Meiosis and genetic variation
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Meiosis and genetic variation
Honors BiologyUnit 5
2011-2012
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Genome: Complete complement of an organism’s DNA.◦ Includes genes (control traits) and non-coding
DNA organized in chromosomes.
Genome
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Eukaryotic DNA is organized in chromosomes.◦ Genes have specific places on
chromosomes.
Genes
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Heredity – way of transferring genetic information to offspring
Chromosome theory of heredity: chromosomes carry genes.
Gene – “unit of heredity”.
Heredity
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Asexual◦ Many single-celled organisms reproduce by
splitting, budding, parthenogenesis.◦ Some multicellular organisms can reproduce
asexually, produce clones (offspring genetically identical to parent).
Reproduction
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Fusion of two gametes to produce a single zygote.
With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.
Sexual reproduction
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23 chromosomes donated by each parent Total = 46 or 23 pairs
Gametes (sperm/ova): haploid (n)◦ Haploid= Contains a single set of chromosomes (23)
Zygote fertilized egg - now diploid (2n).◦ Diploid= Contains a two sets of chromosomes (23x2=46)
Somatic cell: any cell other than gametes, most of the cells in the body. – diploid (2n)
In humans …
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Two kinds of ChromosomesAutosomes:
◦Code for most genes in your body (not sex determining chromosomes)
◦In humans chromosome #1-22
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Two kinds of ChromosomesSex Chromosomes:
◦chromosome that determines what gender you will be.
◦#23 in humans◦Females are: XX◦Males are XY
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Sex Chromosomes Mammals use a chromosomal method of
determining sex: XX is female and XY is male.
Birds use a ZW system: ZZ is male and ZW is female.◦ the evolutionary origin of mammalian and bird sex
chromosomes is different
Some reptiles use developmental temperature to determine sex: depends on the species, but hot is male and cold is female in some.
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Chromosomes exist in homologous pairs in diploid cells.
Homologues
Exception: Sex chromosomes (X, Y).
All autosomes are in homologous pairs.
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Chromosome numbersAll are even numbers – diploid (2n) sets of homologous chromosomes!
Ploidy = number of copies of each chromosome. Diploidy
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Meiosis: cell division process by which the number of chromosomes per cell is cut in half
Why does it occur: Meiosis is used to produce the haploid(n) gametes.
Meiosis: What is it?
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Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis: Key differences from mitosis
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First division of meiosis Prophase 1: Duplicated homologous
chromosomes match up forming tetrads. Crossing-over occurs at the chiasmata.
Metaphase 1: Tetrads align at the equator of the cell.
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.
Meiosis 1
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Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Metaphase 2: Chromosomes line up at the
equatorial. Anaphase 2: Sister chromatids move
separately to opposite ends of the cell. Telophase 2: Cell division is complete. Four
haploid(n) daughter cells are produced.
Meiosis II – Similar to mitosis
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Meiosis KM 20
Animation
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Meiosis KM 21
Mitosis vs. meiosis
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Meiosis KM 22
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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 the unique haploid gametes produces truly unique offspring.
Meiosis creates genetic variation
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Meiosis KM 24
Independent assortment
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Number of combinations: 2n
Independent assortment
e.g. 2 chromosomes in haploid2n = 4; n = 22n = 22 = 4 possible combinations
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In humans
e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!
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Crossing overCrossing over produces recombinant chromosomes, mixing the genes of the mother and father, recombining them.
Chiasmata – sites of crossing over
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Harlequin chromosomes
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Meiosis KM 29
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!
Random fertilization
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Gamete Formation in Animals
In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).
Male gamete formation - Spermatogenesis
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Gamete Formation in Animals
In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg.
The other 3 meiotic cells are small “polar bodies”, which degenerate.
Female gamete formation - Oogenesis
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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.
Why Sexual Reproduction …
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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?
Study Questions
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5. Compare mitosis and meiosis on the following points:
a. number of daughter cells produced.b. the amount of DNA in the daughter v. parent
cell c. mechanism for introducing genetic variation.
6. What is a zygote and how is it formed?