Download Why do cells divide?

Why do cells divide?
 Cells
divide in order to make
more cells – they multiply in
order to create a larger surface
to volume ratio!!!
Chromosomes
 Are
made of chromatin: a mass of
genetic material composed of DNA
and proteins (called histones) that
condense to form chromosomes in
the cell's nucleus.
What exactly are
chromosomes?
Structures within a cell that carry genetic
material in the form of threadlike strands
of DNA bonded to histones in the nucleus
of eukaryotic cells
 Rod-shaped
 Each chromosome consists of two
indentical halves called sister
chromatids which are attached together
at a centromere

 Each
Chromosome
Numbers
species has a set number of
chromosomes in every cell
 Higher number of chromosomes does
NOT mean it is a more complicated
species!
 Humans have 46 chromosomes, but
crayfish have 200!
Cell Division in
Eukaryotes!!!
Division of the nucleus and the cytoplasm
 Involves only one cycle of cell division
 The division of regular cells – 1 cell and 1
set of chromosomes become 2 identical
cells (and two identical sets of
chromosomes) called daughter cells

Introduction to
Mitosis
Apoptosis – “cell death”
The series of events involving the
growth, replication, and division of
a eukaryotic cell
Mitosis
 Division
of cells and cell nuclei
 It’s divided into 4 phases:
Prophase, Metaphase, Anaphase,
Telophase
Before Cell Division
 Interphase
– time for growth and
regular cell activity before cell
division; consists of three phases
 Most of a cell’s life is spent in
Interphase.
Interphase is NOT part of mitosis!
Interphase Phases
G1 (Gap 1) is when the cell grows and
functions normally and during this time,
protein synthesis occurs and the cell
grows producing more organelles,
increasing the volume of the cytoplasm.
 Synthesis (S) is when the cell duplicates
its DNA
 G2 (Gap 2) is where the cell resumes its
growth in preparation for mitosis

Int erphase
Prophase
First stage of Mitosis
 Chromatin condenses into rodshaped chromosomes
 Nuclear membrane and nucleolus
break down and disappear
 Two pairs of dark spots called
centrosomes appear next to the
disappearing nucleus
M o re Pro p hase
 In
animal cells, each centrosome
contains a pair of long, cylindrical
bodies called centrioles
 Centrioles separate to opposite
ends (poles) and spindle fibers
begin to form; they are made of
microtubules of protein
Fibers, Fibers Everywhere
Polar fibers are spindle fibers that extend
from the two poles of a dividing cell
 Kinetchore fibers are microtubules that
connect kinetochores to a centrosome.
 Both of these types of spindle fibers
make up the mitotic spindle
 Aster formation – protein fibers which
radiate from each centriole (not found in
plant cells)

Early Prophase
Late Prophase
Metaphase
Second Phase of Mitosis
 Chromosomes move and line up in
the center of the cell – the equator.
 Each chromosome is connected to a
spindle fiber.
Metaphase
ASTERS
Anaphase
Third Phase of Mitosis
 Centromere
of each pair of
chromatids divide and move
towards opposite poles
 Kinetichore and polar fibers work
together to move chromosomes
 Spindle fibers start to break down
 When chromosomes separate, they
are considered to be individual
chromosomes!
Anaphase
Telophase
4th and Final Stage of Mitosis
 Spindle fibers completely disappear
 Chromosomes gather at opposite ends
of the cell, unwind and lose their
distinct shapes becoming chromatin
again
 Nuclear envelope and nucleolus
reappear.
 Cell membrane begins to pinch in
forming a cleavage furrow (only in
animal cells)
Telophase
Cytokinesis – division of the
cytoplasm
 Two
new cells are formed
 Cells are equal in size
 Each new daughter cell has an
identical set of duplicate
chromosomes
Plant Cell Mitosis
 Has
no asters
 Has a cell plate rather than a
cleavage furrow
Animal Cells
Plant Cells
Regulating the Cycle
A
protein called cyclin helps regulate
the cell cycle in eukaryotic cells.
 These proteins are regulators and
respond to internal and external
events.
When Things Go Wrong
 Cancer
cells do not respond to the
signals that regulate the growth of
most cells.
 As a result, they form masses of cells
called tumors that can damage
surrounding tissues.
Cancer