Transcript Steps of Meiosis - Sonoma Valley High School

Steps of Meiosis
Meiosis I: Prophase I
• Homologous pairs
align with each
other.
• Synapsis occurs:
pairing of
homologues.
• Each pair of
homologues
chromosomes is
called a tetrad.
– Crossing over
occurs.
Crossing Over
• Crossing over
occurs during
tetrad positioning
of chromosomes.
• Results in genetic
recombination
– Alleles are
swapped
Crossing Over
• Actual picture of chromosomes crossing over.
• This creates variation among offspring of a species.
Meiosis I : Metaphase I
• Microtubules
attach only to
outside
kinetochores.
• Homologous
pairs move to
equator of cell
due to
kinetochore
fibers.
Meiosis I: Anaphase I
• Independent
assortment
– Homologues are
pulled apart by
kinetochore
fibers.
– Homologous pairs
are separated
and moved to
opposite poles.
• Note: still two
chromotids
Meiosis I: Telophase I
• Cell goes
through
cytokinesis.
• Cells are now
1n or haploid.
Meiosis II: Prophase II
• Spindle fibers form and begin to move the
chromosomes toward the middle of the dividing cell
Meiosis II: metaphase II
• Chromosomes
are lined up in
the center
• No replication
• No synapsis
• No crossing
over.
Meiosis II:
anaphase II
• Chromotids are split
apart and moved to
opposite poles of cell.
• A second division
occurs.
Cells “want” identical genes
Cells “want” genetic diversity
Comparing Mitosis and Meiosis