Meiosis
Meiosis
• Sexual Reproduction
• The creation of four new cells with ½
(half) the number of chromosomes of
the parents.
– Example: Humans have 46 chromosomes
each new cell only has 23 chromosomes
• Sperm (23) + Egg (23) = 46 chromosomes
Important words to know
related to meiosis:
• Homologous
chromosomes=
– 2 sets of chromosomes
(4 chromosomes total)
that have similar DNA
sequences and come
from separate parents.
– Ex: one set comes from the
mother and the other set
comes from the father.
Important words to know
related to meiosis:
• Diploid= means “two sets.” Cell that contains both sets
of homologous chromosomes.
– Represented by 2N
• Haploid= means “one set.” Cell that contains one set of
chromosomes.
– Represented by N
The stages of
Meiosis
Interphase 1
• Meiosis 1
•
– Prophase 1
– Metaphase 1
– Anaphase 1
– Telophase 1
• Meiosis 2
– Prophase 2
– Metaphase 2
– Anaphase 2
– Telophase 2
Interphase 1
• Chromosomes
Double
• Organelles duplicate
(make copies)
• Cell grows
Prophase 1
• Homologous chromosomes move together to
form Tetrads
– Tetrad= pairs of chromosomes
• Nuclear Envelope (Membrane) disappears
• Centrioles move apart forming Spindle Fibers
Crossing Over
• Occurs during meiosis.
1.Homologous chromosomes form a tetrad.
2.Chromatids cross over one another.
3.The crossed sections of the chromatids are
exchanged.
Crossing Over
Metaphase 1
• Homologous pairs (tetrads) line up at the center
of the cell
• Centrioles are at the poles attached to each
sister chromatid by spindle fibers
Anaphase 1
• Homologous chromosomes move to poles of the
cell
• Sister chromosomes remain attached at the
centromere
•
•
•
•
Telophase 1 &
Cytokinesis
Homologous chromosomes move to poles
Nuclear envelope reforms (sometimes)
Spindle apparatus disappears
Cells divide into TWO new cells (Cytokinesis)
Meiosis 1
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Prophase 2
• Nuclear envelope disappears (if there)
• Spindle fibers reform
Metaphase 2
• Sister Chromatids line up at the center of
the cell
• Centrioles at the ends of the cells
Anaphase 2
• Sister chromatids separate and move
towards the poles
•
•
•
•
Telophase 2
Nucleus reforms
Spindle fibers disappear
Cytokinesis
4 NEW CELLS FORM WITH ½ THE
ORIGINAL NUMBER OF
CHROMOSOMES
Meiosis 2
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis