Meiosis

Review Mitosis• What was the purpose of Mitosis?

– cell division in multi-cellular organisms– Reproduction in uni-cellular organisms

– What were the phases of the cell cycle?

• Interphase – G1, S, G2

• Mitosis Phase - PMAT!• Cytokinesis

Meiosis Yes it is different from Mitosis!!

• Mitosis is division that occurs in any cell in the body called somatic cells (body cells)

• MEIOSIS is cell division by which gamete cells (reproductive cells---sperm and egg) are produced

Remember!!!•Chromosomes = tightly coiled

DNA (chromatid, sister chromatid)

–Contain genes which determine hereditary traits

• We have a total of 46 chromosomes in EVERY cell of our body

–We get…•23 from Mom•23 from Dad

*During the S phase of Interphase,

Chromatids are duplicated to make

sister chromatids

Dad Mom

SO……• We call the pair of matching sister chromatids Homologous

Chromosomes

• Homologous chromosomes contain the same genes for the same traits, but not necessarily the same Alleles

•******This is different than mitosis because the sister chromatids DO NOT pair up.****

– Genes are your traits• Eye color, hair color, height, foot length, ect…

– Alleles are alternate forms your traits can take • Usually one is dominant (brown eyes) vs. recessive (blue eyes)

• DIPLOID (2N) = Somatic cells (Body Cells) – describes a cell that has two copies of each

chromosome: 1 from mom and 1 from dad– Cell starts with 46 chromosomes and each of the 2

new cells has 46 chromosomes.

• HAPLOID (N) = Gametes (Sex Cells)– describes a cell that has half the number of

chromosomes. – Cell starts with 46 chromosomes and each of the 4

new cells has 23 chromosomes.

*Somatic cells and Gametes have different numbers of chromosomes

Sexual Reproduction• During sexual reproduction, an egg is

fertilized by a sperm to create a zygote• The zygote should be diploid, that is, it

should have the same number of chromosomes as all normal human body cells

• Sperm + Egg = 46• (23) + (23)

Sexual Reproduction• What if gametes (sex cells) went through

Mitosis?– Mitosis produces 2 identical daughter cells

so…– Diploid produces diploid– If sperm and egg were diploid they would

create a polyploid (“many sets”) zygote when they came together

– Obviously we do not have “many sets” of chromosomes do we?

SO WHAT’S THE SOLUTION?

MEIOSIS!• Where a Diploid cell produces 4 Haploid

daughter cells• Meiosis is a process of reduction division in

which the number of chromosomes per cell is cut

in half through the separation of homologous

chromosomes in a diploid cell.

Genetic Recombination• So, when a haploid sperm fertilizes a

haploid egg a diploid zygote is the result

• Meiosis helps create Genetic recombination, where 2 different sets of chromosomes come together, keeps you from being an exact copy of one of your parents!

• It’s what creates genetic diversity in our world!!

Stages of Meiosis

•Meiosis I•Meiosis II

Meiosis I

•Prophase I•Metaphase I•Anaphase I•Telophase I•Cytokinesis

Interphase

Same as in Mitosis–G1: cell growth, duplication of organelles, job of cell carried out

–S: DNA synthesis/replication of DNA

–G2: job of cell carried out, further growth

Prophase I• Nuclear membrane and nucleolus disappear• Centrioles begin to separate and spindles

begins to form• The homologous chromosomes pair up creating

a tetrad = 2 sets of sister chromatids = 4 individual chromatids

• The homologous chromosomes pair up, gene for gene, down their entire length.

During Prophase I• Crossing-over can occur

– Involves the exchange of genetic material between homologous chromosomes

– Breaks arms of each chromosome and switches the material

Prophase I

CROSSING OVER!!

Metaphase I• Spindle fibers attach to the tetrads• The tetrads line up along the cell

equator• Each side of the equator has

chromosomes from both parents (mixed up)

Anaphase I• The paired homologous chromosomes

are pulled away and move toward opposite ends of the cell

• Sister chromatids remain attached!!!!

Telophase I and Cytokinesis• The nuclear membrane forms

again in some species, the spindle fibers dissemble and the cell undergoes cytokinesis.

