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Meiosis- cell division that producing reproductive cells called gametes (or egg and sperm)
Meiosis involves replicating DNA once and dividing the nucleus two times in steps called
Meiosis I and Meiosis II.
By dividing twice, the chromosome number is halved in gametes.
Slide 1
- 2 parents
- offspring genetically similar to parents
(combination of parent chromosomes) (offspring not identical)
Gamete: Sex cellFemale: egg
Male: sperm
Zygote: Union of sperm and egg 46
23
23
2n
n
n
(Diploid)
(Haploid)
Gonads: Male – Testes Female – Ovaries
Slide 2
Meiosis: Process of cell division used to producegametes (sex cells)
46SC
46DC
23 DC
23 DC
Interphase
PMAT
23SC
23SC
23SC
23SC
PMAT
IIII
IIIIIIII
Primary Sex Cell
Secondary Sex Cells
Sex Cells
2n
2n
n n
n n n n
Slide 3
What happens in PMAT is the same in meiosis as in mitosis
EXCEPT in…….
Prophase I
Anaphase I
Slide 4
Prophase I of Meiosis I
Sister chromatids pair with their homologous pair, forming a tetrad
*Double chromosomes made during interphase*
Slide 5
Homologous ChromosomesHumans have 23 chromosome pairs, each chromosome in the pair contains
the same kind of traits, like eye color; they are called homologous chromosomes. (Humans have 23 homologous chromosomes for a total of 46 chromosomes
Having two copies of every gene can be an advantage. For example, if the one gene from Dad was damaged, Mom’s good gene could take over.
Slide 6
A Way Meiosis Makes Lots of Different Sex Cells A Way Meiosis Makes Lots of Different Sex Cells Crossing-OverCrossing-Over
Slide 10
Crossing Over: - increases variation
-changes chromosome package -(not same as either parent)
Pieces of homologous chromosomes switch locations
P
Bld
Blu
Fair
M
Brn
Brn
Drk
P
Bld
Blu
Drk
M
Brn
Brn
Fair
Slide 11
Anaphase I of Meiosis I
Spindle fibers pull the homologues (tetrad)
away from each other during disjunction.
Slide 13
Telophase I of Meiosis I
The chromosome number in these cells divides in half; starting with an initial four (2n) and ending with two (n) in each new cell.
Slide 14
Meiosis II: Similar to Mitosis
In Meiosis II each chromosome lines up and sister chromatids separate from each other.
Prophase II Metaphase II Anaphase II Telophase II
Slide 15
http://www.biologyinmotion.com/cell_division/
Meiosis in males Meiosis in femaleOccurs in testes Occurs in ovaries
2n 2n
2n 2n
n n n n
n n n n n n n4 sperm 1 egg polar bodies
1:4 1:1
n3 polar bodies
Slide 16
Meiosis does two things -Meiosis does two things -
1) Meiosis takes a cell with 1) Meiosis takes a cell with two copiestwo copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copysingle copy of every chromosome (haploid). of every chromosome (haploid).
This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.
In meiosis, In meiosis, one diploid cells produces four one diploid cells produces four haploid cells.haploid cells.
Slide 17
2) Meiosis 2) Meiosis scramblesscrambles the specific forms of the specific forms of each gene that each sex cell (egg or sperm) each gene that each sex cell (egg or sperm) receives. receives.
This makes for a lot of This makes for a lot of genetic diversitygenetic diversity. This . This trick is accomplished through trick is accomplished through independent independent assortmentassortment and and crossing-overcrossing-over..
Genetic diversity is important for the Genetic diversity is important for the evolution evolution of populations and speciesof populations and species..
Slide 18
MeiosisMeiosis
Parent cell – chromosome pair
Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes
What if Meiosis doesn’t happen as planned?!?!
Slide 19
Meiosis error - fertilizationMeiosis error - fertilization
Should the gamete with the Should the gamete with the chromosome pair be fertilized chromosome pair be fertilized then the offspring will not be then the offspring will not be ‘normal’.‘normal’.
In humans this often occurs In humans this often occurs with the 21with the 21stst pair – producing pair – producing a child with Downs Syndromea child with Downs Syndrome
Slide 21
Trisomy 21– Downs SyndromeTrisomy 21– Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?
Slide 22
Anaphase I
Disjunction: Separation of tetrad at the centromere
Nondisjunction: Uneven separation of a tetrad during disjunction
Ex. Down’s Syndrome (nondisjunction of 21st chromosome tetrad)
Polyploidy: Nondisjunction of all tetrads-gamete has complete extra set of chromosomes
Animal cells – fatalPlant cells – larger more vigorous variety
Slide 23
Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”
X chromosomeX chromosomeY chromosomeY chromosome
Slide 24
How does the image in the first picture become the person in the last picture?
Slide 26
http://www.pbs.org/wgbh/nova/baby/divide.html#
Type of cells that perform:
Uses:
Number of cell divisions:
Chromosome counts:
Genetic Variation?:
Number of daughter cells:
Mitosis: Meiosis:
Somatic
1
2n2n
None
Gametes
Sexual Reproduction
2
2nn
Yes (crossing over)
Growth, Repair,Asexual reproduction
2 4 sperm/1 egg
Slide 27
- Organism that normally contains both sexes (ovaries and testes) - Usually does not self fertilize
Example: earthworm
-Development of an unfertilized egg to a complete organismExample: male drones bees
- Fertilized eggs give rise to queen and female workers
Slide 28
Union of the sperm and the egg forming a zygote
Occurs outside body of female
Occurs inside bodyof female
Produces more eggs (1000+) Produces less eggs (1-12)
Occurs in water Secretes fluid to providewatery environment
FishAmphibians
BirdsReptilesMammals
Slide 29
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