Meiosis

**MEIOSIS QUIZ**
1. A reciprocal exchange of corresponding segments of DNA is called **CROSSING OVER** 2. When chromosomes condense there are **TWO SISTER CHROMATIDS** connected by a centromere 3. A spore is a **//haploid//** reproductive cell produced by **MEIOSIS** 4. **Fertilization** (or fusion of the //**haploid**// sperm and egg) results in the restoration of the **DIPLOID** number of chromosomes in the zygote 5. The stage of meiosis during which homologues line up along the equator of the cell is called **METAPHASE I** 6. Sexual reproduction increases the **GENETIC VARIATION** of the population 7. During crossing-over a **CHIASMATA** forms between non-sister chromatids 8. The process called **MEIOSIS** guarantees that the number of chromosomes in gametes is half the number of chromosomes in body cells 9. The haploid cells formed in the male at the end of meiosis, develop tails as they mature and are called **SPERM** 10. The process in which sperm and egg cells join is called **FERTILIZATION** 11. Meiosis is a necessary step to facilitate **SEXUAL** reproduction 12. The cells resulting from meiosis in either males or females are called **GAMETES**

**Why?**
//Sexual Reproduction is significantly more expensive for the organism in terms of energy Why do it?//
 * Increases the genetic diversity in the sexual reproducing population.
 * Evolutionary advantage outweighs the cost in energy.

**Meiosis**
//-Body cells (somatic cells ) = Diploid (2n) -Gametes (sex cells) = Haploid (n)// -//Male - testes - spermatogenesis -Female - ovaries - oogenesis//
 * Type of **cell division**
 * Produces **gametes**
 * **Halves** number of chromosomes
 * **Meiosis** prepares for **sexual reproduction.**
 * Two divisions (Meiosis I and Meiosis II).
 * Sex cells divide to produce **gametes** (sperm or egg).
 * **Gametes** have //half// the number of chromosomes (2n to n)
 * Occurs only in gonads

**Meiosis**

 * Purpose**:
 * To reduce the number of chromosomes by half
 * Where**:
 * Only occurs in sex cells
 * Result**:
 * Cells differ from original cells in genetic make-up

**Mitosis**

 * Purpose**:
 * to maintain the number of chromosomes in the cell
 * Where**:
 * Occurs in all cells except sex cells
 * Result**:
 * cells identical to original cell in genetic make-up
 * diploid somatic cells become haploid gametes with half the original number of chromosomes

**Homologous Chromosomes**

 * Pair of chromosomes (maternal and paternal)
 * Similar in shape and size.
 * Carry genes controlling the same inherited traits.
 * Humans have 23 pairs of homologous chromosomes (22 pairs of autosomes, 1 pair of sex chromosomes)
 * Interphase I**
 * Nucleus and nucleolus visible.
 * Similar to mitosis interphase.
 * Chromosomes replicate (S phase).**
 * Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
 * Meiosis I**
 * Cell division that reduces the chromosome number by one-half (2n-> n)
 * 4 Phases:**
 * Prophase I
 * Metaphase I
 * Anaphase I
 * Telophase I

**Prophase I**

 * Longest and most complex phase (90%)
 * Chromosomes condense
 * Homologous chromosomes come together to form a tetrad
 * Tetrad is 2 chromosomes or 4 chromatids (2 sister chromatids with their homologous pair).
 * Crossing over:**
 * segments of non-sister chromatids exchange homologous regions.
 * Also known as Genetic recombination

**Metaphase I**
How many unique combinations of chromosomes are possible in a gamete?
 * Shortest phase
 * Tetrads (a pair of 2 homologous chromosomes) align on the equator (mitosis each chromosome is on the equator)
 * Independent Assortment occurs:**
 * This is the cellular mechanism responsible for the principle that Mendel called Independent assortment
 * Orientation of homologous pair to poles is random which leads to random (distribution) of different genes to different poles:
 * Formula: 2n
 * Example: If an organism has a diploid state of 4 chromosomes then **2n=4**, therefore n=2 and 2^n or 2^2=4 unique combinations of chromosomes


 * IMPORTANT VOCABULARY:**
 * 1.** Crossing over (Genetic Recombination): A reciprocal exchange of corresponding segments of DNA
 * 2.** Metaphase I-The stage of meiosis during which homologues line up along the equator of the cell
 * 3.** Gamete: A haploid reproductive cell that joins with another haploid reproductive cell to form a zygote.
 * 4.** Chiasmata: An observable region in which nonsister chromatids of homologous chromosomes cross-over each other.
 * 5.** Synapsis: Pairing of homologous chromosomes.
 * 6.** Tetrad: A pair of 2 homologous chromosomes. There are 2 chromosomes and 4 chromatids.
 * 7.** Spindle: A network of microtubules that moves chromatids to the poles of a cell.
 * 8.** Karyotype: An arrangement of the chromosomes found in an individual's cell during the metaphase stage. It is arranged in homologous pairs from largest to smallest.
 * 9.** Centriole: Pair of cylindrical bodies in an animal cell that forms spindle. Made of microtubules.


 * Product of Meiosis:** 4 haploid daughter cells produced.