Meiosis

Review Mitosis• What is the purpose of Mitosis?

– cell division

• 2 daughter cells produced that are exact copies of the parent cell

– What are the parts of the cell cycle? • Interphase

– G1, S, G2

• M-Phase

– Mitosis – Cytokinesis

– What were the phases of Mitosis?

• PMAT – Prophase, Metaphase, Anaphase, & Telophase

Meiosis Yes, it is different from Mitosis!!

• Mitosis occurs in any cell in the body that was NOT a reproductive cell– Body cells are called somatic cells

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

Remember!!!•Chromosomes = tightly coiled

DNA (sister chromatids)

–Contain genes which determine hereditary traits

TOTAL NUMBERS• 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,

Those chromosomes are duplicated

Dad Mom

So each chromosome duplicates to make an identical one; these structures are now called SISTER CHROMATIDS!!!!!

So each pair of chromosomes will have 2 “X” (sister chromatids) shapes

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

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

– 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– Contains 2 sets of alleles for each gene– Chromosome # is 46

• HAPLOID (N) = Gametes (Sex Cells)– describes a cell that has only one set of chromosomes– Contains 1 set of alleles for each gene– Each egg or sperm has 22 autosomes and 1 sex

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

*Somatic cells and Gametes have different numbers of chromosomes

Chromosome NumberExample: Humans

Chromosome # (N) = 23

Diploid Cells = 2N = 2x23 = 46

Haploid Cells = N = 23

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 makes 4 genetically

different 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. This 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

–S: DNA synthesis/replication of DNA

–G2: preparing for division

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

begins to form• The homologous chromosomes pair up creating

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

• The chromosomes pair up precisely, 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

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

metaphase plate• Each side of the equator has

chromosomes from both parents

Anaphase I• The paired homologous chromosomes are

pulled apart 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 disassemble and the cell undergoes cytokinesis.

• 2 daughter cells are produced– 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

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

– Ends with 2 haploid2 haploid cells

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 DOES NOT

duplicate– The nuclear membrane

disappears– Centrioles move to

opposite sides of the cell– Spindle fibers start to

assemble

Metaphase II• Spindle fibers attach to

the chromosomes at the centromere

• The 23 chromosomes line up along the metaphase plate

• Each chromosome still has two sister chromatids

Anaphase II• Centromeres break,

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

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– A “reduction division” has

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

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

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), haploid

• What happens to the chromosome number in the cells that are produced from meiosis?– Goes through reduction division

– Cell chromosome number is cut in half - Start with 2N and end with 1N

– 4 haploid gametes are produced, each genetically different from the parents

• What do genes code for?

– traits

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

Comparison Animation

http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

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