Meiosis is a one way process Meiosis is a one way process unlikeunlike mitosis.mitosis.

If you recall, in If you recall, in mitosismitosis, the cell enters a , the cell enters a cycle in which is replicated, grows, then cycle in which is replicated, grows, then replicates again, grows and so on….replicates again, grows and so on….

MeiosisMeiosis begins with 1 parent cell and begins with 1 parent cell and ends with the formation of gametes (sex ends with the formation of gametes (sex cells).cells).

We have 46 chromosomes in each body cell.

These 46 chromosomes are paired up making 23 pairs. These pairs are called homologous chromosomes.

The first 22 pairs (of the 23 pairs) are called autosomes and code for the traits that make us Homo sapiens.

During meiosis, one diploid eukaryotic cell divides twice to generate four haploid cells.

In animals, meiosis always results in the formation of gametes (sex cells).

Each gamete contains one single set of chromosomes, or half of the genetic

content of the original cell.

Chromosome structure

• The # of chromosomes in an organism are referred to as the diploid (2N) number.

• In humans, 2N = 46.

• The chromosomes in the gametes are NOT in pairs.

• We have 23 individual chromosomes in each egg and sperm.

• The number of chromosomes in a gamete is known as the haploid number (1N).

• In humans, 1N =23.

Sex ChromosomesSex Chromosomes

The last pair (the 23The last pair (the 23rdrd pair) pair) are our sex chromosomes.are our sex chromosomes.

The sex chromosomes are The sex chromosomes are different in males and different in males and femalesfemales..

• The sex chromosomes are XX

The sex The sex chromosomes chromosomes are XYare XY

•Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually.

InterphaseInterphase In a cell which will

eventually become a sperm or an egg, the chromosomes double in the S Phase of Interphase just like in any other cell.

A sex cell will not undergo MITOSIS, however, like the other cells.

Sex cells undergo MEIOSIS.

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Parent Cell

The Phases of MeiosisThe Phases of Meiosis Homologous Homologous

chromosomes match chromosomes match upup and form and form pairs pairs (synapsis)(synapsis), a step , a step unique to meiosis. unique to meiosis.

The paired The paired chromosomes are chromosomes are called called tetradstetrads (with (with one chromosome one chromosome coming from each coming from each parent).parent).

Step 1: Step 1: PROPHASE 1PROPHASE 1::

At this stage, non-sister chromatids may At this stage, non-sister chromatids may cross-overcross-over at points called at points called chiasmatachiasmata, which , which

increase genetic randomness (variation).increase genetic randomness (variation).

Crossing overCrossing over http://youtube.com/watch?v=5x_Rp1http://youtube.com/watch?v=5x_Rp1

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v=f18U__0nBxQ&feature=relatedv=f18U__0nBxQ&feature=related

Step 2: Metaphase IStep 2: Metaphase I The homologous chromosomes The homologous chromosomes

(called tetrads) align along the (called tetrads) align along the equator.equator.

Step 3: Anaphase I Step 3: Anaphase I Whole chromosomes are pulled Whole chromosomes are pulled

toward opposing poles, forming two toward opposing poles, forming two haploid sets. haploid sets.

Each chromosome still contains a Each chromosome still contains a pair of sister chromatids. pair of sister chromatids.

• Step 4: Telophase I• The first meiotic division

effectively ends when the centromeres arrive at the poles.

• Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids.

Meiosis I 46

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Now we go to MEIOSIS II, also known as REDUCTION DIVISION.

Parent Cell

Reduction Division: Meiosis IIReduction Division: Meiosis II

• Meiosis II is the second part of the meiotic process.

• Much of the process is similar to mitosis and meiosis I.

• There is NO INTERPHASE in between Meiosis I and Meiosis II therefore the chromosomes are NOT doubled.

Step 5: Prophase IIStep 5: Prophase II Centrioles move to Centrioles move to

the polar regions and the polar regions and arrange spindle fibers arrange spindle fibers for the second meiotic for the second meiotic division. division.

Step 6: In Step 6: In MMetaphase IIetaphase II, the , the centromeres centromeres contain two contain two kinetochores, that kinetochores, that attach to spindle attach to spindle fibers from the fibers from the centrosomes centrosomes (centrioles) at each (centrioles) at each pole.pole.

