Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells.

Why do we need meiosis? Meiosis is necessary to halve the number of chromosomes going into the sex cells Meiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell 4 gametes 1 st division 2 nd division

The Stages of Meiosis: aka: Reduction Division

Meiosis I : Separates Homologous Chromosomes Interphase Interphase – Each of the chromosomes replicate – The result is two genetically identical sister chromatids which remain attached at their centromeres

Prophase I This is a crucial phase for mitosis. This is a crucial phase for mitosis. During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over is the exchange of segments during synapsis. Crossing Over is the exchange of segments during synapsis.

Metaphase I The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. – Still in homologous pairs

Anaphase I The spindle guides the movement of the chromosomes toward the poles The spindle guides the movement of the chromosomes toward the poles – Sister chromatids remain attached – Move as a unit towards the same pole The homologous chromosome moves toward the opposite pole The homologous chromosome moves toward the opposite pole – Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

Telophase I This is the end of the first meiotic cell division. This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells. The cytoplasm divides, forming two new daughter cells.

Cytokinesis Occurs simultaneously with telophase I Occurs simultaneously with telophase I – Forms 2 daughter cells Plant cells – cell plate Plant cells – cell plate Animal cells – cleavage furrows Animal cells – cleavage furrows NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

Figure 13.7 The stages of meiotic cell division: Meiosis I

Question of the Day 11/13/2014 During the process of Meiosis I, homologous chromosomes line up in tetrads. These chromosomes can then undergo a process that leads to genetic diversity. What is this process called? Crossing-Over

Meiosis II : Separates sister chromatids Proceeds similar to mitosis Proceeds similar to mitosis THERE IS NO INTERPHASE II ! THERE IS NO INTERPHASE II !

Prophase II Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis- like fashion The chromosomes are positioned on the metaphase plate in a mitosis- like fashion

Anaphase II The centromeres of sister chromatids finally separate The centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles The sister chromatids of each pair move toward opposite poles – Now individual chromosomes

Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occurs Nuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cells After completion of cytokinesis there are four daughter cells – All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 = 6,000,0000.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?

What each of the human chromosomes look like

Karyotype : A photomicrograph of chromosomes arranged according to a standard classification

In other words… Chromosomes are digitally arranged so that they are matched with their homologue or “partner” chromosome. Homologue chromosomes are the same size, shape, and carry the same genes, and one is inherited from each parent. They are numbered according to size.

Sex determination with karyotype This karyotype has 23 exact pairs, which means the person is female. Note that #23 chromosomes are both X.

Normal human male Note that #23 chromosomes are X and Y.

Is this person female or male?

Trisomy 21 Abnormality shown in karyotype Note that there are three copies of #21 chromosome. This person has Down Syndrome.

Monosomy X Abnormality shown in karyotype Note this person only has 1 copy of the X chromosome. This female has Turner’s syndrome.

XXY Male (Extra X)

How are DNA samples obtained for karyotypes?

Are you red-green color blind? Yes, if you have a difficult time distinguishing a number from this picture