The document summarizes key aspects of the cell cycle and cell division: 1) The cell cycle consists of interphase, where normal cell growth occurs, and mitosis, where the cell divides. 2) During interphase, DNA is replicated in S phase to produce identical copies, or sister chromatids. 3) Mitosis involves the division of the nucleus and separation of the sister chromatids into two daughter cells with identical genetic material.

1. 1
The Cell Cycle and How Cells Divide

2. 2
Phases of the Cell Cycle
• The cell cycle consists of
– Interphase – normal cell activity
– The mitotic phase – cell divsion
INTERPHASE
Growth
G 1
(DNA synthesis)
Growth
G2

3. 3
Functions of Cell Division
20 µm
100 µm 200 µm
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
organism (LM).
(b) Growth and development.
This micrograph shows a
sand dollar embryo shortly after
the fertilized egg divided, forming
two cells (LM).
(c) Tissue renewal. These dividing
bone marrow cells (arrow) will
give rise to new blood cells (LM).

4. 4
Cell Division
• An integral part of the cell cycle
• Results in genetically identical daughter cells
• Cells duplicate their genetic material
– Before they divide, ensuring that each daughter
cell receives an exact copy of the genetic
material, DNA

5. 5
DNA
• Genetic information - genome
• Packaged into chromosomes
50 µm
Figure 12.3

6. 6
DNA And Chromosomes
• An average eukaryotic cell has about 1,000
times more DNA then an average
prokaryotic cell.
• The DNA in a eukaryotic cell is organized
into several linear chromosomes, whose
organization is much more complex than the
single, circular DNA molecule in a
prokaryotic cell

7. 7
Chromosomes
• All eukaryotic cells store genetic information
in chromosomes.
– Most eukaryotes have between 10 and 50
chromosomes in their body cells.
– Human cells have 46 chromosomes.
– 23 nearly-identical pairs

8. 8
Structure of Chromosomes
• Chromosomes are composed of a
complex of DNA and protein called
chromatin that condenses during cell
division
• DNA exists as a single, long, double-
stranded fiber extending chromosome’s
entire length.
• Each unduplicated chromosome contains
one DNA molecule, which may be
several inches long

9. 9
 Every 200 nucleotide pairs, the DNA wraps twice around a
group of 8 histone proteins to form a nucleosome.
 Higher order coiling and supercoiling also help condense
and package the chromatin inside the nucleus:
Structure of Chromosomes

10. 10
The degree of coiling can vary in different
regions of the chromatin:
Heterochromatin refers to highly coiled
regions where genes aren’t expressed.
Euchromatin refers to loosely coiled regions
where genes can be expressed.
Structure of Chromosomes

11. 11
• Prior to cell division each
chromosome duplicates
itself.
• During this time, only the
heterochromatin is visible, as
dense granules inside the
nucleus.
• There is also a dense area of
RNA production called the
nucleolus:
Structure of Chromosomes

12. 12
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Karyotype
• An ordered, visual representation of the chromosomes in a cell
• Chromosomes are photographed when they are highly condensed, then photos
of the individual chromosomes are arranged in order of decreasing size:
• In humans each somatic cell has 46 chromosomes, made up of two sets, one
set of chromosomes comes from each parent

13. 13
Chromosomes
• Non-homologous chromosomes
– Look different
– Control different traits
• Sex chromosomes
– Are distinct from each other in their
characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being
female, XY being male
• In a diploid cell, the chromosomes occur in pairs.
The 2 members of each pair are called
homologous chromosomes or homologues.

