Transcript Meiosis II - Solon City Schools

Meiosis – Why do we look
different from our parents?
Where do we begin?
• What type of cells does mitosis take
place in?
• What type of cells does Meiosis take
place in?
• What is a chromosome?
– http://learn.genetics.utah.edu/content/begin
/tour/index.html
Meiosis – A Source of Distinction
Meiosis does two things 1) Meiosis takes a cell with two copies of every
chromosome (diploid) and makes cells with a single
copy of every chromosome (haploid).
In meiosis, one diploid cell produces four haploid cells.
2) Meiosis scrambles the specific forms of each
gene that each sex cell (egg or sperm) receives.
Meiosis
a cell division forming gametes
Goal: reduce genetic material by half.
Why?
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Karyotype
Replication of chromosomes
• Replication is the
process of duplicating
a chromosome
• Occurs prior to
division
• Replicated copies are
called sister
chromatids
• Held together at
centromere
What phase is this called?
Interphase
Meiosis: cell division in two parts
Homologs
separate
Meiosis I
(reduction
division)
Sister
chromatids
separate
Meiosis II
(equational
division)
Diploid
Haploid
Result: one copy of each chromosome in a gamete.
A replicated chromosome
Gene X
sister chromatids
homologs
same genes
maybe different alleles
same genes
same alleles
Homologs separate in
meiosis I and therefore
different alleles separate.
Meiosis I : the reduction division
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Prophase I
Early prophase
Late prophase
Homologs pair-up.
Crossing over occurs.
Chromosomes condense
Chromosomes form a tetrad.
Spindle fibers form.
Nuclear membrane fragments.
Recombination (crossing over)
• Occurs in prophase
of meiosis I
A
A
B
B
C
• Generates diversity
b
C
D D
E
F
E
F
a
a
b
c
c
d
d
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
e
f
Recombination (crossing over)
• Occurs in prophase
of meiosis I
A
a
B
b
C
• Generates diversity
Letters denote genes
Case denotes alleles
C
D D
E
F
c
c
d
E
F
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
d
e
f
Recombination (crossing over)
• Occurs in prophase
of meiosis I
a
A
B
b
C
• Generates diversity
Letters denote genes
Case denotes alleles
D
E
F
A
a
B
b
C
D
E
F
e
c
c
d
d
e
f
•Creates chromosomes with new
combinations of alleles for genes A to F.
f
Metaphase I
• 1.) Tetrads line-up in middle of cell
• 2.) Spindle fibers attach to centromere
region of each homologous
chromosome pair
Anaphase I
Homologs separate and
move to opposite poles.
Sister chromatids remain
Attached at their centromeres.
Telophase I
Nuclei reform.
Spindle fibers break down.
Cytokinesis divides cell into two.
Meiosis II : the equational division
Prophase II
Metaphase II
Anaphase II
Telophase II
Four
nonidentical
haploid
daughter cells
Prophase II
Nucleus breaks down.
Centrioles move to the poles
Spindle fibers form.
Metaphase II
Chromosomes align
along equator of cell.
Spindle fibers attach
to the centromeres
Anaphase II
Sister chromatids separate
and move to opposite poles.
Telophase II
Chromosomes unwind.
Nuclei reform.
Spindle fibers break down.
Cytokinesis occurs.
Results of meiosis
Gametes
Four haploid cells
One copy of each
chromosome
One allele of each gene
Different combinations of
alleles for different
genes along the
chromosome
Oscar Winning Meiosis
Click on image for link to animation
More Meiosis
• http://www.cellsalive.com/meiosis.htm
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter3/animation__
how_meiosis_works.html
• http://www.sumanasinc.com/webcontent/anim
ations/content/meiosis.html
• http://www.biology.arizona.edu/CELL_BIO/tut
orials/meiosis/page3.html
Meiosis – A Source of Distinction
Meiosis does two things 2) Meiosis scrambles the specific forms of each gene that
each sex cell (egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is
accomplished through independent assortment and crossingover.
Genetic diversity is important for the evolution of
populations and species.
One Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –
Independent Assortment
Independent assortment produces 2n distinct
gametes, where n = the number of unique
chromosomes.
In humans, n = 23 and 223 = 6,000,0000.
That’s a lot of diversity by this mechanism
alone.
http://www.sumanasinc.com/webcontent/ani
mations/content/independentassortment.html
http://learn.genetics.utah.edu/content/
begin/tour/index.html
Another Way Meiosis Makes Lots of Different Sex Cells –
Crossing-Over
Crossing-over multiplies the already huge number of different gamete types
produced by independent assortment.
Between Independent Assortment and
Crossing-Over, No Two Gametes Are
Identical.
The Mechanics of Meiosis Following
a Single Chromosome Pair
Crossing over
Mitosis vs. Meiosis
This step is the key
difference
Independent
assortment occurs
at this step
The Key Difference Between Mitosis and Meiosis is the Way
Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and
distinguishing)
division of meiosis
Mitosis
Meiosis
Number of
divisions
1
2
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Germline cells
When
Throughout life
At sexual maturity
Genetically
identical?
Chromosome #
Role Growth and repair
Sexual reproduction
Gametogenesis – formation of gametes
1) Spermatogenesis
2) Oogenesis
Spermatogenesis: sperm
formation
Oogenesis
First polar body
may divide
(haploid)
a
a
X
a
X
a
Meiosis I
A
Oogonium
(diploid)
X
X
Mitosis
X
Primary
oocyte
(diploid)
Polar
bodies
die
Meiosis II
(if fertilization
occurs)
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
A
X
Second
polar body
(haploid)
Mature
egg
Oogenesis: ovum formation
• One of four meiotic products becomes an
ovum.
• The three remaining meiotic products die.
Oogenesis versus Spermatogenisis
Fertilization
• Fertilization is the joining of
SPERM (n) and EGG or OVUM (n)
to form a Zygote (2n).
Chromosomal
Mutations
Mutations
• Deletion
– Occurs when part of a chromosome is left out
– Most are lethal
Mutations
• Insertion
– Segment of a chromosome is removed and
inserted into another one
Mutations
• Duplication
– Segment of DNA is copied twice
Mutations
• Inversion
– Occurs when part of a chromosome breaks off
and is reinserted backwards
Mutations
• Translocation
– Occurs when segments of DNA on 2
chromosomes are rearranged