MeiosisChapter 10.2 (Pages 263 – 273)

Dude, meiosis starts in five minutes . . .

I can’t believe you’re not condensed yethttp://www.promega.com/

Learning Outcome

Discuss the role of meiosis and sexual reproduction in producing genetic variability in offspring.

Include: crossing-over, randomness

Chromosome Images

http://library.thinkquest.org/28751/review/division/1.html www.geneticusb.blogfa.com/post-140.aspx

Chromosome Images

http://hopes.stanford.edu/basics/dna/f_b11homolgs.jpg

http://home.wxs.nl/~gkorthof/images/chromosome_structure.jpghttp://faculty.ksu.edu.sa/17257/Pictures%20Library/Forms/DispForm.aspx?ID=5

Meiosis

In complex plants and animals where sexual reproduction occurs, there are two types of cells.

Cell Types

Somatic Cells These are the

cells that make up our bodies (muscles, bones, blood, etc.).

Diploid These cells

reproduce by mitosis.

http://www.healthystemcells.org/stemcellbenefits.htm

Cell Types (cont’d)

Reproductive Cells These cells are specialized for reproducing

offspring through sexual reproduction. The gametes have half the DNA (haploid,

single stranded) compared to the regular body cells.

neurophilosophy.wordpress.com/.../

Chromosomes (p 264)

Humans have 23 pairs of chromosomes for 46 chromosomes in each cell nucleus. These cells are called diploid (“di” means

double).

23 from mom homologous pairs

23 from dad

Autosomes vs Chromosomes

Twenty-two of these pairs are called autosomes.

The twenty-third pair are the sex chromosomes (determine gender – more on this later).

Autosomes

Reproductive cells have a haploid (means “half” the number of) number of chromosomes. They contain 23 chromosomes not 23 pair Sperm – male gamete Egg – female gamete (see Fig 10.10 p 266)

Chromosomes (cont’d)

Each pair of homologous chromosomes has genes for the same traits, such as plant height. (not necessarily identical) These genes are arranged in the same

order (see Fig 10.10 p 265) Do Problem Solving Lab 10.2 p 264

Purpose of Meiosis

If mom gives the baby 46 chromosomes and dad gives the baby 46 chromosomes we HAVE A PROBLEM! Junior gets 92! Oops!

Moms and dads need a method of making special cells with half the DNA.

Purpose of Meiosis (cont’d)

There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents.

Meiosis has two purposes:

To make gametes with half the normal number of chromosomes (haploid gametes).

To make gametes that have variations in their DNA code.

The first half of Meiosis makes diploid cells into two haploid cells(Meiosis I).

NOTE:

The first half of Meiosis makes diploid cells into two haploid cells(Meiosis I).

Second half of meiosis splits double strands to single strands. (See Fig 10.12 p 267)

Crossing Over

Some variation is achieved in anaphase 1, but not enough

Further variation is achieved through a process known as Crossing Over.

Crossing Over (cont’d)

The X’mes, in a tetrad (two homologous pairs), pair so tightly that non – sister chromatids from homologous pairs actually break off and exchange genetic material.

This can occur at any location on a chromosome (random).

Crossing Over (cont’d)

It is estimated that there are 2-3 cross-overs in human X’mes during M1.

What happens in one chromosome is completely independent from what happens in another chromosome.

This is what results in so much variation

Crossing Over (cont’d)

http://pl-bioblog.blogspot.com/

AM I really unique?

So how many different gametes can be produced?

How many different offspring can be produced?

Crossing Over (cont’d)

This process results in endless possibilities because each pair of X’mes can line up one of 2 ways (see p 269-270)

(2n where n = # of X’me prs.)

“I can’t live without you Martha.”

Crossing Over (cont’d)

Peas with 7 prs. of X’mes (2n = 27 = 128) Remember, any egg can be fertilized by

any sperm 128 x 128 = 16 384 different possible offspring

Human with 23 prs (2n = 223 = over 8 million) 223 x 223 = about 70 trillion

Gametogenesis

The formation of gametes from germ cells

Different for sperm and egg cells Oogenesis vs. spermatogenesis More about this later

Gametogenesis

http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-20/2009.jpg

In Summary

Three processes guarantee variation in gamete formation Independent orientation

Mixing of mom and dad’s x’mes

Crossing over Random fertilization

GeneticVariation stage of meiosis Animation: Unique Features of Meiosis

Activity/Assignment

Read p 266 – 270 and fill in meiosis chart.

Play Dough Meiosis (MiniLab 10.2 – p268 text & worksheet p3)

Finish heredity key terms

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