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MEIOSIS Making Sex Cells
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Somatic cell = body cell
Get the Lingo Down!!! Somatic cell = body cell Skin Nerve Blood
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Human Chromosomes How many chromosomes? 46
How many pairs of chromosomes? 23
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Homologous Chromosomes
Two sister chromatids joined at the centromere From MOTHER plus Two sister chromatids joined at the centromere From FATHER = All Four Chromatids carrying genes controlling the same inherited characteristics
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Loci The place on a chromosome where a specific gene is located
The plural is "loci," not "locuses." EXAMPLE: gene for hair color Gene for height
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Autosomes Any chromosome that is not a sex chromosome
Humans - #1 through #22
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Sex Chromosomes Female = XX Male = XY Chromosome #23
Is this a male or female?
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Is this a male or female?
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Where do we get the pair of chromosomes?
One set from our father One set from our mother
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Human Cells Diploid number 2n 46 somatic cells Haploid number n 23 sex cells
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Sex Cells “Gametes” EX: Egg and Sperm
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Single set of chromosomes
Are Gametes n or 2n? N Single set of chromosomes
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Are Gametes haploid or diploid?
-n 23 chromosomes egg 23 sperm
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Fertilization Fusing of Egg and Sperm Woo-Hoo! What’s the
Fertilized egg called? Zygote
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Where are the sex cells made?
Testes make sperm Ovaries make eggs Only in reproductive organs
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What is the process… In which sex cells are made? MEIOSIS
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How many cells formed? Haploid or Diploid?
4 haploid diploid
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MEIOSIS INTERPHASE Yes, the chromosomes duplicate
Still chromatin, loose
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Meiosis I: Prophase I 90% of meiosis time
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Meiosis I: Prophase I Synapsis: formation of a tetrad (4 homologous chromosomes)
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Let’s Review: All DNA and Protein
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With Jonathan Edwards…whatever happened to him, anyway?
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Let’s See a Close-up: Crossing Over
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Chiasma Sites of crossing over Appear as X-shaped regions
Site where two homologous chromosomes are attached to each other
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Genetic Recombination
The production of gene combinations different from those carried by the original chromosomes ADVANTAGE: increases diversity
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What else happens Prophase I?
What dissolves? Nuclear Membrane and Nucleolus
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What else happens Prophase I?
What also forms? spindles
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Meiosis I: Metaphase I Where are the tetrads lined up? Metaphase plate
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Meiosis I: Anaphase I Where are the sister chromatids going?
Apart to the poles NOTE: the double strands
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Meiosis I: Telophase I Note the double strands at the poles
Cytokinesis begins
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Interkinesis/Cytokinesis
Some cells will go into a temporary chromatin-forming interphase (like in humans) called interkinesis
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Interkinesis/Cytokinesis
Some will immediately go into Prophase II NOT ALL CELLS DO THIS AT THE SAME TIME…BUT ALL WILL EVENTUALLY GO INTO PROPHASE II
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Interkinesis/Cytokinesis
(Different than mitosis interphase, however) NO NEW DUPLICATION OF CHROMOSOMES… FURTHER DIVISION OF THE CHROMOSOMES WILL OCCUR
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Meiosis II: Prophase II
How many cells are there? 2 What is dissolving? Nuclear membrane What’s forming? New spindles
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Meiosis II: Metaphase II
How many cells? 2 Where are the chromatids lined up? Metaphase (equatorial plate)
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Meiosis II: Anaphase II
How many cells? 2 What are the single chromosomes doing? Moving to poles
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Meiosis II:Telophase II
How many cells will result at the end? 4 Will the cells be haploid or diploid? Haploid
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Name the Phase (Meiosis I)
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Name the Phase (Meiosis II)
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Sperm Meiosis All cells same size
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Sperm maturing in a seminiferous tubule
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Egg Meiosis One large egg, 3 small polar bodies
“oogenesis”
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Follicle and egg
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Results in 2 daughter cells
Contrast MITOSIS 1 division (PMAT) Results in 2 daughter cells MEIOSIS 2 divisions (PMAT) I and (PMAT) II Results in 4 daughter cells
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Contrast MITOSIS Diploid offspring Makes somatic cells MEIOSIS
Haploid offspring Makes sex cells
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Contrast MITOSIS Makes identical cells Same number of chromosomes MEIOSIS Makes similar cells Half the number of chromosomes
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Independent Assortment
This means that traits are transmitted to offspring independently of one another. Independent Assortment Animation
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Independent Assortment
In other words, RANDOM COMBINATION OF ALLELES APPEAR IN THE GAMETES
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How many possible combinations are there?
n = haploid number 22 = ? 8
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When one man (223) combines with one woman (223)?
