 Meiosis is a one way process unlike mitosis.  If you recall, in mitosis, the cell enters a cycle in which is replicated, grows, then replicates again, grows and so on….  Meiosis begins with 1 parent cell and ends with the formation of gametes (sex 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 Chromosomes  The last pair (the 23 rd pair) are our sex chromosomes.  The sex chromosomes are different in males and females.

The sex chromosomes are XX

The sex chromosomes are XY The sex chromosomes are XY

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

Interphase   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 Parent Cell

The Phases of Meiosis  Homologous chromosomes match up and form pairs (synapsis), a step unique to meiosis.  The paired chromosomes are called tetrads (with one chromosome coming from each parent).  Step 1: PROPHASE 1:

At this stage, non-sister chromatids may cross-over at points called chiasmata, which increase genetic randomness (variation).

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Step 2: Metaphase I  The homologous chromosomes (called tetrads) align along the equator.

 Step 3: Anaphase I  Whole chromosomes are pulled toward opposing poles, forming two haploid sets.  Each chromosome still contains a 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 Now we go to MEIOSIS II, also known as REDUCTION DIVISION. Parent Cell

Reduction 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 II Step 5: Prophase II Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.

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

 Step 7: Anaphase II  The centromeres are cleaved, and the sister chromatids are pulled apart.  The sister chromatids are now called sister chromosomes as they move toward opposing poles.

 Step 8: The process ends with telophase II.  It is marked by uncoiling and lengthening of the chromosomes and the disappearance of the microtubules.  Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.  Meiosis is now complete.

Telophase 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, results in the formation of 1 ovum and 3 polar bodies.  The polar bodies disintegrate and are reabsorbed by the body.  The process of meiosis in females occurs during oogenesis. oogenesis

Sex 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 cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur.

The union of egg and sperm forms a zygote. The union of egg and sperm forms a zygote. 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 II is not normal, it is called nondisjunction.  This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy.

 This is a cause of several medical conditions in humans, including:  Down Syndrome - trisomy of chromosome 21 Down Syndrome Down Syndrome  Patau Syndrome - trisomy of chromosome 13 Patau Syndrome Patau Syndrome  Edward Syndrome - trisomy of chromosome 18 Edward Syndrome Edward Syndrome  Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY Klinefelter Syndrome Klinefelter Syndrome  Turner Syndrome - atypical X chromosome dosage in females - ie XO, XXX, XXXX Turner Syndrome Turner Syndrome  XYY Syndrome - an extra Y chromosome in males XYY Syndrome XYY Syndrome

Meiosis Meiosis square dance: Youtube: watch lang