Meiosis Objectives: 1. To Compare Mitosis with Meiosis 2. To recognize the mechanisms of Meiosis...

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Meiosis

Objectives:1. To Compare Mitosis with Meiosis

2. To recognize the mechanisms of Meiosis that increase genetic diversity.

3. To understand the effects of abnormal Meiosis

Mitosis Review

• IPMAT

Big Events of Mitosis• Interphase: DNA replicates• Prophase: Chromatid pairs form• Nuclear envelope

disappears.• Spindle forms• Metaphase: Chromatid pairs align at the equator of the spindle.• Anaphase: Chromatid pairs separate and Chromosomes are pulled to

0pposite sides of the cell.• Telophase: “Opposite” of Prophase• Cytokinesis

• Mitosis Review• Mitosis Rap.....You Have to See This

Reasons for Mitosis?

• Production of 2 daughter cells that are genetically identical

• Provides a) a means of asexual reproduction for unicellular organisms

• b) replacement of worn out and damaged cells

• c) growth of an organism

Replication

Chromosomes, Chromatids and Centromeres

Centromere

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes

From Zygote to Embryo

Zygote2n

Zygote

Cleavage

Blastula

Meiosis

• Special cell division that produces “haploid” sex cells or gametes.

• Consists of 2 stages, Meiosis I and Meiosis II• One replication and two cell divisions• For every special reproductive cell (germ cell)

that undergoes Meiosis, four haploid sex cells are produced

Meiosis I• Interphase I• - DNA Replicates(This cell: 2n = 4)

• Prophase I

– Nuclear envelope disappears– Replicated Chromatin coils into chromatid pairs– Spindle forms– Synapsis forms Tetrads

Synapsis

• Animation

• Chromatid pairs join to form Tetrads• After tetrad formation, arms of the chromatid

pairs exchange segments in a process called Crossing Over

Crossing over

• Where the chromatid pairs cross over is called the Chiasma.

• Increases genetic diversity in sex cells

Prophase I:Tetrad formation/

crossing over

Crossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

Metaphase I

• Tetrads align at the equator of the spindle.

• Note: These tetrads should still be connected!!

Anaphase I

• Tetrads separate in an event called Disjunction.

• Chromatid pairs are pulled to opposite sides of the cell

Telophase I

• Cytokinesis splits the cell in two and ends Meiosis I

Meiosis II

• Cells skip Interphase II and move right into Prophase II

• Meiosis II follows the stages of Mitosis

Prophase II

• Nuclear envelope disappears• Spindle forms• Chromatids are floating throughout the

cytoplasm

Metaphase II

• Chromatids align at the equator of the spindle

Anaphase II

• Chromatid pairs split and chromosomes are pulled to opposite sides of the cells

Telophase II

• Cytokinesis splits the 2 cells into 4 Haploid sex cells. (n=2)

InterphaseInterphase

Mother cell Stages Of Meiosis: Meiosis I

Meiosis IIMeiosis II

crossing over

crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Stages Of Meiosis: Meiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

The products of mitosis are 2 diploid cells with identical chromosomes.

The products of meiosis are 4 haploid cells each with a unique set of chromosomes.

Prophase IIProphase II

Meiosis ReviewMeiosis

Meiosis 2 (The quiz)

Meiosis in a 2n = 2 Cell

Meiosis and Crossing Over

Here it is!

Another look!

Meiosis Square Dance

Spermatogenesisvs

Oogenesis

• Spermatogenesis• - produces sperm cells• - ratio of germ cells to

sex cells is 1:4

• Oogenesis• - produces egg cells• - ratio of germ cells to

sex cells is 1:1 (3 “polar bodies die each time)

Spermatogenesisvs

Oogenesis

• Spermatogenesis • Oogenesis

Terms to Remember:

• Synapsis• Tetrads• Crossing Over• Disjunction• Spermatogenesis• Oogenesis• Germ Cells• Haploid• Diploid

Meiosis Dry Lab

• Using a special germ cell with a 2n = 6, draw the stages of Meiosis I and Meiosis II as a review of this special cell division.

• Draw your diagrams neatly. Make sure any important structures or events are clearly labelled.

• Your finished review will go in your lab book.• I will provide paper and “cell simulators”

Nondisjunction

• Occasionally, during Anaphase I, the tetrads do not separate in an event called Nondisjunction.

• The result is sex cells with either 1 extra chromosome or 1 less chromosome

Nondisjunction

• Nondisjunction

• An extra chromosome creates a Trisomy (3 chromosomes where there should be 2)

• One less chromosome creates a Monosomy (1 chromosome where there should be 2)

Chromosomes

• Autosomal Chromosomes– Chromosomes other than sex chromosomes

– Homologous – same size, shape and have the same gene arrangement

– Pairs 1-22 in humans– Pairs 1-3 in fruit flies

• Sex Chromosomes– Homologous in females XX– Non-homologous in males XY

– SRY Gene found on the Y chromosome directs the development of a male.

Chromosomal Disorders

• Autosomal Disorders:– Down Syndrome (Trisomy 21)– Edward’s Syndrome (Trisomy 18)– Patau’s Syndrome (Trisomy 13)

• Sex Chromosome Disorders:– Turner’s Syndrome (X0)– Kleinfelter’s Syndrome (XXY)

Chromosome Number Disorders

• Trisomy 21 – Down Syndrome

• Trisomy 18 – Edward’s Syndrome

• Trisomy xxy – Kleinfelters Syndrome

• Monosomy xo – Turners Syndrome• (single x female)

Abnormal Chromosomal Number Disorders

• Turner's Syndrome• Edwards Syndrome Elliott• Patau's Syndrome• Down Syndrome• Kleinfelters Syndrome

• Angelman Syndrome

Meiosis Objectives: 1. To Compare Mitosis with Meiosis 2. To recognize the mechanisms of Meiosis...

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