Chapter 7 Section 1: Meiosis Grade 10 Biology Fall 2010

Objectives Summarize the events that occur during meiosis Relate crossing over, independent assortment, and random fertilization to genetic variation Compare spermatogenesis and oogenesis

Bell Ringer Some organisms reproduce by joining gametes to form a new individual The gametes are haploid- contain one set of chromosomes If chromosome number wasn’t reduced (stayed diploid) how many chromosomes would the third generation offspring have if the original parents started with 20 chromosomes?

Bell Ringer Parent 20 chromosomes Parent 20 chromosomes Offspring 1 40 chromosomes Offspring 1 40 chromosomes Offspring 2 80 chromosomes Offspring 2 80 chromosomes Offspring chromosomes Offspring chromosomes Parents First Generation Second Generation Third Generation

Formation of Haploid Cells Meiosis: is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells ▫Such as gametes or spores Meiosis involves two divisions of nucleus ▫Meiosis I ▫Meiosis II

Formation of Haploid Cells Before meiosis, DNA is replicated Meiosis starts with homologous chromosomes ▫Chromosomes similar in size, shape, and genetic content

Stages of Meiosis: Meiosis I Stage 1: Prophase I ▫Chromosomes condense, nuclear envelope breaks down ▫Homologous chromosomes pair up ▫Crossing over occurs: portions of chromatid on one homologous chromosomes are broken and exchanged with the corresponding chromatid portions of the other homologous chromosomes. ▫Cell is diploid at this time

Stages of Meiosis: Meiosis I Stage 2: Metaphase I ▫Pairs of homologous chromosomes are moved by the spindle to the equator of the cell ▫Homologous chromosomes remain together

Stage of Meiosis: Meiosis I Stage 3: Anaphase I ▫Homologous chromosomes separate ▫Chromosomes in each pair are pulled to opposite poles of the cell by spindle fibers ▫Chromatids do not separate at their centromeres- each chromosome is still made up of two chromatids ▫Genetic material has recombined!

Stages of Meiosis: Meiosis I Stage 4: Telophase I ▫Individual chromosomes gather at poles ▫Cytoplasm divides (cytokinesis), forming two new cells ▫Both cells contain one chromosome from each pair of homologous chromosomes ▫2 new cells are haploid

Stages of Meiosis Chromosomes DO NOT replicate between meiosis I and meiosis II!

Stages of Meiosis: Meiosis II Stage 5: Prophase II ▫New spindle forms around the chromosomes ▫2 cells

Stages of Meiosis: Meiosis II Stage 6: Metaphase II ▫Chromosomes line up along equator and are attached at their centromeres to spindle fibers

Stages of Meiosis: Meiosis II Stage 7: Anaphase II ▫Centromeres divide ▫Chromatids (now called chromosomes) move to opposite poles of the cell

Stages of Meiosis: Meiosis II Stage 8: Telophase II ▫Nuclear envelope forms around each set of chromosomes ▫Spindle breaks down, cell undergoes cytokinesis ▫4 new cells are haploid

Graphic Organizer- diploid or haploid 2n n n nn n n Mitosis Meiosis

Independent Assortment In humans, each gamete receives one chromosome from each of 23 pairs of homologous chromosomes Which of the two chromosomes that an offspring receives from each of 23 pairs is a matter of chance

Independent Assortment Independent assortment: random distribution of homologous chromosomes during meiosis

Crossing Over and Random Fertilization DNA exchange that occurs during crossing- over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis Zygote that forms a new individual is created by random joining of two gametes (egg + sperm)

Importance of Genetic Variation Meiosis and joining of gametes are essential to evolution No genetic process generates variations more quickly Why is genetic variation important?

Meiosis in Males Spermatogenesis: process by which sperm are produced in male animals ▫Occurs in the testes (male reproductive organ) Diploid cell (germ cell)  meiosis I  2 cells produced  meiosis II  four haploid cells Four cells change in form and develop a tail to become male gamete ▫Sperm cells!

Spermatogenesis

Meiosis in Females Oogenesis: process by which gametes are produced in female animals ▫Occurs in ovaries (female reproductive organs) After meiosis I, cytoplasm divides unevenly One of cells gets almost all of cytoplasm This cell will give rise to the egg cell Other cell is small, is called polar body

Oogenesis

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