Meiosis Overview
Sexual Reproduction
Defined: Gametes from two separate parents
unite
Meiosis: Diploid (2n) to haploid (n) cells
Mechanism that allows for genetic diversity
• Many eukaryotes reproduce in this manner
– Plants, animals and fungi
Meiosis: Prophase I
• Prophase Ichromosomes become
visible and nuclear
membrane breaks down.
• Crossing over occurs
• portions of one sister
chromatid are broken
and exchanged with
portions of the
corresponding chromatid
of homologous
chromosome.
Meiosis: Metaphase I
• Homologous
chromosomes move to
the middle of the cell
and stay together.
Meiosis: Anaphase I
• The homologous
chromosomes are
pulled apart to opposite
sides of the cell via
spindle fibers.
• Notice that the sister
chromatids are still
linked together at the
centromere.
Meiosis: Telophase I/ Cytokinesis
• A nuclear membrane
reforms around the
separated
chromosomes
• Two new
__________ diploid
cells are formed
Meiosis II: Prophase II
• The nuclear membrane dissolves around the
chromosomes
Meiosis II: Metaphase II
• Chromosomes line up in the middle of the cell and
spindle fibers attach to sister chromatids’
centromeres
Meiosis II: Anaphase II
• The sister chromatids are pulled to opposite sides of
the pole by the spindle fibers.
Meiosis II: Telophase II
• The nuclear envelope forms around each set of
chromosomes
• Four unique haploid (1n) cells are formed
Genetic Variation Created
• ____________ ____________: the random
distribution of homologous chromosomes
• Each of the 23 pairs of chromosomes separate
independently of each other
• Thus, 223 (over 8 million) gametes can be
produced by a single cell
• Gamete formation endless possibilities 223 x 223
= over 64 trillion unique zygotes
• Extremely important to the evolution of all
species.
Meiosis
Males
• Spermatogenesis: sperm
production
Females
• Oogenesis: egg (ova)
production
Fertilization
• Gametes of opposite sexes join to make a
diploid (2n) zygote
• Zygote: cell resulting from the fusion of
gametes