How many combos? When one man (223) combines with one woman (223)? 246
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How many combos from independent assortment:
HUMANS 223 = ? About 8 million (8,388,608)
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If only one crossover occurs, how many possible combinations?
(70,368,744,000,000)
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With fertilization and crossing-over, how many combos possible?
(4 23) 2 = 4,951,760,200,000,000,000,000,000,000
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Wait a second… What are alleles?
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Alleles Different Versions of Genes
FOR EXAMPLE: B = brown eye color b = blue eye color FOR EXAMPLE: T = Tall t = short
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More Allele Examples (fruit)
Hi = Hard rind inhibitor Hr = Hard rind l = light fruit color Rd = dominant = Red skin color of fruit St = stripped fruit lengthwise Rd = dominant = Red skin color of fruit
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Dog Breeds: Lots of Alleles
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Dog Color Alleles in order of decreasing dominance:
Ay - agouti "red" (black, if any, appears in restricted areas only) Aw - "white-bellied" agouti A - solid agouti at - black & tan a – non agouti (recessive black) Ea - dominant black Em - black mask
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Why are the cells formed in meiosis different from the parents?
Random Fertilization (who ya have sex with) Independent Assortment Crossing Over
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So an advantage of Meiosis…
A tremendous amount of genetic variation is possible!
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Let’s Review Three sources of genetic variability:
1. Crossing over during prophase I of meiosis 2. Independent orientation of chromosomes at metaphase I 3. Random fertilization
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Karyotype An orderly display of magnified images of the individual’s chromosomes Shows the chromosomes as they appear in metaphase
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What is a Normal Karyotype?
We are supposed to have 46 total chromosomes in each cell (22 pairs of autosomes = 44, + 2 sex chromosomes).
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Preparing a Karyotype 1. Use lymphocytes (white blood cells)
2. Chemical to stimulate division 3. Chemical to stop in metaphase (stop spindle fibers forming)
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Preparing a Karyotype (cont)
4. Centrifuged to remove white blood cells 5. Chromosomes spread out in hypotonic solution. 6. Drop on a slide. 7.Sort by size and shape.
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Resulting Photograph Sort by size and shape Largest to smallest
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Amniocentesis Take fluid from amniotic fluid around the baby
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Spectral Karyotype
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Normal Karyotype WHY?
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Down Syndrome Karyotype
Trisomy 21
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Down Syndrome Trisomy 21 Folds over eyes Sluggish muscles Mental Problems
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Down Syndrome The most common chromosome number abnormality Round face
flattened nose bridge small, irregular teeth
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Down Syndrome Short Stature heart defects
susceptibility to respiratory infection , leukemia and Alzheimer’s
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Does the mother’s age matter?
As the age of the mother increases above 30, the frequency of Trisomy 21 also increases
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Abnormal Sex Chromosomes
47 XXY syndrome male testes small (sterile) breast enlargement feminine body contours also XXYY, XXXY, XXXXY Klinefelter’s
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TURNER SYNDROME XO (only one X) short
often web of skin between neck and shoulders sterile poor breast development
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Turner Karyotype
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Why are the cells formed in meiosis different from the parents?
Random Fertilization (who ya have sex with) Independent Assortment Crossing Over
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How many possible combinations are there?
n = haploid number 22 = ? 8
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DELETION Fragment of the chromosome is lost
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Duplication Fragment of one chromosome attaches to a homologous chromosome
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Translocation Fragment reattaches in reverse direction (less likely to produce harm)
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INVERSION The chromosome breaks in two places, a piece of the chromosome is removed and the chromosome pieces remaining rejoin. Less likely to remove harm
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The left one is normal and the right one is inverted near the centromere. Inversions, by definition, do not involve loss or gain of chromosomal material. INVERSION 46,XY,inv(16)
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45,XX,rob(13,14) A Robertsonian translocation
(an end to end fusion of #13 and #14 There is no net gain or loss of genetic material in this person so they would have a normal phenotype. 45,XX,rob(13,14) A Robertsonian translocation
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Locus
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Broccoli + Cauliflower = Broccoflower
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Male Lion X Female Tiger
Liger
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