Mitosis, Meiosis

Cell Division, part 1
I. Prokaryotes – Binary Fission
II. Eukaryotes – Mitosis
A. Prophase
B. Metaphase
C. Anaphase
D. 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
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II. Eukaryotes:
Mitosis
1. Karyokinesis –
nuclear division
2. 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
mitosis
1) interphase
2) Prophase, prometaphase
3) Metaphase
4) Anaphase
5) Telophase
6) interphase
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MITOSIS
Mitosis at the DNA level:
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
Prometaphase –
chromosomes begin to move
Once the spindle
apparatus is in place it
attaches to the
kinetochore on the
centromere.
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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
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
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SIGNIFICANCE:
2n = 4
Know the number of chromosomes/chromatids during each phase
2n = 12, 24, 46… etc.
http://www.pbs.org/wgbh/nova/miracle/divide.html
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
Cell Division, part 2 - Meiosis
I. Sexual reproduction requires gametes
II. Meisos v. mitosis
III. Prophase I
A. Letoneme stage
B. Zygoneme stage
C. Pachyneme stage
D. Diploneme stage
E. Diakinesis
IV. Metaphase, Anaphase, Telophase I
V. The second meiotic division
VI. Gamete development in animals
A. Spermatogenesis
B. 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
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Gametes:
• Haploid Gametes – isogamous vs.
heterogamous
II. Meiosis, different from mitosis:
1) Two successive nuclear divisions, MI & MII, which
produces haploid gametes that differ genetically
• 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
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III. Prophase I
A. Leptotene stage
• Chromatin begins to condense,
chromosomes become visible
• Chromomeres develop along each
chromosome
– Localized condensations
• Homology search underway
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
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C. Pachytene stage
• Further development of synaptonemal complex
occurs between the two members of each
bivalent…leading to
• Chromomeres align in the bivalents, producing a
distinctive pattern for each pair (completes the
“homology search”)
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 =
Crossing over at the DNA level –
Holliday Structure
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Additional Crossing over info:
Significance of CROSSING OVER:
• 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
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IV. Metaphase, Anaphase & Telophase I
• Metaphase I – tetrads move to the
metaphase plate,
• Anaphase I –
• 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
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nondisjunction
Review questions
1. A cell containing chromatids at the start of
mitosis would, at its completion, produce cells
containing how many chromosomes?
2. What if the same cell undergoes meiosis –how
many chromatids would there be in the daughter
cells
3. When ½ of the gametes produced are ,
what has occurred during meiosis? (be specific)
4. Which of the following is FALSE in comparing
prophase I of meiosis and prophase of mitosis?
Random assortment
Meiosis produces new Combinations of genes in 3 ways:
1. Random assortment of maternal and
paternal chromosomes, and the alleles of
genes they contain
2. Recombination due to crossing over and
exchange of chromosome parts between
non-sister chromatids
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The number of possible combinations of maternal and
paternal homologues is 2 n , where n = the haploid number of
chromosomes. In this diagram, the haploid number is 3, and 8
(2 3 ) 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
• Secondary oocyte undergoes MII, producing 1
large haploid ootid & 1 small polar body
• Ootid differentiates into mature ovum
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