Meiosis All the cells except the sex cells (sperm

and eggs) of organisms that reproduce sexually contain 2 sets of chromosomes, one from the mother, one from the father.

A cell that contains both sets of chromosomes is a somatic cell. It is said to be diploid because it has 2 sets of chromosomes (one from the father, one from the mother). Diploid can be represented by the symbol 2N (2 sets).

Diploid cells contain 2 complete sets of chromosomes therefore 2 copies of a each gene (except the X and Y chromosomes which have genes for different things.)

The gametes, or sex cells, of sexually-reproducing organisms contain only a single set of chromosomes, therefore only a single set of genes.

These cells are called haploid cells (means one set).

Haploid is represented by N which means one set.

For example, the egg of a Drosophila (fruit fly) has 4 chromosomes. So does a sperm. This can be written as:

N = 4The haploid number is 4.

How many chromosomes are there in a somatic cell of a fruit fly?

So, how does an organism make egg and sperm cells, with only half the number of chromosomes of regular somatic cells?

By a process called meiosis

MeiosisREDUCTION DIVISIONThe division of the cells which are involved in sexual reproduction so that the number of chromosomes is reduced by half. This results in either sperm or egg.

sperm + egg = zygote

n + n = 2n

MeiosisMeiosis is a process of reduction division. In a human, at the beginning of meiosis, there is a cell with 46 chromosomes; at the end, a cell with half that.

Through meiosis, one diploid cell becomes 4 haploid cells. In animals, these haploid cells are sperm or eggs.

Where do sperm cells come from?

They are created from diploid cells called spermatocytes in the testes.

The sperm cell itself is a haploid cell.

Egg cells Egg cells come from diploid cells call

oocytes located in the ovaries.

The egg cell itself is haploid.

The process of meiosis involves 2 distinct divisions called meiosis I and meiosis II.

Before you go any further, look at page 277 of your text book and watch the Crash Course on meiosis using the link below.

http://www.youtube.com/watch?v=qCLmR9-YY7o

Understanding the vocabulary of meiosis

Homologous chromosomes

In each diploid cell there are two chromosomes that have genes on them that code for the same things. One of the pair came from the mother; the other chromosome in the pair came from the father. These pairs of chromosomes that code for the same thing are called a homologous pair.

Each chromosome in a homologous pair is: the same size the same shape the same gene arrangement (generally

genes that code for the same trait)but

they are not identical because the actual code is different. (have different alleles)

For example, both chromosomes in a homologous pair might code for “curling of your tongue” but one might let you curl your tongue and the other might not.

Homologous pairs of chromosomes

Remember that DNA is replicated during the S phase of interphase. This replication also occurs in the cells that will ultimately produce sperm and egg cells.

So, after replication, each homologous pair has a matching homologous pair.

Crossing Over

Synapsis is the process in which homologous pairs of chromosomes pair up during Prophase I to form what is called a tetrad.

(Remember that in a diploid cell, there is a copy of a chromosome from each parent)

This results in exchange of genetic material between the maternal and paternal chromosomes. This process is called crossing over. The result of crossing over is genetic recombination.

The point at which the chromosomes cross over is called the chiasma. (see diagram in 2 slides)

During synapsis the chromatids of chromosomes within a homologous pair may twist around each other, break off and attach to the other chromosome in a process called crossing over.

Crossing Over

Make sure that you know what happens in each phase of meiosis by watching the Crash Course video, reading your text and looking at the following slides.

Law of Independent AssortmentDuring metaphase, as the

homologous pairs line up, the orientation of maternal and paternal chromosomes is randomNot all maternal go to one side, and paternal the other….it is mixed.

Law of independent assortment

As the pairs of chromosomes are separated in anaphase I, the maternal and paternal chromosomes have random separation which results in genetic variation.

Formation of Gametes Meiosis only occurs in the reproductive

cells…all other cells that divide undergo mitosis.

Ending products of meiosis are haploid gametes Eggs in female - oogenesis Sperm in male - spermatogenesis

Fusion of sperm and egg results in a diploid cell

http://www.youtube.com/watch?v=iCL6d0OwKt8

Karyotype

DNA v. Gene