mrs. griffin. the birds and the bees… humans and most other animals reproduce sexually. this...
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Mrs. Griffin
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The birds and the bees…
Humans and most other animals reproduce sexually.
This means that two cells fuse to make a new “baby” organism.
These specialized cells are called gametes or sex cells
Sperm and egg cells are human gametes
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Meiosis What is it?
Who does it?
Where does it happen?
Why does it happen?
Why is a different process needed?
Making sex cells (sperm and egg)
All sexually reproducing organismsIn males- testes
In females- ovaries
To all organisms to reproduce
1) Chromosome # needs to be cut in half2) Creates genetic diversity
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Meiosis
MITOSISDiplo
idzygot
e
Diploid
adult
MEIOSIS
Haploidgamete
s
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Remember, humans have 23 pairs of chromosomes
Pairs 1-22 are said to beHomologous chromosomes(they go together becausethey have same traits but arenot identical)
Gene for eye color
Gene for hair color
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MeiosisMeiosis is a special kind of cell
divisionthat halves the number of
chromosomesin gametes. This insures that the
correctnumber of chromosomes are
passed on.
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The goal is to makehaploid sex cells
First division
Second division
MotherFather
Polar Bodies (not
functional)
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What would happen if twocells with 46 chromosomes
fused?
46 chromosomes+
46 chromosomes
92 chromosomes….
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Nondisjunction =chromosomes fail to “dis-join” or separate:
Trisomy=gamete with 3 of one type
Monosomy=gamete with 1 of one type
Tripoidy= zygote with three of one type
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Down Syndrome= 3 of #21
Karyotype
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Klinefelter’s = XXY
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Some symptoms… Delayed puberty Loss of libido - reduced interest in sexual activity Impotence Reduced facial hair - reduction in need to shave Tiredness Reduced muscle power and stamina Change in body shape - increased fat and breast
development Infertility Osteoporosis - brittle bones resulting in fractures Depression and sometimes dissocial behavior Early heart disease
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Turner’s Syndrome
XO sex chromosome
Instead of XX female
or XY for male
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Meiosis has two stages:Meiosis 1and Meiosis 2
Meiosis 1: This is the stage where the chromosome number is halved
Includes 4 stages:Prophase 1Metaphase 1Anaphase 1Telophase 1
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Meiosis 1 Prophase 1 is
similar in some ways to prophase in mitosis.Chromosomes
condense
Spindle fibers appear
Nucleus and nucleolus disappear
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UNLIKE in mitosis, homologouschromosomes line up next to each other during prophase
•This process iscalled synapsing
•Lined uphomologues arecalled tetrads
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When homologous chromosomeshang out so close to each
other,they sometimes swap parts.This is called “crossing over”
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Synapsis and Crossing Over in Action
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Crossing over increases
genetic diversity
Why is this important
for a species’long-term survival?
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During metaphase 1, tetradsline up on a metaphase plate
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Then spindle fibers pull thehomologous chromosomes apart inanaphase 1
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In telophase 1, two daughter cells are formed. They are NOT identical! (Why?)
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After Meiosis 1…
Homologous chromosomes have been separated.
Two non-identical daughter cells have been formed.
The chromosome number has been cut in half.
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The big picture
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Chromosomes are NOT
duplicated again between
Meiosis 1 and Meiosis 2
Why not?
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Meiosis 2 is similar to mitosis Two identical daughter cells are
formed from each of the cells created in Meiosis 1
Includes 4 Stages: Prophase 2 Metaphase 2Anaphase 2Telophase 2
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Meiosis II
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Meiosis 2 Prophase 2: spindle reforms and
chromosomes move toward the metaphase plate
Metaphase 2: sister chromatids lined up on the metaphase plate
Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell
Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells
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All together now…
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Let’s compare mitosiswith meiosis…
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