14. 14
Chromosomes
• A diploid cell has two sets of each of its chromosomes
• A human has 46 chromosomes (2n = 46)
• In a cell in which DNA synthesis has occurred all the chromosomes are
duplicated and thus each consists of two identical sister chromatids
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere

15. 15
Homologues
• Homologous chromosomes:
• Look the same
• Control the same traits
• May code for different forms of each trait
• Independent origin - each one was inherited
from a different parent

16. 16
Chromosome Duplication
0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Sister
chromatids
Centrometers Sister chromatids
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
• In preparation for cell division, DNA is replicated and the chromosomes condense
• Each duplicated chromosome has two sister chromatids, which separate during cell
division

17. 17
• Because of duplication, each condensed chromosome
consists of 2 identical chromatids joined by a centromere.
• Each duplicated chromosome contains 2 identical DNA
molecules (unless a mutation occurred), one in each
chromatid:
Chromosome Duplication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicated
chromosomes
Centromere
Sister
chromatids
Sister
chromatids
Duplication
Non-sister
chromatids
Two duplicated chromosomes

18. 18
Structure of Chromosomes
• The centromere is a constricted region of the chromosome containing a
specific DNA sequence, to which is bound 2 discs of protein called
kinetochores.
• Kinetochores serve as points of attachment for microtubules that move
the chromosomes during cell division:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metaphase chromosome
Kinetochore
Kinetochore
microtubules
Centromere
region of
chromosome
Sister Chromatids

19. 19
Structure of Chromosomes
– Diploid - A cell possessing two copies of each chromosome
(human body cells).
 Homologous chromosomes are made up of sister
chromatids joined at the centromere.
– Haploid - A cell possessing a single copy of each
chromosome (human sex cells).

20. 20
Phases of the Cell Cycle
• Interphase
– G1 - primary growth
– S - genome replicated
– G2 - secondary growth
• M - mitosis
• C - cytokinesis

21. 21
Interphase
• G1 - Cells undergo majority of growth
• S - Each chromosome replicates (Synthesizes) to
produce sister chromatids
– Attached at centromere
– Contains attachment site (kinetochore)
• G2 - Chromosomes condense - Assemble
machinery for division such as centrioles

22. 22
Mitosis
 Some haploid & diploid cells divide by mitosis.
 Each new cell receives one copy of every
chromosome that was present in the original cell.
 Produces 2 new cells that are both genetically
identical to the original cell.
DNA duplication
during interphase
Mitosis
Diploid Cell

23. 23
Mitotic Division of an Animal Cell
G2 OF INTERPHASE PROPHASE PROMETAPHASE
Centrosomes
(with centriole pairs) Chromatin
(duplicated)
Early mitotic
spindle
Aster
Centromere
Fragments
of nuclear
envelope
Kinetochore
Nucleolus Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
microtubule
Nonkinetochore
microtubules

24. 24
METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS
Spindle
Metaphase
plate Nucleolus
forming
Cleavage
furrow
Nuclear
envelope
forming
Centrosome at
one spindle pole
Daughter
chromosomes
Mitotic Division of an Animal Cell

25. 25
G2 of Interphase
• A nuclear envelope bounds
the nucleus.
• The nucleus contains one or
more nucleoli (singular,
nucleolus).
• Two centrosomes have
formed by replication of a
single centrosome.
• In animal cells, each
centrosome features two
centrioles.
• Chromosomes, duplicated
during S phase, cannot be
seen individually because
they have not yet condensed.
The light micrographs show dividing lung cells
from a newt, which has 22 chromosomes in its
somatic cells (chromosomes appear blue,
microtubules green, intermediate filaments
red). For simplicity, the drawings show only
four chromosomes.
G2 OF INTERPHASE
Centrosomes
(with centriole pairs) Chromatin
(duplicated)
Nucleolus Nuclear
envelope
Plasma
membrane

26. 26
Prophase
• The chromatin fibers become
more tightly coiled, condensing
into discrete chromosomes
observable with a light
microscope.
• The nucleoli disappear.
• Each duplicated chromosome
appears as two identical sister
chromatids joined together.
• The mitotic spindle begins to form.
It is composed of the centrosomes
and the microtubules that extend
from them. The radial arrays of
shorter microtubules that extend
from the centrosomes are called
asters (“stars”).
• The centrosomes move away from
each other, apparently propelled
by the lengthening microtubules
between them.
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids

