Bio 178 Lecture 22Meiosis

http://homepage1.nifty.com/scilla/sonota/ityou/ityou.html

Reading

• Chapter 12

Quiz Material

• Questions on P 240

• Chapter 12 Quizzes on Text Website (www.mhhe.com/raven7)

Outline

• Sexual Reproduction The Sexual Life Cycle

Meiosis

The Sexual Life cycle• Alternation of Generations

All sexual life cycles alternate between a haploid and diploid state. In some organisms the diploid stage is dominant and in others the haploid stage is dominant.

This is a consequence of mitosis and meiosis:

• MitosisMaintains chromosome number.

• Meiosis

Halves chromosome number.

The 3 Types of Sexual Life Cycle

Unique Features of Meiosis

McGraw-Hill Video

Genetic Variation in the Sexual Life Cycle• Maternal and Paternal ChromosomesIf there was no exchange of genetic material between the homologues each gamete would receive an entire homologue that is either maternal or paternal.

• The Process of Crossing OverPurposeGenetic Variation

SynapsisPairing of chromosomes along their length (after DNA replication).Homologues attached to nuclear envelope.

Synaptonemal Complex

Synaptonemal Complex

http://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.html

(Enzymes)

Spermatocyte Synaptonemal Complex

http://7e.devbio.com/article.php?ch=19&id=188

Green = SCP 1

Blue = DNA

Red/Yellow = Centromere

Top = Normal

Bottom = SCP 3 deficient

Genetic Variation in the Sexual Life CycleCrossing Over• DNA is exchanged between the non-sister chromatids at the chiasmata.

• Recombination nodules break and rejoin homologous chromatids.

http://www.blc.arizona.edu/courses/181summer/10.html

Post Crossing Over

• Synaptonemal complex breaks down.

• Homologues are now joined only by common centromeres and chiasmata.

• Homologues released from nuclear envelope.

Comparison of Meiosis and Mitosis

McGraw-Hill Video

Meiosis• Cell Divisions

2 divisions after the chromosomes have replicated:

Meiosis I

Separation of homologous chromosomes.

Meiosis II

Separation of sister chromatids.

• Ploidy

Diploid Haploid

Meiosis I• Prophase IChromosomes finish condensing.

Synapsis and crossing over occur.

Centrioles (animal cells) move to opposite poles and spindle fibers form between them.

Nuclear envelope breaks down.

Chiasmata move to the ends of the chromosomes ( terminal chiasmata).

Prophase I

Meiosis I (Cntd.)• Metaphase I

Homologous chromosomes align on the metaphase plate, attached to the kinetochore microtubules. These are attached only to the outward facing kinetochores.

Independent assortment - The homolgue orientation is random.

Metaphase I

Independent Assortment

McGraw-Hill Video

Meiosis I (Cntd.)• Anaphase IHomologues separate and are pulled to opposite poles.

• Telophase I

Homologues reach opposite poles.

Nuclear envelopes may form.

• Cytokinesis

May occur.

Anaphase I

Telophase I

Meiosis II

• Prophase II

There may be a brief interphase, but NO DNA SYNTHESIS occurs.

Centrioles move to opposite poles and spindle fibers form.

If the nuclear envelope reformed it breaks down.

• Metaphase IIChromosomes align at the metaphase plate. Kinetochore microtubules are attached to both sides of the centromere.

Prophase II

Metaphase II

Meiosis II (Cntd.)• Anaphase II

Sister chromatids separate and are pulled apart ( chromosomes).

• Telophase II

Chromosomes reach opposite poles.

Nuclear envelopes form.

• Cytokinesis

Results in 4 daughter cells.

Anaphase II

Telophase II

Biodidac. http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=19-7D&File_type=GIF

?

Sources of Genetic Variation in Meiosis

• Crossing over

• Independent assortment