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Meiosis The Formation of

Gametes

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Meiosis

Meiosis- a type of cellular reproduction that produces gametes.

Like mitosis, meiosis only occurs in eukaryotic cells.

Biggest difference between meiosis and mitosis is the two cell division process.

Meiosis produces more cells total

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Things that occur during meiosis

Produce cells that contain half the number of chromosomes as the original cell

The daughter cells are not all alike

The number of daughter cells produced is 4 rather than 2 for mitosis

Meiosis Meiosis produces gametes.

Gametes are egg and sperm (animals)

Gametes are pollen and ovum (plants)

Meiosis

There are 2 types of Meiosis. Spermatogenesis: Occurs in the testes of males. (sperm)

Oogenesis: Occurs in the ovaries of females. (eggs)

Spermatogenesis (in the testes of males)

Oogenesis (in the ovaries of females)

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Every gamete, contains the genetic information for the organism to function properly. In humans, each gamete contains 23 chromosomes. In humans all the body cells contain two sets of 23 chromosomes (46 chromosomes)

Chromosome Number

Chromosome number

Any cell that contains any two complete sets is called a diploid cell

The diploid number can also be written as 2N.

A higher diploid number does not mean a more complex organism.

Lettuce =14 Goldfish =94 Human =46 Dog = 78

Chromosome number A cell with only one complete set of chromosomes is call a haploid cell.

The haploid number can also be written as N.

Each human sperm and egg has 23 chromosomes.

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, Meiosis is the fundamental basis of sexual reproduction

Egg (1N) and sperm (1N) unite through fertilization to restore the original chromosome number (2N) for the organism.

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Fertilization of a 6 chromosome organism

1n =3

2n = 6

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Meiosis: Two Part Cell Division

Homologs separate

Sister chromatids separate

Diploid

Meiosis I

Meiosis II

Diploid

Haploid

Homolog

Homologs are “like” chromosomes or the same sized chromosomes that code for the same traits.

Homologs

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Prophase I

Early prophase Crossing over occurs. Homologs pair up.

Late prophase Spindle forms. Nuclear envelope fragments.

Crossing Over The exchange of genetic material between homologous chromosomes.

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Metaphase I

Homologous pairs of chromosomes align along the equator of the

cell (aka:Tetrads)

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Anaphase I

Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres.

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Telophase I

Nuclear envelopes reassemble.

Spindle disappears. Cytokinesis divides cell into two.

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Meiosis II: Reducing Chromosome Number

Prophase II

Metaphase II

Anaphase II Telophase

II 4 Genetically Different

haploid cells

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Prophase II

Nuclear envelope fragments. Spindle forms.

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Metaphase II

Chromosomes align along equator of cell.

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Anaphase II

Sister chromatids separate and move to opposite poles.

Equator

Pole

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Telophase II

Nuclear envelope assembles. Chromosomes de-condense. Spindle disappears. Cytokinesis divides cell into two.

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Results of Meiosis

Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome Different combinations of genes

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The 2 ways the gametes can become genetically diverse

1.When the sister chromatids separate

Meiosis I

Meiosis II

2. When crossing over occurs

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Mitosis Meiosis

Number of divisions

1 2

Number of daughter cells

2 4

Genetically identical?

Yes No

Chromosome # Same as parent Half of parent

When Throughout life At sexual maturity

Role Growth and

repair Sexual

reproduction

Comparison of Divisions