Meiosis All the cells except the sex cells

Meiosis All the cells except the sex cells (sperm and eggs) of organisms that reproduce sexually contain 2 sets of chromosomes, one from the mother, one from the father.

In a eukaryote, a cell other a gamete, gametocyte, germ cell or stem cell, is a somatic cell. It is said to be diploid because it has 2 sets of chromosomes (one from the father, one from the mother). Diploid can be represented by the symbol 2 N (2 sets). Diploid cells contain 2 complete sets of chromosomes therefore 2 copies of each gene (except the X and Y chromosomes which each have genes for different things. )

The gametes, or sex cells of sexually-reproducing organisms, contain only a single set of chromosomes, therefore only a single set of genes. These cells are called haploid cells, (means one set). Haploid is represented by N.

For example, the egg of a Drosophila (fruit fly) has 4 chromosomes. So does a sperm. This can be written as: N=4 The haploid number is 4. How many chromosomes are there in a somatic cell of a fruit fly?

So, how does an organism make egg and sperm cells, with only half the number of chromosomes of regular somatic cells? By a process called meiosis

Meiosis is said to be a process of reduction division The division of the cells which are involved in sexual reproduction so that the number of chromosomes is reduced by half. This results in either sperm or egg.

sperm + n + egg n = = zygote 2 n

Meiosis is a process of reduction division. In a human, at the beginning of meiosis, there is a cell with 46 chromosomes; at the end, a cell with half that. Through meiosis, one diploid cell becomes 4 haploid cells. In animals, these haploid cells are sperm or eggs.

Where do sperm cells come from? They are created from diploid cells called spermatocytes in the testes. The sperm cell itself is a haploid cell.

Egg cells come from diploid cells call oocytes located in the ovaries. The egg cell itself is haploid.

The process of meiosis involves 2 distinct divisions called meiosis I and meiosis II. The phases have the same names as the phases in mitosis except the name is followed by “I” or “II”, depending on whether the phase is in meiosis I or meiosis II. Before you go any further, look at page 277 of your text book and watch the Crash Course on meiosis using the link below. http: //www. youtube. com/watch? v=q. CLm. R 9 YY 7 o

Understanding the vocabulary of meiosis

Homologous chromosomes In each diploid cell there are two chromosomes that have copies of genes on them that code for the same things. One of the pair came from the mother; the other chromosome in the pair came from the father. These pairs of chromosomes that code for the same thing are called homologous pairs.

Each chromosome in a homologous pair is: § the same size § the same shape § the same gene arrangement (generally genes that code for the same trait) but § they are not identical because the actual code is different. (have different alleles) For example, both chromosomes in a homologous pair might code for “curling of your tongue” but one might let you curl your tongue and the other might not.

Homologous pairs of chromosomes

Remember that DNA is replicated during the S phase of interphase. This replication also occurs in the cells that will ultimately produce sperm and egg cells.

So, after replication, each homologous pair has a matching homologous pair.

Meiosis I In Prophase I of Meiosis I, each homologous chromosome pair joins with its corresponding homologous pair to form a tetrad. See Figure 11 -16 page 276 in your book!

Synapsis is the process in which homologous pairs of chromosomes pair up during Prophase I to form what is called a tetrad.

During Prophase I , genetic material is exchanged between the maternal and paternal chromosomes. This process is called crossing over. The result of crossing over is genetic recombination. The point at which the chromosomes cross over is called the chiasma.

During synapsis the chromosomes within a homologous pair may twist around the chromosomes of the other homologous pair in the tetrad, break off and attach to the other chromosomes in a process called crossing over.

Tetrad Remember, these are the homologous chromosome pairs, together. There are 4 sister chromatids in a tetrad. Prophase I of Meiosis I is where crossing over takes place!

Make sure that you know what happens in each phase of meiosis by watching the Crash Course video, reading your text and looking at the following slides.

Meiosis I Interphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Go to Section: Prophase I Metaphase I Each chromosome pairs with Spindle fibers attach to the its corresponding homologous chromosomes. chromosome to form a tetrad. Anaphase I The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis II The 2 cells produced in Meiosis 1 then go through another meiotic division. There is no DNA replication between Meiosis I and Meiosis II. The end result of Meiosis II is 4 cells with 2 sister chromatids in them.

Meiosis II Prophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of Go to chromosomes as the Section: original. Metaphase II The chromosomes line up in a similar way to the metaphase stage of mitosis. Anaphase II The sister chromatids separate and move toward opposite ends of the cell. Telophase II Meiosis II results in four haploid (N) daughter cells.

Law of Independent Assortment During metaphase I, as the tetrads line up, the orientation of maternal and paternal chromosomes is random Not all maternal go to one side, and paternal the other…. it is mixed.

Law of independent assortment As the tetrads are separated in anaphase I, the maternal and paternal chromosomes have random separation which results in genetic variation. This random alignment and separation occurs again in metaphase II and anaphase II, except pairs are separated into individual chromosomes.

Formation of Gametes Meiosis only occurs in the reproductive cells…all other cells that divide undergo mitosis. Ending products of meiosis are haploid gametes Eggs in female - oogenesis Sperm in male - spermatogenesis Fusion cell of sperm and egg results in a diploid

Review – If you understand this, you understand meiosis. http: //www. youtube. com/watch? v=i. CL 6 d 0 Ow Kt 8

Chromosomal Disorders Non-disjunction: when this occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.

Nondisjunction Section 14 -2 Homologous chromosomes fail to separate Go to Section: Meiosis I: Nondisjunction Meiosis II

Karyotype

DNA v. Gene