1.2 CELL CYCLE
Cell Division and Mitosis
WHY DO CELLS DIVIDE?
When organisms grow
 cells divide (more cells)
 Why?
 So each cell stays small, and manageable
 To replace old or damaged cells
 For reproduction
 For multicellular organisms to grow (all cells require
nutrients and must get rid of waste)

CHROMOSOMES
 Structures
found in the nucleus
 Made up of DNA (looks like a twisted ladder)
 DNA contains the genes that provide the
instructions the cell
 Before division, a copy of the DNA is made so each
new cell has its own copy
 Humans have 46 chromosomes
CHROMOSOME TERMINOLOGY
23 + 23 = 46
You have one copy
of each chromosome
from each parent!
Eye Colour
Eye Colour
Hair Colour
Hair Colour
CELL CYCLE
Interphase
 Cell carries out
Its life functions
 DNA is duplicated
 Mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis
 Cells divide into two identical cells

INTERPHASE
Period between cell divisions
 growth phase
 DNA replication
 preparation for division
 ~ 18 hr

MITOSIS  PROPHASE
 chromatin
coil changing
the long strands to short
and thick chromosomes
 Form a pair of identical,
copied strands
(chromatid) joined at the
centromere
MITOSIS  PROPHASE
Nuclear membrane and nucleolus disappear
 Centrioles separate and travel to opposite poles
(plant cells do not contain centrioles)

MITOSIS  METAPHASE
Chromosomes line up at equator (middle of cell)
 spindle fibres attach to centromeres

MITOSIS  ANAPHASE
Sister chromatids are pulled to opposite poles
 Centromeres divide in half

MITOSIS  TELOPHASE
Sister chromatids are at opposite poles
 DNA becomes thin again (chromatin)
 Nuclear membrane reassembles

CYTOKINESIS
Division of cytoplasm
 Animals: Cell membrane pinches
in and forms two cells
 Plants: Cell plate forms between
the two daughter cells and will
eventually become a cell wall

RECAP-- MITOSIS UNDER
THE MICROSCOPE
Prophase
Metaphase
Anaphase
Telophase
UNCONTROLLED DIVISION: CANCER
 Chemical
signals from the nucleus tell
the cell when to divide, not to divide, or
deconstruct itself.
 Tumour: mass of cells that divide
uncontrollably.
 Mutation: permanent change in cell’s
DNA which affect cells response to
division signals.
CANCER OR NOT?
Malignant (cancer)
Benign
Cell divides uncontrollably (fast
rate)
Cell divides uncontrollably
(moderate rate)
Damages and destroys
surrounding cells
Does not interfere with
neighbouring cell but can
overcrowd them
Can spread to other parts of the
body
Does not spread to other parts of
the body
Interferes with the function of
other cells, can result in death if
not destroyed or removed
Harmless unless it becomes so
large it interferes with body
functions
NORMAL VS
CANCEROUS
CELL DIVISION
WATCH THIS!
 Go
to http://www.cellsalive.com/mitosis.htm
for a mitosis interactive animation
HOMEWORK

Pg 14-19