Mitosis and Meiosis

1
Cell Division

• Mitosis and Meiosis

2
Binary Fission
3
Human Life Cycle
4
Cellular Organization of the Genetic Material

• All the DNA in a cell constitutes the cells
  genome
• A genome can consist of a single DNA molecule
  (common in prokaryotic cells) or a number of DNA
  molecules (common in eukaryotic cells)
• DNA molecules in a cell are packaged into
  chromosomes

5

• Every eukaryotic species has a characteristic
  number of chromosomes in each cell nucleus
• Somatic cells (nonreproductive cells) have two
  sets of chromosomes
• Gametes (reproductive cells sperm and eggs) have
  half as many chromosomes as somatic cells
• Eukaryotic chromosomes consist of chromatin, a
  complex of DNA and protein that condenses during
  cell division

6
Distribution of Chromosomes During Eukaryotic
Cell Division

• In preparation for cell division, DNA is
  replicated and the chromosomes condense
• Each duplicated chromosome has two sister
  chromatids, which separate during cell division
• The centromere is the narrow waist of the
  duplicated chromosome, where the two chromatids
  are most closely attached

7
0.5 µm
Chromosomes
DNA molecules
Chromo- some arm
Chromosome duplication (including DNA synthesis)
Centromere
Sister chromatids
Separation of sister chromatids
Centromere
Sister chromatids
8

• Eukaryotic cell division consists of
• Mitosis, the division of the nucleus
• Cytokinesis, the division of the cytoplasm
• Gametes are produced by a variation of cell
  division called meiosis
• Meiosis yields nonidentical daughter cells that
  have only one set of chromosomes, half as many as
  the parent cell

9
Phases of the Cell Cycle

• The cell cycle consists of
• Mitotic (M) phase (mitosis and cytokinesis)
• Interphase (cell growth and copying of
  chromosomes in preparation for cell division)
• Interphase (about 90 of the cell cycle) can be
  divided into subphases
• G1 phase (first gap)
• S phase (synthesis)
• G2 phase (second gap)
• The cell grows during all three phases, but
  chromosomes are duplicated only during the S
  phase

10
Fig. 12-5
INTERPHASE
S (DNA synthesis)
G1
Cytokinesis
G2
Mitosis
MITOTIC (M) PHASE
11
Phases of Mitosis
BioFlix Mitosis

• Mitosis is conventionally divided into four or
  five phases
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis is well underway by late telophase

12
Phases of Mitosis
Metaphase
Anaphase
Telophase and Cytokinesis
G2 of Interphase
Prophase
Prometaphase
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Early mitotic spindle
Aster
Centromere
Fragments of nuclear envelope
Nonkinetochore microtubules
Metaphase plate
Cleavage furrow
Nucleolus forming
Daughter chromosomes
Nuclear envelope forming
Centrosome at one spindle pole
Nuclear envelope
Kinetochore
Spindle
Nucleolus
Plasma membrane
Chromosome, consisting of two sister chromatids
Kinetochore microtubule
13
Fig. 12-6b
G2 of Interphase
Prometaphase
Prophase
Chromatin (duplicated)
Nonkinetochore microtubules
Fragments of nuclear envelope
Aster
Centromere
Early mitotic spindle
Centrosomes (with centriole pairs)
Nuclear envelope
Plasma membrane
Chromosome, consisting of two sister chromatids
Kinetochore
Kinetochore microtubule
Nucleolus
14
Fig. 12-6d
Telophase and Cytokinesis
Metaphase
Anaphase
Cleavage furrow
Nucleolus forming
Metaphase plate
Daughter chromosomes
Nuclear envelope forming
Centrosome at one spindle pole
Spindle
15
Cytokinesis

• In animal cells, cytokinesis occurs by a process
  known as cleavage, forming a cleavage furrow
• In plant cells, a cell plate forms during
  cytokinesis

16
Mitosis in plant cells
17
Mitosis in plant cells
Fig. 12-UN2
18
INTERPHASE
Mitosis Review
G1
S
Cytokinesis
Mitosis
G2
MITOTIC (M) PHASE
Prophase
Telophase and Cytokinesis
Prometaphase
Anaphase
Metaphase
19
Cancer is the result of uncontrolled cell division
20

