Cells need to divide for three main reasons: growth and repair of tissues, reproduction, and replacement of dying cells. There are two main types of cell division: mitosis, which produces identical body cells, and meiosis, which creates gametes for sexual reproduction. The cell cycle consists of interphase, where the cell grows and its DNA is replicated, and the M phase where the cell divides through mitosis or meiosis. Strict controls regulate the cell cycle to ensure cells only divide as needed. Cancer occurs when cells lose their ability to regulate division.

2. Why do cells need to divide?Why do cells need to divide?
o Depends on organismDepends on organism
o Unicellular organisms, likeUnicellular organisms, like
bacteria, divide to reproduce.bacteria, divide to reproduce.

3. Unicellular Cell DivisionUnicellular Cell Division
o ““Binary FissionBinary Fission””
o Result = 2Result = 2
identical cellsidentical cells
o What has toWhat has to
happen?happen?

4. Reasons Multicellular OrganismsReasons Multicellular Organisms
Need Cell DivisionNeed Cell Division
1) Growth (mitosis)1) Growth (mitosis)
2) Repair/replacement of cells (mitosis)2) Repair/replacement of cells (mitosis)
oSome cells die quickly (skin cells).Some cells die quickly (skin cells).
oSome cells never get replaced (nerveSome cells never get replaced (nerve
cells).cells).
3) Create reproductive cells (meiosis)3) Create reproductive cells (meiosis)

5. 2 Types of Cells2 Types of Cells
o ““Body cells” (Body cells” (somaticsomatic) vs. reproductive) vs. reproductive
cells (cells (gametesgametes).).
o Examples of somatic cellsExamples of somatic cells: skin cell, liver: skin cell, liver
cell, brain cell, red blood cell…cell, brain cell, red blood cell…
o Examples of gametesExamples of gametes: egg cell or sperm: egg cell or sperm
cellcell

6. How much DNA is in our cells?How much DNA is in our cells?
o DNADNAchromosomeschromosomes
o How manyHow many chromosomes in a humanchromosomes in a human
somatic cell?somatic cell?
46 chromosomes or 23 homologous pairs46 chromosomes or 23 homologous pairs
o WhatWhat is theis the originorigin of these pairs?of these pairs?
mom and dadmom and dad
o Homologous pairHomologous pair- chromosomes that- chromosomes that
carry thecarry the samesame genesgenes
o These cells areThese cells are diploiddiploid..
o The diploid number in humans isThe diploid number in humans is 4646..

8. How much DNA is in our cells?How much DNA is in our cells?
o How many chromosomes are in a gameteHow many chromosomes are in a gamete
(reproductive cell)?(reproductive cell)?
-23 chromosomes-23 chromosomes
o Process of cell division (meiosis) dividesProcess of cell division (meiosis) divides
homologous pairs. Allows parent to passhomologous pairs. Allows parent to pass
on one chromosomes from eachon one chromosomes from each
homologous pair.homologous pair.
o These cells areThese cells are haploidhaploid..
o The haploid number in humans is 23.The haploid number in humans is 23.

11. The Story of Life:The Story of Life:
o Organisms get one copy of a traitOrganisms get one copy of a trait
(chromosome) from mom, the other from dad.(chromosome) from mom, the other from dad.
o Passed from parents to kids in chromosomesPassed from parents to kids in chromosomes
of reproductive cells.of reproductive cells.

12. The Story of Life:The Story of Life:
o Gametes-Gametes-
reproductive cellsreproductive cells
o Two gametes joinTwo gametes join
(fertilization)(fertilization)
o Egg (23) + SpermEgg (23) + Sperm
(23) = You (46) –(23) = You (46) –
ZygoteZygote
o If egg (46) + SpermIf egg (46) + Sperm
(46) = NO you (92)(46) = NO you (92)

13. If all body cells contain theIf all body cells contain the
same DNA, why are they sosame DNA, why are they so
different?different?
o Different cells make differentDifferent cells make different
proteins.proteins.
o Different “active segments” ofDifferent “active segments” of
DNA.DNA.
o During early stages of development,During early stages of development,
cells become “specialized.”cells become “specialized.”

