Chapter five covers cell division, focusing on mitosis and meiosis, highlighting processes such as chromosome replication, alignment, and separation. Mitosis results in two genetically identical daughter cells, while meiosis produces four haploid gametes with genetic variation through crossing over. The chapter explains the significance of cell division in growth, reproduction, and genetic diversity.

1. Chapter five :
Cell Division Cycle
Part 1: MITOSIS

2. 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

3. The Cell Cycle
G1 G2
S
Two
daughter
cells
M
Cytokinesis
T
e
l
o
p
h
a
s
e
A
n
a
p
h
a
s
e
M
e
t
a
p
h
a
s
e
P
r
o
m
e
t
a
p
h
a
s
e
Prophase
Mitosis
Interphase
Gap 1 Gap 2
Synthesis
Growth
Gene expression
Differentiation
DNA Synthesis
Gene expression
Quality control
Actual division process

4. Three Little Words Geneticist Need to Hear… Homolog,
Loci, Allele
Homologous
pair of
chromo-
somes
Gene loci (location)
A b c
A B c
AA Bb cc
Genotype:
Homozygous
for the
dominant
allele
Heterozygous Homozygous
for the
recessive
allele
Unreplicated chromosome pair

5. Replicated Chromosome
Pair of sister chromatids
Kinetochore
proteins
Centromere
(DNA that is
hidden beneath
the kinetochore
proteins)
One
chromatid
(dark blue)
One
chromatid
(light blue)
(b)
(a)

6. • 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…

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

8. Prophase
• Nuclear envelope
dissociates
• Centrosomes move
to opposite poles
• mitotic spindle
apparatus forms
Microtubules
forming mitotic spindle Sister
chromatids
Centromere

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Polar
microtubule
Kinetochore
proteins attached
to centromere Kinetochore
microtubule
Astral
microtubule
Metaphase
plate
(d) METAPHASE

10. 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

11. Figure 3.8
Kinetochore Spindle Fibers

12. Prometaphase
• 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

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

14. 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

15. 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

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

17. Cell division
Part 2:
MEIOSIS

18. Organisms that reproduce Sexually are made
up of two different types of cells.
1. Somatic Cells are “body” cells and contain the normal
number of chromosomes ….called the “Diploid”
number (the symbol is 2n). Examples would be … skin
cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½ the
normal number of chromosomes…. called the
“Haploid” number (the symbol is n)….. Sperm cells and
ova are gametes.
n = number of chromosomes in the set… so….2n means 2 chromosomes in the
set…. Polyploid cells have more than two chromosomes per set… example: 3n (3
chromosomes per set)

19. Gametes
• The Male Gamete is the Sperm and is produced
in the male gonad the Testes.
• The Female Gamete is the Ovum (ova = pl.) and is
produced in the female gonad the Ovaries.

20. During Ovulation the ovum is released from the ovary and transported to an area where
fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans,
occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized
egg)
Sperm + Ovum (egg) Zygote
fertilization

21. Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

22. Chromosomes
• If an organism has the Diploid number (2n) it has two
matching homologues per set. One of the homologues
comes from the mother (and has the mother’s DNA).…
the other homologue comes from the father (and has the
father’s DNA).
• Most organisms are diploid. Humans have 23 sets of
chromosomes… therefore humans have 46 total
chromosomes….. The diploid number for humans is 46
(46 chromosomes per cell).

23. Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are similar in
shape and size.
• Homologous pairs (tetrads) carry genes controlling the same
inherited traits.
• Each locus (position of a gene) is in the same position on
homologues.
• Humans have 23 pairs of homologous chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes

24. Homologous Chromosomes
(because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

25. Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue

26. Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22

27. Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male.
XX chromosome - female XY chromosome - male
In Humans the “Sex
Chromosomes” are the
23rd
set

28. Sex Chromosomes
“Sex Chromosomes”
…….the 23rd
set
23
This person has 2 “X”
chromosomes… and is a
female.

29. Meiosis
is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are
produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n)  Haploid (n)
If Meiosis did not occur the chromosome number in
each new generation would double…. The
offspring would die.

30. Meiosis
Meiosis is Two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes.

31. Meiosis in males is called spermatogenesis
and produces sperm.
Meiosis in females is called oogenesis and
produces ova.

32. Spermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
4 sperm cells are produced
from each primary
spermatocyte.
Primary Spermatocyte
Secondary Spermatocyte
Secondary Spermatocyte

33. Oogenesis
*** The polar bodies die… only one ovum (egg) is
produced from each primary oocyte.

34. Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical
sister chromatids attached at their centromeres.
• Centriole pairs also replicate.

35. Interphase I
• Nucleus and nucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin

36. Meiosis I (four phases)
• Cell division that reduces the chromosome number
by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

37. Prophase I
• Longest and most complex phase.
• 90% of the meiotic process is spent in Prophase I
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together
to form a tetrad.
• Tetrad is two chromosomes or four chromatids (sister and
nonsister chromatids).

38. Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

39. During Prophase I
“Crossing Over” occurs.
• During Crossing over segments of nonsister
chromatids break and reattach to the other
chromatid. The Chiasmata (chiasma) are the
sites of crossing over.
Crossing Over is one of the Two major occurrences of Meiosis
(The other is Non-disjunction)

40. Crossing Over
creates variation (diversity) in the offspring’s traits.
nonsister chromatids
chiasmata: site of
crossing over
variation
Tetrad

41. Question:
• A cell containing 20 chromosomes (diploid) at
the beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?

42. Answer:
• 10 chromosomes (haploid)

43. Question:
• A cell containing 40 chromatids at the beginning of
meiosis would, at its completion, produce cells
containing how many chromosomes?

44. Answer:
• 10 chromosomes

45. Prophase I
centrioles
spindle fiber
aster
fibers

46. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22
= 4 combinations

47. Anaphase I
• Homologous chromosomes separate and move
towards the poles.
• Sister chromatids remain attached at their
centromeres.

48. Anaphase I

49. Telophase I
• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
are formed.

50. Telophase I

51. Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis

52. Prophase II
• same as prophase in mitosis

53. Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate

54. Anaphase II
• same as anaphase in mitosis
• sister chromatids separate

55. Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells
produced.
gametes = sperm or egg

56. Telophase II

57. Non-disjunction
• Non-disjunction is the failure of homologous
chromosomes, or sister chromatids, to separate
during meiosis.
• Non-disjunction results with the production of
zygotes with abnormal chromosome numbers……
remember…. An abnormal chromosome number
(abnormal amount of DNA) is damaging to the
offspring.
Non-disjunction is one of the Two major occurrences of Meiosis
(The other is Crossing Over)