Transcript Why do Cells Divide?

Why do Cells
Divide?
Cell Size Limits
Why do cells divide?
A. If they get too big
– Surface area to volume ratio, etc.
• Diffusion is fast over short distances but slow over long
distances
• So, b/c of slow rate of diffusion, cells can’t be giants b/c they
would starve to death or be poisoned from the build up of
wastes!!
• When you increase the size of the cell, the vol.
increases faster than the surface area of the cell
membrane!!
Why do cells divide?
B. Growth of organism
– In order for an organism to grow, it needs more cells
C. Repair
– New cells replace old/damaged ones
D. Reproduction
– ASEXUAL- offspring is identical to parent
– SEXUAL- offspring is a combo of both parents
Cell Division
• Mitosis
– Occurs in somatic
cells (body cells)
– Daughter cells are
exactly like parent
– Diploid
• 2n
• In humans n=23
Meiosis
– Produces gametes
(germ cells/sex cells)
– Daughter cells have
half the chromosomes as parent
“Reduction division”
– Haploid
1n
Why?
Mitosis- divides somatic cells
to get tissues and organs
built; makes individual grow
into an adult
DNA VOCAB
1. Chromatin– when a cell is not dividing it’s DNA is a tangled mass of thin threads.
2. Chromosome– When the DNA is ready to divide it condenses into individual
chromosomes
• In humans- there are 23 HOMOLOGOUS PAIRS of chromosomes
• The two chromosomes in a pair are SIMILAR but not exactly alike
• One is from mom and one from dad
3. Sister Chromatid– When the DNA replicates (to make one copy for each cell) the two
copies join together by a centromere and each copy is called a SISTER
CHROMATID
– These sisters are EXACTLY ALIKE!
TYPICAL CHROMOSOME PAIR
The Cell Cycle
(Somatic Cells)
A. INTERPHASE: growth and
preparation
1.
2.
3.
G1
S
G2
B. MITOSIS: Nucleus divides
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
C. CYTOKINESIS: Division of
cytoplasm
Reminder from cell theory
• All cells come from pre-existing cells!!
-Cell Division follows this b/c it’s the
process where new cells are made from
1 cell
• Cell Division makes 2 new daughter cells
that are identical to the parent cell
Cell Cycle- The life of a cell
Cell Cycle- Interphase
Interphase (Nucleus enclosed in nuclear
membrane, nucleolus present,
chromosomes appear as chromatin (in
other words, chromosomes are not yet
visible))
•
a) G1 phase- intense cellular
activity and growth; organelles
double
•
b) S phase- DNA synthesischromosomes replicate
(copy)
Chromatin to Chromosomes
Interphase continued
• c) G2 phase- spindle fibers are
assembled/ centrioles
replicate
- cell prepares for cell division
Interphase Diagram
MITOSIS PHASES
Prophase (1st stage)
Prophase- 1st stage of actual cell division
• a.) Chromosomes condense and shorten,
become distinguishable
b.) Chromatids attached near center by
centromere
• c.) Centrioles separate, migrate to the poles
• d.) Spindle fibers form between centrioles,
asters form
• e.) Nuclear membrane disintegrates,
nucleolus disappears (in order to
separate contents of nucleus, it must
disappear)
Prophase
Early prophase
Late prophase
Metaphase- 2nd stage (shortest)
• a.) chromosomes line up in pairs
at equator (center of cell)
• b.) centromere of each
chromosome is attached to a
separate spindle fiber
Metaphase diagram
Metaphase
Anaphase- 3rd stage
• a.) Centromere of each chromosome
separates
• b.) Separation of the chromatids in each
pair
• c.) Spindle fibers appear to shorten,
pulling the chromatids apart at the
centromere (now called
chromosomes)
• d.) migration of the chromosomes ends
with the arrival at the poles and the
formation of clusters
Anaphase diagram
Anaphase
Telophase – 4th stage
• a.) Cleavage furrow forms b/w 2 cells
and spindle fibers disappear
• b.) Nuclear membrane forms around
each set of chromosomes (forms 2
new nuclei)
• c.) Chromosomes uncoil to form
chromatin net (getting ready for
interphase)
• d.) Nucleoli reorganized
Telophase Diagram
Telophase
Cytokinesis
• Cytokinesis- division of the cytoplasm
• In animal cells- cell membrane pinches
together, furrow forms along the
equator.
• -In plant cells- cell plate forms in the
middle of the dividing cell