Meiosis Reduction Division Meiosis Sexual reproduction involves continuity

Meiosis Reduction Division

Meiosis Sexual reproduction involves continuity of biological traits from one generation to the next – HEREDITY p Offspring acquire traits from their parents p Each parent contributes ½ of the traits to their offspring p

Meiosis In meiosis the offspring are different from their parents and from each other p Meiosis occurs ONLY in the male and female gonads n Male – testes n Female - ovaries p

Asexual reproduction vs sexual reproduction p Asexual (mitosis) n n Single individual is sole parent Offspring genetically identical to the parent Results in a clone identical to parent Genetic differences occur rarely as a result of mutations p Sexual (meiosis) n n Two parents give rise to offspring Each parent contributes ½ of its genes to offspring Offspring are different from their parents Results in genetic variation in the form of mutations

Meiosis includes steps that closely resemble corresponding steps in mitosis p Like mitosis, meiosis is preceded by replication of chromosomes in interphase S stage p Meiosis differs from mitosis in that a single replication consists of two consecutive cell divisions called reduction division p These cell divisions produce 4 daughter cells instead of 2 as in mitosis p

Meiosis These 4 daughter cells have ½ the number of chromosomes as the original cells – they are called haploid (monoploid) p The 2 daughter cells of mitosis have the same number of chromosomes as the parent cell – they are called diploid p

1 st division Interphase p Chromosomes replicate -

Prophase 1 p Chromosomes condense to form chromatids

synapsis p homologous chromosomes pair resulting in a TETRAD (group of 4 similar chromatids)

Crossing over During synapsis in prophase I p Homologous chromosomes can exchange pieces resulting in more variation p

Crossing over p p p The chromosomes can wind around each other Chromosome pieces can break off and be reattached This creates unique gene combinations

Metaphase I p Tetrads line up on the metaphase plate at the equator

Anaphase I p The centromere splits and sister chromotids are pulled toward the poles

cytokinesis p Disjunction occurs – homologous pairs of chromosomes are separated by sister chromatids remain attached

cytokinesis p Results in 2 diploid cells

2 nd division The cells do not re-enter interphase p They go directly into prophase II p

Prophase II Centrioles move toward the poles p Spindle fibers re-form p

Metaphase II p Sister chromosomes line up at the equater

Anaphase II p Centromeres split and the sister chromatids are separated

Telophase p New nuclear membranes form and cytokinesis occurs for the 2 nd time

Result of meiosis 4 monoploid daughter cells p Each cell contains 1 of each chromosome p

http: //www. sumanasinc. com/webcontent/ animations/content/meiosis. html p http: //highered. mcgrawhill. com/sites/0072495855/student_view 0 /chapter 28/animation__how_meiosis_wor ks. html p http: //www. johnkyrk. com/meiosis. html p http: //www. cellsalive. com/meiosis. htm p

Comparison of mitosis and meiosis p Mitosis p Meiosis p Cytokinesis occurs 1 X 2 diploid daughter cells formed No synapsis or disjunction NO crossing over Occurs in all somatic body cells Daughter cells have same number of chromosomes as the parent cell p Cytokinesis occurs 2 X 4 monoploid gametes formed Synapsis occurs forming tetrads then disjunction Crossing over takes place Occurs only in gonads Gametes have ½ the normal chromosome number p p p p p

Oogenesis p p p The production of egg cells (ova) in the ovary of the female When the 1 st division takes place, the cytoplasm divided unequally One of the daughter cells receives most of the cytoplasm – the small cell is called the first polar body

Oogenesis p p During the 2 nd division, the cells divide unequally into 1 large ova and 2 polar bodies The ova must nourish the developing embryo until it implants in the uterus

Oogenesis p p The polar bodies break apart and die For each primary sex cell that undergoes meiotic division, there is only 1 ova produced

Spermatogenesis p p The production of sperm in the testes The sperm are produced by meiosis The division of the cytoplasm is equal For each primary sex cell there are 4 sperm produced