meiosis and genetic variation - chippewa falls middle schoolmeiosis km 16 meiosis – key...
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Meiosis KM 1
Meiosis and genetic variation
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Meiosis KM 2
Genome
• Genome: Complete complement of an organism’s DNA.– Includes genes (control traits) and non-
coding DNA organized in chromosomes.
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Meiosis KM 3
Genes
• Eukaryotic DNA is organized in chromosomes.– Genes have specific places
on chromosomes.
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Meiosis KM 4
Heredity
• Heredity – way of transferring genetic information to offspring
• Chromosome theory of heredity: chromosomes carry genes.
• Gene – “unit of heredity”.
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Meiosis KM 5
Reproduction
• Asexual
– Many single-celled organisms reproduce
by splitting, budding, parthenogenesis.
– Some multicellular organisms can
reproduce asexually, produce clones
(offspring genetically identical to parent).
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Meiosis KM 6
Sexual reproduction
• Fusion of two gametes to produce a single zygote.
• Introduces greater genetic variation, allows genetic recombination.
• With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.
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Meiosis KM 7
Fertilization
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Meiosis KM 8
Chromosomes
• Karyotype:
– ordered display of an individual’s
chromosomes.
– Collection of chromosomes from mitotic
cells.
– Staining can reveal visible band patterns,
gross anomalies.
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Meiosis KM 9
Karyotyping
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Meiosis KM 10
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Meiosis KM 11
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Meiosis KM 12
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Meiosis KM 13
Homologues
• Chromosomes exist in homologous pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes,
they have homologues.
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Meiosis KM 14
In humans …
• 23 chromosomes donated by each parent
(total = 46 or 23 pairs).
• Gametes (sperm/ova):
– Contain 22 autosomes and 1 sex chromosome.– Are haploid (haploid number “n” = 23 in humans).
• Fertilization results in zygote with 2 haploid
sets of chromosomes - now diploid.– Diploid cell; 2n = 46. (n=23 in humans)
• Most cells in the body produced by mitosis.
• Only gametes are produced by meiosis.
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Meiosis KM 15
Chromosome numbers
All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of copies of each chromosome. Diploidy
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Meiosis KM 16
Meiosis – key differences from mitosis
• Meiosis reduces the number of chromosomes by half.
• Daughter cells differ from parent, and each other.
• Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
– Synapsis – homologous chromosomes pair up. Crossing over of non-sister chromatids).
– In Metaphase I, homologous pairs line up at metaphase plate.
– In Anaphase I,overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome (sister chromatids do NOT
separate).
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Meiosis KM 17
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Meiosis KM 18
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Meiosis KM 19
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Meiosis KM 20
MEIOSIS: Putting it all
together
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Meiosis KM 21
Meiosis 1
First division of meiosis
• Prophase 1: Each chromosome dupicatesand remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.
• Metaphase 1: Homologous chromosomes align at the equatorial plate.
• Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
• Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
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Meiosis KM 22
Meiosis II
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate.
• Metaphase 2: Chromosomes align at the equatorial plate.
• Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
• Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.
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Meiosis KM 23
Mitosis vs. meiosis
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Meiosis KM 24
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Meiosis KM 25
5Meiosis creates genetic variation
• During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
• Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over.
No daughter cells formed during meiosis are genetically identical to either mother or father
During sexual reproduction, fusion of theunique haploid gametes produces truly unique offspring. STOP HERE 2-14-11
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Meiosis KM 26
Independent assortment
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Meiosis KM 27
Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 22n = 22 = 4 possible combinations
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Meiosis KM 28
In humans
e.g. 23 chromosomes in haploid
2n = 46; n = 232n = 223 = ~ 8 million possible combinations!
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Meiosis KM 29
Crossing over
Chiasmata – sites of crossing over, occur in synapsis.
Exchange of genetic material
between non-sister chromatids.
Crossing over produces recombinant chromosomes.
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Meiosis KM 30
Harlequin chromosomes
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Meiosis KM 31
Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!
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Meiosis KM 32
Meiosis & sexual life cycles
• Life cycle = sequence
of stages in organisms
reproductive history;
conception to
reproduction.
• Somatic cells = any
cell other than
gametes, most of the
cells in the body.
• Gametes produced by
meiosis.
Generalized animal life cycle
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Meiosis KM 33
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Meiosis KM 34
Sex is costly!
• Large amounts of energy required to find a
mate and do the mating: specialized
structures and behavior required
• Intimate contact provides route for infection
by parasites (AIDS, syphillis, etc.)
• Genetic costs: in sex, we pass on only half of
genes to offspring.
• Males are an expensive luxury - in most
species they contribute little to rearing
offspring.
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Meiosis KM 35
But …
• More genetic diversity: more potential for survival of species when environmental conditions change.– Shuffling of genes in meiosis
– Crossing-over in meiosis
– Fertilization: combines genes from 2 separate individuals
• DNA back-up and repair.– Asexual organisms don't have back-up copies of
genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged.
– Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.
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Meiosis KM 36
Vocabulary
• centriole
• centromere:
• crossing over:
• gamete:
• meiosis:
• zygote:
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Meiosis KM 37
Study Questions
• 1. What happens as homologous
chromosomes pair up during prophase I of
meiosis?
• 2. How does metaphase of mitosis differ
from metaphase I of meiosis?
• 3. What is the sole purpose of meiosis?
• 4. What specific activities, involving DNA,
occur during interphase prior to both
mitosis and meiosis?
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Meiosis KM 38
5. Compare mitosis and meiosis on the following points:
• a. number of daughter cells produced.
• b. the amount of DNA in the daughter cells in contrast to the original cell.
• c. mechanism for introducing genetic variation.
• 6. What is a zygote and how is it formed?
7. Draw a nucleotide and then draw a 10 nucleotide linear sequence of DNA.