1. No category

Cell Division, part 1
Prokaryotes – Binary Fission
Eukaryotes – Mitosis
I.
II.
A.
B.
C.
D.
Prophase
Metaphase
Anaphase
Telophase
Cellular Division Overview
Cell division requires

Duplication, organization and sorting of
chromosomes
Single-celled organisms: asexual reproduction
Multi-celled organisms: growth, replacement
I. Prokaryotes – Binary Fission
Binary Fission: Prokaryotes, Yeast & Amoeba
II. Eukaryotes:
Mitosis
1.
2.
Karyokinesis –
nuclear division
Cytokinesis –
cytoplasmic division
Interphase
Comprises G1, S, G2 phases of cell cycle
Key event = replication of DNA during S,
each chromosome will become two sister
chromatids
Sister chromatids are not visible during
this phase, & the nucleus remains in tact.
mitosis
1)
2)
3)
4)
5)
6)
interphase
Prophase, prometaphase
Metaphase
Anaphase
Telophase
interphase
Mitosis at the DNA level:
MITOSIS
whitefish blastula cells
A. Prophase, prometaphase
First part of M phase
Chromatin condenses into visible
chromosomes, sister chromatids are joined at
the centromere (genetically identical to one
another)
Nucleoli disappear, and nuclear membrane
begins to break down
Centrosomes appear; Spindle fibers form
Prometaphase –
chromosomes begin to move
Once the spindle
apparatus is in place it
attaches to the
kinetochore on the
centromere.
Homologues move toward
the “equator”
B. Metaphase
 Each chromosome lines up on
the equatorial plane
(metaphase plate), led by the
centromere via the spindle
apparatus
C. Anaphase
 Sister chromatids of each chromosome
separate from each other and migrate
to opposite ends of the cell
 Each centromeric region is split in two,
once this occurs each chromatid is
referred to as a daughter chromosome
 Movement of each chromosome made
possible by the spindle apparatus
D. Telophase
Cleavage furrow forms in
animal cells,
Cell plate in plants…
Final stage of mitosis
 Cytokinesis occurs – cytoplasm is partitioned to each half,
cleavage furrow forms in animal cells; cell plate forms in plant
cells
 Chromatids de-condense into chromatin
 Nuclear membrane reforms around each nucleus; cell divides
into two genetically identical daughter cells
SIGNIFICANCE:
results in 2 new
identical daughter
cells, the exact
distribution of the DNA
(via chromosomes) to
the daughter cells
ensures the stability of
cells and the
inheritance of traits
from one generation to
the next.
http://www.pbs.org/wgbh/nova/miracle/divide.html
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
2n = 4
Know the number of chromosomes/chromatids during each phase
2n = 12, 24, 46… etc.
Cell Division, part 2 - Meiosis
I. Sexual reproduction requires gametes
II. Meisos v. mitosis
III. Prophase I
A.
B.
C.
D.
E.
Letoneme stage
Zygoneme stage
Pachyneme stage
Diploneme stage
Diakinesis
IV. Metaphase, Anaphase, Telophase I
V. The second meiotic division
VI. Gamete development in animals
A.
B.
Spermatogenesis
Oogenesis
I. Sexual reproduction requires gametes
Meiosis = cell division that halves the
genetic content for sexual reproduction
Meiosis is critical to the successful sexual
reproduction of all diploid organismsMechanism by which 2n is reduced to n
Leads to the formation of gametes
Basis for the production of extensive
genetic variation among members of a
population
Gametes:
Haploid Gametes – isogamous vs.
heterogamous
In many species, haploid (n) gametes
are descended from germ cells that are
originally diploid (2n) (via meiosis)
 hapliod – they contain ½ the genetic
content
Gametes then combine in fertilization to
reconstitute the diploid complement
found in parental cells
II. Meiosis, different from mitosis:
1) Two successive nuclear divisions, MI & MII, which
produces haploid gametes that differ genetically
2) Homologous chromosomes pair up (synapse),
forming tetrads
3) Non-sister chromatids exchange homologous
sections of DNA (crossing over)
III. Prophase I
A. Leptotene stage
Chromatin begins to condense,
chromosomes become visible
Chromomeres develop along each
chromosome

