CELL DIVISION
Types of Cell Division
• Mitosis – makes new body
cells (in eukaryotes)
• Meiosis – makes new sex
cells (in eukaryotes)
WHY DIVIDE?
• Why do multicellular organisms do mitosis?
– To repair or heal
– To grow and develop
– To replace cells that die
• Why do unicellular organisms do mitosis
(eukaryotes)?
– To reproduce asexually (cloning)
• Why do multicellular organisms for meiosis?
– To reproduce sexually
Important part of Cell Division
• Nucleus - contains genetic information
(DNA)
• Chromosomes – strands of DNA
• When a cell divides, the new cells made must
have the correct number of chromosomes.
• Most of the time, DNA looks like a lump of
spaghetti, and is called chromatin.
Cell Cycle
Cell Cycle
• Interphase
– G0 – cell at rest; is not preparing
for division
– G1 – cell gets ready for division
– S – chromosomes copy themselves
– G2 – cell checks for mistakes and
prepares for division
• Mitosis – cell divides
• Most of the time cells are not
dividing.
• Several proteins are involved
in starting division.
• Some cells can divide in as
little as 20 minutes.
• Cells typically stop dividing
when:
– Nutrient levels drop (no
food)
–They come into contact
with other cells (no room)
–Receive chemical signals
to stop.
INTERPHASE
• Cells spend most of their life in
interphase.
• Normal cell activities occur.
• To prepare for cell division, the
chromosomes make copies of
themselves.
• In other words, the DNA replicates.
• Recall, DNA polymerase… etc.
MITOSIS: CELL DIVISION
FOR EUKARYOTES
Four Stages
1.Prophase
2.Metaphase
3.Anaphase
4.Telophase
Some animations….
Cells Alive animation
McGraw animation
John Kyrk animation
PROPHASE
• Chromatin condenses and
becomes visible as
chromosomes.
• The nucleus and its various
parts disappear.
• Spindle fibers are made and
attach to the chromosomes.
• Centrioles separate and
move to opposite ends (poles)
of the cell.
• Each copy of the chromosome
is called a chromatid.
Chromosome
chromatids
centromere
METAPHASE
• The chromosomes line up
at the middle of the cell
(equator).
ANAPHASE
• The joined chromatids separate
and move to opposite ends of the
cell.
TELOPHASE
• The chromatids arrive at the
opposite poles of the cell.
• Spindle fibers disappear.
• New nuclei form in the two new
cells.
• Cytokinesis - pinching in of cell
membrane (formation of cleavage)
in order to form two new cells
END RESULTS OF MITOSIS
• Two new “daughter” cells are made
with each containing the same
number and kinds of chromosomes
as the original (parent) cell.
• They have the same genetic code.
CANCER
• Caused by uncontrollable mitosis.
• Cells either fail to stop dividing or get
signals to continue dividing.
• An increased number of cells makes a tumor.
• Tumors can be benign (non-cancerous) or
malignant (cancerous).
• Cancer cells spread (metastasis) throughout
body, unlike normal cells!
Some Causes of Cancer
•
•
•
•
•
Genetics
Radiation
Carcinogens (mutate DNA)
Old Age
Viruses
– Can mutate DNA causing some cancers
– Example: Human papillomavirus (HPV)
associated with cervical cancer in women
• Poor diets
MEIOSIS:
Making sex cells
CHROMOSOME NUMBER
• The number of chromosomes
varies from one kind of
organism to another.
• Humans have 46 chromosomes
& fruit flies have 8.
• Some plants have hundreds.
• Karyotype – a picture of
chromosomes that shows the
number and the size of
chromosomes that a cell has
• Amniocentesis and Chorionic
Villi Sampling– tests done on
pregnant women in order to
look at the chromosomes of
the developing baby
• Chromosomes of animal cells
are usually paired. WHY?
• Humans have 23 pairs, fruit
flies have 4 pairs.
• Haploid - the # of pairs or n
• Diploid - the total number of
chromosomes or 2n
• For humans: n=23 and 2n=46
SEXUAL REPRODUCTION
• Fertilization - the combination of
chromosomes (genetic
information) from two sex cells
(combining sperm and egg)
• Each sex cell gives half of the
total chromosomes.
• Sex cells are also called
gametes.
–Female sex cell = egg or
ovum
–Male sex cell = sperm
• Gametes are haploid
(half the chromosomes).
Gamete Production
• Gametes are produced in gonads.
• Eggs are made in ovaries.
– Females begin making their eggs
before they are born!
– They stop making eggs during
menopause.
• Sperm are made in testes.
– Males make sperm from
approximately puberty until death.
• After fertilization, the
developing organism is called a
zygote and is diploid.
• After fertilization the zygote
grows bigger by doing
MITOSIS.
• A zygote becomes an embryo
and later a fetus.
MEIOSIS END RESULT
• Meiosis makes 4 haploid, sex cells.
• These 4 cells are sex cells (sperm
and egg).
• Each cell has half the number of
chromosomes as the parent cell
(haploid).
• It increases genetic diversity in
the offspring.