How Cells Divide – Mitosis and Meiosis
Chapters 11&12
1
Cell Division in Prokaryotes
•
Prokaryotic cell division occurs as binary
fission in which cell divides into two halves.
– Genetic information exists as a single,
circular double-stranded DNA molecule.
 Copying begins at replication origin, and
proceeds bi-directionally.
 One genome ends up in each
daughter cell.
2
Binary Fission
3
Discovery of 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
4
Structure of Chromosomes
•
•
Chromosomes are composed of a complex of
DNA and protein, chromatin.
– heterochromatin - not expressed
– euchromatin - expressed
DNA exists as a single, long, double-stranded
fiber extending chromosome’s entire length.
– forms nucleosome every 200 nucleotides
 DNA coiled around histone proteins
5
Eukaryotic Chromosomal Organization
6
Structure of Chromosomes
•
Karyotype - Individual’s particular array 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).
7
Karyotype & Chromosomes
8
Phases of the Cell Cycle
•
Five phases of cell division:
– G1 - primary growth phase
– S - genome replicated
– G2 - secondary growth phase
 collectively called interphase
– M - mitosis
– C - cytokinesis
9
Cell Cycle
10
Interphase
•
•
•
G1 - cells undergo majority of growth
S - each chromosome replicates to produce
sister chromatids
– attached at centromere
 contains attachment site (kinetochore)
G2 - chromosomes condense
– assemble machinery such as centrioles
11
12
Mitosis
•
Prophase
– spindle apparatus assembled
 Microtubules connect kinetochores on
each pair of sister chromatids to the
spindle poles.
– nuclear envelope breaks
13
14
Mitosis
•
Metaphase
– chromosomes align in cell’s center
 metaphase plate
 spindle
15
16
Mitosis
•
•
Anaphase
– sister chromatids pulled toward poles
 poles move apart
 centromeres move toward poles
 microtubules shorten
Telophase
– spindle disassembles
– nuclear envelope forms around each set
of sister chromatids
17
18
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
19
Cytokinesis
20
21
Cell Cycle Control
•
•
Two irreversible points in cell cycle:
– replication of genetic material
– separation of sister chromatids
Cell can be put on hold at specific
checkpoints.
22
Cell Control Cycle
•
•
•
G1 / S - primary division decision point
G2 / M - commitment to mitosis
Spindle checkpoint - all chromosomes are
attached to spindle
23
24
Growth Factors and the Cell Cycle
•
Each growing cell binds minute amounts of
positive regulatory signals (growth factors)
that stimulate cell division.
– If neighboring cells use up too much
growth factor, there is not enough left to
trigger cell division.
 Growth factors trigger intercellular
signaling systems.
25
26
Sexual Reproduction and Meiosis
Chapter 12
27
Reduction Division
•
In sexual reproduction, gametes fuse
(fertilization) to produce a zygote.
– Gamete formation involves a mechanism
(meiosis) that reduces the number of
chromosomes to half that found in other
cells.
 Adult body cells are diploid.
 Gamete cells are haploid.
 alternation of generations
28
Sexual Life Cycle
•
Diploid cells carry chromosomes from two
parents
– 2 haploid cells join to form diploid cell
29
Sexual Life Cycle
•
Three types of sexual life cycles.
– In sexual reproduction, haploid cells or
organisms alternate with diploid cells or
organisms
30
Sexual Life Cycle
31
Meiosis
•
•
•
Synapsis
– Homologues pair along their length.
Homologous recombination
– Genetic exchange (crossing over) occurs
between homologous chromosomes.
Reduction division
– Meiosis involves two successive divisions,
with no replication of genetic material
between them.
32
Unique Features of Meiosis
33
Prophase I
•
Homologous chromosomes become closely
associated in synapsis, exchange segments
via crossing over, and then separate.
– Presence of a chiasma indicates crossing
over has occurred.
34
Metaphase I
•
•
Terminal chiasmata holds homologous pair
together.
– Spindle microtubules attach to kinetochore
proteins on the outside of each
centromere.
Joined pairs of homologues lines up on
metaphase plate.
– orientation of each pair is random
35
36
Completing Meiosis
•
Anaphase I
– Spindle fibers begin to shorten and pull
whole centromeres toward poles.
 Each pole receives a member of each
homologous pair.
 complete set of haploid
chromosomes
 random orientation results in
independent assortment
37
38
Completing Meiosis
•
Telophase I
– Chromosomes are segregated into two
clusters; one at each pole.
 Nuclear membrane re-forms around each
daughter cell.
 Sister chromatids are no longer
identical due to crossing over.
39
40
Second Meiotic Division
•
•
Meiosis II resembles normal mitotic division.
– prophase II - nuclear envelope breaks
down and second meiotic division begins
– metaphase II - spindle fibers bind to both
sides of centromere
– anaphase II - spindle fibers contract and
sister chromatids move to opposite poles
– telophase II - nuclear envelope re-forms
Final result - four haploid cells
41
42
Sex
•
•
Asexual reproduction - individual inherits all its
chromosomes from a single parent
– parthenogenesis - development of an adult
from an unfertilized egg
Sexual reproduction - produces genetic
variability.
– Segregation of chromosomes tends to
disrupt advantageous combinations.
 Only some progeny maintain advantages.
43
Origin and Maintenance Of Sex
•
Theories
– DNA repair hypothesis
 Only diploid cells can effectively repair
certain kinds of chromosomal damage.
– Contagion hypothesis
 A secondary consequence of the
infection of eukaryotes by mobile
genetic elements.
44
Origin and Maintenance Of Sex
•
•
Red Queen hypothesis
– Current recessive alleles can be stored in
reserve for future use.
Miller’s Ratchet
– Sexual reproduction may be a method of
keeping the mutational load low.
45
Evolutionary Consequences of Sex
•
Evolutionary process is revolutionary and
conservative.
– pace of evolutionary change is
accelerated by genetic recombination
– evolutionary change not always favored by
selection
 may act to preserve existing gene
combinations
46
Independent Assortment
47