major objectives: 1. understand heredity and genes and basic information on dna 2. differences...
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Major Objectives:
1. Understand heredity and genes and basic information on DNA
2. Differences between asexual and sexual life cycles
3. Understand role of meiosis in sexual
life cycles
4. Know the major steps of meiosis
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Major Objectives:
5. Know where the major sources of genetic variability come from during meiosis
6. Know when the different phases occur in humans (females and males)
7. Know what can go wrong in meiosis
8. Know major differences between mitosis and meiosis
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What is heredity?
The transmission of traits from one generation to the next
How are these traits passed on?
By offspring acquiring genes from parents by inheriting chromosomes
(SEXUAL REPRODUCTION)
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Karyotype
Ordered display of an individual's chromosomes Homologous Chromosome
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ASEXUAL REPRODUCTION
* A single individual passes along copies of all its genes to offspring via binary fission in prokaryotes
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Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image is copyright Dennis Kunkel
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MITOSIS REVIEW QUESTIONS
1. What type of cells does mitosis occur in?2. How many times does DNA replicate in mitosis?3. How many cellular division(s) occur in mitosis?4. How many daughter cells are formed by mitosis?5. What is the chromosome number in daughter cells formed by mitosis from diploid parent cells?6. In mitosis, are daughter cells identical to or different from parent cells?
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SEXUAL REPRODUCTION
Reproduction where two parents give rise to offspring that have unique combinations of genes inherited from the parental gametes
Gametes need 1) motility to be able to meet and unite and 2) food to nourish the developing embryo
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Why Meiosis?
Reduces genetic information that is passed on to offspring; fertilization of sperm and ovum recombine genetic information to form new organism (zygote)=GENETIC VARIABILITY
New organism
Gametes
Original organisms
Haploid (n)
Diploid (2n)
Diploid (2n)
Note: The actual process of meiosis is more complicated than this drawing—we will get to that in a bit
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Why Meiosis?
New organism
Gametes
Original organisms
Haploid (n)
Diploid (2n)
Diploid (2n)
Genetics Problem—Color of leaves:
G=chlorophyll; g=no chlorophyll
gg GG
g G Gg
Gg
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Where does meiosis occur?
Only in Reproductive Organs:
Males-testes-sperm cell(s)
Females-ovaries-egg cell (ovum; ova)
Human gametes only have one set of chromosomes- 22 autosomes and an X (female) or an X or Y (male) sex chromosome= HAPLOID
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REVIEW--HAPLOID/DIPLOID
1. If an organism has 2N=24 chromosomes is this the haploid or diploid number of chromosomes?
2. How many chromosomes will be found in a gamete produced by this organism?
3. How many chromosomes will be found in a zygote produced by this organism?
4. If the zygote of an organism has 12 chromosomes, N= __ chromosomes.
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Life Cycle-Generation to generation sequence of stages on the reproductive history of an organism—from conception to production of own offspring
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Gametophyte
Sporotophyte
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SPOROPHYTE: DOMINANT: In ferns, rhizome with roots & leaves (fronds); sporangia in sori on the underside of fronds
Pteridophytes
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GAMETOPHYTE: In ferns, INDEPENDENT, heart-shaped prothallus with antheridia and archegonia
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Meiosis-Replicates once, divides twice (reduces amount of genetic material)
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Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur.
During this stage, each individual chromatid replicates, similar to mitosis.
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Prophase I is one of the most important stages of meiosis.
1) DNA of the chromosomes begin to twist and condense, making the DNA visible to the microscope.
2) Each chromosome actively seeks out its homologous pair (which also has a sister chromatid).
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Cells in Prophase I
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At Metaphase I, the homologous pairs and their sister chromatids prepare for separation.
During metaphase, the chromosomes are lined by the spindle fibers at the metaphase plate.
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Cell in Metaphase I
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Anaphase I pulls apart the tetrad, separating each homologous chromosome.
It is by random chance that a certain chromosome is pulled to a certain pole.
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Cell in Anaphase I
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In Telophase I two nuclear envelopes begin to surround the separate chromosomes.
Interkinesis follows, which is a resting period from Telophase I to Prophase II. This differs from mitosis because DNA replication does not occur.
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Cell in Telophase I
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MAIN EVENTS OF MEIOSIS I
The number of chromosomes has been cut in half, HOMOLOGOUS CHROMOSOMES SEPARATE. End result is two haploid daughter cells.
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During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids connected by a centromere.
The centrosomes (replicated during Telophase I) produce the spindle fibers and start to move toward the poles of the cell.
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Cells in Prophase II
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Metaphase II is similar to Metaphase I in that the dyads (sister chromatids) are lined up at a metaphase plate by the spindle fibers.
