APBIO- CHPT 13 MEIOSIS OVERVIEW

Warm-Up:

The ends of DNA molecules form telomeres that help keep the ends of chromosomes from sticking to each other. Why might this be especially important in germ cells, which go through meiosis and make haploid gametes?

Objectives

• (1) The student will be able to compare and contrast the two rounds of division in meiosis.

• (2) The student will be able to describe how haploid cells develop into mature gametes.

KEY CONCEPT: During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

Cells go through two rounds of division in meiosis.

• Meiosis reduces chromosome number and creates genetic diversity.

Meiosis I and meiosis II each have four phases, similar to those in mitosis.

• Pairs of homologous chromosomes separate inmeiosis I.

• Homologous chromosomes are similar but not identical.• Sister chromatids divide in meiosis II.• Sister chromatids are copies of the same chromosome.

Process of Meiosis• Meiosis I occurs after DNA has been replicated.• Meiosis I divides homologous chromosomes in four

phases.

Process of Meiosis• Meiosis II divides sister chromatids in four phases.• DNA is not replicated between meiosis I and

meiosis II.

MEIOSIS MICROSCOPE SLIDE DEMONSTRATION

• The next set of slides for Meiosis show lily anthers and ovules.

• Meiosis in plants occurs in sporangia. • The sporangia of flowering plants are

located in the flower. • Flowers have both male and female

sporangia. • Male sporangium- anther • Female sporangium- ovule

• Since the lily is a plant, the meiotic products will not directly form gametes, but will develop by mitosis into structures (gametophytes) that will eventually contain sperm (the pollen grain) and egg (the embryo sac).

Meiosis I• Recall that prior to meiosis, during interphase, DNA was

duplicated so that all of the chromosomes in the cells about to do meiosis are duplicated.

Meiosis I

• Prophase I- chromosomes condense, homologous chromosomes begin to pair up, nuclear envelope breaks down, spindle fibers form.

Meiosis I

• Metaphase I- spindle fibers align homologous chromosomes along the cell equator

Meiosis I

• Anaphase I- Homologous chromosomes separate to opposite sides of the cell, sister chromatids remain attached together.

Meiosis I

• Telophase I- Spindle fibers fall apart, nuclear membrane may form again, cell undergoes cytokinesis.

Meiosis II• Meiosis II is just like mitosis; duplicated chromosomes are

distributed equally into new cells, each with the same number of chromosomes as the original cell.

• The differences are that you are starting with two cells, and forming a total of four new cells, and the two cells contain no homologous chromosomes.

Meiosis II

• Prophase II- nuclear envelope breaks down if necessary, spindle fibers form

Meiosis II

• Metaphase II- spindle fibers align chromosomes along the cell equator.

Meiosis II

• Anaphase II- chromatids separate to opposite sides of the cell

Meiosis II

• Telophase II- nuclear membrane forms around chromosomes, they begin to uncoil, spindle fibers fall apart, cell undergoes cytokinesis.

Reflection

• (1) Compare and contrast the two rounds of division in meiosis.

Meiosis vs. Mitosis• Meiosis differs from mitosis in significant ways.• Meiosis has two cell divisions while mitosis has one.• In mitosis, homologous chromosomes never pair up.• Meiosis results in haploid cells; mitosis results in diploid

cells.

Haploid cells develop into mature gametes.

• Gametogenesis- production of gametes.

• Gametogenesis differs between females and males.• Sperm become streamlined

and motile.• Sperm primarily contribute

DNA to an embryo.• Eggs contribute DNA,

cytoplasm, and organelles to an embryo.

• During meiosis, the egg gets most of the contents; the other cells form polar bodies.

Reflection

• (2) Describe how haploid cells develop into mature gametes.

Online Resources• Directions: Log onto a computer and visit the following

websites. Read over the material and create an outline of the key points.

• Life Cycles of Animals and Plants• http://www.wisc-online.com/objects/ViewObject.aspx?ID=BIO104

• Plant Sexual Life Cycles• http://taggart.glg.msu.edu/bot335/sex.htm