Download Prophase 1

Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
23
meiosis
23
egg
23
46
23
zygote
fertilization
sperm
46
46 46
46 46 46
4646 46
mitosis
development
Homologous chromosomes

Paired chromosomes
 both chromosomes of a pair carry “matching” genes
 control
same inherited characters
 homologous = same information
diploid
2n
2n = 4
single stranded
homologous
chromosomes
double stranded
homologous chromosomes
Sister chromatids: DRAW
Before we begin…Interphase
Each chromosome duplicates itself to
form sister chromatids
Prophase 1
Homologous chromosomes pair up
 Crossing over occurs – pieces of
homologous chromosomes switch places
 Nuclear envelope disappears

Metaphase 1

Each pair of homologous chromosomes
lines up in the middle of the cell
Anaphase 1

Each pair of homologous chromosomes
are split up and pulled to opposite sides
Telophase 1

Cell splits into two cells
Prophase 2
Nuclear envelope breaks down
 Spindle fiber forms

Metaphase 2

Chromosomes (sister chromatids) line
up in the middle
Anaphase 2

Sister chromatids are separated
Telophase 2

Cells divide into two
In summary…
One diploid cell produces four haploid
cells (gametes)
 Meiosis makes sex cells (sperm/eggs)
 Each gamete has a different combination
of genes

Remember this!!!
PMATPMAT
Overview of meiosis
I.P.M.A.T.P.M.A.T
2n = 4
interphase 1
prophase 1
metaphase 1
anaphase 1
n=2
n=2
prophase 2
telophase 1
metaphase 2
anaphase 2 telophase 2
n=2
Double division
of meiosis
DNA replication
Repeat
I can’t
after
hear you!
me!
1st division of
meiosis separates
homologous pairs
2nd division of
meiosis separates
sister chromatids
Meiosis 1
Meiosis 2
Meiosis 1

2n = 4
single
stranded
1st division of meiosis separates
homologous pairs
prophase 1
2n = 4
double
stranded
metaphase 1
2n = 4
double
stranded
synapsis
tetrad
reduction
telophase 1
IRepeat
can’t
after
hear you!
me!
1n = 2
double
stranded
Meiosis 2

2nd division of meiosis separates sister
chromatids
prophase 2
What does
this division
look like?
1n = 2
single
stranded
1n = 2
double
stranded
metaphase 2
4
telophase 2
1n = 2
double
stranded
Meiosis video
Meiosis Animation Short
 https://www.youtube.com/watch?v=Y4s7CyzFU0k
Meiosis Extended
 https://www.youtube.com/watch?v=rqPMp0U0HOA
Three factors that increase genetic
variation:
1.
2.
3.
Crossing over
Independent assortment
Law of segregation
Law of segregation

The two alleles for each gene separate during
meiosis
 Gametes
have equal chance of inheriting either allele
(but NOT both)
Nondisjunction
• failure of homologous chromosomes or sister
chromatids to separate properly during cell division
This leads to Down
Syndrome!
The value of sexual reproduction

Sexual reproduction introduces genetic variation

genetic recombination

independent assortment of chromosomes


crossing over


mixing of alleles across homologous chromosomes
random fertilization


random alignment of homologous chromosomes in Metaphase 1
which sperm fertilizes which egg?
Driving evolution

providing variation for natural selection
metaphase1
For more practice….




Visit the class website: southernbio.weebly.com
Go to “Webquests” and scroll down to where it says
“Meiosis Webquest”
Download the file named “Meiosis Interactive PPT”
Once you open the PowerPoint, click on “Now for
Meiosis”