Chapter 10 – Cell Growth and Division
Why Cells Divide
When a living thing grows what happens to its cells? (Do
the cells get larger, or do they increase in number)
– Living things produce more cells as they grow.
Why do cells divide rather than grow?
– Exchanging Materials: it takes long to move
materials in and out of larger cell then it does for a
smaller cell
Why Cells Divide
• Surface to volume ratio
– As a cell grows volume increases faster than the
surface area
– The cell uses materials faster than it can get them in
• Town analogy: A small
town has only a 2 lane
street. As the town grows
there is more traffic on the
2 lane street. It takes
longer to travel crowded
streets.
Why Cells Divide
Size of
the cube
Large
3 cm x
3cm x 3cm
Medium
2 cm x
2cm x 2cm
Small
1 cm x
1cm x 1cm
Surface
Area
(cm2)
Volume
(cm3)
SA:V
___:1
Chromatin, Chromatids, and Chromosomes
During Cell Division
• The chromatin coils
up into chromosomes.
Before cell division
• Chromatin is a mixture
of DNA and protein in
the nucleus.
• The DNA is copied
(replicated)
Chromatin, Chromatids, and Chromosomes
• Each chromosome has two
chromatids
– The chromatids are held together
by a centromere
– Each chromatid contains identical
DNA
– The chromatids will split apart
and end up in different cells
– When the chromatids split, each
chromatid will be considered a
chromosome
Eukaryotic Cell Cycle
The cell cycle represents the
events in the life of a cell.
Interphase: the time between
cell divisions.
Interphase occurs in 3 steps
– G1 Phase: cell grows
– S Phase: DNA replication
– G2: Cell prepares for
mitosis
Eukaryotic Cell Cycle
Cell Division
(also known as the M phase)
Cell Division occurs in two
steps
– Mitosis: division of the
nucleus.
– Cytokinesis: division
of the cytoplasm
Stages of Mitosis
Mitosis occurs in 4 steps:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Prophase
• DNA condenses into
chromosomes
• 2 centrioles take positions on
opposite sides of the nucleus
– Centrioles control the
development and attachment
of microtubules
– Microtubules are thin hollow
tubes that are part of the
cytoskeleton.
Prophase
• A spindle begins to form
from each microtubule
• Spindle: A structure that
helps move and separate
chromosomes.
• Chromosomes attach to
the spindle at the
centromere
• Nucleolus disappears
• Nuclear envelope breaks
down
Metaphase
• Chromosomes line up
across the middle of the
cell
Anaphase
• The chromosomes in the middle of the cells are split
and each chromatid is pulled in opposite directions.
• When the chomatids separate, each chromatid becomes
a chromosome.
• The chromatids move
away from each other
to opposite sides of the
cell.
Telophase
• Spindle breaks down
• The nucleus begins to reform in each end of the cell.
– Chromosomes uncoil
and relax
– Nuclear envelope
reforms around each
cluster of
chromosome
– A nucleolus becomes
visible in each
daughter nucleus.
Cytokinesis
• 2 new nuclei are now in one cytoplasm
• Cytokinesis: Division of the cytoplasm
– Animal Cells: The membrane pinches
inward and splits the cell in two
– Plant Cells: A cell plate forms across
the center of the cell to divide the cell
wall.
• Cytokinesis usually occurs the same time
as telophase
Meiosis
Homologous chromosomes – sets of chromosomes where
one comes from the male parent and the other from the
female.
– Humans have 46
chromosome
– 23 from Mom + 23 from
Dad = 23 homologous
pairs
– Homologous chromosomes
are the same size
– Homologous chromosomes
contain the same genes
Meiosis
• Diploid: cells that have a full set of chromosomes.
– Abbreviated: 2N
– In humans, diploid cells have 46 chromosomes.
– Almost all cells in the human body are diploid.
Meiosis
• Haploid: cells that have ½
the number of chromosomes.
– Cells contain only 1
chromosome from each
homologous
chromosomes.