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Cells in Metaphase II
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Anaphase II separates the dyads into individual chromatids. Each sister chromatid ends up on one side of the cell.
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Cells in Anaphase II
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In Telophase II, the nuclear envelopes forms around each set of DNA and the cytoplasm divides.
Four haploid cells have formed from one diploid cell. The chromosomal content is 1/2 the chromosomal content of a diploid cell.
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Cells in Telophase II
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MAIN EVENTS OF MEIOSIS II
Separate the sister chromatids of the replicated chromosomes.
End result is four haploid daughter cells. GAMETES in animals or SPORES in plants.
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MEIOSIS REVIEW QUESTIONS
1. What type of cells does meiosis occur in?2. How many times does DNA replicate in meiosis?3. How many cellular division(s) occur in meiosis?4. How many daughter cells are formed by meiosis?5. What is the chromosome number in daughter cells formed by meiosis from diploid parent cells?6. In meiosis, are daughter cells identical to or different from parent cells?
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MECHANISMS FOR GENETIC VARIATION
1. Independent assortment.
2. Crossing over.
3. Random fertilization.
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Independent assortment
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Independent assortment
*Variation due to the random orientation of homologous chromosomes during meiosis I and nonidentical sister chromatids in meiosis II.# of possible combinations when chromosomes are independently assorted is 2n, where n=haploid #.
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HOW MANY POSSIBLE COMBINATIONS FOR HUMANS??
2n,2(23 power)= ~8 MILLION
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Crossing over
Occurs in Prophase I; homologous portions of two nonsister chromatids trade spaces.
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Crossing over*Occurs in humans, 2-3 times per chromosome pair.
*Crossing over does not create new genes. It simply recombines existing genes in new ways.
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Random fertilization
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HOW MANY POSSIBLE COMBINATIONS FOR HUMANS??
2n,2(23 power)= ~8 MILLION in ovum2n,2(23 power)= ~8 MILLION in spermZygote composed of 1 in 70
trillion possible combinations
Crossing over and random fertilization add more diversity
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Gametogenesis-process of sperm or ovum production via meiosisWHAT:
WHERE:
WHEN:
END RESULT:
MALESSpermatogenesis-production of sperm
in testes, 100,000,000/hr/teste
puberty until death
4 haploid spermatids that will differentiate into 4 mature sperm cells
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Spermatogenesis
Entire process takes ~74 days
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FEMALES
WHAT:
WHERE:
WHEN:
Oogenesis-production of ova (eggs)
in ovaries
Initial steps occur PRIOR to birth. By week 25, cells develop into primary oocytes and begin meiosis until late prophase I and then STOP (~700,000-2 million).
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FEMALES
WHEN: Oocytes recommence at puberty (~20-40,000 remain). Oocytes have been in prophase I for 12-40 years. During female lifetime ~480 may complete meiosis I (one/month). Meiosis II will NOT be completed unless fertilized by a sperm (cell cycle restarted by sperm).
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END RESULT:MEIOSIS I: large secondary oocyte and small polar body; MEIOSIS II: ovum and three polar bodies
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ERRORS IN MEIOSIS1. Non-disjunction-When
homologous chromosomes fail to separate or disjoin in Meiosis I. 25-50% of miscarriages result from zygotes with abnormal chromosome numbers. 1 in 118 newborns have chromosomal abnormalities.Aneuploidy-If effect is limited to one or a few chromosomes;
one more=TRISOMY,
one less=MONOSOMY
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ChromosomalAbnormality
Number amongSpontaneous
Abortions
Number among
Live BirthsTrisomy for 1 0 02 159 03 53 04 95 05 0 0
6 - 12 561 013 128 1714 275 015 318 016 1,229 017 10 018 223 13
19 - 20 52 021 350 113
Paute's Syndrome
Edward's Syndrome
Down Syndrome
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TRISOMY 21-Down Syndrome
*Occurs once in every 650 to 1,000 live births
*Risk increases with maternal age (above 35 for mother): age 30:1 in 1,000 and at age 40:9 in 1,000
*Possible cause due to "suspended" division that female gametes undergo
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Egg is in center of pictureMany cumulus cells from the ovary are seen around the egg. This is a low quality, oocyte from a woman 41 years oldEgg is irregularly shaped and dark.
A "good" egg from a 32 year old woman.
Photos from Advanced Fertility Clinic in Chicago
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Sex Chromosomes
XYY 4 46
XXY 4 44
X0 1,350 8
XXX 21 44
Translocations 239 216
Polyploidy
Triploidy 1,275 0
Tetraploidy 450 0
Klinefelter Syndrome
Turner Syndrome
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2. Alteration of Chromosome Structure
Four Types of Change:1) Deletion2) Duplication3) Inversion4) Translocation
ERRORS IN MEIOSIS
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REVIEW CELL DIVISION
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