Genes & Chromosomes Organisms have tens of thousands of genes that determine individual traits Genes are lined up on chromosomes A thousand or more genes are arranged on a 1 chromosome. Alleles – alternative forms of a gene for each variation of a trait of an organism. Trait – any characteristic that is inherited.

Diploid Cells In body cells, chromosomes come in pairs (Mitosis) A cell with two of each kind of chromosome is called a diploid cell. It is called 2n or diploid number of chromosomes. Contains 2 chromosomes so there are 2 alleles for any given trait.

Haploid Cells In Organisms that produce gametes (for sexual reproduction) or sex cells have cells that contain one of each kind of chromosome A cell that contains one of each kind of chromosome is called a haploid cell. It is called n or haploid number of chromosomes. Contains 1 chromosome so there is only 1 allele for any given trait.

Homologous Chromosomes Two chromosomes of each pair in a diploid cell help determine what the individual organism looks like. Homologous Chromosomes – The paired chromosomes. They are not always identical due to variations of alleles.

What does this person pass on to the next generation? What sex will this person be? What does this person pass on to the next generation?

Why Meiosis? When cells divide by Mitosis, the new cells have the exact same number and kind of chromosomes If sex cells had the same number of chromosomes as a body cell, with each fertilization, the chromosome number would double! Meiosis – A different type of cell division where gametes have half the number of chromosomes as the parents. Therefore, when sex cells combine, they’ll create an offspring with the same number of chromosomes as the parents.

Meiosis: Reduction/Division Two divisions: Meiosis I & Meiosis II

Meiosis I & Crossing Over Meiosis occurs in 2 stages: Meiosis I & II Meiosis I – Begins with one diploid (2n) cell Meiosis I is very similar to Mitosis except for what happens to the chromosomes Prophase I – each pair of homologous chromosomes comes together to form a tetrad. Tetrad – 2 homologous chromosomes, each made of 2 sister chromatids (4 total) These chromosomes exchange genetic material in a process called crossing over. Can occur at any location on a chromosome and several locations at the same time.

Prophase I Unlike Mitosis: Homologous chromosomes group with other homologous chromosomes to form a “tetrad” A “chiasma” occurs at the point where the chromosomes cross over Like Mitosis: Nuclear membrane disappears Spindles form Chromatin condenses into chromatids

Metaphase I Homologous chromosomes line up at the metaphase plate. Microtubules (spindles) attach to the kinetochore

Anaphase I Homologous chromosomes separate. Sister chromatids remain attached.

Telophase I Two daughter cells are formed with each one containing only one chromosome of the homologous pair. The daughter cells are now diploid.

Meiosis I Re-cap… Metaphase I The Centromere of each chromosome becomes attached to a spindle fiber Spindle fibers pull the tetrads to the equator Unlike mitosis, the chromosomes are together Anaphase I The homologous chromosomes separate to opposite ends of the cell This ensures each new cell has 2 chromosomes Telophase I The cell divides with a new genetic combination unique to the parent cell

Meiosis II: Gamete formation Prophase II: DNA does not replicate. Prophase II is just like mitotic prophase.

Metaphase II: All of the chromosomes line up at the metaphase plate.

Anaphase II: Centromeres divide Sister chromatids migrate separately to each pole.

Telophase II: Cell division is complete. Four haploid daughter cells are obtained.

Meiosis II - Recap Meiosis II – ends with 4 haploid (n) cells Meiosis II is identical to Mitosis Prophase II Spindle forms and attaches to the chromosomes Metaphase II Spindle attaches to the chromosomes and pulls to them to the equator Anaphase II Sister chromatids separate and are pulled to the poles Telophase II Spindle breaks down Cytoplasm divides

Summary of Meiosis

Comparison Table of Mitosis and Meiosis Trait Mitosis Meiosis Chromosome Number Same (Diploid) Half (Haploid) Genetic Similarities Same Diverse Type of Cell Body Cell Sex Cells (eggs & sperm) Number of Divisions I (one) I & II (two) Type of Reproduction Asexual Sexual