• Most animal cells exhibit density-dependent
  inhibition, in which crowded cells stop dividing
• Most animal cells also exhibit anchorage
  dependence, in which they must be attached to a
  substratum in order to divide
• Cancer cells exhibit neither density-dependent
  inhibition nor anchorage dependence

21
Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
25 µm
25 µm
(a) Normal mammalian cells
(b) Cancer cells
22
Loss of Cell Cycle Controls in Cancer Cells

• Cancer cells do not respond normally to the
  bodys control mechanisms
• Cancer cells may not need growth factors to grow
  and divide
• They may make their own growth factor
• They may convey a growth factors signal without
  the presence of the growth factor
• They may have an abnormal cell cycle control
  system

23

• A normal cell is converted to a cancerous cell by
  a process called transformation
• Cancer cells form tumors, masses of abnormal
  cells within otherwise normal tissue
• If abnormal cells remain at the original site,
  the lump is called a benign tumor
• Malignant tumors invade surrounding tissues and
  can metastasize, exporting cancer cells to other
  parts of the body, where they may form secondary
  tumors

24
Fig. 12-20
Lymph vessel
Tumor
Blood vessel
Cancer cell
Glandular tissue
Metastatic tumor
Cancer cells invade neigh- boring tissue.
A tumor grows from a single cancer cell.
Cancer cells spread to other parts of the body.
Cancer cells may survive and establish a
new tumor in another part of the body.
1
2
3
4
25
Cell Division

• And Reproduction
• And Meiosis

26
Inheritance of Genes

• Genes are the units of heredity, and are made up
  of segments of DNA
• Genes are passed to the next generation through
  reproductive cells called gametes (sperm and
  eggs)
• Each gene has a specific location called a locus
  on a certain chromosome
• One set of chromosomes is inherited from each
  parent

27
Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, one parent produces
  genetically identical offspring by mitosis
• A clone is a group of genetically identical
  individuals from the same parent
• In sexual reproduction, two parents give rise to
  offspring that have unique combinations of genes
  inherited from the two parents

28
Sets of Chromosomes in Human Cells

• Human somatic cells (any cell other than a
  gamete) have 23 pairs of chromosomes
• A karyotype is an ordered display of the pairs of
  chromosomes from a cell
• The two chromosomes in each pair are called
  homologous chromosomes, or homologs
• Chromosomes in a homologous pair carry genes
  controlling the same inherited characters

29
Fig. 13-3
APPLICATION
TECHNIQUE
5 µm
Pair of homologous replicated chromosomes
Centromere
Sister chromatids
Metaphase chromosome
30
Autosomes and Sex chromosomes

• The sex chromosomes are called X and Y
• Human females have a homologous pair of X
  chromosomes (XX)
• Human males have one X and one Y chromosome
• The 22 pairs of chromosomes that do not determine
  sex are called autosomes

31

• Each pair of homologous chromosomes includes one
  chromosome from each parent
• The 46 chromosomes in a human somatic cell are
  two sets of 23 one from the mother and one from
  the father
• A diploid cell (2n) has two sets of chromosomes
• For humans, the diploid number is 46 (2n 46)

32
Haploid cells

• A gamete (sperm or egg) contains a single set of
  chromosomes, and is haploid (n)
• For humans, the haploid number is 23 (n 23)
• Each set of 23 consists of 22 autosomes and a
  single sex chromosome

33
Review

• In a cell in which DNA synthesis has occurred,
  each chromosome is replicated
• Each replicated chromosome consists of two
  identical sister chromatids

34
Fig. 13-4
Diploid Cell
Key
Maternal set of chromosomes (n 3)
2n 6
Paternal set of chromosomes (n 3)
Two sister chromatids of one replicated chromosome
Centromere
Two nonsister chromatids in a homologous pair
Pair of homologous chromosomes (one from each set)
35
Meiosis reduces the number of chromosome sets
from diploid to haploid

• Like mitosis, meiosis is preceded by the
  replication of chromosomes
• Meiosis takes place in two sets of cell
  divisions, called meiosis I and meiosis II
• The two cell divisions result in four daughter
  cells, rather than the two daughter cells in
  mitosis
• Each daughter cell has only half as many
  chromosomes as the parent cell

