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Meiosis
Overview of Meiosis
• Sexual reproduction
• Gametes, with half the # of chromosomes, are
produced
• 2n1n
• Ensures that the next generation will have a
combination of traits from both parents
Overview of Meiosis
• Homologous chromosomes: similarly
constructed chromosomes with the same
shape and that contain genes for the same
traits (homologues)
• Just like in mitosis, meiosis occurs after
interphase, when the cell grows, DNA is
replicated, and chromosomes are duplicated.
• Genetic recombination: production of
offspring with combinations of traits that
differ from those found in either parent.
– Crossing over
– Independent assortment
Meiosis I
• Prophase I
– Synapsis occurs (pairing of
homologous chromosomes)
– Spindle appears
– Nuclear membrane fragments
– Nucleolus dissappears
Meiosis I
• Prophase I
– Synapsis
• Homologous chromosomes line up side by side
– Crossing-over: exchange of genetic material
between non-sister chromatids
– Result= chromatids held together by centromere
are no longer identical
– Daughter cells will receive chromosomes with
recombined genetic material
Meiosis I
• Metaphase I
– Homologous pairs line up at equator
• Maternal and paternal member may be oriented
toward either pole.
• In humans, results in 223 (8,388,608) combinations of
maternal and paternal chromosomes in gametes
• Anaphase I
– Homologous chromosomes
separate, pulled to opposite poles
by spindle fibers
– Independent assortment: all
possible combinations of
chromosomes occur in the
daughter nuclei.
• Telophase I
– Nuclear membrane reforms
– Nucleoli appear
– Cytokinesis produces two
haploid daughter cells
• Interkinesis: between meiosis I and II
– No DNA replication since chromosomes are
already dublicated
• Prophase II: spindle appears, nuclear membrane
disassembles, nucleolus disappears
• Metaphase II: chromosomes attach to spindles
and align at equator
• Anaphase II: sister chromatids separate becoming
daughter chromosomes
• Telophase II: spindle disappears, nuclei form,
nuclear membrane reforms
• Cytokinesis occurs
• Meiosis results in four haploid daughter cells
• Nondisjunction
– Failure of homologous chromosomes to separate
during anaphase I, or sister chromatids to
separate at anaphase II
– Results in gametes with abnormal number of
chromosomes
– Causes various syndromes
• Trisomy 21 (Down Syndrome)—extra copy of
chromosome 21
• Turner Syndrome—absence of second X chromosome
• Daughter cells mature into gametes (eggs,
sperms) that fuse during fertilization
– Restores the diploid number of chromosomes in a
cell that will develop into a new individual.