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CELL DIVISION
MITOSIS:
Reproducing the nucleus of a somatic cell.
MEIOSIS:
Producing sex cells (gametes).
CYTOKINESIS:
Division of cytoplasm and organelles.
MITOSIS & MEIOSIS, con’t.
Both mitosis and meiosis consist of a
sequence of phases:
•
interphase (preparation for division)
•
prophase
•
metaphase
•
anaphase
•
telophase
MITOSIS & MEIOSIS, con’t.
Result of Mitosis:
•formation of two daughter nuclei
•have the same chromosome number as
original (parent) cell
MITOSIS & MEIOSIS, con’t.
Result of Meiosis:
•formation of 1 to 4 daughter nuclei
•have half the chromosome number as original
(parent) cell
Concludes with cytokinesis
•division of the cytoplasm and
organelles
I. MITOSIS
A. Terminology
1. Chromatin
2. Chromosome
3. Diploid (46 or “2N” in humans)
4. Haploid (23 or “N” in humans)
5. Homologous chromosomes
6. Somatic cells
MITOSIS, con’t.
B. PHASES: arbitrarily assigned
1. INTERPHASE:
a. NO division. Cell appears to be
“resting”
b. the metabolic phase
of the cell
Interphase, continued …
c. Chromatin visible
d. Nucleus, nuclear membrane, nucleolus
visible
e. Centrioles located at poles
f. Longest phase
1. ~96% of the cell’s life cycle
2. average of 23/24 hours
Interphase
of Mitosis
MITOSIS, con’t.
2. Prophase
a. Centrioles move to opposite poles of
cell
b. Chromatin condenses into
chromosomes
c. Nucleoli disappear
d. Spindle apparatus is organized
Late
Prophase of
Mitosis
MITOSIS, con’t.
:
Prophase
e. Toward end of phase, nuclear
membrane dissipates
f. Phase ends as spindle fibers attach
to centromeres
g. Each chromosome now consists of
two chromatids
Chromosome at Metaphase
A single
metaphase
chromosome
with two
chromatids
MITOSIS, con’t.
3. Metaphase
a. Chromosomes arranged around
equatorial plate
b. “Single file”
c. Two spindle fibers attached at each
centromere
MITOSIS, con’t.: Metaphase
d. A relatively short phase
e. Chromosomes easily counted (KARYOTYPE)
f. Phase ends as chromatids separate
g. Centromere splits as DNA replication is
completed
Metaphase
of Mitosis
MITOSIS, con’t.
4. ANAPHASE
a. Begins as chromatids separate
b. Spindles move chromosomes to
opposite poles
c. Polar spindles elongate cells
d. Phase ends when chromosomes
have arrived at poles
Anaphase
of Mitosis
MITOSIS, con’t.
5. Telophase
a. “Reverse” of prophase
b. Identical sister chromosomes have
arrived at opposite poles
c. Nuclear membrane reassembles
d. Spindle apparatus dissipates
e. Nucleolus reassembles
MITOSIS, con’t.: Telophase
f. Chromosomes relax  chromatin
g. Centrioles replicate
h. Each chromosome consists of a single
complete strand of DNA
i. Phase ends when nuclear membrane is
completely reassembled
Telophase
of Mitosis
MITOSIS, con’t.
…
C. CYTOKINESIS
1. Begins during anaphase
2. Completed during telophase
3. Result: two daughter cells
a. Genetically identical to parent cell
b. Contain ~1/2 the cytoplasm, organelles
Mitosis
An
overview of
Mitosis
showing all
stages
II. MEIOSIS:
• A special case of mitosis
• Occurs during maturation of sex cells
(gametes)