• 2 daughter cells are produced that are not identical– A “reduction division” has occurred

because each daughter cell has half of the number of chromosomes the original parent cell had

Important Note:

Meiosis II will occur in both of the daughter cells created in

Meiosis I

Review

• Meiosis IMeiosis I – Homologous Chromosomes separate– Begins w/ 1 diploid1 diploid cell – 46 chromosomes (in homologous pairs)

– Prophase I, Metaphase I, Anaphase I, Telophase I

– Ends with 2 Haploid2 Haploid cells– 23 sister chromatids in each new cell.

Meiosis I

Interphase Prophase I Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis II

•Prophase II•Metaphase II•Anaphase II•Telophase II•Cytokinesis

Prophase II• The DNA HAS NOT been

duplicated– The nuclear membrane

disappears– centrosomes and centrioles

move to opposite sides of the cell

– Spindle fibers start to assemble

Metaphase II• Spindle fibers attach to

the sister chromatids at the centromere

• The 23 sister chromatids (in each cell) line up along the align at the cell equator

Anaphase II• Centromeres break,

spindle fibers shorten, and the sister chromatids are pulled apart from each other and move to opposite ends of the cell

• So 23 individual chromatids go to each side!!!

Telophase II and Cytokinesis• Spindle fibers break down• Nuclear envelopes and nucleolus

reform• Cell membrane pinches in and

cytoplasm is divided• 4 haploid daughter cells are

produced with 23 individual chromatids– A “reduction division” has

occurred

Results of Meiosis• The 4 haploid daughter

cells…– Have only 1 set of

genes– Are not considered to be

in any phase because they will not grow and divide

– Now referred to as gametes (sex cells)

Review

• Meiosis IIMeiosis II – Sister chromatids separate

– NO DNA replicationNO DNA replication

– Prophase II, Metaphase II, Anaphase II, Telophase II

– Ends with 4 haploid4 haploid cells

– 23 individual chromatids in each cell

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Review Questions

• What type of cells are produced from meiosis?

– Gametes (sex cells)

• What happens to the chromosome number in the cells that are produced from meiosis?

– Cell chromosome number is cut in half - Start with 2n and end with 1n (end with 4 haploid gametes each genetically different from the parents)

• What do genes code for?

– traits

Meiosis Mistakes

Mistakes in Meiosis

• Nondisjunction: failure of homologous chromosomes to separate properly during meiosis I

– both chromosomes of a homologous pair move to the same pole of the cell rather than separating

Mistakes of MeiosisEffects of Nondisjunction:

1. Trisomy: zygote will have an extra chromosome

– The gamete with an extra chromosome is fertilized by a normal gamete resulting in a zygote with an extra chromosome

– Organisms usually survive

– Example: Down syndrome – flat face, short neck, some degree of mental retardation

2. Monosomy: zygote will be missing a chromosome

– a gamete with a missing chromosome fuses with a normal gamete during fertilization resulting in a zygote that lacks a chromosome

– Organisms usually do not survive

– Example: Turner syndrome – human females only have one x-chromosome rather than two

• This is called a karotype– it is a picture of your 23 pairs of chromosomes

• Autosomes – pairs 1-22 • Sex chromosome - pair 23

Meiosis Visuals

http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Spermatogenesis• Meiosis in males to

produce sperm• Thousands of

reproductive cells undergo meiosis each day to produce large numbers of sperm

Oogenesis• Meiosis in females to

produce ova (eggs)• Only occurs in one cell

once a month• The cytokinesis at end

of meiosis I and meiosis II is uneven and cytoplasm is divided unequally

• One cell gets the majority of the cytoplasm and becomes the egg

• The other cells are called polar bodies and do not participate in reproduction

Comparing Mitosis and Meiosis

Mitosis vs. Meiosis

MitosisMitosis MeiosisMeiosis

Starts as one cell, ends as two cells

Starts as one cell, ends as four cells

Starts 2N (diploid), ends 2N Starts 2N, ends 1N (haploid)

New cells are identical to each other AND to parent

New cells NOT identical to each other OR parent

Used to produce body (somatic) cells

Used to produce gametes (sex cells)

Mitosis results in the production of two

genetically identical diploid cells, whereas meiosis

produces four genetically different haploid cells