
(n).
• There are two stages in meiosis.
i.
Meiosis I (reduction division) is the
process by which homologous
chromosomes separate.
ii. Meiosis II is like mitosis. In meiosis I,
each chromosome pairs up precisely
with its homologue by a process called
synapsis and forms a structure known
as a tetrad.
• Synapsis is important for two reasons.
i.
It ensures that each daughter cell will
receive one homologue from each
parent.
ii.
It makes possible the process of
crossing-over by which homologous
chromatids exchange genetic material.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Meiosis I
PROPHASE I
Synapsis, the pairing of homologues, occurs.
Crossing-over, the exchange of homologous bits of
chromosomes, occurs.
Chiasmata, the visible manifestations of the crossover events, are visible.
This is the longest phase.
METAPHASE I
The homologous pairs of chromosomes are lined up
double file along the metaphase plate.
Spindle fibers from the poles of the cell are
attached to the centromeres of each pair of
homologues.
ANAPHASE I
Homologous chromosomes are separated as they
are pulled by spindle fibers and migrate to opposite
poles.
TELOPHASE I
Homologous pairs continue to separate until they
reach the poles of the cell. Each pole has the
haploid number of chromosomes.
CYTOKINESIS I
Cytokinesis usually occurs simultaneously with
telophase I.
Meiosis II
• Meiosis II is functionally
the same as mitosis and
consists of the same
phases:
• prophase, metaphase,
anaphase, telophase,
and cytokinesis.
MEIOSIS AND GENETIC VARIATION
Three types of genetic variation result from the
processes of meiosis and fertilization.
• independent assortment of chromosomes,
• crossing-over
• random fertilization of an ovum by a sperm
Independent Assortment of
Chromosomes
• During meiosis, homologous pairs of chromosomes
separate depending on the random way in which they
line up on the metaphase plate during metaphase I.
• Each pair of chromosomes can line up in two possible
orientations. There is a 50 percent chance that a
particular gamete will receive a maternal chromosome
and a 50 percent chance it will receive a paternal
chromosome.
• Given that there are 23 pairs of chromosomes in
humans, the number of possible combinations of
maternal and paternal chromosomes in each gamete is
223, or about 8 million.
Crossover
• Crossover produces recombinant chromosomes
that combine genes inherited from both parents.
• For humans, an average of two or three
crossover events occur in each chromosome pair.
• In addition, at metaphase II, these recombinant
chromosomes line up on the metaphase plate in
random fashion.
• This increases the possible types of gametes
even more.
Random Fertilization
• One human ovum represents one of
approximately 8 million possible chromosome
combinations.
• The same is true for the human sperm. Thus,
when one sperm fertilizes one ovum, 8 million
x 8 million recombinations are possible.
THE CELL CYCLE
• A cell cycle control system regulates the rate at
which cells divide.
• Several checkpoints act as built-in stop signals
that halt the cell unless they are overridden by
go-ahead signals.
• Three checkpoints exist in G1, G2, and M.
• The G1 checkpoint is known as the restriction
point and is the most important one in mammals.
If it receives a go-ahead, the cell will most likely
complete cell division. On the other hand, if it
does not get the appropriate signal, the cell will
exit the cycle and become a non-dividing cell
arrested in the Go (G zero) phase.
The Cell cycle cont.
•
•
a)
b)
c)
d)
Since the activity of a cell varies, the rate at which it needs to
divide also varies. This timing of cell division is controlled by two
kinds of molecules: cyclins and cyclin-dependent kinases or CDKs.
The first CDK discovered was MPF, which stands for M-phase
promoting factor. In humans, the frequency of cell division varies
with the cell type.
Bone marrow cells are always dividing in order to produce a
constant supply of red and white blood cells.
Cells in the human intestine normally divide twice per day to
renew tissue that is destroyed during digestion.
Nerve and muscle cells are arrested in Go and do not divide or
regenerate at all, hence the great danger from spinal cord injury.
Liver cells are also arrested in Go but can be induced to divide
when liver tissue is damaged.