Step 7: Step 7: Anaphase IIAnaphase II

The centromeres are The centromeres are cleaved, and the cleaved, and the sister chromatids are sister chromatids are pulled apart. pulled apart.

The sister chromatids The sister chromatids are now called are now called sister sister chromosomeschromosomes as as they move toward they move toward opposing poles.opposing poles.

Step 8: The process ends with Step 8: The process ends with telophase II.telophase II.

It is marked by uncoiling and It is marked by uncoiling and lengthening of the chromosomes and lengthening of the chromosomes and the disappearance of the the disappearance of the microtubules.microtubules.

Nuclear envelopes reform and Nuclear envelopes reform and cleavage or cell wall formation cleavage or cell wall formation eventually produces a total of four eventually produces a total of four daughter cells, each with a haploid daughter cells, each with a haploid set of chromosomes. set of chromosomes.

Meiosis is now complete.Meiosis is now complete.

Telophase IITelophase II

•Meiosis in males, results in 4 sperm cells from every parent cell.

•The process of meiosis in males occurs during spermatogenesis.

Meiosis in females, Meiosis in females, results in the results in the formation of 1 formation of 1 ovum and 3 polar ovum and 3 polar bodies.bodies.

The polar bodies The polar bodies disintegrate and disintegrate and are reabsorbed by are reabsorbed by the bodythe body..

The process of The process of meiosis in females meiosis in females occurs during occurs during oogenesis.oogenesis.

Sex CellsSex Cells

Sex cells are called gametes and are found in the gonads.

In females, the gametes are eggs (ovum) and the gonads are the ovaries.

In males, the gametes are sperm (or pollen) and the gonads are the testes (or the anthers)

If meiosis produces gametes, these If meiosis produces gametes, these cells must fuse during cells must fuse during fertilizationfertilization to to create a new diploid cell, or create a new diploid cell, or zygotezygote before any new growth can occur. before any new growth can occur.

The union of egg and sperm forms a The union of egg and sperm forms a zygotezygote. . The very first body (2N) cell.

• Fertilization begins with the binding of a sperm head to the outer coating of the egg (called the zona pellucida).

• The tip of the sperm head releases enzymes that digest a path through the zona and enable the sperm head to bind to the cell membrane of the egg.

• Fusion of their respective membranes allows the entire contents of the sperm to be drawn into the cytosol of the egg.

• (Even though the sperm's mitochondria enter the egg, they are almost always destroyed and do not contribute their genes to the embryo. So human mitochondrial DNA is almost always inherited from mothers only.)

• Within moments, enzymes released from the egg cytosol act on the zona making it impermeable to the other sperm that arrive.

When the separation during Anaphase I or When the separation during Anaphase I or II is not normal, it is called II is not normal, it is called nondisjunctionnondisjunction. .

This results in the production of gametes This results in the production of gametes which have either more or less of the which have either more or less of the usual amount of genetic material, and is a usual amount of genetic material, and is a common mechanism for common mechanism for trisomytrisomy or or monosomymonosomy. .

This is a cause of several medical conditions in This is a cause of several medical conditions in humans, including:humans, including:

Down Syndrome Down Syndrome - trisomy of chromosome 21- trisomy of chromosome 21 Patau Syndrome Patau Syndrome - trisomy of chromosome 13 - trisomy of chromosome 13 Edward Syndrome Edward Syndrome - trisomy of chromosome 18- trisomy of chromosome 18 Klinefelter Syndrome Klinefelter Syndrome - extra X chromosomes in - extra X chromosomes in

males - ie XXY, XXXY, XXXXYmales - ie XXY, XXXY, XXXXY Turner Syndrome Turner Syndrome - atypical X chromosome - atypical X chromosome

dosage in females - ie XO, XXX, XXXXdosage in females - ie XO, XXX, XXXX XYY Syndrome - XYY Syndrome - an extra Y chromosome in malesan extra Y chromosome in males

MeiosisMeiosishttp://www.youtube.com/watch?v=D1_-mQS_FZ0http://www.youtube.com/watch?v=D1_-mQS_FZ0

Meiosis square dance:Meiosis square dance:Youtube: watch langYoutube: watch lang