Download Notes on Meiosis

MEIOSIS
Ch. 11

Definition: a process of reduction division that
produces gametes ( sex cells) in which the
number of chromosomes per cell is cut in half
(haploid) through the separation of homologous
chromosomes.
WHAT ARE HOMOLOGOUS CHROMOSOMES?
chromosomes that have a corresponding
chromosome from the opposite-sex parent.

A cell that contains both sets of homologous
chromosomes is diploid, which means “two sets”.
(2n)
 A cell that contains only one set of chromosomes
is haploid. (n)

CELL CYCLE:



Interphase
Meiosis I and Meiosis II
Cytokinesis
INTERPHASE: just like interphase before
mitosis

G1: cells grows, protein production

S: DNA replication

G2: productions of new organelles including
centrioles

MEIOSIS I:
PROPHASE I:
 Chromatin coils into chromatids and the
homologous pairs of chromatids come together
to form a tetrad.
 *Crossing over occurs were portions of the
homologous chromosomes are exchanged.


The rest of prophase is very similar to mitosis.
CROSSING OVER DURING PROPHASE
Why is it so important?

Crossing over allows for the random mixing of
genes (genetic recombination) which adds genetic
variety to a species so that no two individuals are
exactly the same.

METAPHASE I:
 Tetrads line up at the equator of the spindle

ANAPHASE I:
 Homologous chromosomes separate and are
pulled to the poles (the centromeres do NOT
split).

TELOPHASE I:
 Chromosomes uncoil, spindle breaks down. The
nuclear membranes and nucleoli reform.
 Cytoplasm divides (cytokinesis) to form two cells.

NO INTERPHASE????
NOTE: There is NO interphase between meiosis I
and meiosis II.
 Why is this important?
 So that each new cell will only have half
(haploid) the number of chromosomes as
the original parent cell.

PROPHASE II:
*just like mitosis
 METAPHASE II:
except that there
 ANAPHASE II:
is only half the
 TELOPHASE II/
number of
 CYTOKINESIS:
chromosomes.

WHAT IS THE END RESULT OF MEIOSIS?
Four genetically different haploid
cells that contain half the
number of chromomsomes as the
original parent cell.
WHAT WAS THE END RESULT OF
MITOSIS?

Two genetically identical diploid cells that
are identical to the original parent cell.