• Result: daughter nuclei receive one half the
number of chromosomes of somatic
cells
• Makes sexual reproduction possible
II. MEIOSIS, con’t.
A. Terminology
1. Haploid: having a single set of
chromosomes per
individual or cell (N)
2. Diploid: having two full sets of
chromosomes per
individual or cell (2N)
II. MEIOSIS, con’t.: Diploid
a. Usually find sexual reproduction in
diploid organisms
b. Provides enormous adaptive
advantages:
1. allows for population variations
2. Increases chances for survival
II. MEIOSIS, con’t.: Terminology
3. Homologous chromosomes:
a. Chromosomes that pair during
meiosis
b. Have the same morphology
c. Contain genes governing the same
characteristics
MEIOSIS, con’t.: Terminology
4. Allele: one of the two or more alternate forms of a
gene
a. dominant, recessive, co-dominant
b. examples:
B = Brown hair (dominant)
b = blonde hair (recessive)
c. Different alleles arise by mutations
MEIOSIS, con’t.: Terminology
5. Gene:
a. sequence of DNA nucleotides
b. unit of heredity transmitted in the
chromosome
6. Chromosome: sequence of genes
II. MEIOSIS, con’t.: Terminology
7. One gene – One enzyme theory:
•
A gene is that part of a DNA molecule
responsible for the synthesis of one
enzyme (polypeptide)
II. MEIOSIS, con’t.:
B. Phases of Meiosis
1. Two “sets” of events
2. Same names as mitosis
3. Must indicate phase ‘I’ or ‘II’
I = Reductional Division
II = Equational Division
II. MEIOSIS, con’t.:
C. Reductional Division
1. Interphase I: Like mitosis
2. Prophase I:
a. Long, complex
b. Three subdivisions
II. MEIOSIS, con’t.: Prophase I
1. Early: like mitosis
2. Middle: important
a. homologs pair
b. crossing over occurs
c. exchange of genetic info
d. called “tetrads”
3. Late: like mitosis
Prophase I
of Meiosis
II. MEIOSIS, con’t.:
3.
Metaphase I
a. Similar to mitosis
b. Homologous pairs align
c. One spindle
fiber/centromere
d. Ends as one of each pair is
pulled to opposite poles
e. NO DNA replication!!
II. MEIOSIS, con’t.:
4. Anaphase I
a. Similar to mitosis
b. Members of homologous
pairs move to poles
c. Moved by spindle fibers
Anaphase II
of Meiosis
II. MEIOSIS, con’t.:
5. Telophase I
a. Nuclear membrane
reorganizes
b. Chromosomes “relax”
6. Cytokinesis: may not occur
Telophase I
of Meiosis
II. MEIOSIS, con’t.:
7. End result
a. Nuclei are haploid (N)
b. Chromosome number is
reduced by one-half
II. MEIOSIS, con’t.:
D. Equational Division:
1. Interphase II
a. NO DNA replication!!
b. “Coffee break” for nuclei
c. May not occur
Interphase II
of Meiosis
II. MEIOSIS, con’t.:
2. Prophase II
a.
b.
c.
d.
e.
Chromosomes condense
Spindle apparatus forms
Two fibers/centromere
Membrane dissipates
Like mitosis
II. MEIOSIS, con’t.:
3. Metaphase II
a.
b.
c.
d.
Like mitosis
Chromosomes on equator
Single file
Ends with centromere
replication
Metaphase II
of Meiosis
II. MEIOSIS, con’t.:
4. Anaphase II
a. Centromeres replicate
b. Chromatids pulled to
opposite poles
Anaphase II
of Meiosis
II. MEIOSIS, con’t.:
5. Telophase II
a. Nuclear membranes
reorganize
b. Nucleolus reorganizes
c. Chromosomes relax
d. Centrioles replicate
e. Like mitosis
Telophase II
of Meiosis
II. MEIOSIS, con’t.:
5. Cytokinesis occurs
6. Result:
a. Females: unequal division
of cytoplasm
-one ovum, 3 polar bodies
b. Males: equal division of
cytoplasm
-four spermatozoa
Telophase II
of Meiosis:
Four new
cells
II. MEIOSIS, con’t.:
7. Cells produced are:
a. Different from parent cell
b. Different from each other
c. Haploid
Spermatogenesis
Spermatogenesis
results in four
spermatozoa
Oogenesis
Oogenesis
results in one
Ovum and two
or three polar
bodies