Download The Key Roles of Cell Division

BSC 2010 Integrated Principles of Biology I - Genetics
• Lecturer: Dr. J. C. Herrera, Zoology
• Office: 3175 McCarty Hall
• Office Hours: TWTh 3rd and 5th periods, and by appointment
• Phone: 213-2498, Email: [email protected]
• Course Home Page: http://nersp.nerdc.ufl.edu/~herrera/bsc.html
• Lectures
• Tuesday, Wednesday, Thursday, McCarty C 100 (MCC 100)
• Discussions
• Friday, MCC 100; May include lectures as needed.
Exam II Lectures and Text Pages
• I. Cell Cycles
–
Mitosis (218 – 228)
–
Meiosis (238 – 249)
• II. Mendelian Genetics
• III. Chromosomal Genetics
• IV. Molecular Genetics
–
Replication
–
Transcription and Translation
• V. Microbial Models
• VI. DNA Technology
The Key Roles of Cell Division
• The continuity of life
– Is based upon the reproduction of cells, or cell
division
Figure 12.1
1
Unicellular organisms
• reproduce by cell division
100 µm
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
Figure 12.2 A
organism (LM).
Multicellular organisms
• depend on cell division for
– Development from a fertilized cell (zygote)
– Growth
– Maintenance and repair
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
give rise to new blood cells (LM).
after the fertilized egg divided,
forming
two
cells
(LM).
Figure 12.2 B, C
The Cell Cycle
• The cell cycle is the life of the cell from its
formation to its own division.
• Cell division is just a portion of the cell’s lifecycle
(cell cycle)
– Some cells go through repeated cell cycles.
– Other cells never or rarely divide once they are
formed (e.g., vertebrate nerve and muscle cells).
2
Mitotic cell division
• produces genetically identical daughter cells
• Cells precisely duplicate their genetic
material
• They allocate the two copies to opposite
ends of the cell
• Then they divide into two new daughter cells
Cellular Organization of Genetic Material
• A cell’s total endowment of genetic information
– Is called its genome
• The DNA in a cell
– Is packaged into
chromosomes
Figure 12.3
50 µm
Eukaryotic chromosomes
• Eukaryotic chromosomes
– Are supercoils of chromatin, a complex of
DNA and protein that condenses durnig cell
division
– Allow duplication and distribution of large
genomes
• In animals
– Somatic cells have two haploid sets of
chromosomes
– Gametes have one haploid set of
chromosomes
3
Chromosomes
• Each chromosomes has:
–
A single, long, molecule of DNA, segments of which are genes.
–
Proteins which maintain the structure of the chromosome or are
involved with the expression of genes, DNA replication, and DNA
repair
• Each species: has a characteristic number of chromosomes
• The chromosomes: are in different states at different stages of the
cell cycle.
–
Interphase: loosely folded; not visible.
–
Mitotic phase: highly folded and condensed; visible with a light
microscope (basis of karyotype)
Distribution of Chromosomes During Cell Division
• During S-phase of interphase, DNA is replicated and
the chromosomes condense
• Each duplicated
chromosome
– Has two sister
chromatids,
attached at the
centromere
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.
Figure 12.4
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Sister
chromatids
Centromeres
Sister chromatids
Important Note
• chromosome, chromatid, chromatin
• The course includes several sets of words
which are very similar – be sure you know the
differences among each set of similar words.
• Another example: centromere, centrosome,
centriole
4
Two Types of Eukaryotic Cell Division
• Mitotic cell division (occurs only in eukaryotes) consists of
–
Mitosis - the division of the nucleus - sister chromatids pulled
apart into two sets of chromosomes, one at each end of the cell.
–
Cytokinesis - the division of the cytoplasm into two separate
daughter cells, each containing 1 nucleus with 1 diploid set of
single-copy chromosomes.
• Animal cells - cytokinesis = cleavage
• Plant cells - cytokinesis = cell plate formation
• Not all cells undergo cytokinesis following mitosis.
• Meiotic cell division
–
Gametes are produced after a reduction in chromosome number
–
Gametes each have 1 haploid set of chromosomes
Human Life Cycle
•
Adult inherits 46 chromosomes (2n)
•
Meiosis in gonads halves
chromosome number (n)
– Sperm cell (23 chromosomes)
– Ovum (23 chromosomes)
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
•
Fertilization restores the chromosome
number to 46 and results in a zygote
Ovary
Mitosis produces genetically identical
daughter cells. Mitosis is responsible
for growth and development.
