Meiosis

Today: We are going to start talking about the process of meiosis. You will take notes first, then you will have an activity to work on.

Figure 13.9 Independent Assortment

I. Heredity Heredity - the transmission of traits from one generation to the next Genetics – the scientific study of heredity and hereditary variation Genes – hereditary units endowed from parents Segments of DNA Divided into Chromosomes 46 in humans A gene’s specific location on a chromosome is called its locus

II. Reproduction – 2 modes Asexual reproduction – a single individual is the sole parent and passes copies of all its genes to its off spring Sexual reproduction – two parents give rise to offspring that have unique combinations of genes inherited from each parent

1. Asexual Reproduction 1 parent Binary Fission in bacteria Single cell eukaryotes : mitotic cell division DNA is copied and divided equally between daughter cells Multicellular organisms – Budding Hydra : Buds break off – are genetically identical to its parent Each offspring in asexual reproduction is called a Clone

Figure 13.1 The asexual reproduction of a hydra

2. Sexual Reproduction 2 parents Results in greater variation than asexual reproduction Offspring vary genetically from siblings and both parents Behavior of chromosomes during the sexual lifecycle

Figure 13.2 Two families

Human Lifecycle Somatic cells (any cell but sperm or ovum cells) have 46 chromosomes Can be visualized with a light microscope during mitosis Are two of each type Arranged in pairs Karyotype –ordered display of an individuals chromosomes Homologous chromosomes (homologues) – chromosomes that make up a pair that have the same length , centromere position and staining pattern

Figure 13.3 Preparation of a human karyotype

Human Lifecycle Autosomes – somatic chromosomes If a gene for a trait is located at a particular locus on a certain chromosome, then the homologue of that chromosome will also have a gene for the same trait at the same locus EXCEPTION: SEX CHROMOSOMES X and Y – only a small part are homologous Y is much shorter than the X X has few Y counterparts , Y is lacking many X genes XX (female) XY (male)

Karyotype The occurrence of homologous pairs of chromosomes in our karyotype is a consequence of our sexual origins A maternal set (23) and a parental set (23)

Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

https://www.youtube.com/watch?v=rqPMp0U0HOA

Next: Use the list of important terms to create flash cards. Simply, fold a piece of paper into 6-8 pieces and cut them out. Write the term on one side, and the definition on the other/ We have covered all terms except for the meiosis ones.

III. Chromosome Types Autosome= chromosome that contains genes for characteristics not directly related to the sex of the organism. (#1-22) Sex Chromosomes= chromosome that directly controls the development of sexual characteristics. Determines if the organism is female or male. (#23)

IV. Sperm and Ova Gamete= sex cell; an egg or a sperm cell. Have a chromosome count of 23 22 autosomes – in a single set Autosome = chromosome that contains genes for characteristics not directly related to the sex of the organism. Plus a single sex chromosome (X or Y) HAPLOID (n)

Discussion So we have 46 chromosomes or 23 pairs in everyone of our cells. But half came from your mom and half came from your dad. How does this happen? In Mitosis, we start with 46 and at the end we have a new cell with 46. So shouldn’t we have 92 chromosomes if we get some from mom and some from dad? WWWWHHHHAAAATTTTTT?

V. Sperm and Ova – A haploid sperm reaches and fuses with a haploid ovum Fertilization or syngamy Results in a fertilized egg or zygote The zygote contains the two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines Diploid n + n = 46 (2n)

How did the Gametes become Haploid? The process of Meiosis!!

VI. Meiosis Meiosis= form of nuclear division that divides a diploid cell into haploid cells; important in forming gametes for sexual reproduction. The process that halves the number of chromosomes in a cell Occurs in Ovaries or Testes Gametogenesis= process by which gametes are produced through the combination of meiosis.

