Meiosis Is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes).

Meiosis Involves two divisions of the nucleus – meiosis I and Meiosis II Starts with one diploid cell, and ends with four haploid cells.

Review of Terms Haploid (n)-- one set chromosomes Diploid (2n)-- two sets chromosomes (46 chromosomes) Chromatid – Two exact copies of DNA (each single copy called a chromatid) makes up a chromosome Centromere – point at which the two chromatids are attached Homologous chromosomes – chromosomes similar in size, shape, and genetic content.

Meiosis

Meiosis I

Prophase I Crossing – over occurs Homologous Chromosomes synapse (pair together) This group of 2 chromosomes together is called a tetrad (because there are 4 chromatids) Crossing – over occurs When portions on one homologous chromosome are broken and exchanged with the corresponding portion on one of the chromatids of the other homologous chromosome.

Crossing - Over

Metaphase I Paired Homologous Chromosomes line up in the middle of the cell.

Anaphase I Paired Homologus Chromosomes separate Daughter cells are now HAPLOID

Telophase I Nuclear membrane forms around each set of chromosomes Cytokinesis begins -division of cytoplasm Left with two haploid daughter cells.

Between Meiosis I & II The chromosomes do NOT replicate between Meiosis I and Meiosis II. Start of Meiosis I Start of Meiosis II DNA copy made during Interphase

Meiosis II – Same as Mitosis Prophase II Metaphase II Anaphase II Telophase II

Prophase II A new spindle forms around the chromosomes Spindles – are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division Centrosomes (Centrioles) - two cylindrical cellular structures that form at the poles of a cell during meiosis.

Metaphase II Chromosomes line up in the middle of the cell

Anaphase II Centromeres divide Chromatids move to opposite poles of the cell * Remember centromeres are what hold the two chromatids together to make a chromosome.

Telophase II Nuclear membrane forms around each set of chromosomes Cytokinesis begins Left with four haploid cells.

Sexual vs. Asexual Reproduction Male and Female reproduce offspring (USES MEIOSIS) Advantages Genetic variability – offspring are genetically different than parents; Species can rapidly adapt to new environments Disadvantages More energy; Slower generation time Only 1 parent (NO MEIOSIS) Ex: (1) Binary Fission (bacteria) and (2)Budding (yeast) Advantages Less Energy; Faster Reproduction Time Disadvantages Offspring are clones (identical) of parent; no genetic variability

Karyotype Ordered display if an individuals chromosomes arranged in pairs, starting with the longest.

Trisomy 21 Three number 21 chromosomes, making 47 total. Causes down syndrome.

Errors Nondisjunction – a member of a chromosome pair fail to separate. Deletion – a fragment of a chromosome is lost. Duplication – a fragment of one chromosome joins to a sister chromatid. Inversion – a fragment reattaches to the original chromosome, but in the reverse direction. Translocation – a fragment attaches to a nonhomologous chromosome.