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Meiosis and Sexual Reproduction > The Process of Meiosis
The Process of Meiosis
• Introduction
• Meiosis I
• Meiosis II
• Comparing Meiosis and Mitosis
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Meiosis and Sexual Reproduction > The Process of Meiosis
Introduction
• Sexual reproduction is the production of haploid cells and the fusion of two of
those cells to form a diploid cell.
• Before sexual reproduction can occur, the number of chromosomes in a diploid
cell must decrease by half.
• Meiosis produces cells with half the number of chromosomes as the original cell.
• Haploid cells used in sexual reproduction, gametes, are formed during meiosis,
which consists of one round of chromosome replication and two rounds of nuclear
division.
• Meiosis I is the first round of meiotic division, while meiosis II is the second round.
Offspring Closely Resemble Their Parents
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Meiosis and Sexual Reproduction > The Process of Meiosis
Meiosis I
• Meiosis is preceded by interphase which consists of the G1 phase (growth), the S
phase (DNA replication), and the G2 phase.
• During prophase I, the homologous chromosomes condense and become visible
as the x shape we know, pair up to form a tetrad, and exchange genetic material
by crossing over.
• During prometaphase I, microtubules attach at the chromosomes' kinetochores
and the nuclear envelope breaks down.
• In metaphase I, the tetrads line themselves up at the metaphase plate and
homologous pairs orient themselves randomly.
• In anaphase I, centromeres break down and homologous chromosomes separate.
Crossover between homologous chromosomes
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• In telophase I, chromosomes move to opposite poles; during cytokinesis the cell
separates into two haploid cells.
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Meiosis and Sexual Reproduction > The Process of Meiosis
Meiosis II
• During prophase II, chromsomes condense again, centrosomes that were
duplicated during interphase I move away from each other toward opposite poles,
and new spindles are formed.
• During prometaphase II, the nuclear envelopes are completely broken down, and
each sister chromatid forms an individual kinetochore that attaches to
microtubules from opposite poles.
• During metaphase II, sister chromatids are condensed and aligned at the equator
of the cell.
• During anaphase II sister chromatids are pulled apart by the kinetochore
microtubules and move toward opposite poles.
Meiosis I vs. Meiosis II
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• During telophase II and cytokinesis, chromosomes arrive at opposite poles and
begin to decondense; the two cells divide into four unique haploid cells.
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Meiosis and Sexual Reproduction > The Process of Meiosis
Comparing Meiosis and Mitosis
• For the most part, in mitosis, diploid cells are partitioned into two new diploid cells,
while in meiosis, diploid cells are partitioned into four new haploid cells.
• In mitosis, the daughter cells have the same number of chromosomes as the
parent cell, while in meiosis, the daughter cells have half the number of
chromosomes as the parent.
• The daughter cells produced by mitosis are identical, whereas the daughter cells
produced by meiosis are different because crossing over has occurred.
• The events that occur in meiosis but not mitosis include homologous
chromosomes pairing up, crossing over, and lining up along the metaphase plate
Comparing Meiosis and Mitosis
in tetrads.
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• Meiosis II and mitosis are not reduction division like meiosis I because the
number of chromosomes remains the same; therefore, meiosis II is referred to as
equatorial division.
• When the homologous chromosomes separate and move to opposite poles during
meiosis I, the ploidy level is reduced from two to one, which is referred to as a
reduction division.
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Appendix
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Meiosis and Sexual Reproduction
Key terms
• chromatid either of the two strands of a chromosome that separate during meiosis
• crossing over the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes
• diploid of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the
spermatozoon
• equatorial division a process of nuclear division in which each chromosome divides equally such that the number of
chromosomes remains the same from parent to daughter cells
• gamete a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
• haploid of a cell having a single set of unpaired chromosomes
• meiosis II the second part of the meiotic process; the end result is production of four haploid cells from the two haploid cells
produced in meiosis I
• ploidy the number of homologous sets of chromosomes in a cell
• reduction division the first of the two divisions of meiosis, a type of cell division
• tetrad two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis
process
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Meiosis and Sexual Reproduction
Offspring Closely Resemble Their Parents
<em>In kind</em> means that the offspring of any organism closely resemble their parent or parents.The hippopotamus gives birth to hippopotamus
calves (a).Joshua trees produce seeds from which Joshua tree seedlings emerge (b).Adult flamingos lay eggs that hatch into flamingo chicks (c).
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Meiosis and Sexual Reproduction
Crossover between homologous chromosomes
Crossover occurs between non-sister chromatids of homologous chromosomes.The result is an exchange of genetic material between homologous
chromosomes.
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Meiosis and Sexual Reproduction
Synapsis holds pairs of homologous chromosomes together
Early in prophase I, homologous chromosomes come together to form a synapse.The chromosomes are bound tightly together and in perfect alignment
by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
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Meiosis and Sexual Reproduction
Meiosis I ensures unique gametes
Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2).In this case,
there are two possible arrangements at the equatorial plane in metaphase I.The total possible number of different gametes is 2n, where n equals the
number of chromosomes in a set.In this example, there are four possible genetic combinations for the gametes.With n = 23 in human cells, there are
over 8 million possible combinations of paternal and maternal chromosomes.
