Cell Reproduction Meiosis How Do Cells Reproduce Most

Cell Reproduction (Meiosis)

How Do Cells Reproduce? • Most of the time cells simply divide in half to make new cells that are identical, this is called mitosis. • This is known as asexual reproduction (no partner needed).

How do Cells Reproduce? • Other times cells join together to create new combinations of DNA, this is called meiosis. • This is known as sexual reproduction (partner needed).

Asexual vs. Sexual Reproduction • Asexual reproduction occurs quickly, and makes identical copies of cells. • Sexual reproduction occurs much slower, but allows for more diversity.

Meiosis = Sexual Reproduction • Meiosis is important for two reasons: A) Meiosis reduces the number of chromosomes in a cell by ½. B) Meiosis creates opportunities for genes to be exchanged (cross over).

Meiosis = Sexual Reproduction • Meiosis is sometimes called “Reduction Division” because it reduces the normal chromosome number in half. DIPLOID CELL ---------> HAPLOID CELL 46 Chromosomes 23 Chromosomes

Mitosis = Asexual Reproduction • This is unlike in Mitosis where the chromosome number stays the same. DIPLOID CELL ---------> DIPLOID CELL 46 Chromosomes

The Meiosis Process • Meiosis begins in Interphase where the cell is going through G 1, S, and G 2. INTERPHASE

The Meiosis Process • When directed by certain chemicals the cell begins meiosis, instead of mitosis. • Meiosis is divided into two stages: A) Meiosis I B) Meiosis II

Meiosis I – Prophase I • The nuclear envelope of the cell begins to break up and starts to disappear. • Chromosomes become visible.

Meiosis I – Prophase I • Homologous chromosomes pair up (similar chromosomes from mom & dad). • This is called a tetrad. Mom’s Chromosome Dad’s Chromosome

Meiosis I – Prophase I • When tetrads get too close segments of genes can be exchanged (cross over. ) • This allows for more combinations of DNA to occur and even more diversity. Mom’s Chromosome Dad’s Chromosome

Meiosis I – Metaphase I • The tetrads (homologous chromosomes) move to the center (equator) of the cell.

Meiosis I – Anaphase I • Tetrads (homologous chromosomes) get pulled apart and moved to opposite sides.

Meiosis I – Telophase I • The chromosomes gather at opposites sides and the cytoplasm divides to make two new cells.

Meiosis I – Cytokinesis I • Two cells are made. • Chromosomes stay wound up.

So far… • The sister chromatids have NOT been pulled apart at their centromeres. • Interphase does NOT occur again. • The chromosomes are NOT copied.

Meiosis II – Prophase II This occurs in both of the cells made during Meiosis I. • Chromosomes are still visible (wound up). • Spindle fibers surround the chromosomes.

Meiosis II – Metaphase II This occurs in both of the cells made during Meiosis I. • The sister chromatids move to the center (equator) of the cell.

Meiosis II – Anaphase II This occurs in both of the cells made during Meiosis I. • Sister chromatids get pulled apart at their centromeres and move to opposite sides.

Meiosis II – Telophase II This occurs in both of the cells made during Meiosis I. • The chromosomes gather at opposites sides and the cytoplasm divides.

Meiosis II – Cytokinesis II This occurs in both of the cells made during Meiosis I. • Two more cells are made (4 total). • The chromosomes unwind and Interphase begins again.

Meiosis Animation l http: //biology. clc. uc. edu/graphics/bio 104/meiomove. gif

Meiosis Summary • We start with 1 diploid cell and make 4 haploid cells. • A cell goes through 2 separate cell division processes (Meiosis I and II).

Meiosis Summary • Parent cells begin with 46 chromosomes. • In S phase the 46 chromosomes make identical copies to become sister chromatids (92 chromosomes).

46 chromosomes 92 chromosomes

Meiosis Summary • During Meiosis I homologous chromosomes are separated into 2 cells that now have 46 chromosomes. 92 chromosomes 46 chromosomes

Meiosis Summary • Both cells from Meiosis I, go to Meiosis II. • Now sister chromatids are separated into cells that each have 23 chromosomes. 46 chromosomes 23 23

Meiosis Summary • A grand total of 4 cells each containing 23 chromosomes are formed!

Diversity in Meiosis • When tetrads are formed during Meiosis I they can cross over segments of DNA. http: //www. tokyo-med. ac. jp/genet/anm/mimov. gif

More Diversity in Meiosis • The tetrads (homologous chromosomes) randomly line up at the equator and can separate independently. • This is called independent assortment.

Even More Diversity in Meiosis • A zygote is made by a random sperm joining with a random egg.

Diversity! • Combined all three processes: 1) crossing over, 2) independent assortment, 3) random fertilization, …create an almost infinite number of possible outcomes.

Meiosis in Males • The process of Meiosis in males is known as spermatogenesis. • It occurs in the testes. • It creates 4 functional sperm.

Meiosis in Females • The process of Meiosis in females is known as oogenesis. • It occurs in the ovaries. • It creates 1 functional egg and 3 polar bodies.

Mitosis vs. Meiosis

Mitosis vs. Meiosis • In sexual and asexual reproducing organisms. • Occurs in body cells. • Cells divide 1 time and make 2 cells. • No crossing over • Cells are diploid • Only in sexual reproducing organisms. • Occurs in sex cells. • Cells divide 2 times and make 4 cells. • Yes, crossing over • Cells are haploid

Mitosis vs. Meiosis • No crossing over. • Yes crossing over. • Cells are diploid (46) • Cells are haploid (23) • All new cells made are identical. • Not all new cells made are identical (DIVERSITY!).

Any Questions? “Like a ten-speed bike, most of us have gears we do not use. ” --Charles Schulz “The desire to do something is the proof of our potential to do it. ” --Eric Butterworth