Meiosis
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Terminology
Heredity – continuity of biological traits from one
generation to the next:
Results from transmission of hereditary units (genes)
from parents to offspring.
Variation – inherited differences among individuals of the
same species
Genetics - Scientific study of heredity and hereditary
variation
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Terminology
DNA – type of nucleic acid that is a polymer of four
different kinds of nucleotides
Genes – units of hereditary information that are made of
DNA and are located on chromosomes.
Chromosomes – organizational unit of heredity material
in the nucleus of eukaryotic organisms
Consist of a single long highly folded DNA molecules
that are coiled along with proteins
Contain genetic information arranged in a linear
sequence and coded by genes
Locus – specific location on a chromosome that contains
a gene
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Comparison of asexual versus sexual reproduction
Single individual is the sole
parent
Two parents give rise to
offspring.
Single parent passes on all its
genes to its offspring
Each parent passes on half its
genes, to its offspring
Offspring are genetically
identical to the parent
Offspring have a unique
combination of genes inherited
from both parents
Results in a clone, or
genetically identical individual.
Rarely, genetic differences
occur as a result of mutation, a
change in DNA
Results in greater genetic
variation; offspring vary
genetically from their siblings
and parents
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Asexual reproduction
of hydra
In asexual
reproduction – a
single individual is
the sole parent and
passes copies of
all its genes on to
its offspring.
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Definitions
Sexual reproduction occurs only in eukaryotes.
During the formation of gametes (reproductive cells),
the number of chromosomes is reduced by half (haploid),
and returned to the full amount (diploid) when the two
gametes fuse during fertilisation.
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Definitions
Ploidy is a term referring to the number of sets of
chromosomes.
Haploid organisms have one set of chromosomes; diploid –
two.
Meiosis is a special type of nuclear division which
segregates one copy of each homologous chromosome into
each new "gamete".
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Definitions
Somatic (most) cells in the human body are produced by
mitosis.
Germ line cells (gametes) are produced by meiosis.
The vast majority of cell divisions in the human body are
mitotic, with meiosis being restricted to the gonads.
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Life cycle
In each generation, the doubling of chromosome number that results
from fertilization is offset by the halving of chromosome number that
results from meiosis.
For humans, the number of chromosomes in a haploid cell is 23 ( n=23);
the number of chromosomes in the diploid zygote and all somatic cells
arising from it is 46 (2n = 46).
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Life cycle
Three sexual life cycles differing in the timing of meiosis and
fertilization.
The common feature of all three cycles is the alternation of these two
key events, which contribute to genetic variation among offspring.
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Overview of meiosis
Synapsis
Maternal
chromatid Replicated
Paternal
homolog
chromosomes
homolog
Cell division I
Premeiotic cell
Daughter cells
Cell division II
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Meiosis I (reduction)
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Prophase I
Prophase I has a unique event - the
pairing of homologous
chromosomes.
The process of linking of the
replicated homologous
chromosomes is called synapsis.
The resulting chromosome
composed of two chromatids from
each chromosome is termed a
tetrad.
At this point crossing-over may
occur.
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During crossing-over chromatids break and may be
reattached to a different homologous chromosome.
Metaphase and anaphase
In metaphase tetrads line-up along the equator of the
spindle. Spindle fibers attach to the centromere region of
each homologous chromosome pair.
In anaphase I the tetrads separate, and are drawn to
opposite poles by the spindle fibers. The centromeres in
Anaphase I remain intact.
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Meiosis II (division)
During Prophase II, nuclear
envelopes (if they formed
during Telophase I) dissolve,
and spindle fibers reform.
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Meiosis II (division)
During Anaphase II, the centromeres split and the former
chromatids (now chromosomes) are segregated into opposite
sides of the cell.
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Meiosis II (division)
Meiosis II divides the remaining set of chromosomes in a
mitosis-like process (division).
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Comparison of mitosis and meiosis
Mitosis maintains ploidy level, while meiosis reduces it.
Meiosis may be considered a reduction phase followed by a
slightly altered mitosis.
Meiosis occurs in a relative few cells of a multicellular
organism, while mitosis is more common.
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Comparison of mitosis and meiosis
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Comparison of mitosis and meiosis
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Homologous chromosome rearrangements
In this figure we consider the consequences of meiosis in a
hypothetical organism with a diploid chromosome number of 4
(2n = 4).
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Homologous chromosome rearrangements
The arrangement of chromosomes at metaphase I determines
which chromosomes will be packaged together in the haploid
daughter cells.
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The results of crossing
over during meiosis
During prophase of meiosis I,
nonsister chromatids of
homologous chromosomes
exchange corresponding
segments.
Following these chromosomes
through meiosis, we can see
that crossing over gives rise to
individual chromosomes that
have some combination of DNA
originally derived from two
different parents.
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Gametogenesis (spermatogenesis and
oogenesis)
Gametogenesis is the process of forming gametes (by
definition haploid, n) from diploid cells of the germ line.
Spermatogenesis is the process of forming sperm cells by
meiosis (in animals, by mitosis in plants) in specialized organs
known as gonads (termed testes in males ). After division the
cells undergo differentiation to become sperm cells.
Oogenesis is the process of forming an ovum (egg) by meiosis
(in animals, by mitosis in the gametophyte in plants) in
specialized gonads known as ovaries.
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Gametogenesis (spermatogenesis and
oogenesis)
In spermatogenesis all 4 meiotic products develop into
gametes;
in oogenesis most of the cytoplasm is placed into one
large egg and the other cells, the polar bodies, do not
develop.
Human males produce 200,000,000 sperm per day, while
the female produces one egg (usually) each menstrual
cycle.
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Gametogenesis
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Gametogenesis
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Summary
1. Offspring derive from parents by inheriting chromosomes
2. Asexual versus sexual reproduction: offspring either from one
or two parents
3. Meiosis is a special type of nuclear division which segregates
one copy of each homologous chromosome into each new
"gamete".
4. Meiosis reduces chromosome number from diploid to haploid.
5. Sexual life cycles produce genetic variation among offspring:
• independent assortments of chromosomes
• crossing over between homologous chromosomes
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Reading
Ch. 13, pp. 248-261
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