This document provides an overview of cell division and the cell cycle. It begins with acknowledging various universities and organizations. It then lists the course objectives which are to explain the importance of cell division, discuss the cell cycle stages of mitosis and interphase, describe the four stages of mitosis, and discuss cell cycle control mechanisms. The document proceeds to define key terms, describe the stages of interphase and mitosis, and explain chromosomes, the mitotic spindle apparatus, and control of the cell cycle through cyclins and CDKs. It concludes by noting some key points about mitosis and introducing meiosis.

1. Chapter three
Cell division

2. Acknowledgment
• Addis Ababa University
• Jimma University
• Hawassa University
• Haramaya University
• University of Gondor
• American Society of Clinical Pathologists
(ASCP)
• Center for Disease Control-Ethiopia (CDC-
Ethiopia)

3. Course objective of mitosis
At the end of this chapter, the students will be able to
• Explain the importance of cell division
• Discuss about cell cycle stage in mitosis
(interphase, and mitosis)
• Describe the characteristic features of the four
developmental stages of mitosis ( Prophase,
metaphase, anaphase, and telophase)
• Discuss about cell cycle control mechanisms

4. Course outline

5. • One purpose of cell division is asexual
reproduction
– This is the means by which some unicellular
organisms produce the next generation of the
organism
– Examples
• Bacteria
• Amoeba
• Yeast
– Saccharomyces cerevisiae (Baker’s yeas
3.2 CELLULAR DIVISION

6. • A second purpose for asexual reproduction is
to produce multi-celled organisms and to
enable these organisms to grow and repair
damaged tissue
– Plants, animals and certain fungi are derived
from a single cell that has undergone repeated
asexual cell divisions
– For example
• Humans start out as a single fertilized egg
• End up as an adult with several trillion cells
3.2 CELLULAR DIVISION

7. Prokaryotes Reproduce
Asexually by Binary Fission
• The capacity of bacteria to divide is really quite
astounding
– Escherichia coli, for example, can divide every 20
minutes
• Prior to division, the bacterial cell replicates its
chromosome
• Then the cell divides into two daughter cells by
a process termed binary fission

8. Figure 3.4

9. A Generalized Cell
Golgi
body
Nuclear
envelope
Chromosomal
DNA Nucleus
Nucleolus
Polyribosomes
Ribosome
Rough ER
Cytoplasm
Membrane protein
Plasma membrane
Smooth ER
Mitochondrion
Centrioles
Microtubules
Microfilaments
Lysosome
(b) Animal cell

10. Cell division
Cell division composed of two processes:
• Nuclear division (Karyokinesis)-that includes
mitosis, and meiosis
• Cytoplasm division (Cytokinesis

11. • Kinotochores-are the attachment points of the sister
chromatides, and the constriction in the chromosome where
the attachment occur, are called centromers.
• Chromosomes may grouped, according to the position of
the centromers as:
- Metacentric: where centromers are in the middle of
chromosomes
- Telocentric: where centromers are in the end of
chromosomes
- Acrocentric: where centromers are in the very end of
chromosomes
- Other terminologies include: subtelocentric, or
submetacentric

12. Figure 3.6 (b)
Identical
twin
sisters!!

13. • Karyotype: the total chromosome complement of a
cell
• Ideogram: a photograph that showmitotic division,
and the rearrangement of pair of chromosomes

14. Chromosomes
• Key features of a chromosome: centromere (where spindle
attaches), telomeres (special structures at the ends), arms
(the bulk of the DNA).
• Chromosomes come in 2 forms, depending on the stage of
the cell cycle. The monad form consists of a single
chromatid, a single piece of DNA containing a centromere
and telomeres at the ends. The dyad form consists of 2
identical chromatids (sister chromatids) attached together
at the centromere.
• Chromosomes are in the dyad form before mitosis, and in
the monad form after mitosis.
• The dyad form is the result of DNA replication: a single
piece of DNA (the monad chromosome) replicated to form
2 identical DNA molecules (the 2 chromatids of the dyad
chromosome).

15. More Chromosomes
• Diploid organisms have 2 copies of each
chromosome, one from each parent. The two
members of a pair of chromosomes are called
homologues.
• Each species has a characteristic number of
chromosomes, its haploid number n. Humans have
n=23, that is, we have 23 pairs of chromosomes.
Drosophila have n=4, 4 pairs of chromosomes.

