Download 5. Cell Division

The Cell Cycle and How Cells Divide
1
Phases of the Cell Cycle
•
The cell cycle consists of
– Interphase – normal cell activity
– The mitotic phase – cell divsion
INTERPHASE
Growth
G1
(DNA synthesis)
Growth
G2
2
Functions of Cell Division
100 µm
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
organism (LM).
200 µm
20 µm
(b) Growth and development.
(c) Tissue renewal. These dividing
This micrograph shows a
bone marrow cells (arrow) will
sand dollar embryo shortly after
give rise to new blood cells (LM).
the fertilized egg divided, forming
two cells (LM).
3
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
4
DNA
•
•
Genetic information - genome
Packaged into chromosomes
Figure 12.3
50 µm
5
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
6
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
7
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, doublestranded fiber extending chromosome’s
entire length.
Each unduplicated chromosome contains
one DNA molecule, which may be
several inches long
8
Structure of Chromosomes
 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:
9
Structure of Chromosomes
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.
10
Structure of Chromosomes
•
•
•
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:
11
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
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids
12
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.
13
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)
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)
14
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
15
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
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.
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Sister
chromatids
16
Centrometers
Sister chromatids
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:
Non-sister
chromatids
Centromere
Duplication
Sister
chromatids
Two unduplicated
chromosomes
Sister
chromatids
Two duplicated chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
17
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:
Metaphase chromosome
Centromere
region of
chromosome
Kinetochore
Kinetochore
microtubules
Sister Chromatids
18
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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).
19
Phases of the Cell Cycle
•
•
•
Interphase
–
G1 - primary growth
–
S - genome replicated
–
G2 - secondary growth
M - mitosis
C - cytokinesis
20
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
21
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
22
Mitotic Division of an Animal Cell
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
23
Mitotic Division of an Animal Cell
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nucleolus
forming
Nuclear
envelope
forming
24
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.
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Chromatin
(duplicated)
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.
Nucleolus
Nuclear
Plasma
envelope membrane
25
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
26
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
Metaphase
plate
Spindle
Centrosome at
one spindle pole
27
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
28
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
Aster
Microtubules
Sister
chromatids
Chromosomes
Centrosome
Metaphase
plate
Kinetochores
Centrosome
1 µm
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm
29
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 fullfledged 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
30
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
Cleavage
furrow
Nucleolus
forming
Nuclear
envelope
forming
31
Mitosis in a plant cell
Chromatine
Nucleus
Nucleolus condensing
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.
Chromosome
Metaphase. The
2 Prometaphase.
3
4
spindle is complete,
We now see discrete
and the chromosomes,
chromosomes; each
attached to microtubules
consists of two
at their kinetochores,
identical sister
are all at the metaphase
chromatids. Later
plate.
in prometaphase, the
nuclear envelop will
fragment.
5
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.
32
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
33
Cytokinesis In Animal And Plant Cells
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Vesicles
forming
cell plate
Wall of
patent cell
1 µm
Cell plate
New cell wall
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)
34
35
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
36
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
37
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
38
Asexual Reproduction
•
In asexual reproduction, one parent
produces genetically identical offspring by
mitosis
Parent
Bud
Figure 13.2
0.5 mm
39
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
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
FERTILIZATION
Zygote
2n
Diploid
multicellular
organism
2n
Mitosis
(a) Animals
40
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
41
Sexual Reproduction - The Human Life Cycle
Haploid gametes (n = 23)
•
•
During fertilization,
sperm and ovum fuse
forming a diploid
zygote
The zygote develops
into an adult organism
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
42
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)
43
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.
44
Meiosis
Interphase
•
•
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
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
45
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.
46
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
Nonsister
chromatids
Prophase I
of meiosis
Tetrad
Chiasma,
site of
crossing
over
47
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
PROPHASE I
Sister
chromatids
Tetrad
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Chiasmata
Metaphase
plate
Spindle
Microtubule
attached to
kinetochore
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Homologous
chromosomes
separate
Tetrads line up
Pairs of homologous
chromosomes split up
48
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
PROPHASE I
Sister
chromatids
Tetrad
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Chiasmata
Metaphase
plate
Spindle
Microtubule
attached to
kinetochore
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Homologous
chromosomes
separate
Tetrads line up
Pairs of homologous
chromosomes split up
49
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
50
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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
51
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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
52
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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
53
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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
54
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
55
A Comparison of Mitosis and Meiosis
•
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
56
A Comparison Of Mitosis And Meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
57
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)
58
Sexual Reproduction - The Human Life Cycle
Haploid gametes (n = 23)
•
•
During fertilization,
sperm and ovum fuse
forming a diploid
zygote
The zygote develops
into an adult organism
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
59
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
60
Spermatogenesis
Figure 27.8b, c
61
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
62
Oogenesis
63