Diploid
zygote
(2n = 46)
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
Phases of the Mitotic Cell Cycle
• The cell cycle consists of
– Interphase
– The mitotic phase (m-phase)
INTERPHASE
G1
C
M yto
ito ki
si ne
s si
s
•
Testis
S
(DNA synthesis)
G2
MIT
(M OT
) P IC
HA
SE
Figure 12.5
5
Interphase
• Interphase is 90% of the cell cycle and can be
divided into 3 sub-phases
– G1 phase – Growth 1 (Gap 1) cell grows
– S phase – Synthesis phase – DNA replicates
(cell continues to grow)
– G2 phase - Growth 2 (Gap 2) cell grows
The Mitotic (M) Phase
• The mitotic phase - 10% of the cell cycle, and
– Is made up of mitosis (division of the nucleus)
and cytokinesis (division of the cytoplasm)
• Mitosis is unique to eukaryotes and thought to be
an evolutionary adaptation to handle large amounts
of genetic material
• Mitosis is a very reliable process, only 1 error in
about 100,000 cell divisions
• Mitosis is a continuous process, and cytokinesis
usually begins during telophase
Mitosis
• Mitosis begins with a cell just leaving G2 of
Interphase
Nucleus: well-defined
Nucleoli: visible
Centrosomes: two,
adjacent to nucleus
A pair of centrioles: in
each centrosome
An aster: a microtubular
array around each
centrosome
G2 OF
INTERPHASE
Centrosomes
Chromatin
(with centriole pairs)
(duplicated)
PROPHASE
Early mitotic
spindle
Aster
Centromere
PROMETAPHASE
Fragments Kinetochore
of nuclear
envelope
Nonkinetochore
microtubules
Chromosomes:
duplicated, but cannot be
distinguished individually.
Figure 12.6
Nucleolus
Nuclear
Plasma
envelope membrane
Chromosome, consisting
of two sister chromatids
Kinetochore
microtubule
6
The Five Phases of Mitosis - Prophase
In the nucleus:
Nucleoli disappear
Chromatin fibers condense into visible doubled chromosomes
In the cytoplasm:
Mitotic spindle
(microtubules)
forms.
Figure 12.6
Centrosomes move
apart due to
lengthening of
spindle fibers
between them.
The Five Phases of Mitosis - Prometaphase
Nucleus: envelope disintegrates
Spindle fibers: extend from centrosome at each pole toward the cell’s equator.
Each chromatid: has a kinetochore, at the centromere region = spindle attachment site.
By the end of prometaphase the chromosomes are aligned at the center of the cell.
Spindle microtubules:
Kinetochore
microtubules:
become attached to
the kinetochores.
Figure 12.6
Nonkinetochore
microtubules:
radiate from each
centrosome toward
the equator without
attaching to
chromosomes. They
overlap with those
from opposite
pole. May be
attached by protein
bridges.
The Five Phases of Mitosis - Metaphase
Centrosomes: already at
opposite poles.
Chromosomes: already
lined up at the
metaphase plate.
Centromeres: aligned
on metaphase plate.
The long axis of each
chromosome: at a
right angle to the
spindle axis
METAPHASE
Figure 12.6
ANAPHASE
Metaphase
plate
Kinetochores: (structures
of proteins and DNA
that are part of the
centromere) of sister
chromatids face
opposite poles.
Spindle
Centrosome at Daughter
one spindle pole chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nucleolus
forming
Nuclear
envelope
forming
7
The Five Phases of Mitosis - Anaphase
Paired centromeres: of
each chromosome
separate and move
apart.
Sister chromatids: split
apart - move towards
opposite poles.
Kinetochore microtubules:
shorten at the
kinetochore end.
Figure 12.6
Nonkinetochore
microtubules: move cell
ends further apart.
The Five Phases of Mitosis - Telophase
Nonkinetochore microtubules further elongate cell.
Daughter nuclei form, from fragments of the parent nucleus, at poles.
Nucleoli reappear
There is a set of chromosomes at each pole of the cell, and the chromatin uncoils and
chromosomes no longer visible.
By the end of
telophase:
Mitosis is
complete.
Cytokinesis has
begun.
Figure 12.6
The Mitotic Spindle
• Events of mitosis depend on the spindle,
which forms in the cytoplasm from
microtubules and other proteins.
• Microtubules of the cytoskeleton: partially
disassembled during spindle formation.
– alpha- and beta-tubulin
– Fibers: elongate by adding tubulin subunits at
the end away from the centrosome
• Assembly begins in the centrosome
(microtubule organizing center).
