Transcript Cell Division

Chapter 9
Chromosomes, the Cell
Cycle and Cell Division
Homework: (due Thursday) BABOON TEXT
 Draw the phases of mitosis
 Answer the Recap questions using complete
sentences: Section 9.1, 9.2, 9.3 and 9.6 (we are
NOT covering Meiosis at this point)
 Read The Immortal Cells of Henrietta Lacks
p180-181. Summarize and discuss the
importance of HeLa Cells.
Chromosomes and DNA- the
genetic material
Prokaryotes
Single circular
chromosome
contains DNA
folded with
help of
proteins
Eukaryotes
Many
chromosomes
containing
DNA wrapped
around
proteins and
folded into
many tightly
packed coils
within the
nucleus
Cell Division
Cells reproduce to
form genetically
identical daughter
cells
 “Omnis cellula e
cellula” – Virchow

– “all cells from cells”
Functions of Cell Division

Reproduction:
– Making more cells

Growth:
– Enables multicellular organisms to grow and
develop from a single cell

Repair:
– Replacing cells that die from normal wear and
tear and accidents
Prokaryotic Cell Division

Prokaryotes divide by
Binary Fission
– “division in half”

Steps in Binary
Fission
– Bacterial DNA is
replicated
– Cell grows
– Cell splits in two
Eukaryotic Chromosomes

Chromosomes
– DNA wound around
histone proteins
– 46 chromosomes in
all human cells
except gametes
– Chromosomes
come in pairs (23
pairs in humans)
 One copy from
mom, one from dad
Eukaryotic Cell Division- Cell Cycle
 The
Cell Cycle
–Interphase
G1, S, and G2
phases
–Mitotic Phase
Mitosis
Cytokinesis





The cell spends 90%
of its time in
interphase
G1- Gap 1
– Cell growth
S: Synthesis
– DNA is replicated
G2- Gap 2
– Cell growth
– Preparation for
division
G0- cells NOT going
through division
Interphase
Mitotic Phase

5 subphases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase

Followed by Cytokinesis
Prophase
Nuclear membrane
dissolves
 Chromatin condense
into chromosomes
 Mitotic spindle begins
to form in the
cytoplasm
 Centrosomes (with
centrioles that form
spindle fibers) move
toward the poles

Prometaphase
Nuclear envelope
disappears
 Bundles of
microtubules extend
from each pole
towards the equator
of the cell
 Some microtubules
attach to kinetochores

Metaphase
Centrosomes are at
opposite poles
 Chromosomes line up
on metaphase plate
 Mitotic spindle is fully
formed at this point

Anaphase
Sister chromatids
separate at the
centromere
 Chromatids move
towards the poles
 Cell becomes more
ovoid in shape

Telophase
Daughter nuclei begin
to form at the poles
of the cell
 Nuclear envelopes
reform
 Nucleoli reappear
 Chromatin becomes
less dense 
chromosomes
disappear

Cytokinesis
Division of the
cytoplasm
 Animal Cells:

– Cleavage furrow
forms
– Pinches the cell into
two

Plant Cells:
– Cell plate forms
– Divides the cell in 2
Animal Cells
Plant Cells
Controlling Cell Division
Cells rely on internal and external cues to
control cell division
 Restriction Checkpoints- sites where cell
division are either prevented or stopped
 Growth factor

– Substance that is required for certain cells to
divide

Density of cells
– If cells become too dense in their container,
they will stop dividing  they’re competing
for resources!
Restriction Point

Late in G1, before S phase, the cell passes the
“point of no return”
– Can we go ahead and divide?
– If yes… cell division occurs!
– If no, the cell enters G0 phase
 Essentially exits the cell cycle and just
stops
 Most grown human cells are in this phase
Cell Cycle Control System

Checkpoints are controlled by protein activity (2 main)
– Cyclins- proteins that control progression of
cells through the cell cycle by activating
cyclin-dependent kinase
– Kinase Proteins- proteins that activate or
inactivate target proteins by phosphorylating
them
 Phosphorylation: breaking down ATP and
adding a phosphate group
–This changes the shape of the target protein
that directly regulates the cell cycle
MPF
 “Maturation
Promoting Factor”
–A specific type of cyclin-dependent
kinase (cdk)
–Cdks are regulatory proteins
Helps control the cell cycle
–The “master switch” for a cell’s
passage from interphase to mitosis
Cell Cycle Control System


Specific Example at the G1 checkpoint
Proteins involved
– Cyclin
– Cyclin dependent kinase (Cdk)
– Rb (target protein) normally inhibits the cell cycle at G1 checkpoint
How it works
When Cdk binds with a cyclin, it becomes activated
Cdk/cyclin complex phosphorylates Rb protein (Rb is a tumor suppressor)
Rb becomes inactive and can no longer inhibit the cell cycle
Cell proceeds to stage Synthesis
NOTE: RB and other target proteins in the cell cycle control system act as
tumor suppressors by stopping uncontrolled cell division.
LOOK AT FIGURE 9.6 IN YOUR BABOON TEXT
Normal v. Cancer Cells
Have densitydependent inhibition
 Will only divide when
attached to a
substrate (anchored)


Must have sufficient
growth factors or
nutrients for division
Lack densitydependent
inhibition
 Will divide in
solution or when
not anchored
 Unaffected by
presence of
external growth
factors or nutrients

Cancer CellsCarcinomas- uncontrolled cell division in
endodermal and ectodermal tissue
 Sarcomas- malignant tumors of bone,
cartilage, fat, muscle, vascular
 Leukemias- cancer of the white blood cells
or bone marrow
 Lymphomas- cancer of the lymph tissue
(WBC, lymph nodes, spleen, red bone
marrow)

Tumors
Tumor:
– When cancerous cells divide, they form a mass of
cancer cells within an otherwise normal tissue
 Benign tumor:
– Cancerous cells remain in one spot
 Malignant tumor:
– Becomes invasive enough to impair the functions
of one or more organs
 Metastasis:
– The spread of cancer cells beyond their original
site

Treatment
Chemotherapy- (chemo) is the use of
medicines or drugs to treat cancer
 Radiation- high-energy particles or waves
to destroy or damage cancer cells
