Cell Reproduction: Meiosis (September 18th)

Recombination

Changes composition of chromosomes (usually homologous)

Mechanisms exist to PREVENT recombination in mitosis and PROMOTE it in meiosis

Meiosis

Creates Gametes and consists of Meiosis I and Meiosis II

Meiosis I (like Mitosis)

Prophase I

Metaphase I

Anaphase I

Telophase I and Cytokinesis

Meiosis II (where cell goes from 2n to 1n)

Prophase II

Metaphase II

Anaphase II

Telophase II and Cytokinesis

Prophase I

DNA condenses into sister chromatids

Nuclear membrane is broken down

Centrosomes move to poles

This is where recombination happens in Meiosis

Metaphase I

Microtubules push chromosomes NEXT to each other in a line

This is where Independent Assortment happens

Anaphase I

Chromosome pairs are pulled apart

Sister Chromatids are still attached, and cell elongates

Telophase I and Cytokinesis

Cell membrane pinches into the two new cells

Nuclear membrane DOES NOT reform

Prophase II

Centrosomes move to poles

Metaphase II

Sister Chromatids line up at metaphase plate

Now only one line of single sets

Anaphase II

Sister Chromatids are pulled apart

Cell elongates

If crossing over had occurred, the chromosomes are not identical

Telophase II and Cytokinesis

Membrane pinches to divide cells

Nuclear membrane DOES reform

Now the cells are 1n

Nondisjunction

An uneven splitting of chromosomes, usually during Meiosis

Consequences of Nondisjunction

Trisomy (3 copies of a chromosome)

Monosomy (1 copy of a chromosome)

Monosomies

Except for gametes, monosomies are not viable

Meiosis mistakes:

Mistakes will be seen in all cells

Generate first cell from which all other cells are made

If lethal, organism dies

Mitosis mistakes:

Effects only observed in cells derived from the mother cell

Can result in cancer