14. Cell DifferentiationCell Differentiation
o The first few cells that develop from theThe first few cells that develop from the
zygote are called “stem cells.”zygote are called “stem cells.”
o Stem cells-Stem cells- undifferentiated (generic) cellsundifferentiated (generic) cells
that become specialized cells…that become specialized cells…

15. The Cell CycleThe Cell Cycle
o Different cells have different “lifeDifferent cells have different “life
spans.”spans.”
o This depends on the type of cell and theThis depends on the type of cell and the
DNA that is “active.”DNA that is “active.”
o Each cell goes through a series ofEach cell goes through a series of
phases through out its life.phases through out its life.

16. The Cell CycleThe Cell Cycle
o There are 2 major parts in the life ofThere are 2 major parts in the life of
a cell.a cell.
1.1. GG11 PhasePhase
S PhaseS Phase INTERPHASEINTERPHASE
GG22 PhasePhase
2. Division (Mitosis and Cytokinesis)2. Division (Mitosis and Cytokinesis)

17. The Cell CycleThe Cell Cycle

18. Step #1 “G1 Phase”Step #1 “G1 Phase”
o First growth phase.First growth phase.
o 46 chromosomes present (two #7’s).46 chromosomes present (two #7’s).
o Newly formed cells that go throughNewly formed cells that go through
intense growth.intense growth.

19. Step 2 “S Phase”Step 2 “S Phase”
o ““S” = SynthesisS” = Synthesis
o All chromosomes copied duringAll chromosomes copied during
o (DNA replication)(DNA replication)
o 92 chromatids present (four #7’s).92 chromatids present (four #7’s).
Duplicated
chromosome
Sister
chromatids
Chromosome

20. Step #3 “G2 Phase”Step #3 “G2 Phase”
o Second growth phase.Second growth phase.
o Cell grows some more.Cell grows some more.
o Organelles copied.Organelles copied.
o Still 92 chromatids present (four #7’s).Still 92 chromatids present (four #7’s).

21. Interphase SummaryInterphase Summary
o G1, S, G2 all part interphase.G1, S, G2 all part interphase.
o Cell spends most of its life here-Cell spends most of its life here-
growing/preparing to divide.growing/preparing to divide.
o If cell enters “S,” then must continueIf cell enters “S,” then must continue
the cycle and divides.the cycle and divides.

22. Interphase DiagramInterphase Diagram
o ChromosomesChromosomes
NOT visible-NOT visible-
““chromatin”chromatin”

23. Step #4 “M Phase”Step #4 “M Phase”
o Mitosis = “nuclear division”Mitosis = “nuclear division”
o Creates 2 nuclei with equal amounts DNA inCreates 2 nuclei with equal amounts DNA in
one cellone cell
o Four phases: prophase, metaphase,Four phases: prophase, metaphase,
anaphase, telophase.anaphase, telophase.

24. ProphaseProphase
o Nuclear envelope/Nuclear envelope/
nucleolusnucleolus
disappear.disappear.
o ChromosomesChromosomes
visible.visible.
o Centrioles/Centrioles/
spindle fibersspindle fibers
appearappear
o Still 92 chromatids present (four #7’s)Still 92 chromatids present (four #7’s)

25. MetaphaseMetaphase
o Chromosomes middleChromosomes middle
o Spindle fibersSpindle fibers
attach centerattach center
(centromere) each(centromere) each
chromosome.chromosome.
o Still 92 chromatids (four #7’s)Still 92 chromatids (four #7’s)

26. AnaphaseAnaphase
o Spindle fibersSpindle fibers
pull apartpull apart
identical chromatidsidentical chromatids
(separates chromosome(separates chromosome
pairs)pairs)
o Still 92 chromatidsStill 92 chromatids
(four #7’s)(four #7’s)

27. TelophaseTelophase
o Each sideEach side
full setfull set
chromosomes (DNA).chromosomes (DNA).
o Nuclear envelopeNuclear envelope
reforms.reforms.
o Spindle fibersSpindle fibers
disappear.disappear.
o NucleolusNucleolus
reappears.reappears.
o 46 chromosomes per46 chromosomes per
nucleus (two #7’s per nucleus)nucleus (two #7’s per nucleus)

28. CytokinesisCytokinesis
o Division cytoplasm-Division cytoplasm-
creates 2 new cellscreates 2 new cells

30. Interphase vs. MitosisInterphase vs. Mitosis

31. Mitosis StagesMitosis Stages

32. Regulating the CellRegulating the Cell
CycleCycle
o Not all cells move through the cell cycleNot all cells move through the cell cycle
at the same rate.at the same rate.
o Examples:
skin cell 2-3 days
esophagus 2-3 days
large intestine 6 days
RBC 120 days
cardiac muscle no division
brain/nerve cell no division

