Meiosis

Meiosis is the type of cell division for that produces the ______________ cells ( ______________ ) which are also known as gametes.

Two important characteristics of meiosis is that it reduces the number of chromosomes to half of the parent cell and it creates ___________________.

Somatic Cells vs. Gametes

Somatic cells are body cells (nerve cells, muscle cells, epithelial cells…).

These cells all have different appearances and functions but have the same genetic information.

Gametes are the reproductive cells - the egg or the sperm.

Gametes __________________________________

_______________________.

If Meiosis Did NOT Occur

If two parental cells combined that had the parents full set of 46 chromosomes:

The first generation of offspring would have____ chromosomes.

The next generation of offspring would have ____ chromosomes…

Homologous Chromosomes Most cells in a human (and in most eukaryotic organisms)

contain two copies of each chromosome.

These chromosomes are not identical (one is from the

mother and one from the father), but they ________

_______________. These pairs are called homologous

chromosomes.

Homologous chromosomes may contain different forms

of a gene for the same trait.

Brown eyes Green eyes

Humans have 23 pairs of chromosomes, for a

total of 46 chromosomes in each somatic (body)

cell.

Autosomal chromosomes: Chromosomes 1-22, which form homologous pairs in both males and females.

Sex chromosomes: Chromosome 23. Determines gender (XX= female, XY= male). For males, this pair is not homologous (different length and type of genes).

Diploid and Haploid Cells

Diploid cells have homologous

pairs of chromosomes. (2 of each

chromosome)

Human somatic (body) cells are

diploid. (di = 2)

Haploid cells ______________

_______________________.

Gametes (egg and sperm) cells

are haploid.

When fertilization occurs (fusion of egg and

sperm) the diploid number is restored.

Video: Diploid vs. Haploid Cells http://www.youtube.com/watch?v=MU83VW

AvUf4&list=PLF8D6A5276A9A092F

Diploid cells are represented by 2n and

haploid by n, where n is the number of

different chromosomes.

Examples:

Organism Gamete (n) Body cell (2n)

Human 23 46

Pea Plant 7 ___

Fruit Fly __ 8

Dog 39 ___

Indian Fern ____ 1260

Chromosome number in different organisms

http://morgan.rutgers.edu/morganwebframes/level1/page2/ChromNum.html

Discuss with your table partner:

Compare and contrast mitosis with meiosis.

What are homologous chromosomes? What kind of cells have homologous chromosomes?

What do the terms diploid and haploid mean? What kind of cells are diploid? What kind of cells are haploid?

Interpreting Genetics Diagrams

Homologous chromosomes are indicated by length.

Sister chromatids (identical copies produced during S phase of interphase) are shown connected by centromere.

Genes on the chromosome are represented with letters. Example: R/r represents gene for pea seed shape

Different forms of the same gene are represented with capital versus lower case letters.

Example: R represents round shape form of the gene, r represents wrinkled shape form of the gene.

Overview of Meiosis

In meiosis, __________________ of

chromosomes is followed by ___cell divisions,

to produce __ __________cells.

Phases of Meiosis As with mitosis, meiosis is preceded by

an interphase period,

during which the chromosomes are

duplicated.

The two sister chromatids are

___________________.

The homologous pairs have the

same genes, but could have

different alleles (forms of the

gene).

At the start of meiosis, there are

____ copies of each gene.

Prophase I

During Prophase I,

the chromosomes

condense to become

visible.

______________________

_______________________

_______________________

(Homologous pairs never form tetrads during

mitosis).

Metaphase I

In metaphase I,

the _______________

__________ line up along

the middle of the

spindle.

Anaphase I:

HOMOLOGOUS PAIRS

separate.

Telophase I

Nuclei reform, producing

2 haploid daughter cells.

But each cell still has ___

______________________

______________________

Meiosis II separates the sister chromatids.

Prophase II Metaphase II

Anaphase II Telophase/

cytokinesis II

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

Meiosis Animation

http://www.cellsalive.com/meiosis.htm

Genetic Variation from Meiosis

Meiosis is important for providing genetic variation because it allows for genetic recombination- new combinations of genes.

Parental

combinations

Recombinants

Sources of Genetic Variation from Meiosis

Independent Assortment:

Each homologous pair lines up independently on the spindle in Meiosis I. So all of the possible arrangements of chromosomes are ______________ if two genes are on different chromosomes.

In humans

there are 223

possible

chromosome

combinations

from each

parent.

If an organism has gene forms Y and y on one

homologous pair, and R and r on a different homologous

pair, then due to independent assortment the gametes

produced will be 25 % YR, 25% yr, 25% Yr and 25% yR

Independent Assortment Video

http://www.youtube.com/watch?v=kZyK2eWhpfQ

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Sources of Genetic Variation from Meiosis cont.

Crossing Over:

During prophase I, strands of ____________

_____________ can be exchanged while the

homologous pair are joined in tetrads.

Crossing over allows

recombination of

genes on the ______

_________________

Crossing Over Video http://www.youtube.com/watch?v=op7Z1Px8oO4

Crossing over animation

Discuss with your table partner:

Compare and contrast mitosis with meiosis.

Fill in the chart on the next slide for these two different types of cell division.

Comparing Mitosis and Meiosis

MITOSIS MEIOSIS

Purpose

Location

# of DNA

replications

# of cell

divisions

#/type of cells

produced

______

_________

________

__________

_ _

_

__ _______cells __ _______ cells

_

__________

___________

___________

____________

Comparing Mitosis and Meiosis Video http://www.youtube.com/watch?v=Ba9LXKH2ztU

Additional Meiosis Overview Video (5.5 min.) http://www.youtube.com/watch?v=-DLGfd-W

pr4

Comparing Mitosis and Meiosis Cont.

NOVA site comparing mitosis and meiosis.

Spermatogenesis: produces

four ____ sperm cells

through meiosis. A greater

quantity of sperm is

produced, but each sperm

has less stored energy.

Oogenesis (egg production):

meiotic division produces 1

larger egg which receives

______________________

______________ of the

parent cell. The other three

cells produced in the

meiotic division are smaller

polar bodies.

Fewer eggs need to be produced so more energy is invested per egg (mitochondria etc.).