Meiosis
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B.
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D.
Overview of meiosis
Homologous chromosomes
Stages of meiosis
Spermatogenesis & oogenesis
A. Overview of Meiosis
Meiosis:
– Specialized nuclear division in which the number of
chromosomes is reduced by half
– Purpose of meiosis: the formation of gametes
– Occurs only in germ-line tissue
– Diploid number: the number of chromosomes in a
germ-line cell
– Haploid number: the number of chromosomes in a
gamete; ½ The diploid number
A. Overview of Meiosis
Zygote:
– During the process of fertilization, a haploid gamete
from one parent fuses with the haploid gamete from
the other parent
– The resulting diploid cell is called a zygote
A. Overview of Meiosis
Gametes:
– In most sexually reproducing species, there are two distinct
types of gametes
– Spermatozoa (or pollen in plants) are compact, highly motile
gametes that contribute their chromosomes to the zygote;
“Male” gametes
– Ova (or ovules in plants) are much larger and contribute both
chromosomes and cytoplasm (the bulk of the cell mass) to
the zygote; “Female” gametes
A. Overview of Meiosis
Gonads
– In most multicellular species, germ-line tissue is found in
organs called gonads
– Spermatozoa are produced in gonads called testes
– Ova are produced in gonads called ovaries
– Sexually dimorphic species: two separate genders, with each
individual having either male or female gonads
– Sexually monomorphic species (hermaphroditic species):
each individual contains both male and female gonads
B. Homologous Chromosomes
Homologous chromosome pairs:
– For each chromosome in a diploid nucleus, there is
another very similar chromosome in the same
nucleus
– This pair of very similar chromosomes is called a
homologous chromosome pair
– One chromosome in each pair comes from one
parent, and the other chromosome comes from the
other parent
B. Homologous Chromosomes
Homologous chromosomes are similar in:
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Size
Position of the centromere
Banding patterns in staining procedures
The type of genetic information they contain
During meiosis, the homologous chromosomes are
separated, so that a gamete receives only one member
of each homologous chromosome pair
C. Stages of Meiosis
Prior to meiosis:
– The diploid germ-line cell goes through a complete
interphase, including an S phase
– Therefore, at the start of meiosis, each of the
chromosomes is in a replicated state (consisting of
sister chromatids connected at the centromere)
Meiosis is accomplished in two divisions:
meiosis I and meiosis II
C. Stages of Meiosis
Meiosis I:
– At the start of meiosis I, the two chromosomes in each
homologous chromosome pair line up along their lengths
– During meiosis I the homologous chromosomes separate and
move to opposite sides of the cell. (Note that the chromatids
stay together at this point.)
– At the end of meiosis I, the cell divides into two
– Note that each daughter cell formed by meiosis I is haploid,
but the chromosomes are still in their replicated state
C. Stages of Meiosis
Meiosis II:
– Each of the cells from meiosis I can undergo meiosis II
– During meiosis II, the centromeres split, the sister chromatids
separate and become daughter chromosomes, and the
daughter chromosomes move to opposite sides of the cell
– New nuclei form, and the cell divides
– Therefore, meiosis (I and II together) has the potential of
forming four haploid cells, with the chromosomes in an
unreplicated state at the end of the process
C. Stages of Meiosis
Meiosis I is divided into four stages: prophase I,
metaphase I, anaphase I, and telophase I
Prophase I: during prophase I, the nuclear membrane &
nucleolus disperse, and a spindle forms. The
homologous chromosomes condense and pair in five
steps:
– Leptonema: the chromosomes begin to condense and have
the appearance of slender threads
– Zygonema: the homologous chromosomes align completely
along their lengths, forming paired chromosomes called
bivalents. The connection between the chromosomes is
called the synaptonemal complex
C. Stages of Meiosis
Prophase I (continued):
– Pachynema: the bivalent chromosomes continue to
condense, becoming very short & thick
– Diplonema: the chromosomes in each bivalent begin
to partially separate. The two chromosomes in the
bivalent remain connected at X-shaped regions
called chiasmata (singular: chiasma)
– Diakinesis: the chiasmata migrate to the ends of the
bivalents
C. Stages of Meiosis
Metaphase I:
– The bivalents align at the equator of the spindle
Anaphase I:
– The homologous chromosomes separate and move to
opposite poles of the spindle
– Note that the the chromatids do not separate at this time
Telophase I:
– The chromosomes at each pole may decondense, and new
nuclei form
– Cytokinesis takes place, resulting in two cells
C. Stages of Meiosis
Meiosis II is divided into four stages: prophase II,
metaphase II, anaphase II, and telophase II
Prophase II
– Chromosomes condense; Membrane disperses;
Spindle forms
C. Stages of Meiosis
Metaphase II
– Chromosomes align at equator of spindle
Anaphase II
– The sister chromatids separate and move to
opposite poles of the spindle
Telophase II
– Chromosomes decondense; New nuclei form
– Cytokinesis takes place
D. Gametogenesis
Spermatogenesis
– The cytokinesis divisions (cell divisions) after
meiosis I and meiosis II are equal
– This means that one germ line cell in the testes
divides by meiosis to produce four cells of equal size
– Each of these four cells develops into a
spermatozoan
D. Gametogenesis
Spermatogenesis
– Stages
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Spermatogonium
Primary spermatocyte
Secondary spermatocyte
Spermatid
Mature spermatozoan
D. Gametogenesis
Oogenesis
– When the primary oocyte undergoes meiosis I, the
cytokinesis is unequal, resulting in one very large cell (the
secondary oocyte) and one much smaller cell (the first polar
body)
– When the secondary oocyte divides in meiosis II, again the
division is unequal. The result is one very large gamete (the
ovum) and a second polar body
– Therefore, a single diploid germ-line cell in an ovary will
produce only one gamete
D. Gametogenesis
Oogenesis
– Stages
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Oogonium
Primary oocyte
Secondary oocyte
Ootid
Ovum