Meiosis
The Continuation of Life
Meiosis: The Continuation of Life
Homologous Chromosomes

Paired chromosomes

One from each parent

Genes for same trait
– Alleles
– Same order on
chromosome
Homologous Chromosomes
Diploid vs. Haploid

Diploid
– Cell that has TWO of
each kind of
chromosome (parent)
– 2n

Haploid
– Cell that has ONE of
each kind of
chromosome
– 1n
Question

What would happen if Mitosis was the only
way our cells could divide?
23pairs  46pairs  96pairs…etc.

We need a way to divide the Chromosome
# in half! What is this called??
 Meiosis!
Meiosis

Purpose –
make haploid cells

Gametes
– Egg
– Sperm
 Form Zygote
Phases of Meiosis
Meiosis I
•
•
•
•
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
•
•
•
•
•
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Prophase I

Same as in Mitosis…
But…

Homologous
Chromosomes find
each other

Synapsis
– Tetrads
Prophase I

Synapsis

Crossing over occurs

Exchange of Genetic
material
Crossing Over

Crossing Over and its
products!
Metaphase I

Homologous pairs line
up

Tetrads still together
Anaphase I

Homologous
Chromosomes separate

Independent Assortment
Telophase I
Just like Telophase in
Mitosis
 Chromosomes unwind
 Nuclear membrane
forms

Meiosis II

Just like Mitosis!!

Meiosis II
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
Prophase II
Chromosomes appear
 Centrioles migrate
 Spindle fibers

Metaphase II

Chromosomes line up
– Like mitosis
Anaphase II
Chromosomes separate like in mitosis
 Move to ends of cell

Telophase II
Nuclear membrane forms
 Chromatids unwind  Chromatin


Finally…Cytokinesis!
Products

From single diploid (2N)
 4 Haploid (1N)
Cellsalive!

Meiosis: An Interactive Animation

Web Lesson Meiosis
Mitosis vs. Meiosis
Gametes

Why are some of the chromosomes
different colors?
Crossing Over!
Variation

Genetic Variation is provided by meiosis

Because of assortment 70 trillion
possibilities

Crossing Over increases that number
– Reason for great variety of people
Mistakes in Meiosis

Nondisjunction
– the failure of homologous chromosomes to
separate properly
Trisomy – 1 extra chromosome
 Monosomy – 1 less chromosome
 Polyploidy – extra set of all chromosomes

Trisomy
Trisomy 21
 Down’s Syndrome

– Flattened face
– Shorter than normal
– Heart defects
Monosomy
Usually lethal but, but
not in…
Turner’s Syndrome
Look at Karyotype
What is wrong here?
Turner’s Syndrome
Polyploidy

Whole extra set of chromosomes
– (3n) or (4n)

Death – animal zygotes

Often occurs in plants
– Bigger and Healthier
Phase? #1
Question

What separates during Meiosis I?
– Homologous Chromosomes
Question

What type of cell results from meiosis I?
– Haploid!
Question

What separates during Meiosis II?
– Sister Chromatids
Questions?