8
The Mitotic Spindle
• The spindle arises
from the centrosomes
– And includes
spindle fibers and
asters
Aster
Sister
chromatids
Centrosome
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores
microtubules
Microtubules
0.5 µm
Chromosomes
Figure 12.7 Centrosome
1 µm
The Mitotic Spindle
• The kinetochore microtubules of the spindle 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
• Current model: Kinetochore microtubules shorten by
depolymerizing
at the kinetochore
ends,were
and
motor
proteins
The microtubules
of a cell in early anaphase
labeled
with a fluorescent
dye
that glows in the microscope (yellow).
“walk” the chromosomes poleward.
Kinetochore
Spindle
pole
Figure 12.8
The Mitotic Spindle
• Nonkinetechore microtubules from opposite
poles
– Overlap and push against each other,
elongating the cell in preparation for
cytokinesis. Probably attached to each other
by protein bridges.
• Cytokinesis usually begins during telophase
9
Cytokinesis
• In animal cells
– Cytokinesis occurs by a
process known as
cleavage, first forming a
cleavage furrow
– Actin microfilaments form
a ring just under the
plasma membrane. They
constrict and pinch the
cell in two.
Cleavage furrow
Contractile ring of
microfilaments
Figure 12.9 A
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Cytokinesis
• In plant cells
– Vesicles from the Golgi
apparatus carrying cell
wall material converge at
the center of the cell
– The vesicles fuse forming
the cell plate
– The plate grows and
connects to the existing
cell wall forming new cell
wall.
Vesicles
forming
cell plate
1 µm
Wall of
patent cell Cell plate
New cell wall
Daughter cells
Figure 12.9 B (b) Cell plate formation in a plant cell (SEM)
Mitosis in a plant cell
Chromatine
Nucleus
Nucleolus condensing
Chromosome
Metaphase. The
2 Prometaphase.
3
1 Prophase.
spindle is complete, 4
The chromatin
We now see discrete
and the chromosomes,
is condensing.
chromosomes; each
attached to microtubules
The nucleolus is
consists of two
at their kinetochores,
beginning to
identical sister
are all at the metaphase
disappear.
chromatids. Later
plate.
Although not
in prometaphase, the
yet visible
nuclear envelop will
in the micrograph,
fragment.
the mitotic spindle is
staring to from.
Anaphase. The
5
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.
Figure 12.10
10
Binary Fission – Prokaryotic Cell Division
• In binary fission
– The single circular bacterial chromosome replicates
– The two daughter chromosomes actively move apart
Origin of
replication
Cell wall
E. coli cell
1 Chromosome replication begins.
Soon thereafter, one copy of the
origin moves rapidly toward the
other end of the cell.
Two copies
of origin
2 Replication continues. One copy of
the origin is now at each end of
the cell.
Origin
Plasma
Membrane
Bacterial
Chromosome
Origin
3 Replication finishes. The plasma
membrane grows inward, and
new cell wall is deposited.
Figure 12.11 4 Two daughter cells result.
The Evolution of Mitosis
• Since prokaryotes are simpler and preceded
eukaryotes by billions of years
– It is likely that mitosis evolved from bacterial
cell division
• Certain protists
– Exhibit types of cell division that may represent
intermediates between binary fission and the
type of mitosis carried out by most eukaryotic
cells
A hypothetical sequence for the evolution of mitosis
(a) Prokaryotes. During binary fission, the origins of the
daughter chromosomes move to opposite ends of the
cell. The mechanism is not fully understood, but
proteins may anchor the daughter chromosomes to
specific sites on the plasma membrane.
(b) Dinoflagellates. In unicellular protists called
dinoflagellates, the nuclear envelope remains intact
during cell division, and the chromosomes attach to the
nuclear envelope. Microtubules pass through the
nucleus inside cytoplasmic tunnels, reinforcing the
spatial orientation of the nucleus, which then divides in a
fission process reminiscent of bacterial division.
(c) Diatoms. In another group of unicellular protists, the
diatoms, the nuclear envelope also remains intact
during cell division. But in these organisms, the
microtubules form a spindle within the nucleus.
Microtubules separate the chromosomes, and the
nucleus splits into two daughter nuclei.
(d) Most eukaryotes. In most other eukaryotes,
including plants and animals, the spindle forms
outside the nucleus, and the nuclear envelope
breaks down during mitosis. Microtubules separate
the chromosomes, and the nuclear envelope then
re-forms.
Figure 12.12 A-D
Bacterial
chromosome
Chromosomes
Microtubules
Intact nuclear
envelope
Kinetochore
microtubules
Intact nuclear
envelope
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
11