What is Meiosis?
Meiosis: A special type of cell division necessary
for sexual reproduction
 Produces cells called gametes
Gamete: a mature reproductive cell
 Egg (AKA “ova”)
 Sperm
Ova being fertilized by
sperm cell (human)
Chromosome (DNA)
Made of 2 chromatids (singular = chromatid)
 Bound by a centromere
Centromere: where the spindle fibers bind to pull the
sister chromatids apart

Haploid: (n) a single set of unpaired chromosomes
 Gamete cells
 Humans have 23 chromosomes in a gamete
Diploid: (2n) a pair of homologous chromosomes
 Humans have 46 chromsomes (23 pairs)
Polyploid: having more than a pair of homologous
chromosomes
Karyotype: the chromosomes of a cell, usually
displayed as chromosome pairs in
descending order of size.
Human karyotype
Male
Somatic cell: (body cell) A cell that makes the body
(liver, stomach, skin, etc)
Germ Cell: (sex cell/gemete) A cell for sexual
reproduction only.
Autosome: A chromosome that does not code for
gender
 Haploid human #: 22
 Diploid human #: 44
Sex chromosome: A chromosome that codes for
gender
 Haploid human #: 1
 Diploid human #: 2
Before Meiosis Starts …
Interphase: Genetic material (DNA) is replicated
 each chromosome duplicates
 the 46 chromosomes becomes a complex of two
identical sister chromatids.
 the cell is diploid (2n) because it still contains the same
number of centromeres.
Meiosis I Separate the homologues
Prophase 1 exchange of information
Start with 1 cell (parent cell)
 Homologous chromosomes condense
 Matching chromosomes pair up
 Matching chromosomes exchange
information during “crossing over”
 Crossing over: matching regions on
matching chromosomes break and
then reconnect to the other
chromosome.

Crossing over
Metaphase I alignment & attachment
Homologous chromosomes align at
the cell equator
 Spindle fibers attach to the
centromeres and centrioles

Spindle fibers
Centrioles
Anaphase I pulling apart

Spindle fibers shorten pulling
homologous chromosomes apart
 2 haploid sets
 each chromosome has 2 sister chromatids

Cell elongates
Telophase I prepare to divide
Centrioles and spindle fibers vanish
 Nuclear membrane surrounds each set
of haploid sister chromatids
 Chromatids uncoil into chromatin
 Cytokinesis occurs

 2 daughter cells result

Cytokinesis: pinching of cell in half
Meiosis II Separate the sister chromatids
Prophase II separation preparation
Begin with 2 cells
 Nuclear envelope vanishes
 Sister chromatids condense
 Spindle fibers and centrioles form


NO DNA REPLICATION
Metaphase II alignment & attachment
Homologous chromosomes align at
the cell equator
 Spindle fibers attach to the
centromeres and centrioles

Spindle fibers
Centrioles
Anaphase II pulling apart

Sister chromatids are pulled apart
 Sister chromatids are now known as “sister
chromosomes”

Sister chromosomes move toward
opposite ends of the cell
Telophase II prepare to divide
Centrioles and spindle fibers vanish
 Nuclear membrane surrounds each set of
haploid sister chromosomes
 Chromosomes uncoil into chromatin
 Cytokinesis occurs

 4 daughter cells result (23 chromosomes each)

Cytokinesis: pinching of cell in half
MEIOSIS
Diploid (2n)
46
chromosomes
n = 23
2n = 46
Interphase
92
chromosomes
Meiosis I
46
46
Meiosis II
23
Haploid (n)
23
23
23
23

+
23
Only for creating gametes / sex
cells.
=
46
MITOSIS
Diploid (2n)
46
chromosomes
n = 23
2n = 46
Interphase
92
chromosomes
Mitosis
46
Diploid (2n)

Only for creating somatic cells (body
cells) for growth and repair.
46