16. Cell Cycle
• The cell cycle is a theoretical concept that defines the state of the cell
relative to cell division.
• The 4 stages are: G1, S, G2, and M.
• M = mitosis, where the cell divides into 2 daughter cells. The
chromosomes go from the dyad (2 chromatid) form to the monad (1
chromatid) form. That is, before mitosis there is 1 cell with dyad
chromosomes, and after mitosis there are 2 cells with monad
chromosomes in each.
• S = DNA synthesis. Chromosomes go from monad to dyad.
• G1 = “gap”. Nothing visible in the microscope, but this is where the
cell spends most of its time, performing its tasks as a cell. Monad
chromosomes
• G2 (also “gap”). Dyad chromosomes, cell getting ready for mitosis.
• G1, S, and G2 are collectively called “interphase”, the time between
mitoses

17. The Cell Cycle
G1 G2
S
Two
daughter
cells
M
Mitosis
Interphase
Gap 1 Gap 2
Synthesis
Growth
Gene expression
Differentiation
DNA Synthesis
Gene expression
Quality control
Actual division process

18. • At the end of S phase, a cell has twice as many
chromatids as there were chromosomes in G1
phase
– i.e. - human cell
• 46 chromosomes in G1 phase
• 46 pairs of sister chromatids in G2 phase
• chromosome is therefore a relative term
– In G1, anaphase, & telophase it refers to the
equivalent of one chromatid
– In G2, prophase, & metaphase, it refers to a pair of
sister chromatids
Chromatids, Chromosomes… What the…

19. Interphase
• Chromosomes are
decondensed
• chromosomes
replicate
• The centrosome
divides
Nuclear
membrane
Chromosomes
Two centrosomes,
each with centriole pairs

20. Mitosis
• Comes from the Greek word for “ a thread”
referring to chromosome.
• Mitosis is a continuous process. However, for
descriptive purpose it is broken into four stages:
prophase,metaphase, anaphase, and
telophase.(Greek-pro-before; meta-mid; ana-back,
and telo-end).
• The timing for the four stagesin the cell cycle(G1,
S, G2, and M) varies from species to species, from
organ to organ with in a species, even from cell to
cell with in a given cell.

21. • Mitosis is subdivided into four phases
–Prophase
–Metaphase
–Anaphase
–Telophase

22. Prophase
• chromosomes
condense (become
short and thickened) so
that individual
chromosomes become
distinct
• Nuclear envelope
disintegrate
• Centrosomes move to
opposite poles
• mitotic spindle
apparatus forms
• nuclear envelope
disappears
Microtubules
forming mitotic spindle Sister
chromatids
Centromere

23. METAPHASE
• Spindle fibers attached
to the individual
chromosomes at their
kinetochores
• Chromosomes are
positioned in the plane
of equator of the
spindle, called
metaphase plate
Astral
microtubule
Metaphase
plate
Kinetochore
proteins attached
to centromere
Kinetochore
microtubule
Polar microtubule

24. Spindle Apparatus
• Composed of microtubules originated from centrioles
• Microtubules are formed polymerization of tubulin
proteins
• 3 types of spindle microtubules
– Aster microtubules
• Important for positioning of the spindle apparatus
– Polar microtubules
• Help to “push” the poles away from each other
– Kinetochore microtubules
• Attach to kinetochore , at the centromere

25. Figure 3.8
Kinetochore Spindle
Fibers

26. • Spindle fibers bind
kinetochores
• The two kinetochores
on a pair of sister
chromatids are
attached to
kinetochore MTs from
opposite poles
Nuclear membrane
fragmenting
Spindle pole
Mitotic
spindle

27. Metaphase
• Pairs of sister
chromatids align
themselves at the
metaphase plate Polar
microtubule
Kinetochore
proteins attached
to centromere Kinetochore
microtubule
Astral
microtubule
Metaphase
plate

28. Anaphase
• Centromeres separate
• Each chromatid, is
linked to only one pole
• As anaphase proceeds
– Kinetochore MTs shorten
• Chromosomes move to
opposite poles
– Polar MTs lengthen
• Poles themselves move
further away from each
other
Chromosomes