27. 27
Metaphase
• Metaphase is the longest stage of
mitosis, lasting about 20 minutes.
• The centrosomes are now at
opposite ends of the cell.
•The chromosomes convene on the
metaphase plate, an imaginary
plane that is equidistant between
the spindle’s two poles. The
chromosomes’ centromeres lie on
the metaphase plate.
• For each chromosome, the
kinetochores of the sister
chromatids are attached to
kinetochore microtubules coming
from opposite poles.
• The entire apparatus of
microtubules is called the spindle
because of its shape.
METAPHASE
Spindle
Metaphase
plate
Centrosome at
one spindle pole

28. 28
The Mitotic Spindle
• The spindle includes the centrosomes, the spindle
microtubules, and the asters
• The apparatus of microtubules controls
chromosome movement during mitosis
• The centrosome replicates, forming two
centrosomes that migrate to opposite ends of the
cell
• Assembly of spindle microtubules begins in the
centrosome, the microtubule organizing center
• An aster (a radial array of short microtubules)
extends from each centrosome

29. 29
• Some spindle microtubules attach to the kinetochores of
chromosomes and move the chromosomes to the
metaphase plate
• In anaphase, sister chromatids separate and move along
the kinetochore microtubules toward opposite ends of the
cell
Microtubules Chromosomes
Sister
chromatids
Aster
Centrosome
Metaphase
plate
Kineto-
chores
Kinetochore
microtubules
0.5 µm
Overlapping
nonkinetochore
microtubules
1 µm
Centrosome
The Mitotic Spindle

30. 30
Anaphase
• Anaphase is the shortest stage of
mitosis, lasting only a few minutes.
• Anaphase begins when the two sister
chromatids of each pair suddenly part.
Each chromatid thus becomes a full-
fledged chromosome.
• The two liberated chromosomes begin
moving toward opposite ends of the cell,
as their kinetochore microtubules
shorten. Because these microtubules are
attached at the centromere region, the
chromosomes move centromere first (at
about 1 µm/min).
• The cell elongates as the
nonkinetochore microtubules lengthen.
• By the end of anaphase, the two ends of
the cell have equivalent—and
complete—collections of chromosomes.
ANAPHASE
Daughter
chromosomes

31. 31
Telophase
• Two daughter nuclei begin to
form in the cell.
• Nuclear envelopes arise from
the fragments of the parent
cell’s nuclear envelope and
other portions of the
endomembrane system.
• The chromosomes become
less condensed.
• Mitosis, the division of one
nucleus into two genetically
identical nuclei, is now
complete.
TELOPHASE AND CYTOKINESIS
Nucleolus
forming
Cleavage
furrow
Nuclear
envelope
forming

32. 32
Mitosis in a plant cell
1 Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
Prometaphase.
We now see discrete
chromosomes; each
consists of two
identical sister
chromatids. Later
in prometaphase, the
nuclear envelop will
fragment.
Metaphase. The
spindle is complete,
and the chromosomes,
attached to microtubules
at their kinetochores,
are all at the metaphase
plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends
of cell as their
kinetochore
microtubles shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will
divided the cytoplasm
in two, is growing
toward the perimeter
of the parent cell.
2 3 4 5
Nucleus
Nucleolus
Chromosome
Chromatine
condensing

33. 33
Cytokinesis
• Cleavage of cell into two
halves
– Animal cells
 Constriction belt of
actin filaments
– Plant cells
 Cell plate
– Fungi and protists
 Mitosis occurs
within the nucleus

34. 34
Cytokinesis In Animal And Plant Cells
Daughter cells
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
100 µm
1 µm
Vesicles
forming
cell plate
Wall of
patent cell Cell plate
New cell wall
(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

35. 35

36. 36
Meiosis and Sexual Life Cycles
• Living organisms are distinguished by their ability to
reproduce their own kind
• Heredity
– Is the transmission of traits from one generation to the
next
• Variation
– Shows that offspring differ somewhat in appearance
from parents and siblings

37. 37
Inheritance of Genes
• Genes are segments of DNA, units
of heredity
• Offspring acquire genes from
parents by inheriting
chromosomes
• Genetics is the scientific study of
heredity and hereditary variation

