Reducing the Chromosome Number Meiosis Reducing the Chromosome Number

Meiosis Chromosomes carry the DNA, our genetic code that creates all the characteristics of our body. Human’s have 46 chromosomes, goldfish have 94 chromosomes and fruit flies have 8 chromosomes. Now if two human’s mate, why do we not have 92 chromosomes in the offspring? There is a process called Meiosis that happens in our reproduction cells – sperm and eggs. Sexual reproduction is when a female reproduction cell combines with a male reproduction cell.

Meiosis As we have learned, our body has cells that divide. These cells are called somatic cells. This process is called Mitosis. Now, human’s somatic cells have 23 pairs of chromosomes for a total of 46 chromosomes. Our sex cells are called gametes. Male gametes are called sperm, and female gametes are called eggs.

Meiosis Meiosis only happens in cells that produce gametes. Each gametes is then able to produce half of the genetic material for the offspring. Cells with half of the chromosomes are called haploid and are symbolized with a single n. Cells that have a complete set of chromosomes are called diploid symbolized as 2n. Sperm (n) + egg (n) = zygote (2n)

Meiosis During Meiosis Diploid cells (2n)  Haploid cells (n) If human body cells have 46 chromosomes, then gametes (sperm and egg) each have 23 chromosomes.

Homologous Chromosomes A sperm will have 23 chromosomes and an egg will have 23 chromosomes When the sperm and egg meet, the 23 chromosomes will pair up with a corresponding chromosome, this is called homologous chromosomes.

Homologous Chromosomes Example: one sperm chromosome carries the DNA for eye colour, it will pair up with the egg chromosome that has the DNA for eye colour. Thus each parent will contribute one half (n) (haploid) of the chromosomes to create a diploid (2n) somatic cell with the chromosome for eye colour.

Prior to Meiosis - Interphase Recall Chromosomes are what carry our DNA. Before Meiosis begins, DNA replication occurs in the S-phase of Interphase. Each Duplicated Chromosome contains two (2) sister chromatids which are attached at their centromeres.

Meiosis I There are four (4) phases in Meiosis I: Prophase I Metaphase I Anaphase I Telophase I NOTE: For our example we will look at two pairing of chromosomes that have replicated. Remember, the human body has 23 pairs.

Prophase I Is the longest and most complex stage; 90 percent of the meiotic process is spent in prophase Chromatids condense and shorten Nuclear membrane disintegrates During prophase the homologous chromosomes pair up (synapsis) and the non-sister chromatids exchange DNA. The exchange of DNA is referred to as Crossing Over.

Why is Crossing Over Important? During crossing over segments of non-sister chromatids break and reattach to the other chromatid. Non-sister chromatids Tetrad

Why crossing over is important Since one pair can be arraigned two different ways we can create an equation 2pairs? = possible arrangements In this case we would have 21 – 2 arrangements But the human body has 23 pairs of chromosomes, thus 223 = 8,388,608 different arrangements. Now that is a lot of variations. No wonder our siblings do not look like identical to us. or The same pair of chromosomes create another type of arrangement One pair of chromosomes create one arrangement

Why is Crossing Over Important? Multiple crossovers create an infinite number of genetic possibilities for just one gamete. Variation is important and crossing over creates variation in sexual reproduction.

Prophase I Centrioles Spindle Fibers

Metaphase I Shortest phase in Meiosis Pairs of homologous chromosomes lines up at the equator of the cell. Spindle Fibers attach to centromere Independent assortment occurs.

Independent Assortment The process of random segregation and assortment of homologous chromosomes during metaphase resulting in the production of genetically unique gametes.

Metaphase I Equator Equator OR

Anaphase I Homologous chromosomes separate and are pulled to opposite ends (poles) of the cell. This ensures that each new cell will receive only one chromosome from each homologous chromosome. Unlike in mitosis the sister chromatids remain attached.

Anaphase I

Telophase I One chromosome from each homologous pair is at each pole of the cell Two nuclei form Each nucleus contains a complete copy of the cell’s DNA Cell divides, forming two cells

Cytokinesis The cytoplasm divides via cytokinesis to yield two new daughter cells.

Meiosis II Meiosis II is very similar to Mitosis There is no DNA replication, no Interphase prior to Meiosis II. Meiosis II has four (4) stages as well, and this occurs after Meiosis I: Prophase II Metaphase II Anaphase II Telophase II

Prophase II There is one chromosome of each homologous pair in each cell. Chromosomes relax and condense again Nuclear membrane begins to disappear DNA exists as chromosomes

Metaphase II The chromosomes line up at the equator of the cell

Anaphase II Chromosome breaks apart at the centromere and sister chromatids move to opposite poles of the cell. Once the chromosomes separate, each sister chromatids is considered to be a chromosome.

Telophase II Telophase II Spindle fibres begin to breakdown, and a nuclear membrane forms around each set of chromosomes Cytoplasm begins to divide. Telophase II

Cytokinesis Each sex cell divides into 2 producing 4 haploid daughter cells 4 gametes are produced – sperm or eggs Note that each gamete has its own nuclei as well.

The stages of Meiosis

Video on Meiosis https://www.connectschool.ca/mod/mheresource/view.php?id=900290 https://www.youtube.com/watch?v=nMEyeKQClqI Another good video https://www.youtube.com/watch?v=16enC385R0w

Assignment Page 42 in Workbook Textbook page 78 – 1-11, 14, 16, and 17 Note, question 7, do not answer the fruit fly part.