29. Telophase & Cytokinesis
• Chromosomes reach poles
& decondense
• Nuclear membrane reforms
• Quickly followed by
cytokinesis
– In animals
• Formation of a cleavage furrow
– In plants
• Formation of a cell plate

30. Telophase
cytokinesis: cytoplasm divided into 2 separate cells
--chromosomes de-condense
--nuclear envelope re-forms
--spindle vanishes

31. Control of cell cycle
Cell cycle controlled by proteins found in the
cytoplasm. Of these, principle ones include:
A) Cyclines:-cyclin D (G1 cyclin)
- cyclins E &A (S phase cyclin)
- cyclines B &A (mitotic cyclins)
Levels of cyclines rise and fall with the stage of cell
cycle

32. Controlling of cell cycle, continued
B) Cyclin-dependant kinases (CDKs)
- CDK 4, a G1 CDK
- CDK 2, an S-phase CDK
- CDK 1, an M phase CDK
• CDK’s level remain fairly stable, and activated
when combined with respective cyclin
• CDK added phosphate group to variety of protein
substrates that control cell cycle

33. • Mitosis ultimately produces two daughter
cells genetically identical to the mother cell
– Barring rare mutations
• Processes requiring mitotic cell division
– Development of multicellularity
– Organismal growth
– Wound repair
– Tissue regeneration
Some Key Points

34. Controlling of cell cycle, continued
The anaphase-promoting complex (APC, also called
cyclosome, & so designed as APC/C)
-triggers the events leading to destruction of the
cohesions thus allowing the sister chromatids to
separate
-degrades the mitotic cyclin B

35. Summery questions
1. Explain the importance of cell cycle on the life of an
organism
2. Discuss the function of at least 5 organelles of the cells
3. Define the term: kinotochore, metacentrics, telocentric,
acrocentric and ideogram
4. Explain the characteristic feature of the cell at G1, S,
and G2 interphase stages
5. Explain the characteristic feature of mitosis
developmental stages (prophase, metaphase, anaphases,
and telophase)
6. Explain about control mechanisms mitotic cell
division

36. Meiotic Cell Division
Part II

37. Course objective of meiosis
At the end of the course, the students will be
able to
• Describe developmental stages in meiosis
• Explain the difference between mitosis and
meiosis
• Discuss about spermatogenesis
• Discuss about oogenesis

38. • Sexual reproduction is the most common way
for eukaryotic organisms to produce offspring
• Parents make gametes with half the amount of
genetic material (haploid)
• These gametes fuse with each other during
fertilization to generate a new organism
Sexual Reproduction

39. • Simple eukaryotes are isogamous
– They produce gametes that are morphologically similar
• Most eukaryotic species are heterogamous
– These produce gametes that are morphologically different
• Sperm cells
– Relatively small and mobile
• Oocytes or ova
– Usually large and nonmobile
– Store large amounts of nutrients
• Microspores (Pollen)
• Macrospores (Ovules)
Gametes

40. How Does One Make a Haploid
Gamete?
• Answer – meiosis
• Haploid cells are produced from diploid cells during
gametogenesis
• The chromosomes must be distributed to reduce the
chromosome number to half its original value
• but simultaneously sorted to assure that each
chromosome (& its genes) is represented in each
gamete

41. • Meiosis begins after a cell has progressed
through G1, S, & G2
• Meiosis involves two successive divisions
– Meiosis I and II
– Each of these is subdivided into
• Prophase
• Metaphase
• Anaphase
• Telophase
3-44
Meiosis

42. Meiosis
• Prophase I is further subdivided into periods known
as
– Leptotena
– Zygotena
– Pachytena
– Diplotena
– Diakinesis

43. A
recognition
A total of 4
chromatids
Figure 3.11
Periods of Prophase I

44. Synaptonemal Complex

45. A physical exchange
A
tetrad
2
bivalent
s
Periods of Prophase I

46. Spindle apparatus
complete;
pairs of chromatids
attached to kinetochore
microtubules
Stages of Meiosis I

47. • Bivalents are organized along
the metaphase plate
• Homologous pairs of sister
chromatids aligned side by side
– A pair of sister chromatids is
linked to one of the poles
– And the homologous pair is
linked to the opposite pole
– The arrangement is random with
regards to the (blue and red)
homologues
Figure 3.13

