Transcript Meiosis - Weebly

Biology is the only subject in which
multiplication is the same thing as division…
Meiosis
Importance of Meiosis
• Unlike the other 2 trillion cells in your body,
meiosis is reserved just for the formation of
gametes
• Sperm in testes
• Eggs in ovaries
Homologous
• The female egg (ovum) is the largest cell in
the body by mass (neurons can be long)
• Meiosis ensures diversity
Mitosis vs. Meiosis
• Today we talked about how the somatic (body)
cells divide for growth, repair, and reproduction.
• We have twenty-three pairs of chromosomes
that are located in EVERY CELL in the
body…except for….
• GAMETES! Sperm & eggs
• They have twenty three chromosomes (no pairs)
• Two of these chromosomes are the ‘sex
chromosomes’ and will determine gender
Human female karyotype
46 chromosomes
23 pairs
Human male karyotype
46 chromosomes
23 pairs
What’s the difference between these two
karyotypes?
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic similarity &
differences.
Jonas
Brothers
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
Florida Panther
Pause for Case Study
The importance of genetic diversity
2050
2011
"Natural habitats are becoming
islands in a sea of development"
~Florida Fish & Wildlife Commission
Inbreeding Depression (2001 data)
• Arterial septal defect
• Cryptorchidism (55%)
• Vaginal Papilloma (93%)
• Kinked tails (72%)
• High incidence of feline infectious,
peritonitis, calicivirus, feline
panleaukopenia (distemper), rabies virus
CRYPTORCHIDISM IN FLORIDA PANTHERS: PREVALENCE, FEATURES, AND INFLUENCE OF GENETIC RESTORATION. 2002. Mansfield et. al.
Sperm Morphology in Felids
The impact and potential etiology of teratospermia in the domestic cat and its wild relatives. 2006. Budhan S. Pukazhenthi
Florida Panther Strategy
• 20% Genetic introgression
with 8 Texas Puma in 1995
2009
Genetic Restoration of the Florida Panther. 2010. Johnson et. al
Florida Panther Success
Genetic restoration of the Florida Panther. 2010. Johnson, Wharren E. et. al
.
Overview of Meiosis
• One diploid cell (2n) is going to undergo
TWO separate division to produce 4
haploid gametes (1n)
• In the case that the gametes meet
(fertilization) a diploid (2n) cell will result
• In meiosis, there are two cell divisions
instead of just one like in mitosis
Meiosis I
• How is it like
mitosis? How is
it different?
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
S phase has happened
double stranded
homologous chromosomes
Meiosis: production of gametes
• chromosome number must be reduced
• diploid  haploid
• 2n  n
haploid
• humans: 46  23
• meiosis reduces chromosome number
• makes gametes
• fertilization restores chromosome
number
• haploid  diploid
• n  2n
diploid
Double division of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
Meiosis 1
Meiosis 2
2nd division of meiosis
separates sister
chromatids
2n = 4
single
stranded
Meiosis 1
• 1st division of meiosis separates homologous pairs
prophase 1
2n = 4
double
stranded
synapsis
metaphase 1
tetrad
2n = 4
double
stranded
reduction
telophase 1
1n = 2
double
stranded
Crossing Over
• IMPORTANT!!!!!!!
• Homologous chromosomes line up and
exchange genetic information
• Ensures genetic diversity
• The chromatids literally BREAK where they
cross over and are rejoined
• The resulting chromosome (with new DNA) is
called a recombinant chromosome
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
Meiosis 2
• 2nd division of meiosis separates sister chromatids
1n = 2
double
stranded
prophase 2
What does
this division
look like?
1n = 2
single
stranded
1n = 2
double
stranded
metaphase 2
4
telophase 2
Steps of meiosis
• Meiosis 1
•
•
•
•
•
interphase
prophase 1
metaphase 1
anaphase 1
telophase 1
1st division of
meiosis separates
homologous pairs
(2n  1n)
“reduction division”
• Meiosis 2
•
•
•
•
prophase 2
metaphase 2
anaphase 2
telophase 2
2nd division of meiosis
separates sister chromatids
(1n  1n)
* just like mitosis *
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
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
23
meiosis
23
egg
23
46
23
46
46
46 46 46
4646 46
zygote
fertilization
sperm
46
mitosis
development
The value of sexual reproduction
• Sexual reproduction introduces genetic variation
• genetic recombination
• independent assortment of chromosomes
• random alignment of homologous chromosomes in Metaphase 1
• crossing over
• mixing of alleles across homologous chromosomes
• random fertilization
• which sperm fertilizes which egg?
• Driving evolution
• providing variation for natural selection
metaphase1
When Things Go Wrong
• Nondisjunction: when gametes have the wrong number of
chromosomes after the meiotic process
• When this happens we get problems
Example
• Down Syndrome: an extra 21st autosome
• Klinefelter syndrome: XXY sex chromosomes
• Turner’s syndrome: missing a sex chromosome