a.k.a “Reduction Division” Meiosis

Assignment: Read p ASEXUAL Reproduction-how does this occur? Binary Fission Fragmentation Budding Parthenogenesis-The Komodo Dragon story we read The offspring are GENETICALLY IDENTICAL SEXUAL Reproduction Uses gametes The offspring are GENETICALLY DIFFERENT

Germ Cells & Somatic Cells Germ Cells- the cell that will become the haploid cell (sperm/egg) Somatic Cells-regular body cells

The Pros and Cons of Reproduction Asexual Reproduction: PROSCONS *Simplest*Little variation in a *Many offspring, quickly changing environment *Most efficient use of energy Sexual Reproduction: PROS CONS *Genetically different*More complicated

Diploid Cells Di- means TWO Each cell contains two copies of each chromosome These alike chromosomes are called homologous or homologues Where do these two copies come from? “One from Mom & one from Dad”

Diploid Cells We write the number of chromosomes as 2N. 2N means 2 sets of chromosomes For humans N=23 What is the total chromosome number? Hint: 2(N) = 2(23) = ?

Meiosis Meiosis cuts the number of chromosomes in half. It turns diploid cells into haploid cells. (Haploid means half) 2N reduced to 1N “That’s why we call it reduction division!”

Meiosis Haploid cells have 1N chromosomes Haploid cells have many names: sperm and egg cells reproductive cells Sex cells Gametes

Phases of Meiosis Two stages- 1 st and 2 nd meiotic divisions 1 st division is Meiosis 1; Prophase I Metaphase I Anaphase I Telophase I

We’re going to use a pretend cell with 4 chromosomes for the example.

Prophase I DNA replication occurs Homologous (alike)chromosomes match up This creates a tetrad = 4 chromatids Q-How many tetrads are in this cell?

Crossing Over-occurs in Prophase 1 Crossing-over may occur during this stage This is when homologous chromosomes exchange information Exchange of genes creates new combinations “This is why you may look very different from your siblings”.

Metaphase I Homologous chromosomes meet in the middle of cell

Anaphase I Homologous chromosomes separate

Telophase I Two new cells form Each is genetically different

Meiosis II 2 nd Meiotic division Prophase II Metaphase II Anaphase II Telophase II

Prophase II No DNA replication this time Each cell contains only 2 chromosomes (Unlike Prophase I where they contained 4chromosomes)

Metaphase II Chromosomes meet in the middle.

Anaphase II Chromatids separate

Telophase II 4 new haploid cells, called gametes, are now created

Summary Meiosis I gives us the random distribution of genes. Meiosis II creates the haploid cells called gametes. p

Why do we need haploid cells? When the egg meets the sperm you NEED 46 chromosomes. FERTILIZATION 23 chromosomes from Mom & 23 chromosomes from Dad. Otherwise, lots of problems; -Klinefelter,s Turner’s,Down Syn…

Gametes In males they are sperm cells In females they are egg cells Egg cells are much larger than sperm cells (look at picture) Males make 4 sperm Females make 1 egg (pg.257in Holt bk.)