Download Meiosis II

Meiosis Notes
 Cell division to form
the gametes, sperm
(male gamete) and egg
(female gamete).
 Normal cells are
diploid: 2 copies of
every gene.
 Gametes are haploid: 1
copy of every gene
Number of Chromosomes
Overview of Meiosis
 2 cell divisions.
 Starts with 2 copies of each
chromosome (homologous),
each with 2 chromatids
(copies of DNA).
 In meiosis I, homologous
chromosomes are separated
into 2 cells.
 In Meiosis II the chromatids
are separated into 4 cells.
Meiosis I (PMAT I)
 PROPHASE I - The homologous
chromosomes pair together (Sometimes
crossing over occurs).
 METAPHASE I - The pairs of homologous
chromosomes line up along the middle
 ANAPHASE I - Homologous chromosomes
are pulled apart.
 TELOPHASE I - One cell becomes two
cells with one chromosome of the pair
(haploid)
Meiosis II (PMAT II)
PROPHASE II – Prepare to divide
METAPHASE II – Chromosomes line
up in the middle
ANAPHASE II – Chromatids (copies
of DNA) pull apart
TELOPHASE II – The end result is
four cells with one copy of each gene.
Mother cell
Stages Of Meiosis:
Meiosis I
Interphase
Prophase I:
Condensing
Chromosomes
Prophase I:
Tetrad formation/
crossing over
Meiosis II
Telophase I
Metaphase I
Anaphase I
Stages Of Meiosis:
Meiosis II
TelophaseIII
Prophase
The products of meiosis are 4
haploid cells each with a
unique set of chromosomes.
Metaphase II
Anaphase II
Telophase II
Segregation
 In humans meiosis
starts with one cell
containing 46
chromosomes (23
pairs) and results in
four cells containing
23 chromosomes.
 The copies of DNA
are separated when
gametes are formed.
Independent Assortment
 Homologous
chromosomes are
positioned randomly so
any copy can be passed to
the gametes with any
combination of other
chromosomes
 There are 2n combinations
possible during meiosis
with n the haploid number
of chromosomes for the
organism
How many combinations are
possible in human meiosis?
 Possible combinations: 2n
 n=23 in humans
223=about 8,300,000 combinations
Crossing Over
During Prophase I, the exchange
of genetic material between
homologous chromosomes
Crossing Over
Prophase I:
Tetrad formation/
crossing over
Because of crossing over, every
gamete receives a unique set of
genetic information.
Telophase II
Metaphase I
Anaphase I
Telophase I
Fertilization
 The combination of a sperm
and an egg which forms a
zygote.
 1 sperm (1 of 8 million
possible chromosome
combinations) x 1 ovum (1
of 8 million different
possibilities) = 64 trillion
diploid combinations!
Fertilization Results In A
Diploid Zygote
Egg
Haploid
nucleus
Sperm
Haploid
nucleus
Fertilization Results In A
Diploid Zygote
Egg
Haploid
nucleus
Sperm
Haploid
nucleus
Fertilization Results In A
Diploid Zygote
Egg
Haploid
nucleus
Haploid
nucleus
Sperm
Fertilization Results In A
Diploid Zygote
Egg
Haploid
nucleus
Haploid
nucleus
Sperm
From Zygote to Embryo
Zygote
Zygote
Diploid
2n
From Zygote to Embryo
Mitosis
From Zygote to Embryo
Mitosis
From Zygote to Embryo
Mitosis
From Zygote to Embryo
Mitosis
From Zygote to Embryo
Twins
Monozygotic Twins
(Identical)
 1/3 of all twins are identical
 Twins that form from one
zygote (one egg fertilized
by one sperm).
 These twins have identical
genes and must be the same
sex.
 Having identical twins is
random, not genetic
Dizygotic Twins (fraternal)
 2/3 of all twins are fraternal
 Twins that form from two
zygotes (two eggs fertilized
by two sperm)
 Can be the same sex or
different sexes.
 The ability to have fraternal
twins is thought to be
genetic.
Phenotype
The way an
ORGANISM
LOOKS
Genotype
THE
GENETIC
MAKEUP OF
AN
ORGANISM
Dominant
and
Recessive
Traits
Dominant Traits
Characteristics you can
always see if you have the
genes.
Symbol
capital letters
Recessive Traits
Characteristics
you cannot see if
you have a
dominant gene.
Symbol
lower case letters