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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 …

Two kinds of ChromosomesAutosomes:

◦Code for most genes in your body (not sex determining chromosomes)

◦In humans chromosome #1-22

Two kinds of ChromosomesSex Chromosomes:

◦chromosome that determines what gender you will be.

◦#23 in humans◦Females are: XX◦Males are XY

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

Meiosis KM 21

Mitosis vs. meiosis

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

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

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

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

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?