38. 38
Inheritance of Genes
• Each gene in an organism’s DNA has a
specific locus on a certain chromosome
• We inherit one set of chromosomes from our
mother and one set from our father
• Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents - sexual reproduction

39. 39
Asexual Reproduction
• In asexual reproduction, one parent
produces genetically identical offspring by
mitosis
Figure 13.2
Parent
Bud
0.5 mm

40. 40
Sexual Reproduction
• Fertilization and meiosis alternate in sexual life cycles
• A life cycle is the generation-to-generation sequence of
stages in the reproductive history of an organism
Gametes
Diploid
multicellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n
2n
Zygote
Haploid
Diploid
Mitosis
(a) Animals

41. 41
Sex Cells - Gametes
• Unlike somatic cells, sperm and egg cells
are haploid cells, containing only one set of
chromosomes
• At sexual maturity the ovaries and testes
produce haploid gametes by meiosis

42. 42
Sexual Reproduction - The Human Life Cycle
• During fertilization,
sperm and ovum fuse
forming a diploid
zygote
• The zygote develops
into an adult organism
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

43. 43
Meiosis
• Reduces the chromosome number such that
each daughter
• Cell has a haploid set of chromosomes
• Ensures that the next generation will have:
– Diploid number of chromosome
– Exchange of genetic information
(combination of traits
– that differs from that of either parent)

44. 44
Meiosis
• Only diploid cells can divide by meiosis.
• Prior to meiosis I, DNA replication occurs.
• During meiosis, there will be two nuclear divisions, and the result will be
four haploid nuclei.
• No replication of DNA occurs between meiosis I and meiosis II.

45. 45
Meiosis
• Meiosis reduces the
number of chromosome
sets from diploid to
haploid
• Meiosis takes place in
two sets of divisions
– Meiosis I reduces the
number of chromosomes
from diploid to haploid
– Meiosis II produces four
haploid daughter cells
Figure 13.7
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids Diploid cell with
replicated
chromosomes
1
2
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Sister chromatids
separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

46. 46
Meiosis Phases
• Meiosis involves the same four phases seen in
mitosis
 prophase
 metaphase
 anaphase
 telophase
• They are repeated during both meiosis I and
meiosis II.
• The period of time between meiosis I and meiosis
II is called interkinesis.
• No replication of DNA occurs during interkinesis
because the DNA is already duplicated.

47. 47
Prophase I
• Prophase I occupies more than 90% of the time required for meiosis
• Chromosomes begin to condense
• In synapsis, the 2 members of each homologous pair of chromosomes
line up side-by-side, aligned gene by gene, to form a tetrad consisting
of 4 chromatids
• During synapsis, sometimes there is an exchange of homologous parts
between non-sister chromatids. This exchange is called crossing over
• Each tetrad usually has one or more chiasmata, X-shaped regions
where crossing over occurred
Prophase I
of meiosis
Tetrad
Nonsister
chromatids
Chiasma,
site of
crossing
over

48. 48
Metaphase I
• At metaphase I, tetrads line up at the metaphase plate, with one
chromosome facing each pole
• Microtubules from one pole are attached to the kinetochore of one
chromosome of each tetrad
• Microtubules from the other pole are attached to the kinetochore of the
other chromosome
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I METAPHASE I ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Pairs of homologous
chromosomes split up
Tetrads line up

49. 49
Anaphase I
• In anaphase I, pairs of homologous chromosomes separate
• One chromosome moves toward each pole, guided by the
spindle apparatus
• Sister chromatids remain attached at the centromere and
move as one unit toward the pole
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I METAPHASE I ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Pairs of homologous
chromosomes split up
Tetrads line up

50. 50
Telophase I and Cytokinesis
• In the beginning of telophase I, each half of the
cell has a haploid set of chromosomes; each
chromosome still consists of two sister chromatids
• Cytokinesis usually occurs simultaneously, forming
two haploid daughter cells
• In animal cells, a cleavage furrow forms; in plant
cells, a cell plate forms
• No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II
because the chromosomes are already replicated

