Transcript CELL DIVISION

CELL DIVISION
mitosis
•How many cells are you composed of?
•When an organism grows bigger do you get
more cells or just bigger cells or both?
•When do your cells divide the fastest?
Slowest?
•Do cells ever stop dividing?
•Are all cells capable of division and
replacement?
Why does a cell divide?
-As a cell absorbs nutrients and gets larger, the
volume of the cell increases faster than the
surface area.
-Therefore, the demands of the cell (the volume)
exceed the ability of the cell to bring in nutrients
and export wastes. Solution? Divide into two
smaller cells
When is cell division occurring?
GROWTH -increase number of cells
REPAIR -replace lost cells due to injury, disease
CANCER – Abnormally high rates of cell division
due to mutation
Different kinds of cells divide at different rates:
Yeast cell – 2 hours
Amoeba – a few days
Human embryo cell – 15-20 minutes
Human adult cell – 8 hours to 100 days
.
Aging
All cells die after a certain number of divisions
(programmed cell death). At any given time
some cells are dividing and some cells are dying.
Childhood
Cell division > cell death
Adulthood
Cell division = cell death
Aging
Cell division < cell death
Cell Cycle = 90% Interphase and 10% Mitotic Phase
THE CELL CYCLE:
3 phases: Interphase-Mitosis-Cytokinesis
Interphase- 90% of the time.
 G1: Little new cell absorbs nutrients and
grows larger. Does protein synthesis, its job.
 S phase: Synthesis of new DNA (DNA
replication) for daughter cells in preparation
for mitosis.
 G2: Cell continues to grow, does protein
synthesis, its job. Gets too large, needs to
divide.
How long is one cell cycle?
Depends. Eg. Skin cells every 24
hours. Some bacteria every 2 hours.
Some cells every 3 months. Nerve
cells, never. Cancer cells very short.
Programmed cell death: Each cell
type will only do so many cell cycles
then die. (Apoptosis)
MITOSIS
Equal distribution of the 2 sets of DNA
amongst the 2 daughter cells.
4 Stages:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
As cell enters mitosis
from interphase it has
2 complete sets of
chromosomes because
of replication in the S
phase. Each set must
be re-arranged and
distributed into the 2
new daughter nuclei.
This is mitosis.
Prophase
-Chromatin condenses (coils)
into chromosomes. Sister
chromatids joined by
centromere.
-Nuclear membrane dissolves.
-Centrioles divide and move to
opposite poles forming spindle
between them.
Metaphase
-Sister
chromatids line up on metaphase plate.
-Centromeres lock on to spindle fibre
Anaphase
-
Centromeres divide.
-Spindle fibres
contract pulling
sister chromatids
apart to poles.
Telophase:
-New nuclear membranes form around new nuclei
CYTOKINESIS
Cytoplasm splits into 2 cells.
-Animal cells: Cleavage furrow forms from
outside in.
-Plant cells: Division plate forms from inside
out.
Cell now returns to interphase .
The chromosomes uncoil back
into chromatin. The whole cell
cycle starts over again…..
At any point
in time the
cells in a
tissue will
be at
different
stages in
the cell
cycle.
The Guarantee of Mitosis…
Why is this so important?
The 2 daughter cells formed are identical
to each other and identical to the mother
cell.
2n = 46
4n = 92
2n = 46
In Mitosis, each daughter cell is exactly
the same as the original mother cell.
Bacteria Reproduce via Binary Fission
Mitosis is also an ASEXUAL
form of reproduction. These are
other examples of the uses of
mitosis to create new organisms
asexually:
Propogation of
plants by cuttings
Runners from
plants like
strawberries
Budding of Yeast
Homologous pairs of
chromosomes:
Each chromosome has a certain gene sequence on it.
Eg. Chromosome #! Has insulin, foot size, and
lactase on it. You have a chromosome one from
your mom and one from your dad. So you have 2
genes for each trait. One from your mom – one
from your dad. A homologous pair is a pair with
the same gene sequence – one from mom, one from
dad.