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Meiosis and Sexual Reproduction
Meiosis I vs. Meiosis II
The process of chromosome alignment differs between meiosis I and meiosis II.In prometaphase I, microtubules attach to the fused kinetochores of
homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I.In anaphase I, the homologous
chromosomes are separated.In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at
the midpoint of the cells in metaphase II.In anaphase II, the sister chromatids are separated.
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Meiosis and Sexual Reproduction
Complete Stages of Meiosis
An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.
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Meiosis and Sexual Reproduction
Comparing Meiosis and Mitosis
Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.The four daughter cells
resulting from meiosis are haploid and genetically distinct.The daughter cells resulting from mitosis are diploid and identical to the parent cell.
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Meiosis and Sexual Reproduction
Which of the following explains why meiosis is an integral part of
sexual reproduction?
A) Meiosis results in a reduction of chromosomes.
B) Meiosis produces diploid cells from haploid cells.
C) Meiosis fertilizes a sperm and an egg cell.
D) Meiosis doubles the number of chromosomes in a cell.
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Meiosis and Sexual Reproduction
Which of the following explains why meiosis is an integral part of
sexual reproduction?
A) Meiosis results in a reduction of chromosomes.
B) Meiosis produces diploid cells from haploid cells.
C) Meiosis fertilizes a sperm and an egg cell.
D) Meiosis doubles the number of chromosomes in a cell.
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Meiosis and Sexual Reproduction
How is genetic material exchanged during meiosis?
A) Following telophase, cytokinesis seperates the cell into two daughter
cells.
B) Microtubules attach to fused homologous chromosomes.
C) Chromosomes are replicated and produce identical copies during DNA
replication.
D) Segments of homologous chromosomes recombine, creating
recombinant chromosomes
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Meiosis and Sexual Reproduction
How is genetic material exchanged during meiosis?
A) Following telophase, cytokinesis seperates the cell into two daughter
cells.
B) Microtubules attach to fused homologous chromosomes.
C) Chromosomes are replicated and produce identical copies during DNA
replication.
D) Segments of homologous chromosomes recombine, creating
recombinant chromosomes
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Meiosis and Sexual Reproduction
At what stage do homologous chromosomes separate?
A) Metaphase I
B) Telophase I
C) Anaphase I
D) Prometaphase I
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Meiosis and Sexual Reproduction
At what stage do homologous chromosomes separate?
A) Metaphase I
B) Telophase I
C) Anaphase I
D) Prometaphase I
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Meiosis and Sexual Reproduction
Which of the following is a result of meiosis II?
A) Homologous chromosomes separate and four haploid cells are
created.
B) Homologous chromosomes separate and four diploid cells are
created.
C) Sister chromatids separate and four haploid cells are created.
D) Sister chromatids separate and two diploid cells are created.
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Meiosis and Sexual Reproduction
Which of the following is a result of meiosis II?
A) Homologous chromosomes separate and four haploid cells are
created.
B) Homologous chromosomes separate and four diploid cells are
created.
C) Sister chromatids separate and four haploid cells are created.
D) Sister chromatids separate and two diploid cells are created.
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Meiosis and Sexual Reproduction
Which of the following occurs in both mitosis and meiosis?
A) movement of sister chromatids to opposite poles
B) crossing over between sister chromatids
C) formation of two diploid cells
D) two nuclear cell divisions
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Meiosis and Sexual Reproduction
Which of the following occurs in both mitosis and meiosis?
A) movement of sister chromatids to opposite poles
B) crossing over between sister chromatids
C) formation of two diploid cells
D) two nuclear cell divisions
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Meiosis and Sexual Reproduction
Attribution
• Connexions. "Introduction." CC BY 3.0 http://cnx.org/content/m44468/latest/?collection=col11448/latest
• Wiktionary. "diploid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/diploid
• Wiktionary. "gamete." CC BY-SA 3.0 http://en.wiktionary.org/wiki/gamete
• Wiktionary. "haploid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/haploid
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/
• Wikipedia. "crossing over." CC BY-SA 3.0 http://en.wikipedia.org/wiki/crossing%20over
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Wiktionary. "chromatid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/chromatid
• Wiktionary. "tetrad." CC BY-SA 3.0 http://en.wiktionary.org/wiki/tetrad
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Wikipedia. "meiosis II." CC BY-SA 3.0 http://en.wikipedia.org/wiki/meiosis%20II
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/equatorial-division
• Wiktionary. "ploidy." CC BY-SA 3.0 http://en.wiktionary.org/wiki/ploidy
• Wiktionary. "reduction division." CC BY-SA 3.0 http://en.wiktionary.org/wiki/reduction+division
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