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

MeiosisMeiosis– Produces gametes Produces gametes

(germ cells/sex cells)(germ cells/sex cells)– Daughter cells have Daughter cells have

half the chromo-half the chromo-somes as parentsomes as parent

““Reduction division”Reduction division”

– HaploidHaploid1n1nWhy?Why?

Mitosis- divides somatic cells to get tissues and organs built; makes individual grow into an adult

DNA VOCAB1. 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. G12. S3. G2

B. MITOSIS: Nucleus divides1. Prophase2. Metaphase3. Anaphase4. 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 synthesis- chromosomes 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