36
The Stages of Meiosis (overview)

• In the first cell division (meiosis I),
  homologous chromosomes separate
• Meiosis I results in two haploid daughter cells
  with replicated chromosomes it is called the
  reductional division
• In the second cell division (meiosis II), sister
  chromatids separate
• Meiosis II results in four haploid daughter cells
  with unreplicated chromosomes

37
Fig. 13-7-1
Interphase
Homologous pair of chromosomes in diploid parent
cell
Meiosis overview
Chromosomes replicate
Homologous pair of replicated chromosomes
Sister chromatids
Diploid cell with replicated chromosomes
38
Fig. 13-7-2
Interphase
Homologous pair of chromosomes in diploid parent
cell
Meiosis overview
Chromosomes replicate
Homologous pair of replicated chromosomes
Sister chromatids
Diploid cell with replicated chromosomes
Meiosis I
Homologous chromosomes separate
1
Haploid cells with replicated chromosomes
39
Fig. 13-7-3
Interphase
Homologous pair of chromosomes in diploid parent
cell
Meiosis overview
Chromosomes replicate
Homologous pair of replicated chromosomes
Sister chromatids
Diploid cell with replicated chromosomes
Meiosis I
Homologous chromosomes separate
1
Haploid cells with replicated chromosomes
Meiosis II
Sister chromatids separate
2
Haploid cells with unreplicated chromosomes
40

• Meiosis I is preceded by interphase, in which
  chromosomes are replicated to form sister
  chromatids
• The sister chromatids are genetically identical
  and joined at the centromere

BioFlix Meiosis
41
Fig. 13-8
Telophase I and Cytokinesis
Telophase II and Cytokinesis
Metaphase I
Prophase II
Metaphase II
Anaphase II
Prophase I
Anaphase I
Centrosome (with centriole pair)
Sister chromatids remain attached
Centromere (with kinetochore)
Sister chromatids
Chiasmata
Spindle
Metaphase plate
Sister chromatids separate
Haploid daughter cells forming
Homologous chromosomes separate
Cleavage furrow
Homologous chromosomes
Fragments of nuclear envelope
Microtubule attached to kinetochore
42
Meiosis I Phases

• Division in meiosis I occurs in four phases
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I and cytokinesis

43
Fig. 13-8a
Telophase I and Cytokinesis
Metaphase I
Prophase I
Anaphase I
Centrosome (with centriole pair)
Sister chromatids remain attached
Centromere (with kinetochore)
Sister chromatids
Chiasmata
Spindle
Metaphase plate
Cleavage furrow
Homologous chromosomes separate
Homologous chromosomes
Microtubule attached to kinetochore
Fragments of nuclear envelope
44
Fig. 13-8b
Prophase I
Metaphase I
Centrosome (with centriole pair)
Centromere (with kinetochore)
Sister chromatids
Chiasmata
Spindle
Metaphase plate
Homologous chromosomes
Fragments of nuclear envelope
Microtubule attached to kinetochore
45
Prophase I

• Chromosomes begin to condense
• In synapsis, homologous chromosomes loosely pair
  up, aligned gene by gene
• Each pair of chromosomes forms a tetrad, a group
  of four chromatids
• In crossing over, nonsister chromatids exchange
  DNA segments
• Each tetrad usually has one or more chiasmata,
  X-shaped regions where crossing over occurred

46
Metaphase I

• In 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

47
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

48
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

49
Meiosis II Phases

• Division in meiosis II also occurs in four
  phases
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II and cytokinesis
• Meiosis II is very similar to mitosis

50
Prophase II

• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each still
  composed of two chromatids) move toward the
  metaphase plate

51
Metaphase II

• In metaphase II, the sister chromatids are
  arranged at the metaphase plate
• Because of crossing over in meiosis I, the two
  sister chromatids of each chromosome are no
  longer genetically identical
• The sister chromatids attach to microtubules
  extending from opposite poles

52
Anaphase II

• In anaphase II, the sister chromatids separate
• The sister chromatids of each chromosome now move
  as two newly individual chromosomes toward
  opposite poles

53
Telophase II

• In telophase II, the chromosomes arrive at
  opposite poles
• Nuclei form, and the chromosomes begin
  decondensing