A Variety of Sexual Lifecycles Human Life cycle Most fungi and some protists (including some algae) Plants and some other species of algae

Figure 13.4 The human life cycle

Alternation of generations Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy) Alternation of generations

Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number Meiosis Facts: Four daughter cells IMPORTANT: Homologous chromosomes are different than sister chromatids Homologous Chromosome= chromosomes that have the same length, appearance, and copies of genes, although the alleles may differ. 4 Haploid (n) cells instead of 2 diploid cells (2n)

VII. Meiosis I : Separates Homologous Chromosomes 1. Interphase Each of the chromosomes replicate The result is two genetically identical sister chromatids which remain attached at their centromeres

2. Prophase I Lasts longer and is more complex than prophase in mitosis Chromosomes begin to condense and homologues, each consisting of two sister chromatids, pair up

Prophase I Later in prophase, each chromosome pair becomes visible in the microscope as a tetrad A cluster of four chromatids At various places along their length, chromatids of homologous chromosomes are crisscrossed and Crossing Over can take place.

Prophase I Other cellular components prepare for division of the nucleus in a manner similar to that of mitosis Centrosomes move away from each other and spindle microtubules form between them The nuclear envelope and nucleoli disperse The spindle microtubules capture the kinetochores that form on the chromosomes The chromosomes begin moving to the metaphase plate Can last for days or longer (over 90% of meiosis)

3. Metaphase I The chromosomes are now arranged on the metaphase plate Still in homologous pairs Kinetochore microtubles from one pole of the cell are attached to one chromosome of each pair while microtubules from the opposite pole are attached to the homologue

4. Anaphase I The spindle apparatus 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 Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

5. Telophase I The members of each pair of homologous chromosomes continue to move apart until they reach the poles of the cell Two new cells are ready to go through cytokinesis, however, they are NOT identical to the original cell (like in mitosis). Remember, crossing over shuffles the DNA. (recombination)

Why are the 2 cells not IDENTICAL? During Prophase I Crossing Over took place and therefore created a new set of chromosomes that get passed on.

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

Figure 13.7 The stages of meiotic cell division: Meiosis I

Figure 13.7 The stages of meiotic cell division: Meiosis II

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

7. Prophase II A spindle apparatus forms and the chromosomes progress toward the metaphase II plate

8. Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis-like fashion Kinetochores of sister chromatids of each chromosome pointing toward opposite poles

9. Anaphase II The centromers of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles Now individual chromosomes

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

Figure 13.7 The stages of meiotic cell division: Meiosis II

Figure 13.8 A comparison of mitosis and meiosis

Figure 13.8 A comparison of mitosis and meiosis: summary

What type of cells are created at the end of Meiosis What type of cells are created at the end of Meiosis? (based on chromosome number) Haploid

What do we call the cells that go through Meiosis? Gametes

When 2 gametes come together, what do we call the new cell? Zygote

Explain why it is necessary for gametes to become Haploid? A zygote is made up of 2 gametes coming together during reproduction. Both gametes need to be haploid so that when they come together they can then create a diploid zygote.

IX. Origins of Genetic Variation As mentioned earlier, in species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Independent assortment of chromosomes Crossing Over Random Fertilization

Figure 13.9 Independent Assortment

Figure 13.10 Crossing Over

Random Fertilization A human ovum plus a human sperm 1 of 8 million combinations possible for each ovum and sperm 223 X 223 = over 70 billion combinations 70 trillion possible combinations with out considering crossing over YOU REALLY ARE UNIQUE!

X. Gametogenesis 1. Oogenesis= Females undergo oogenesis which usually produces 1 viable ova and 3 polar bodies which cannot be fertilized. 2. Spermatogenesis= Males undergo spermatogenesis which is the production of 4 sperm cells that are roughly the same size.

Next: Take the next 20-30 minutes to work on the chromosomes handout. You can either work together, or on your own. If you finish early, work on your unit 2 whiteboards.

Tomorrow: We will do lab 2 and take our test early! So, we will finish unit 2 tomorrow.

THE END OF MEIOSIS