48. 3-50
Pairs of sister chromatids
separate from each other
The centromere remains
between sister chromatids
Stages of Meiosis I

49. Meiosis
• Telophase I & cytokinesis of meiosis I followed
by meiosis II
• Meiosis I has reduced the number of
chromosomes in the daughter cells to the ½ the
diploid number
• However, each homolog is still composed of 2
recombinant sister chromatids
– The genetic content is still 2n
• Meiosis II reduces the genetic content to n

50. Stages of Meiosis II
1 of each type of
chromosome (n) in each
daughter cell (gamete)

51. Mitosis vs Meiosis
• Produces two diploid
daughter cells
• Produces daughter cells
that ARE genetically
identical
• Produce four haploid
daughter cells
• Produces daughter cells
that are NOT
genetically identical

52. Separation of Alleles During Meiosis
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Haploid cells
Heterozygous (Yy) cell from
a plant with yellow seeds
y
y
y
y y
y
y
Y Y
Y
Y
Y Y
Y

53. Meiosis I
Meiosis II
or
2 Ry : :
Heterozygous diploid
cell (YyRr) to
undergo meiosis
y
y
y
y y
y
y
y
y
Y
Y
Y
Y
Y
Y
Y
Y Y
R
R
R r
r
y
r
r R
R
Y
R
R
R
R
y
R
r
r
r
r
Y
r
Y
y
R
r
R
r
r
2 rY 2 ry 2 RY
Separation of Alleles During Meiosis

54. Spermatogenesis
• The production of sperm
• In male animals, it occurs in the testes
• A diploid spermatogonium cell divides
mitotically to produce two cells
– One remains a spermatogonial cell
– The other becomes a primary spermatocyte
• The primary spermatocyte progresses
through meiosis I and II
– Refer to Figure 3.14a

55. 3-55
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Figure 3.14 (a)
Meiois I yields
two haploid
secondary
spermatocyte
s
Meiois II yields
four haploid
spermatids
Each
spermatid
matures into a
haploid sperm
cell

56. 3-56
• The structure of a sperm includes
– A long flagellum
– A head
• The head contains a haploid nucleus
– Capped by the acrosome
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The acrosome contains
digestive enzymes
- Enable the sperm to
penetrate the protective
layers of the egg
 In human males, spermatogenesis is
a continuous process
 A mature human male produces several
hundred million sperm per day

57. Oogenesis
• The production of egg cells
• In female animals, it occurs in the ovaries
• Early in development, diploid oogonia produce
diploid primary oocytes
– In humans, for example, about 1 million primary
occytes per ovary are produced before birth
3-57
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

58. • The primary oocytes initiate meiosis I
• However, they enter into a dormant phase
– They are arrested in prophase I until the female
becomes sexually mature
• At puberty, primary oocytes are periodically
activated to progress through meiosis I
– In humans, one oocyte per month is activated
• The division in meiosis I is asymmetric
producing two haploid cells of unequal size
– A large secondary oocyte
– A small polar body
3-58
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

59. • The secondary oocyte enters meiosis II but
is quickly arrested in it
• It is released into the oviduct
– An event called ovulation
• If the secondary oocyte is fertilized
– Meiosis II is completed
– A haploid egg and a second polar body are
produced
• The haploid egg and sperm nuclei then fuse
to created the diploid nucleus of a new
individual

60. Summery questions
1. What is the importance of meiotic cell
division?
2. Explain the developmental stages of
meiosis cell division
3. What is the difference between mitotic
and meiotic cell division
4. What is the similarity and differences
between spermatogenesis and oogenesis.

61. Reference
• Robert F. weaver, Philip W. Hedrick.
Genetics.
• Darnel, Lodish, Baltimore. Molecular
Cell Biology
• James D. Watson: Recombinant DNA
• Robert F. Weaver. Molecular biology
• Benjamin Lewin: Genes VI and above

62. Reference continued….
• Richard J. Epistein: Human Molecular Biology
• P.K. Gupta: Cell and Molecular Biology
• Vertualtext erigito.com/Molecular biology
www.ergito.com
• http://www.emporia.edu/biosci/genetics/notegen
e.html
• http://www.biol.wwu.edu/young/321_07a.html