54
Cytokinesis

• 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

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
Fig. 13-9a
MITOSIS
MEIOSIS
MEIOSIS I
Chiasma
Parent cell
Chromosome replication
Chromosome replication
Prophase I
Prophase
Homologous chromosome pair
2n 6
Replicated chromosome
Metaphase I
Metaphase
Anaphase I
Anaphase Telophase
Telophase I
Haploid n 3
Daughter cells of meiosis I
MEIOSIS II
2n
2n
Daughter cells of mitosis
n
n
n
n
Daughter cells of meiosis II
57
Fig. 13-9b
SUMMARY
Meiosis
Mitosis
Property
DNA replication
Occurs during interphase before mitosis begins
Occurs during interphase before meiosis I begins
Number of divisions
One, including prophase, metaphase, anaphase, and
telophase
Two, each including prophase, metaphase,
anaphase, and telophase
Occurs during prophase I along with crossing
over between nonsister chromatids resulting
chiasmata hold pairs together due to sister
chromatid cohesion
Synapsis of homologous chromosomes
Does not occur
Number of daughter cells and genetic composition
Two, each diploid (2n) and genetically identical
to the parent cell
Four, each haploid (n), containing half as many
chromosomes as the parent cell genetically
different from the parent cell and from each other
Role in the animal body
Enables multicellular adult to arise from zygote
produces cells for growth, repair, and, in some
species, asexual reproduction
Produces gametes reduces number of chromosomes
by half and introduces genetic variability among
the gametes
58

• 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, it is homologous chromosomes,
  instead of sister chromatids, that separate

59
Genetic variation produced in sexual life cycles
contributes to evolution

• Mutations (changes in an organisms DNA) are the
  original source of genetic diversity
• Mutations create different versions of genes
  called alleles
• Reshuffling of alleles during sexual reproduction
  produces genetic variation

60
Origins of Genetic Variation Among Offspring

• The behavior of chromosomes during meiosis and
  fertilization is responsible for most of the
  variation that arises in each generation
• Three mechanisms contribute to genetic variation
• Independent assortment of chromosomes
• Crossing over
• Random fertilization

61
Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly
  at metaphase I of meiosis
• In independent assortment, each pair of
  chromosomes sorts maternal and paternal
  homologues into daughter cells independently of
  the other pairs
• The number of combinations possible when
  chromosomes assort independently into gametes is
  2n, where n is the haploid number
• For humans (n 23), there are more than 8
  million (223) possible combinations of chromosomes

62
Fig. 13-11-1
Possibility 2
Possibility 1
Two equally probable arrangements of chromosomes
at metaphase I
63
Fig. 13-11-2
Possibility 2
Possibility 1
Two equally probable arrangements of chromosomes
at metaphase I
Metaphase II
64
Fig. 13-11-3
Possibility 2
Possibility 1
Two equally probable arrangements of chromosomes
at metaphase I
Metaphase II
Daughter cells
Combination 1
Combination 2
Combination 3
Combination 4
65
Crossing Over

• Crossing over produces recombinant chromosomes,
  which combine genes inherited from each parent
• Crossing over begins very early in prophase I, as
  homologous chromosomes pair up gene by gene
• In crossing over, homologous portions of two
  nonsister chromatids trade places
• Crossing over contributes to genetic variation by
  combining DNA from two parents into a single
  chromosome

66
Fig. 13-12-1
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
67
Fig. 13-12-2
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
68
Fig. 13-12-3
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
69
Fig. 13-12-4
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
70
Fig. 13-12-5
Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter cells
Recombinant chromosomes
71
Random Fertilization

• Random fertilization adds to genetic variation
  because any sperm can fuse with any ovum
  (unfertilized egg)
• The fusion of two gametes (each with 8.4 million
  possible chromosome combinations from independent
  assortment) produces a zygote with any of about
  70 trillion diploid combinations

72

• Crossing over adds even more variation
• Each zygote has a unique genetic identity

Animation Genetic Variation
73
The Evolutionary Significance of Genetic
Variation Within Populations

• Natural selection results in the accumulation of
  genetic variations favored by the environment
• Sexual reproduction contributes to the genetic
  variation in a population, which originates from
  mutations