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

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes

From Zygote to Embryo

Zygote2n

Zygote

2n

Cleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Cleavage

From Zygote to Embryo

Blastula

From Zygote to Embryo

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

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

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

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

Chromosome Number Disorders

• Trisomy 18 – Edward’s Syndrome

Chromosome Number Disorders

• Trisomy xxy – Kleinfelters Syndrome

Chromosome Number Disorders

• 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