Topics: 3 & 10
4.2 & 10.1 Meiosis
• What is the difference between meiosis and
mitosis?
1 – Meiosis Preparation
Read & Consider Understandings 3.3.1-3.3.3 & 10.1.1
In meiosis four daughter cells are produced
and each has half of the original number of
chromosomes and is said to be haploid (n).
meiosis consists of two nuclear
divisions of a diploid nucleus (2n)
and only one replication
Meiosis

Halving the
number of
chromosomes in
gametes allows for
fusion of gametes
and a genetically
unique individual.
Sexual Life Cycle
Diploid (2n)= 4; Haploid (n) =2.
INTERPHASE
4.2.3 Outline the process
of meiosis, including
pairing of homologous
chromosomes and crossing
over, followed by two
divisions, which results in
four haploid cells.
• Define homologous chromosome and sister
chromatid.
2 – Meiosis Steps
Read & Consider Understandings 3.3.4-3.3.6 & 10.1.5-10.1.6

Homologous chromosomes are the
same length, have the same shape and
carry the same genes at the same gene
loci.
Meiosis I: This is the first of two sets
of divisions. This division separates
homologous pairs.
PROPHASE I
Prophase resembles
prophase in mitosis in
that the nuclear
membrane breaks
down, spindle fibers
form, and chromatids
condense.
Crossing-over
Metaphase I
Anaphase I
TELOPHASE I
This completes the reduction division and
each cell has one of the pair of
homologous chromosomes, but still two
identical copies of each.
Meiosis II: involves the separation of
the sister chromatids and looks very
much like mitosis.
In meiosis I, homologous chromosomes separate; in
meiosis II sister chromatids separate.
Draw and label the stages of meiosis.
• How is individual uniqueness achieved?
3 – Genetic Variation
Read & Consider Understandings 3.3.7-3.3.8, 10.1.2-10.1.4 & 10.1.7
The point at which homologous
chromosomes join by ‘crossing over’ is
known as the chiasma
 It is common to have at least one or more
chiasmata between homologous pairs.

Crossing Over
Variation
Independent assortment of maternal and
paternal homologous chromosomes in
metaphase I.
 Crossing over of segments of individual
maternal and paternal homologous
chromosomes.

Allele pairs (homologous
chromosomes) separate
independently during the
formation of gametes
 Traits are transmitted to
offspring independently of
one another

Mendel’s Law
The law of independent assortment is
accomplished during metaphase I.
 Allele pairs separate independently
because of the random orientation of
homologous chromosomes when they
align.

Independent Assortment

Non-disjunction is the failure of homologous
chromosomes to separate during anaphase I.
Resulting in too many or too few chromosomes
in a particular gamete.
Non-Disjunction
Chorionic villus sampling
 Amniocentesis

Karyotyping
"CELL DIVISION: MEIOSIS AND SEXUAL
REPRODUCTION." Estrella Mountain. N.p.,
n.d. Web. 13 Aug. 2015.
 "Meiosis: Crossing Over and Variability
[3D Animation]." YouTube. YouTube, 5
Mar. 2014. Web. 13 Aug. 2015.
 Sadava, et al., Life: The Science of
Biology, Ninth Edition, Sinauer Associates
© 2010 Sinauer Associates, W. H.
Freeman & Co., and Sumanas, Inc.

Works Cited