A typical bio lab ……

//www.youtube.com/watch?v=Vc1UqeHhjeo

MeiosisUnlike mitosis, which occurs in somatic cells, meiosis has evolved to make sexual reproduction possible. Meiosis produces sex cells.

Meiosis – the big picture

THIS PROCESS GENERATES

THE DIVERSITY OF ALL

SEXUALLY REPRODUCING

ORGANISMS. Including,

naturally, us.

Asexual reproduction – no meiosis here

Asexual reproduction – NOT meiosis

Chromosome reduction …

How a father with 46 chromosomes in each cell and a mother with 46 chromosomes per cell can have a child with 46 chromosomes per cell.

Diploid and Haploid Diploid cell – contains two of each

kind of chromosome, one from the male parent, one from the female parent ..... It is 2n. (2n = 46 in human)

Haploid cell – contains one of each kind of chromosome, from either of the parents ..... It is n. (n = 23 in human)

Meiosis defined

Meiosis- process where the number of chromosomes per cell is cut in half through the separation of chromosomes in a diploid cell.

Homologous chromosomes are …

A pair of chromosomes having

the same gene sequences, each derived from one parent.

So, the chromosomes have the same genes at the same loci but possibly different alleles.

Gene – DNA sequence coding for protein. Basic unit of heredity.

Allele – One member of a pair of genes occupying a specific spot on a chromosome (locus) that controls the same trait.

Human Karyotype

Haploid or Diploid?

Mitosis produces _________ cells.

Sex cells (eggs, sperm) are _________ cells.

Phases (Stages) of Meiosis

Interphase I -- • cell replicates its DNA, like in mitosis • each chromosome now consists of 2 sister chromatids held together by a centromere

Interphase

Interphase

Meiotic stages (cont’d.)

Prophase I -- • Chromosomes coil up • Spindle forms • Homologous chromosomes pair

up as tetrads • Crossing over occurs – genetic material exchanges between homologues

Homologues in a tetrad … how romantic!

Crossing over – very important

Stirring the pot …

Crossing over - The chromosomes you receive from your mom’s egg and your dad’s sperm are NOT the same chromosomes that your mom and dad have. They are “shuffled” versions.

How do we know this is .....

Meiosis I stages (cont’d.)

Metaphase I – the centromere of each

chromosome becomes attached to a spindle fiber

the spindle fibers pull the tetrads to the equator of the spindle

homologous chromosomes are lined up side by side along the equator

More shuffling …

Independent assortment – homologues line up or “shuffle” randomly on the metaphase plate in Meiosis I. 223 or 8 million possible assortments of chromosomes for every sperm cell. (Random fertilization – the ovum also has 8 million possible chromosome combos, so … 8 mill. X 8 mill. = 64 trillion possible diploid combinations in each zygote!!!)

How do we know this is …..

Metaphase I

Metaphase I -- after crossing over

Meiotic stages (cont’d.)

Anaphase 1: homologous chromosomes

separate and move to opposite ends of the

cell centromeres do not split this ensures that each new cell will

receive only one chromosome from each homologous pair

How do we know this is …..

Meiotic stages (cont’d.)

Telophase 1: the spindle breaks down and the

chromosomes uncoil the cytoplasm divides to yield two

new cells each cell has half the genetic

information of the original cell because it has only one homologous

chromosome from each pair

How do we know this is …..

Cell plate forms …..

Meiosis II – mitosis of the products of Meiosis I

Prophase II- a spindle forms in each of the two

new cells and the fibers attach to the chromosomes

Metaphase II- the chromosomes are pulled to the

center of the cell and line up randomly at the equator

How do we know this is …..

How do we know this is …..

Meiosis II – mitosis of the products of Meiosis I (cont’d.)

Anaphase II- the centromere of each

chromosome splits the sister chromatids separate and

move to opposite poles Telophase II- -nuclei re-form -the spindles break down -the cytoplasm divides

How do we know this is …..

The spindle

How do we know this is …..

What Meiosis produces:

four haploid sex cells from one original diploid cell

-each haploid cell contains one chromosome from each homologous pair

-haploid cells will become gametes transmitting genes to offspring

A different type of tetrad ….

Put it together …

Don’t blink …

Meiosis overview

Spermatogenesis

Spermatogenesis -- in testes. 2 months from start to finish. Every day, several hundred million sperm are made by meiosis! Spermatogonia – 2n, undergo meiosis Primary spermatocytes – 2n, meiosis I. 23 pairs of homologues incl. X,Y Spermatids – n, meiosis II, 23 chromosomes, incl. X or Y Spermatozoa - n, pumped and ready to roll

Oogenesis Oogenesis – in ovaries. 15 – 30

years from start to finish. Developmental arrest in Meiosis I until a girl goes through puberty.

Oogonia - 2n. 2 million are formed in a baby girl before birth!

Oocytes – primary, meiosis I … 2n. 23 pairs of homologues including XX chromosomes. A few progress through meiosis every 28 days. secondary, meiosis II … 23 chromosomes including an X chromosome. All made up and lookin’ to party.

And finally ……

Meiosis videos

http://www.youtube.com/watch?v=uh7c8YbYGqo

http://www.youtube.com/watch?v=D1_-mQS_FZ0

http://bio.rutgers.edu/~gb101/lab10_meiosis/meiosis_web/index10.html

Meiosis phase I.D. lab

Mitosis vs Meiosis

http://www.pbs.org/wgbh/nova/baby/divide.html#

Meiosis Animation Links

Animation How Meiosis Works.mht

Meiosis animation 2.mht

Animation Stages of Meiosis 3.mht

YouTube - Meiosis.mht

Meiosis An Interactive Animation Cells Alive.mht

YouTube - Meiosis.mht

Meiosis Quiz

http://www.biologycorner.com/bio1/qz_meiosis.html