1. Cell Cycle, Cell Division and Mitosis

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

3. Cell Division All cells are derived from pre-existing cells
New cells are produced for reproduction, growth and to replace damaged or old cells
Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)

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

5. 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

6. Cell Devision DNA must be copied or replicated before cell division
Each new cell will then have an identical copy of the DNA

7. Identical Daughter Cells
Two identical daughter cells
Parent Cell

8. Prokaryotic Chromosome
The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane

9. Eukaryotic Chromosomes
All eukaryotic cells store genetic information in chromosomes
Each chromosome is composed of a single, tightly coiled DNA molecule
Chromosomes can’t be seen when cells aren’t dividing and are called chromatin

10. Called Sister Chromatids
Duplicated chromosomes are called chromatids & are held together by the centromere
Called Sister Chromatids

11. Chromosomes Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere

12. Homologues Chromosome
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

13. Chromosome Duplication
In preparation for cell division, DNA is replicated and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
0.5 µm
Chromosome duplication (including DNA synthesis)
Centromere
Separation of sister chromatids
Sister chromatids
Centrometers
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.

14. Non-sister chromatids Two duplicated chromosomes
Chromosome Duplication
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:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicated
chromosomes
Centromere
Sister
chromatids
Duplication
Non-sister chromatids
Two duplicated chromosomes

15. 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
microtubules
Centromere
region of
chromosome
Sister Chromatids

16. 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).

17. Types of Cell Reproduction
Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells
Mitosis & binary fission are examples of asexual reproduction
Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells
Meiosis is an example of sexual reproduction

18. Cell Division in Prokaryotes
Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission
Single chromosome makes a copy of itself
Cell wall forms between the chromosomes dividing the cell
Parent cell
Chromosome doubles
Cell splits
2 identical daughter cells

19. Phases of the Cell Cycle
Interphase
G1 - primary growth
S - genome replicated
(DNA replication)
G2 - secondary growth
M - mitosis
C - cytokinesis

20. Phases of the Cell Cycle

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

22. Interphase - G1 Stage 1st growth stage after cell division
Cells mature by making more cytoplasm & organelles
Cell carries on its normal metabolic activities

23. Interphase – S Stage Synthesis stage DNA is copied or replicated
Two identical copies of DNA
Original DNA

24. Interphase – G2 Stage 2nd Growth Stage
Occurs after DNA has been copied
All cell structures needed for division are made (e.g. centrioles)
Both organelles & proteins are synthesized

25. Cell Divides into Identical cells
DNA Copied
Cells prepare for Division
Cells Mature
Daughter Cells
Cell Divides into Identical cells

26. DNA duplication during interphase
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

27. Mitosis Division of the nucleus Also called karyokinesis
Only occurs in eukaryotes
four stages
Doesn’t occur in some cells such as brain cells

28. Four Mitotic Stages
Prophase
Metaphase
Anaphase
Telophase

29. Early Prophase
Chromatin in nucleus condenses to form visible chromosomes
Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal)
Cytoplasm
Nucleolus
Nuclear Membrane
Chromosomes

30. Late Prophase Nuclear membrane & nucleolus are broken down
Chromosomes continue condensing & are clearly visible
Spindle fibers called kinetochores attach to the centromere of each chromosome
Spindle finishes forming between the poles of the cell

31. Late Prophase
Chromosomes
Nucleus & Nucleolus have disintegrated

32. Spindle Fiber attached to Chromosome
Kinetochore Fiber
Chromosome

33. Review of Prophase

34. Spindle Fibers
The mitotic spindle form from the microtubules in plants and centrioles in animal cells
Polar fibers extend from one pole of the cell to the opposite pole
Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach
Asters are short fibers radiating from centrioles

35. The Spindle

36. Metaphase
Chromosomes, attached to the kinetochore fibers, move to the center of the cell
Chromosomes are now lined up at the equator
Equator of Cell
Pole of the Cell

37. Metaphase Asters at the poles Spindle Fibers
Chromosomes lined at the Equator

38. Chromosomes at Equator
Metaphase
Aster
Chromosomes at Equator

39. Metaphase

40. Anaphase Occurs rapidly
Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers

41. Anaphase
Sister Chromatids being separated

42. Anaphase

43. Telophase Sister chromatids at opposite poles Spindle disassembles
Nuclear envelope forms around each set of sister chromatids
Nucleolus reappears
CYTOKINESIS occurs
Chromosomes reappear as chromatin

44. Comparison of Anaphase & Telophase

45. Mitosis in a plant cell Nucleus Chromatine condensing Chromosome1
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

46. Cytokinesis Means division of the cytoplasm
Division of cell into two, identical halves called daughter cells
In plant cells, cell plate forms at the equator to divide cell
In animal cells, cleavage furrow forms to split cell

47. Cytokinesis In Animal And Plant 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)
Daughter cells

48. Cell plate in animal cell
Cytokinesis
Cleavage furrow in animal cell
Cell plate in animal cell

49. Daughter Cells of Mitosis
Have the same number of chromosomes as each other and as the parent cell from which they were formed
Identical to each other, but smaller than parent cell
Must grow in size to become mature cells (G1 of Interphase)

50. Identical Daughter Cells2
Chromosome number the same, but cells smaller than parent cell

51. 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

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

53. 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 hromosomes 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

54. 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

55. METAPHASE
Spindle
Metaphase plate
Centrosome at one spindle pole
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.

