Meiosis Page 275

Meiosis The most important things to remember in distinguishing Mitosis from Meiosis are: - Mitosis deals in body cells. (SOMATIC CELLS) - Meiosis deals with sex cells. (SEX CELLS) - Mitosis results in the production of 2 genetically identical diploid cells. - Meiosis produces 4 genetically different haploid cells.

Meiosis Every organism must inherit a single copy of every gene from each of its parents. Humans get 23 chromosomes from our mothers and and 23 from our fathers: 46 total. When gametes (sex cells) are produced, 2 sets of genes have to be separated from each other so that each gamete contains just one set of genes.

Meiosis A cell that contains both sets of homologous chromosomes is said to be diploid. This means “two sets”. 2N = 46 for humans Diploid cells contain 2 complete sets of chromosomes therefore 2 complete sets of genes.

Meiosis In contrast, haploid cells, gametes of sexually reproducing organisms, contain only a single set of chromosomes, and only a single set of genes. N = 23 for humans

Meiosis Let’s start with an easier number and separate it. A fruit fly has 8 chromosomes total. 4 from mom and 4 from dad. These chromosomes are homologous, meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. Chromosome 1 from mom matches up with Chromosome 1 from dad. This makes a homologous pair.

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 which means copies of the gene) 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.

Meiosis How are haploid (N) gamete cells produced from diploid (2N) cells? By meiosis 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.

Meiosis Meiosis consists of two separate processes 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, watch the Crash Course video on meiosis. YY7o

Meiosis I Before Meiosis I, the chromosomes are replicated. (Just like before mitosis) In Prophase I of Meiosis I, each homologous chromosome pair joins with its corresponding homologous pair to form a tetrad. See Figure 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 synapsis parts of the chromatids in a tetrad may twist around each other, break off and attach to another chromatid in a process called crossing over.

This results in exchange of genetic material 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.

Section 11-4 Crossing-Over Go to Section:

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

Section 11-4 Crossing-Over Go to Section:

Section 11-4 Crossing-Over Go to Section:

Meiosis I Interphase I 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. Go to Section:

Law of Independent Assortment During metaphase I, as the homologous pairs 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.

As the pairs of chromosomes are separated in anaphase I, the maternal and paternal chromosomes have random separation which results in genetic variation.

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 Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase II Metaphase IIAnaphase IITelophase 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. Meiosis II results in four haploid (N) daughter cells. Go to Section:

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.

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

Review – If you understand this, you understand meiosis. Kt8 Kt8