Meiosis

Sexual Reproduction

Meiosis: What is the point?

• Cell division that produces a sperm cell or an egg cell (Gametes).

• Starts with one cell that contains two copies of each chromosome (Diploid). One chromosome from female parent and one from male parent.

• Ends with 4 Haploid (only one set of chromosomes) daughter cells. 4 cells all genetically different.

Vocabulary• Meiosis- process of reduction division in

which the number of chromosomes per cell is cut in half

• Homologous- chromosomes that each have a corresponding chromosome from the opposite-sex parent

• Diploid – a cell that contains both sets of homologous chromosomes

More Vocab• Haploid- a cell that contains only a single set

of chromosomes. (single set of genes)

• Tetrad- Structure containing 4 chromatids that form during meiosis

• Crossing over- homologous chromosomes exchange portions of their chromatids during meiosis

Mendel’s principles of genetics require at least two things:

1.Each organism must inherit a single copy of every gene from both its “parents”.

2.When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes.

Let’s Look at a Fruit Fly: Drosophila

• A fruit fly has 8 chromosomes• 4 of the chromosomes came from the fruit

fly’s male parent and 4 came from the female parent.

• These two sets of chromosomes are homologous. Meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent

• A cell that contains both sets of homologous chromosomes is said to be diploid. (two sets)

Diploid Cell

• A diploid cell is sometimes represented by the symbol 2N.

• What is the diploid number for the Drosophilia fly?

• Your answer should be 8

2N = 8

Gametes (sperm & egg cells)

• The gametes of sexually reproducing organisms, contain only a single set of chromosomes and therefore only a single set of genes.

• These are called Haploid cells (one set)• Haploid is represented as N.• What is the Haploid number for

Drosophilia?• Your answer should be N = 4

Why are all 4 daughter cells different?

When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.

Crossing-over produces new combinations of alleles.

Crossing Over

Segments ofSegments of homologous homologous chromosomeschromosomes break break and reattach at similar and reattach at similar locations.locations.

Results in new Results in new geneticgenetic combinations combinations of offspring.of offspring.

This is the mainThis is the main advantage advantage of of sexualsexual reproductionreproduction

Why is this an advantage?

• Offspring has more variety

• If an environment undergoes frequent changes it is better for the offspring to have more variety so the species will not die

Meiosis I

• Prior to Meiosis I, each chromosome is replicated.

• The cells then begin to divide similar to mitosis.

• In mitosis, the 4 chromosomes line up individually at the center of the cell.

• The 2 chromatids that make up each chromosome then separate from each other.

The Difference

• In prophase of Meiosis I, however, each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad.

• There are 4 chromatids in a tetrad.

• This pairing of homologous chromosomes is the key to understanding meiosis.

How are Mitosis and Meiosis the same?

• Division of cells

• DNA replication occurs before the division of the nucleus.

SummarySimilarSimilar in many ways to in many ways to mitosismitosisSeveral differences howeverSeveral differences however

Involves Involves 2 cell divisions2 cell divisionsResults in Results in 4 daughter cells 4 daughter cells with with

1/2 the normal genetic 1/2 the normal genetic information.information.

The 4 cells are genetically The 4 cells are genetically DIFFERENTDIFFERENT..

Gametes or cells resulting from Gametes or cells resulting from meiotic division will have HALF as meiotic division will have HALF as many chromosomes.many chromosomes.

Overview of Meiosis

Phases of Meiosis

Meiosis II

Telophase II and Cytokinesis

Prophase IIMetaphase II Anaphase IITelophase I and

Cytokinesis IMeiosis II

Gamete FormationIn male animals, meiosis results in four equal-sized gametes called sperm.

In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in

reproduction.

• http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

• video

Practice questions

Q: What is the cell process that occurs only in organisms that reproduce sexually?___________

Q.A sperm cell of an alligator has 16 chromosomes. What is the total number of chromosomes normally present in a stomach cell of this alligator? _________