– In humans, haploid cells
have 23 chromosomes
– In humans, the only
haploid cells are sperm
and egg cells.
Meiosis
• Meiosis – a process in which sperm
and egg cells are created by cutting
the chromosome number in half.
• Meiosis goes thru 2 rounds of
division:
– Meiosis I and Meiosis II
– Diploid cell become 4 haploid
cells
– Produces sperm and egg cells
(gametes)
Meiosis I
• Interphase: Chromosomes (DNA) are replicated
before division
• Prophase I: each replicated chromosome pairs with its
homologous chromosome.
– Tetrad: a pair of
homologous
chromosomes with 4
chromatids.
– Crossing-over :
portions of
chromatids, which
contain genes, are
exchanged
Meiosis I
• Metaphase I: paired homologous
chromosomes line up in the center.
• Anaphase 1: spindle fibers separate
each pair of chromosome, pulling
them to opposite ends of the cell.
• Telophase 1: Nuclear envelope
forms around each cluster of
chromosomes.
• Cytokinesis: forms two new cells
– The cells are now haploid
– Each cell has a different set of
genes
Meiosis II
• After meiosis I, chromosomes do not
replicate.
• Cells go thru a second round of
divisions (Meiosis II)
– Prophase II – chromosomes form
– Metaphase II – chromosomes line
up at the center
– Anaphase II – chromatids separate
– Telophase II – nucleus reforms
• Result is 4 haploid daughter cells
Mitosis vs.Meiosis
SQUARE
DANCING
List the differences between mitosis and meiosis.
Comparing Mitosis and Meiosis
• Mitosis produces 2 genetically identical
diploid daughter cells
• Meiosis produces 4 genetically different
haploid daughter cells
SQUARE
DANCING
10-3 Regulating the Cell Cycle
• Cell growth and division is very controlled
• True or False – All cells move through the
cell cycle at the same rate.
Controls of Cell Division
• What happens when cells are grown in a petri
dish?
Cells will grow until they form a thin layer
covering the bottom of the dish.
• What does this experiment show?
Controls of cell division can be turned on and off.
• What happens in our body that is similar?
Cells at the edge of an injury are stimulated to
divide rapidly.
Cell Cycle Regulators
• Scientists have been trying to determine
what regulates the cell cycle.
• Found a protein – if they inject it into a nondividing cell, the cell enters mitosis
• What is the protein called? Cyclin
Why? regulates cell cycle
• Cyclins regulate the timing of the cell cycle
in eukaryotic cells.
Cell Cycle Regulators
Internal Regulators
• Proteins that respond to events inside the cell
Ex: Don’t allow mitosis to start until all
chromosomes are replicated.
External Regulators
• Proteins that respond to events outside the cell.
• Growth Factors found on the outside of cells
can speed up or slow down cell division.
• Ex: Cells at the edge of an injury are
stimulated to divide rapidly.
Uncontrolled Cell Growth =
CANCER
• Cancer cells do not
respond to cycle regulators
• Results
– Divide uncontrollably
– Form tumors (masses of
cells) that can damage the
surrounding tissue
Uncontrolled Cell Growth = CANCER
• Cancer cells may break loose from
tumors and spread throughout the
body disrupting normal activities
(metastasis).
• What are some causes of cancer?
smoking tobacco, radiation exposure,
viral infection, defective p53 gene.
Meiosis Activity
• Using the supplies in the bag, create a single diploid cell
with 4 chromosomes.
• Think which of the objects represents the . . .
– Spindle
– Cell Membrane
– Nuclear Membrane
– Chromosomes
– Centrioles
• Show each of the stages of Meiosis I and II
• Describe the cells that are created by the end of this
process.
Inquiry Activity
•
Use a microscope to compare the sizes of
similar cells in large and small plants, then do
the same for large and small animals.
Questions
1. Are the cells of the small plant larger or
smaller than those of the larger plant? Are the
cells of the small animal larger of smaller than
those of the large animal.
2. Can you make a general statement that
compares the number and size of cells in small
organisms to those in large organisms.