33. Controls on Cell DivisionControls on Cell Division
o CyclinsCyclins – proteins that regulate different phases– proteins that regulate different phases
of cell cycleof cell cycle
o 2 Types of Cyclins-2 Types of Cyclins-
1) Internal regulators1) Internal regulators
2) External regulators2) External regulators

34. Internal RegulatorsInternal Regulators
o Respond to events “inside” cellRespond to events “inside” cell
o Control each step of cell cycleControl each step of cell cycle
o Prevent things from happening untilPrevent things from happening until
certain events happen in the cellcertain events happen in the cell
o Ex) Cell will not be allowed to enter “MEx) Cell will not be allowed to enter “M
phase” until all chromosomes have beenphase” until all chromosomes have been
copied.copied.

35. External RegulatorsExternal Regulators
o Respond to events “outside” the cellRespond to events “outside” the cell
o Directs cell to speed up or slow downDirects cell to speed up or slow down
cell cycle based on what is happeningcell cycle based on what is happening
outside the cell.outside the cell.
o Ex) wound healing or growth of embryoEx) wound healing or growth of embryo
developmentdevelopment

36. Controls on Cell DivisionControls on Cell Division
o Cells are “cohesive”Cells are “cohesive”
o If cells are removed, they will “fill inIf cells are removed, they will “fill in
the gaps.”the gaps.”
o Once all cells come in contact with eachOnce all cells come in contact with each
other, cell division stops.other, cell division stops.
o ““Contact InhibitionContact Inhibition””
o Happens in body when cells are injuredHappens in body when cells are injured

38. o Sometimes cells lose ability to regulateSometimes cells lose ability to regulate
cell cycles.cell cycles.
o CancerCancer
o Cancer cells do not respond toCancer cells do not respond to
internal/external regulatorsinternal/external regulators
o Results:Results:
1) uncontrolled cell division1) uncontrolled cell division
2) loss of cell cohesion2) loss of cell cohesion
Abnormal Cell DivisionAbnormal Cell Division

39. Abnormal Cell DivisionAbnormal Cell Division
o Cells grow at an abnormal rate because theCells grow at an abnormal rate because the
DNA was mutated.DNA was mutated.
o This can causeThis can cause tumorstumors..
o BenignBenign tumor- abnormal cells remain attumor- abnormal cells remain at
original siteoriginal site
o MalignantMalignant tumors- abnormal cells becometumors- abnormal cells become
invasive to surrounding tissues due to loss ofinvasive to surrounding tissues due to loss of
“cohesiveness”“cohesiveness”
o MetastasisMetastasis- cancer cells spread to new areas- cancer cells spread to new areas
through blood/lymph vessels and new tumorsthrough blood/lymph vessels and new tumors
formform

40. How are reproductive cells formed?How are reproductive cells formed?
o Meiosis is nuclear division that producesMeiosis is nuclear division that produces
haploid gametes.haploid gametes.
o Chromosomes - no longer in pairs.Chromosomes - no longer in pairs.
o Each parent passes on one chromosome fromEach parent passes on one chromosome from
each homologous pair in the gamete.each homologous pair in the gamete.
o Reproductive cells contain half the number ofReproductive cells contain half the number of
chromosomes that a somatic cell contains.chromosomes that a somatic cell contains.
o 46462323

42. How is Mitosis different fromHow is Mitosis different from
Meiosis?Meiosis?
MITOSISMITOSIS MEIOSISMEIOSIS
Chromosome #Chromosome #
stays samestays same
Chromosome #Chromosome #
reduced halfreduced half
2 new cells produced2 new cells produced 4 new cells produced4 new cells produced
Somatic cellsSomatic cells GametesGametes
IdenticalIdentical VariationVariation

43. What are reproductive cells?What are reproductive cells?
o In males reproductive cells originate
from cells called spermatogonium.
o In females reproductive cells (egg cells)
originate from cells called oogonium.

44. 46
46 46
23 23 23 23
Spermatogonium
Sperm

45. 23 23 23
46
46 46
23
Oogonium
Polar Body
Egg

46. Meiosis in HumansMeiosis in Humans
o How is meiosis different in males andHow is meiosis different in males and
females?females?