Meiosis
Sexual Reproduction
Mitosis Review
• Mitosis: division of somatic (body) cells during
the cell cycle.
– Product = 2 identical daughter cells.
– Start with 46 chromosomes and ends with 46
chromosomes.
– 2n = 2n when n = a single chromosome
• Ie: n = 23 chromosomes and 2n = 46 chromosomes or
23 pairs of chromosomes.
Notes
• Meiosis is: The division of sex cells (gametes)
•
•
•
•
– Humans have 46 chromosomes
– Fruit fly has 8 chromosomes
Diploid = “two sets” 2n Haploid = “one set” n
Meiosis is division of gametes, while mitosis is division
of somatic (body) cells.
There are 2 cell divisions during meiosis.
Gametes (sex cells):
– Males = XY
– Females = XX
• Fertilizations: Joining of a sperm (n) and egg (n)
to create a zygote (2n)
How about the rest of us?
• What if a complex multicellular organism (like
us) wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
No!
if we did, then….
46
egg
+
46
92
sperm
zygote
Doesn’t work!
Karyotype
• A “picture” of our chromosomes used by
genetists.
– If you remember: our parents gave each of us 1/2 set
of chromosomes (23 of them to be exact) to pair up
and complete the set (46).
– Used by scientists and doctors to look for
abnormalities and gender
– Pictures are taken during the prophase/metaphase
stage of Mitosis. When chromosomes become visible.
• Each chromosome is paired up with it’s match and
then laid out in order by size and shape.
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
How do we make sperm & eggs?
• Must reduce 46 chromosomes  23
– must reduce the number of chromosomes by half
23
46
meiosis
46
zygote
23
egg
46
23
23
sperm
gametes
fertilization
Steps of meiosis
• Meiosis 1
– interphase
– prophase 1
– metaphase 1
– anaphase 1
– telophase 1
• Meiosis 2
– prophase 2
– metaphase 2
– anaphase 2
– telophase 2
1st division of
meiosis separates
homologous pairs
(2n  1n)
“reduction division”
2nd division of meiosis
separates sister
chromatids
(1n  1n)
* just like mitosis *
I.P.M.A.T.P.M.A.T
Overview
of
meiosis
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
Preparing for meiosis
• 1st step of meiosis
– Duplication of DNA
– Why bother?
• meiosis evolved after mitosis
• convenient to use
“machinery” of mitosis
• DNA replicated in
S phase of interphase
of MEIOSIS
(just like in mitosis)
M1 prophase
2n = 6
single
stranded
2n = 6
double
stranded
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
I can’t
Repeat
after you!
hear
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 1 & 2
Trading pieces of DNA
• Crossing over
– during Prophase 1, sister chromatids
intertwine
• homologous pairs swap
pieces of chromosome
– DNA breaks & re-attaches
synapsis
tetrad
prophase 1
Mitosis vs. Meiosis
Mitosis vs. Meiosis
• Mitosis
– 1 division
– daughter cells
genetically identical to
parent cell
– produces 2 cells
– 2n  2n
– produces cells for
growth & repair
– no crossing over
• Meiosis
– 2 divisions
– daughter cells genetically
different from parent
– produces 4 cells
– 2n  1n
– produces gametes
– crossing over
Pedigrees
• Pedigrees are family trees that show specific
traits passed down through each generation.
• Ms. Greb’s family tree:
How to read a pedigree