MEIOSIS Meiosis

Review of Mitosis Mitosis is the division of a parent cell into two genetically identical daughter cells. Mitosis occurs during the M Phase of the Cell Cycle and is essential for growth and repair.

The Four Stages of Mitosis PROPHASE METAPHASE ANAPHASE TELOPHASE In mitosis, a cell that begins with 46 chromosomes divides into two daughter cells each with 46 chromosomes.

Overview of Meiosis The function of meiosis is to produce our sex cells that are used for reproduction. Sex cells are also called GAMETES. In humans, our gametes are egg and sperm.

Fertilization of Gametes When a sperm cell penetrates an egg cell, this is called FERTILIZATION and the result is a zygote, a fertilized egg which will then grow and divide and develop into an embryo.

Overview cont. All other cells in your body are somatic cells and use the cell cycle which includes mitosis to replicate.

Why do we need a separate process for sex cells? Eggs and sperm cells must have half the number of chromosomes as the parent cell, so that when they are put together (during sexual reproduction egg+sperm) they will have the same number of chromosomes as the parent. Egg and sperm cells are NOT genetically identical to the parent cell. (If they were, you would be identical to your siblings!)

AMOEBA SISTERS CHROMOSOME NUMBERS Everyone has 2 sets of chromosomes: A set from Mom and a set from Dad, these two sets are homologous chromosomes. AMOEBA SISTERS CHROMOSOME NUMBERS https://www.youtube.com/watch?v=gcz1FOWw0Cg The term sister chromatids refers to the identical copy of the chromosomes that is temporarily attached by a centromere after replication. Sister chromatids have identical DNA, homologous chromosomes do not as they came from different parents.

Diploid Cells A cell containing both sets of homologous chromosomes is said to be diploid, which means “two sets.” This number is usually represented by the symbol 2N. Example: Humans have 46 chromosomes or 2 sets of 23 2N = 46 EXAMPLES: Skin Cells, Lung Cells, Heart Cells, Blood Cells, a Zygote

Haploid Cells A cell containing only 1 set of chromosomes is said to be haploid, which means “one set.” This number is usually represented by the symbol N. Example: Humans 2N = 46 then N =23 EXAMPLES: Sperm Cell, Egg Cell

Important Information about Meiosis To produce gametes, specific cells (called germ cells) must undergo the cell cycle and replicate their DNA during the S phase. While in the M Phase (Cell Division) instead of Mitosis, these cells use Meiosis I and II where they will divide twice before entering into G0. The result is four (instead of two) daughter cells that are all haploid (N).

The Stages of Meiosis Meiosis is divided into two rounds: Meiosis I and Meiosis II. It is simply 2 rounds of phases very similar to mitosis.

MEIOSIS I Prophase I Interphase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. CROSSING OVER occurs in Prophase I Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes.

MEIOSIS I Metaphase I Anaphase I Spindle fibers pull the homologous chromosomes apart and away toward opposite ends of the cell Metaphase I Homologous Chromosomes align in the middle and spindle fibers attach to their centromeres

MEIOSIS I Telophase I and Cytokinesis Two new nuclear membranes form The cell separates into two cells

Meiosis I

WHY TWO ROUNDS? Meiosis I results in two diploid (2N) daughter cells with the same number of chromosomes as the parent cell. So if an egg and sperm were to join there would be too many chromosomes!!! To solve this issue, the two daughter cells DO NOT continue on in the cell cycle with Interphase. The DNA is NOT replicated again. Instead, the two daughter cells progress right into meiosis II.

MEIOSIS II Metaphase II Prophase II Chromosomes align in the middle and spindle fibers attach to their centromeres Prophase II Nuclear membrane disappears and chromosomes once again begin to pair up

MEIOSIS II Telophase II and Cytokinesis Anaphase II Nuclei reform and cells split. Meiosis II results in four haploid (N) daughter cells. Anaphase II Spindle fibers pull the sister chromatids apart and away toward opposite ends of the cell

Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Telophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells.

SUMMARY OF MEIOSIS By the end of Meiosis, the original diploid parent cell has now become FOUR sex cells. These gametes are haploid cells because they have only a single set of chromosomes. 3D SUMMARY OF THE PHASES OF MEIOSIS: https://www.youtube.com/watch?v=vgCTI0_LJJM

How Meiosis Encourages Genetic Differences Crossing Over and Independent Assortment are two processes that happen during Meiosis I and II that ensure daughter cells are unique and different from the parent cell.

Crossing Over Crossing-over happens during PROPHASE 1 when homologous chromosomes exchange portions of their chromatids, resulting in new and unique combinations of genes in the gametes. CROSSING OVER CLIP https://www.youtube.com/watch?v=pdJUvagZjYA

Independent Assortment Independent Assortment ensures that chromosomes are randomly assorted. Chromosomes align randomly in the middle during Meiosis I and II, and they separate independently. This gives countless possibilities of gene combinations.

MITOSIS VS MEIOSIS Type of Reproduction Asexual Sexual Number of times cell divides 1 2 Number of daughter cells made at completion of division(s) 4 Cells where this type of division occurs Body Cells (somatic) Gametes Daughter cells are haploid/diploid Diploid Haploid Daughter cells are genetically similar or different from parent Identical Different Daughter cells are genetically similar or different from each other If there were 6 chromosomes in the parent cell, how many would be in the daughter cells? 6 3 Type of Reproduction Asexual Sexual MEIOSIS SQUARE DANCE https://www.youtube.com/watch?v=iCL6d0OwKt8

46 Chromosomes 46 Chromosomes 46 Chromosomes 92 Chromosomes MITOSIS 46 Chromosomes 46 Chromosomes 92 Chromosomes AMOEBA SISTERS MEIOSIS https://www.youtube.com/watch?v=33VbhIENEJI 23 Chromosomes 23 Chromosomes MEIOSIS 23 Chromosomes 46 Chromosomes 92 Chromosomes 23 Chromosomes 46 Chromosomes