Why Meiosis?
The purpose of meiosis is to reduce the
chromosome number so that parents can
reproduce unique offspring--chromosomal
number of offspring would double with each
generation without meiosis!
Why Meiosis?
Which species has 92 chromosomes?
Aquatic rats
13-2
Basics
Gametes -½ the genetic material as
somatic cells and are genetically unique.
Haploid is term for ½ set of genes. Examples
are sperm and egg. (Not somatic body cells)
New offspring formed after sexual reproduction
(fertilization). Sperm + egg = zygote
Basics
The goals/purpose of meiosis are two-fold 1)
to create diversity and 2) to reduce the number
of total chromosomes in the cell to ½ that of
the parent cell.
Large numbers of people
remained healthy during an
Ebola outbreak and showed
antibody resistance to the
virus.
71% percent of individuals with
positive Ebola antibody tests
had not gotten sick; in another,
46 % remained healthy despite
testing positive for Ebola
antibodies (Huffington post
10/2014 & picture from NPR)
Basics
Some organisms (bacteria and yeast) go
through a type of mitotic reproduction called
binary fission.
Some organisms only contain one gender and
reproduce asexually so there is less genetic
variation in female populations (examples
include some lizard species and fish). For
instance, desert whiptails do not have male
genders and meiosis is limited in its ability to
diversify genes before egg laying.
Check your understanding
What is the difference between haploid
and diploid?
What is a somatic cell?
What is a zygote?
What are the 2 main goals and
advantages of meiosis and sexual
reproduction?
A. *DNA synthesis —all cells
have 2 x the DNA (enough for
92 chromosomes).
B. Meiosis phases are very
similar to mitosis (PMAT)
with a few exceptions:
1) -Prophase 1 of meiosis:
a) 2 copied chromosomes
come in contact with each
other. They form a structure
called a tetrad.
Prophase 1 and Crossing Over
b) Homologous pairs of
chromosomes physically
touch
c) Crossing over is the
exchange and
recombination of genes
(pieces of DNA) from
one chromosome to the
other.
http://highered.mheduc
ation.com/sites/98340
92339/student_view0/c
hapter11/meiosis_with_
crossing_over.html
Crossing over during meiosis: With exchange
of genetic information, the DNA sequence is
changed in each chromosome. Genetic
recombination during crossing over leads to
more genetic diversity and the possibility of
new traits.
Haploid formation Meiosis I
2---(PMAT) At the end of meiosis 1 there are
46 unique haploid chromosomes (copied) in
each cell and the homologous pairs have
been independently separated—each
genetically unique new cell contains 23x2
and 23x2 .
1/25/2016
11
Haploid Gamete
formation: Meiosis II
Meiosis II-another
cell division that
results in 4 haploid
cells with only 23
chromosomes in
each.
1/25/2016
12
Meiosis II
Notice the haploid cells separated from
homologous pairs at the beginning (P1)
Review and practice:
http://www.phschool.com/science/biology_pl
ace/biocoach/meiosis/overview.html
Haploid Gamete formation
Polar Bodies
In males, all 4 haploid cells become sperm
In females, only 1 of the haploid cells become an
egg. That one egg takes all of the cytoplasm and
organelles from the other 3 cells.
Polar body is the
term for each of
the other 3 cells
produced as
waste and reabsorbed by the
female. Why do 3
of the haploid
cells produced in
female meiosis
not make it to
maturity??
Egg cell/ovum
“polar body”
Revisit Earlier question
Which stage of meiosis accomplishes
each of the 2 goals?
Meiosis 1 = Genetic Diversity
Meiosis 2 = Reduce chromosome
number
Similarities
1.
2.
3.
4.
5.
6.
Types of cell division
Occur in humans and other Eukaryotes
Involve parent cells with 46 chromosomes
Produce new daughter cells
Cytokinesis
PMAT sequence is the same in mitosis and meiosis II
Differences between meiosis and mitosis
1.
Prophase of meiosis I creates genetic
diversity/recombination. Mitosis doesn’t create
diversity-only cloned cells.
2. What are the only cells in the mammal body that
undergo meiosis? Why?
http://www.youtube.com/watch?v=qCLmR9YY7o&edufilter=cILBN4dlLF7R2bE8S5GdlA&safe=acti
ve
Which picture (left or right) represents meiosis and
which represents mitosis? How can you tell?
Compare Mitosis and Meiosis
http://highered.mheducation.com/sites/9834092339/student_v
iew0/chapter11/comparison_of_meiosis_and_mitosis.html
DIFFERENCES:
Mitosis
# of times through the
stages:
# of daughter cells produced:
Chromosome number in
daughter cells:
Chromosome number in
parent cell:
Purpose:
Meiosis
1
2
2
4
46
23
46
46
Growth and
repair
Gamete
production
(sexual
reproduction)
Meiosis vs Mitosis Visual Summary
This diagram is courtesy of Stacy Zimmermann of Rutgers
University. Quiz yourself by using your fingers—1 for
haploid, 2 for diploid etc.