Vocabulary: Homologous Diploid Haploid Meiosis I and II Tetrad Crossing-over Gametes Key Concepts: What happens during the process of meiosis? How is meiosis different than mitosis?

Genes, Chromosomes, and Numbers ● Organisms have tens of thousands of genes that determine individual traits; ● Where are genes??...  lined up on chromosomes! ● a typical chromosome can contain a thousand or more genes

Chromosomes and Genes ● in the “body cells” of animals and most plants, chromosomes occur in pairs; ● one chromosome in each pair came from the female parent, and the other came from the male parent

VOCABULARY: ● Homologous chromosomes = paired chromosomes; 1 came from mom, and the corresponding chromosome came from dad ● Diploid = has 2 sets of chromosomes (2n); all “body cells” are diploid ● Haploid = one set of chromosomes (n) -in the gametes (sex cells) -when egg joins with sperm, the diploid # of chromosomes is restored!

● MEIOSIS is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm. ● Meiosis appears much more complicated than mitosis… -it is really just two divisions in sequence -each one of which has strong similarities to mitosis.

INTERPHASE: ● Interphase prior to meiosis is identical to interphase prior to mitosis ● This includes the copying of DNA in the S phase! ● Meiotic division will only occur in cells associated with male or female sex organs.

MEIOSIS I ● Meiosis I, the first of the two divisions, is often called reduction division  it is here that the chromosome number is reduced from 2n (diploid) to n (haploid). Ex: In humans, the diploid number = 46 (haploid = 23)

PROPHASE I ● Prophase I of meiosis is similar to prophase in mitosis  chromosomes condense and become visible  spindle fibers develop  nuclear envelope breaks down

Prophase I - Important Events ● Homologous chromosomes pair up to form a TETRAD ● The sister chromatids press together at points along their length ● It is during this alignment that chromatid arms may overlap and temporarily fuse and exchange segments resulting in crossing over  Result: chomosomes (consisting of 2 sister chromatids) are no longer just from “mom” or “dad”—the chromosomes are a mixture!!!

More VOCABULARY: Tetrad—each chromosome pairs with homologous chromosome –4 sister chromatids! Crossing over– when chromosomes are in tetrads, they may exchange parts –This happens in Prophase I –Result – increased variety in offspring (a good thing)

Result: INCREASED genetic variety!!!

METAPHASE I Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis.

METAPHASE I In MITOSIS, all the chromosomes line up in single file on the metaphase plate in no particular order. In Metaphase I (of Meiosis), the chromosome PAIRS are aligned on either side of the metaphase plate.

ANAPHASE I ● During Anaphase I the spindle fibers shorten -the homologous pairs are pulled away from each other toward each pole of the cell.

TELOPHASE I ● spindle fibers break down; ● the nuclear membrane (envelope) may or may not reform, and the chromosomes do not disappear.  At the end of Telophase I: ● each daughter cell has a single set of chromosomes ● half the total number in the original cell where the chromosomes were present in pairs. ● chromosomes are still in doubled state (1 chromosome = 2 sister chromatids)

● While the original cell was diploid (2n), the daughter cells are now haploid (1n). ● This is why Meiosis I is often called reduction division. Example: if the -Diploid # = 4, then the -Haploid # = 2 It was 4, now it will be 2 in each cell!!

MEIOSIS II ● Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I. ● There is no interphase between Meiosis I and Meiosis II… ● Meiosis II begins with…

…PROPHASE II ● A new set of spindle fibers forms ● the chromosomes begin to move toward the center of the cell ● no longer with their homologous partner!

METAPHASE II ● The chromosomes in each haploid cell align in the center of the cell… this time in single file!!

ANAPHASE II ● The centromeres split ● The spindle fibers shorten -pulls the single chromosomes toward each pole of the cell.

TELOPHASE II ● the nuclear envelope reforms ● the chromosomes begin to uncoil, converting back to chromatin ● total of four daughter cells, each with half the total number of chromosomes as the original cell. ● followed by CYTOKINESIS

NOVA video

● MALE GAMETES: -all four haploid cells will eventually develop into mature, functional sperm cells.

● FEMALE GAMETES: (as seen in life cycles in “higher” organisms) -the cytoplasm and cellular organelles are divided unequally -three of the cells will typically abort -leaves a single cell to develop into a mature egg cell **usually much larger than a typical sperm cell.

Gametes