Localized condensations
Homology search underway – essential to
the initial pairing of homologs
B. Zygotene stage
Continued chromosomal condensation
Homologous chromosomes undergo initial
alignment, wherein "pairing sites" on the
chromosomes are matched
Synaptonemal complex begins to form
between the homologs
Upon completion of this stage, paired
homologs are referred to as bivalents
C. Pachytene stage
Further development of synaptonemal complex
occurs between the two members of each
bivalent…leading to Synapsis
Chromomeres align in the bivalents, producing a
distinctive pattern for each pair (completes the
“homology search”)
Physical exchange between non-sister
chromatids occurs (but not visible yet)
An identical pattern of
twinned loops occurs on
both pairs of sister
chromatids
D. Diplotene stage
Tetrads highly visible, each consisting of
two pairs of sister chromatids
Within each teterad, each pair of sister
chromatids begins to separate
One or more areas remain in contact
where chromatids are intertwined =
Chiasma (crossing over visible)
Crossing over at the DNA level –
Holliday Structure
Significance of CROSSING OVER:
Crossing over results in recombinant chromosomes
that now have paternal alleles at some loci and
maternal alleles at other loci.
Additional Crossing over info:
The exchange is reciprocal, such that
each chromosome gets the same region of
the chromosome segment from the other
parent that it donated to the other
chromosome.
Crossovers are frequent and there is
usually at least one on each chromosome
and 3-4 on the larger chromosomes.
E. Diakinesis
Final stage of prophase I
Chromosomes pull farther apart, but
nonsister chromatids remain loosely
associated via the chaismata
Terminalization occurs = Chiasmata move
toward the ends of the tetrad
Nucleolus and nuclear envelope break
down & the two centromeres of each tetrad
attach to spindle fibers
IV. Metaphase, Anaphase & Telophase I
Metaphase I – tetrads move to the
metaphase plate, alignment is random
Anaphase I – ½ of each tetrad (dyad) is
pulled to one or the other pole at
random, and the the other ½ moves to
the opposite pole [disjunction]
Telophase I – nuclear membrane forms
around the dyads, cleavage furrow
forms, nucleus enters short interphase
tetrad
dyad
V. Second
meitotic
division
Prophase II
Metaphase II
Anaphase II
Telophase II
monad
nondisjunction
Review questions
1. A cell containing 64 chromatids at the start of mitosis
would, at its completion, produce cells containing how
many chromosomes? 32
2. What if the same cell undergoes meiosis – how many
chromatids would there be in the daughter cells after MI,
and how many chromosomes after MII? 32, 16
3. When ½ of the gametes produced are trisomic, what has
occurred during meiosis? (be specific)First division nondisjunction
4. Which of the following is FALSE in comparing prophase
I of meiosis and prophase of mitosis?
a.
b.
c.
d.
e.
The chromosomes condense
Homologs pair up side by side
The nuclear envelope disassembles
A spindle apparatus forms from centrosomes
Each chromosome has two chromatids
The significance of meiosis is that it produces
genetic variation by reducing the genetic material
by ½ that is recombined, to novel produce
gametes for fertilization
Meiosis produces new Combinations of genes in 3 ways:
1. Random assortment of maternal and
paternal chromosomes, and the alleles of
genes they contain

each monad is a combination of maternal &
paternal genetic information
2. Recombination due to crossing over and
exchange of chromosome parts between
non-sister chromatids
3. Random fertilization
Random assortment
The number of possible combinations of maternal and
paternal homologues is 2n, where n = the haploid number of
chromosomes. In this diagram, the haploid number is 3, and 8
(23) different combinations are produced.
VI. Gamete development in animals
A. Spermatogenesis
takes place in testes
Germ cell = spermatogonium, enlarges
to become a primary spermatocyte
Primary spermatocyte undergoes MI,
producing 2 secondary spermatocytes –
producing haploid spermatids
Spermatids undergo modifications –
becoming spermatozoa
B. Oogenesis
Occurs in ovary
Germ cell = oogonium, enlarges to form primary
oocyte
Primary oocyte undergoes MI, producing 1 large
secondary oocyte & 1 small polar body

Unequal division of cytoplasm results in polar body
Secondary oocyte undergoes MII, producing 1
large haploid ootid & 1 small polar body
Ootid differentiates into mature ovum
2n = 8, what phase of mitosis, MI or MII is this cell in?
This cell is in
metaphase II, if it
were in mitosis there
would be 8 pairs of
sister chromatids!
2n = 4, what phase of cell division is each cell in?
Metaphase II
Metaphase
of mitosis
Interphase
Metaphase I