Meiosis

Quick Review of MITOSIS

• What happens to the DNA BEFORE mitosis?• DNA synthesis (replication)

• Why? • Needs an exact copy for the new “daughter

cells”

2 types of cells:

• Somatic Cell = body cell– Example: skin, muscle,

lung cells– Diploid cells

• Gamete = sex cell– Example: egg, sperm,

pollen– Haploid cells

Haploid Cells vs. Diploid Cells

• Haploid = have 1 of EACH kind of chromosome (n)

• Diploid = have 2 of EACH kind of chromosome (2n)

• Meiosis = cell division that produces ½ the chromosomes of the parent cell; makes gametes

What does meiosis produce?

• SEX CELLS • (aka: gametes, haploid

cells)

Why do we need Meiosis?

How is MITOSIS different

from MEIOSIS?

Meiosis Vocabulary

• Homologous Chromosome = 2 pairs of sister chromatids (form a tetrad); same order and letter of gene

How do we get diploid cells?

Mitosis!

How do we get haploid cells?

MEIOSIS!

Will Meiosis or Mitosis occur in these cells?

• Skin cells• Pollen • Ovum/ovules• Muscle cells

What is genetic variation?

• Genetic variation = the differences between genes of individuals of a species or populations of different species.

• Which two individuals will have the least genetic variation in their genes?

A. Brother and sisterB. Brother and cousinC. Sister and cousinD. Mother and nephew

• Which two species will have the greatest genetic variation between species.

1. Lancelet and Lamprey2. Lancelet and Leopard3. Salamander and Leopard4. Turtle and Leopard

Meiosis provides genetic variation among organisms

• 2 sources of genetic variation:

1. Random alignment of chromosomes during metaphase

2. Crossing Over

Chromosome alignment can vary!

• Crossing Over = exchange of genetic information during prophase I of meiosis– Provides MORE

variation– Basis for evolution– This is a form of genetic

recombination (or the recombining of genes!)

Chromosome Action Explains Mendel’s Work

• Law of segregation– Traits are separately inherited from parents

• Law of independent assortment– Traits are inherited independently of each other

• Rule of dominance– If its dominant it will appear; recessive will only

appear if there is a “double dose”

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

QUIZ TIME!

• What are some examples of gametes?• Egg• Sperm

Haploid or Diploid?

• Sex cells• Somatic cells• Body cells• Sperm• Egg

QUIZ TIME!

• How can you tell if a homologous chromosome will be homologous?

Mitosis or Meiosis?

• Gametes produced• Diploid cells produced• Haploid cells produced• Half the number of chromosomes produced• Same number of chromosomes produced

QUIZ TIME!

• During what phase of meiosis does crossing over occur?

QUIZ TIME!

• If a cell starts with 20 chromosomes how many chromosomes will be present in the cells following meiosis?

• How many chromosomes would be present following mitosis in the same original cell?

QUIZ TIME!

• How many cells are produced during mitosis?• 1 2

• How many cells are produced during meiosis?• 1 4

Mistakes in Meiosis

What do the chromosomes have to do in meiosis in order for it to be successful?

• One chromosomes from each homologous pair moves to each pole of the cell…

What if the chromosomes fail to separate?

• Nondisjunction = the failure of homologous chromosomes to separate properly during meiosis

• What does nondisjunction do to the chromosome count?

• One cell has too many chromosomes• Another cell has too few chromosomes

When would you notice the effects of nondisjunction?

• Seen after gametes fuse fertilization

• When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome…

• It should have 2, but has three!• This is called trisomy

What if an organism is lacking chromosomes?

• Organisms with an extra will often survive BUT organisms lacking one or more, usually do not survive

• When a gamete with a missing chromosome fuses with a gamete with normal chromosome number, you get monosomy

What if BOTH gametes have extra sets of chromosomes?

• Produces offspring with 4 sets of chromosomes

• This is called a tetraploid• Organisms with more than the usual number

of chromosomes are called polyploids

Polyploids in plants

• Very useful commercially– Tetraploid wheat (4n)– Triploid apples (3n)

• Plant breeders use chemicals to artificially cause nondisjunction to get these polyploid plants.

• Why could this be an area of concern for humans?