56. 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

57. The Mitotic Spindle
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
1 µm

58. 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 non kinetochore microtubules lengthen.
By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes.
ANAPHASE
Daughter chromosomes

59. TELOPHASE AND CYTOKINESIS
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

60. Meiosis Formation of Gametes (Eggs & Sperm)

61. Meiosis Reduction in amount of genetic material in the daughter cell
A type of cell division that results in four daughter cells each with half number of chromosome of the parent cell, as in the production of gametes and plant spores
The process by which the nucleus divides in all sexually reproducing organisms during the production of spores and gametes
The process by which the cell divides producing daughter cell that have a single set of chromosomes and are called haploid as opposed to diploid cells with two sets of chromosome

62. Facts About Meiosis
Preceded by interphase which includes chromosome replication
Two meiotic divisions:
1. Meiosis I
2. Meiosis II
(Reduction- division)
Original cell is diploid (2n)
Four daughter cells produced that are monoploid (1 n)

63. Facts About 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.

64. Facts About Meiosis
Daughter cells contain half the number of chromosomes as the original cell
Produces gametes (eggs & sperm)
Occurs in the testes in males (Spermatogenesis)
Occurs in the ovaries in females (Oogenesis)

65. Meiosis It is the fundamental basis of sexual reproduction
Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

66. Fertilization
2n = 6
1n =3
“Putting it all together”

67. Stages of Meiosis Meiosis II Meiosis I Begins with Interphase
Chromosomes replicate
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I

68. Replication of Chromosomes
Replication is the process of duplicating a chromosome
Occurs prior to division
Replicated copies are called sister chromatids
Held together at centromere
Occurs in Interphase
Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.

69. A Replicated Chromosome
Gene X
Sister Chromatids
(same genes,
same alleles)
Homologs
(same genes,
different alleles)
Homologs separate in meiosis I and therefore different alleles separate.

70. Meiosis Forms Haploid Gametes
Meiosis must reduce the chromosome number by half
Fertilization then restores the 2n number
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right number!

71. Meiosis: Two Part Cell Division
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Haploid
Haploid

72. Meiosis I: Reduction Division
Nucleus
Spindle
fibers
Nuclear
envelope
Early Prophase I
(Chromosome number doubled)
Metaphase I
Late Prophase I
Telophase I
Anaphase I

73. Prophase I Late prophase Early prophase Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Early prophase
Homolog’s pair.
Crossing over occurs.

74. Tetrads Form in Prophase I
Homologous chromosomes (each with sister chromatids)  
Join to form a TETRAD
Called Synapsis

75. Crossing-Over Homologous chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces Genetic recombination in the offspring

76. Homologous Chromosomes During Crossing-Over

77. Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

78. Metaphase I
Homologous pairs of chromosomes align along the equator of the cell

79. Anaphase I Homolog’s separate and move to opposite poles.
Sister chromatid’s remain
attached at their centromers.

80. Telophase I Nuclear envelopes reassemble. Spindle disappears.
Cytokinesis divides cell into two.

81. Meiosis II
Only one homolog of each chromosome is present in the cell.
Gene X
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry
identical genetic
information.

82. Meiosis II 4 Identical haploid cells Prophase II Metaphase II
Telophase II
4 Identical haploid cells
Anaphase II

83. Prophase II
Nuclear envelope fragments.
Spindle forms.

84. Metaphase II
Chromosomes align
along equator of cell.

85. Anaphase II Sister chromatids separate and move to opposite poles.
Equator
Pole
Sister chromatids separate and move to opposite poles.

86. Telophase II Nuclear envelope assembles. Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.

87. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome

88. Oogenesis or Spermatogenesis
Gametogenesis
Oogenesis or Spermatogenesis

89. Spermatogenesis Occurs in the testes
Two divisions produce 4 spermatids
Spermatids mature into sperm
Men produce about 250,000,000 sperm per day

90. Spermatogenesis in the Testes
Spermatid

92. Oogenesis Occurs in the ovaries
Two divisions produce 3 polar bodies that die and 1 egg
Polar bodies die because of unequal division of cytoplasm
Immature egg called oocyte
Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

93. Oogenesis in the Ovaries

94. Oogenesis Oogonium (diploid) Mitosis Primary oocyte Meiosis I
Secondary
(haploid)
Meiosis II
(if fertilization
occurs)
First polar body
may divide
Polar
bodies
die
Ovum (egg)
Second
polar body a A X
Mature
egg

95. Mitosis Meiosis Mitosis & Meiosis Functions Function
Asexual reproduction
Growth, repair
Occurs throughout plant
Produces clones
Diploid parents and offspring
Meiosis
Function
Sexual reproduction
Occurs only in cells that give rise to sperm and eggs
Produces variable offspring
Diploid parents, haploid offspring

96. Mitosis & Meiosis Mitosis Meiosis Number of divisions 1 2
Number of daughter cells 4
Genetically identical?
Yes No
Chromosome number
Same as parent
Half of parent
Where
Somatic cells
Germ cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

97. Mitosis and Meiosis Mitosis Two diploid cells produced
Each identical to parent
Meiosis
Four haploid cells produced
Differ from parent and one another