51. 51
Prophase II
• Meiosis II is very similar to mitosis
• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each still composed of
two chromatids) move toward the metaphase plate
Cleavage
furrow
PROPHASE II METAPHASE II ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
Sister chromatids
separate
Haploid daughter cells
forming

52. 52
Metaphase II
• At metaphase II, the sister chromatids are at the metaphase plate
• Because of crossing over in meiosis I, the two sister chromatids of each
chromosome are no longer genetically identical
• The kinetochores of sister chromatids attach to microtubules extending
from opposite poles
Cleavage
furrow
PROPHASE II METAPHASE II ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
Sister chromatids
separate
Haploid daughter cells
forming

53. 53
Anaphase II
• At anaphase II, the sister chromatids separate
• The sister chromatids of each chromosome now move as
two newly individual chromosomes toward opposite poles
Cleavage
furrow
PROPHASE II METAPHASE II ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
Sister chromatids
separate
Haploid daughter cells
forming

54. 54
Telophase II and Cytokinesis
• In telophase II, the chromosomes arrive at opposite poles
• Nuclei form, and the chromosomes begin decondensing
• Cytokinesis separates the cytoplasm
• At the end of meiosis, there are four daughter cells, each with a haploid
set of unreplicated chromosomes
• Each daughter cell is genetically distinct from the others and from the
parent cell
Cleavage
furrow
PROPHASE II METAPHASE II ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
Sister chromatids
separate
Haploid daughter cells
forming

55. 55
A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome
sets, producing cells that are genetically identical
to the parent cell
• Meiosis reduces the number of chromosomes sets
from two (diploid) to one (haploid), producing cells
that differ genetically from each other and from the
parent cell
• The mechanism for separating sister chromatids is
virtually identical in meiosis II and mitosis

56. 56
• Three events are unique to meiosis, and all three
occur in meiosis l:
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
– At the metaphase plate, there are paired homologous
chromosomes (tetrads), instead of individual replicated
chromosomes
– At anaphase I of meiosis, homologous pairs move
toward opposite poles of the cell. In anaphase II of
meiosis, the sister chromatids separate
A Comparison of Mitosis and Meiosis

57. 57
MITOSIS MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n 2n
Daughter cells
of mitosis
2n = 6
A Comparison Of Mitosis And Meiosis

58. 58
Comparison
• Meiosis
• DNA duplication
followed by 2 cell
divisions
• Sysnapsis
• Crossing-over
• One diploid cell
produces 4
haploid cells
• Each new cell
has a unique
combination of
genes
• Mitosis
• Homologous
chromosomes do not
pair up
• No genetic exchange
between homologous
chromosomes
• One diploid cell
produces 2 diploid
cells or one haploid
cell produces 2
haploid cells
• New cells are
genetically identical to
original cell (except for
mutation)

59. 59
Sexual Reproduction - The Human Life Cycle
• During fertilization,
sperm and ovum fuse
forming a diploid
zygote
• The zygote develops
into an adult organism
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

60. 60
Spermatocytes to Spermatids
• Primary spermatocytes undergo meiosis I, forming
two haploid cells called secondary spermatocytes
• Secondary spermatocytes undergo meiosis II and
their daughter cells are called spermatids
• Spermatids are small round cells seen close to the
lumen of the tubule
• Late in spermatogenesis, spermatids are nonmotile
• Spermiogenesis – spermatids lose excess
cytoplasm and form a tail, becoming motile sperm

61. 61
Spermatogenesis
Figure 27.8b, c

62. 62
Oogenesis
• Production of female sex cells by meiosis
• In the fetal period, oogonia (2n ovarian stem cells)
multiply by mitosis and store nutrients
• Primordial follicles appear as oogonia are transformed
into primary oocytes
• Primary oocytes begin meiosis but stall in prophase I
• From puberty, each month one activated primary oocyte
completes meiosis one to produce two haploid cells
– The first polar body
– The secondary oocyte
• The secondary oocyte arrests in metaphase II and is
ovulated
• If penetrated by sperm the second oocyte completes
meiosis II, yielding:
– One large ovum (the functional gamete)
– A tiny second polar body

63. 63
Oogenesis