MRS. STIPANOVICH

11. 2 MEIOSIS

MEIOSIS

• Takes place only in the reproductive cells• Sperm Cells

• A male child does not have sperm cells until puberty

• Egg Cells• A female child is born with all of the eggs she will ever need

MEIOSIS

• Homologous Chromosomes• 1 from Dad• 1 from Mom

Somatic Cells (body cells) contain two identical sets of chromosomes (copies).

They are called diploid.

MEIOSIS

• 2N-A diploid cell can be represented by

“2n”Example: Fruit flies have 8 chromosomes, so we say 2N=8

MEIOSIS

• Gamete cells (reproductive) contain only one set of chromosomes. They are called haploid.

Example: A fruit fly’s sex cells would be represented by “N”.

If 2N=8, then N=4

MEIOSIS

• We all start out as diploid cells (fertilized egg cells). How do we make haploid cells from diploid cells?

• Answer: Meiosis!• Meiosis-the process of reduction division in which the number of chromosomes in a cell is cut in half. Only takes place in germ cells.

Fig. 9.4a p. 142

plasma membrane

newly forming microtubules in the cytoplasm

spindle equator (midway between the two poles)

one pair of homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I

MEIOSIS I

MEIOSIS 1

• Interphase 1: Gamete cells follow normal path—replicating DNA and forming more organelles

• Prophase 1: Each replicated chromosome finds its homologous pair.

• Crossing-over: The homologous pairs wrap around each other and pass genetic information back and forth.

MEIOSIS 1

• Metaphase 1: • Homologous pairs line up next to each other in the middle of the cell

• Spindle fibers attach to each pair of chromatids

MEIOSIS 1

• Anaphase 1: • Spindle Fibers pull the paired chromatids apart from each other

• Pull pairs toward opposite ends of the cell

MEIOSIS 1

• Telophase 1 and Cytokinesis:• Nuclear membrane forms• Cell separates into two new cells

MEIOSIS 1

• We end up with 2 new cells with very different genetic information from it’s parent cell (due to crossing over)• This is why you do not look exactly like your mother or exactly like your father.

• This is also why you might look like an extended relative, such as a grandfather or aunt

MEIOSIS II

• Now, we need to split the cells one more time.

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

MEIOSIS II

there is no DNA replication between the two divisions

MEIOSIS II

• Interphase: NOTHING

MEIOSIS II

• Prophase II:• Spindle re-emerges

MEIOSIS II

• Metaphase II• Sister chromatids line up in the middle of the cells (like Mitosis)

• Spindle fibers attach

MEIOSIS II

• Anaphase II• Each CHROMATID is pulled to a separate pole (opposite ends)

MEIOSIS II

• Telophase II• Nuclear membrane re-formsCytokinesis OccursWe have 4 haploid cells

SPERM

• Spermatogenesis

• Spermatocyte to 4 spermatids

EGGS

• Oogenesis

• Oocyte to 3 polar bodies and 1 ovum

2n

n

2n

germ cell germ cell

each chromosome duplicated during

interphase

MEIOSIS Iseparation of homologues

MEIOSIS IIseparation of

sister chromatids

diploid number restored at fertilization

zygote

gametes gametes