Cell Division: Meiosis Ch 11-4; p

Types of Reproduction Asexual – mitosis; results in clones or genetically identical offspring (2N) Sexual – meiosis; results in offspring genetically different from parent (N)

Easy Genetic variety; Adaptability Energy; Must have mate Genetically Identical Mitosis Meiosis

Vocabulary Diploid – 2 copies of each chromosome (2N) Homologous Chromosomes – paired like chromosomes Haploid – 1 copy of each chromosome (N)

Haploid vs. Diploid Humans have a diploid (2N) number of 46. What is the haploid (N) number? Fruit flies have a haploid (N) number of 4. What is the diploid (2N) number?

Meiosis Double Mitosis Two divisions (same steps) Reduction Division Chromosome number is reduced by half Diploid to haploid Sexual Reproduction Produces 4 genetically UNIQUE cells

Stages of Meiosis Interphase I Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Meiosis I Duplicated homologous chromosomes are separated into two daughter cells. Chromosome number cut in half (HAPLOID). Each cell receives one homologue of each pair of chromosomes. Crossing over may occur; so cells are genetically DIFFERENT. Sister chromatids DO NOT separate.

Quick Questions: Interphase What do the chromosomes look like in interphase I? What important event occurs during this stage?

Stages of Meiosis Prophase I Chromosomes thicken and condense disintegrates Nuclear membrane disintegrates Spindle fibers form (Tetrad) Homologous chromosomes pair. (Tetrad) Crossing over occurs Crossing over occurs.

Stages of Meiosis Prophase I: Homologous Pairs MATERNAL and PATERNAL chromosomes find each other and pair TETRAD forms = 4 sister chromatids

Stages of Meiosis Prophase I : Crossing Over Crossing Over occur as chromatids in tetrad exchange parts Genetic recombination – new combination of alleles (form of gene) on chromosome

Quick Questions What event makes crossing over possible? What is a tetrad?

Stages of Meiosis Homologous pairs Homologous pairs of chromosomes line up at equator (mitosis is not pairs) Spindle fibers attach centromere Metaphase I

Quick Question Where are the chromosomes in metaphase I?

Stages of Meiosis TETRADS TETRADS separate Each chromosome still contains two chromatids (mitosis has sister chromatids split) Anaphase I

Quick Question What happens to each homologous pair of chromosomes during anaphase I?

Stages of Meiosis Spindles disappear Two clusters of chromosomes Diploid Only one of homologous pair, therefore, Diploid Telophase I

Quick Question How many cells are formed at the end of telophase I? Each chromosomes consists of how many chromatids or strands? Describe the genetic composition of the cells formed by meiosis I.

Stages of Meiosis Interphase I Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

No Interphase No Replication of Chromosomes What stage of cell division is missing? Look back at first few slides. Without interphase, what important event does not occur?

Meiosis II Sister chromatids are separated Resembles mitosis Four cells Four cells are produced, they are: Haploid Genetically different

Stages of Meiosis (cont.) Chromosomes recondense if necessary Spindle fibers reform Prophase II Do chromosomes pair up?

Prophase II

Stages of Meiosis (cont.) Chromosomes line up along equator Spindle fibers have attached to sister chromatids Metaphase II Describe the position of the chromosomes.

Stages of Meiosis (cont.) Sister chromatids Sister chromatids separate and move to opposite poles Anaphase II

Stages of Meiosis (cont.) Telophase II Sister chromatids uncoil Nuclear membrane reforms 4 GENETICALLY DIFFERENT HAPLOID CELLS (mitosis is 2 identical diploid cells)

Telophase II

Quick Question How many cells are formed at the end of telophase II? Compare the number of chromosomes in the newly formed cells to the number in interphase I.

Meiosis Movie

Web Sites 80/Lectsked/Supple2/meiosis.html 80/Lectsked/Supple2/meiosis.html (meiosis graphics) Comparison Graphics

Meiosis vs. Mitosis 4 cells 2 divisions Sexual Haploid Only in sex cells Chromosomes replicate and pair Shuffling of genes produce cells that are genetically different than parent 2 cells 1 division Asexual Diploid Occurs all over Chromosomes replicate;DON’T pair All generations maintain exact genetic identity of parent.

Vocabulary Haploid Diploid Homologous Chromosomes Crossing over

Errors in Meiosis Nondisjunction – extra copies or missing copies of chromosomes when homologous pairs or sister chromatids DO NOT separate

Quick Question How do errors here compare with errors in mitosis?

Why Sex? Asexual reproduction takes less energy – more offspring, quickly without variation. Why change what works? Genetically different offspring may be more successful. What are the advantages of sexual reproduction?

Gametogenesis Formation of sex cells (GAMETES) Spermatogenesis Oogenesis Sperm – haploid (N) male gamete (sex cell) Egg – haploid (N) female gamete (sex cell) Zygote – diploid (2N) fertilized egg

Veeerrry Interesting… Sperm begins meiosis at end of gestation. Arrested at prophase I. When does it restart? At 5 months of gestation ovary contains 6-7 million oogonia. Production at birth, and by puberty this is reduced to only 300, ,000.

More interesting ideas… Meiosis I in females completes during preparation for ovulation Unequal division WHY? Polar body – contains haploid set of chromosomes; discarded nucleus. Stops at metaphase II

Oogenesis in Ovary Oocyte (egg) – Completes meiosis II only at fertilization. If female has 2 kids how many times has she completed meiosis? Remember that ¾ fertilizations are lost

Emry ology links