Remember! Homologous Chromosomes Paired chromosomes; both chromosomes in a pair carry the same genes Control the same inherited characteristics Homologous – same information Diploid (2n) 2n=4 1 copy from mom & 1 copy from dad! Human cells are 2n = 46! (23 from mom & 23 from dad)

Cell Division & Asexual Reproduction Mitosis produces cells with the same information! Identical daughter cells Clones Same amount of DNA

Asexual Reproduction Binary Fission Budding Prokaryotes (bacteria) Mostly single-celled eukaryotes (yeasts, amoebas, etc.)

Are egg & sperm cells made by mitosis? What about us? What if a complex, multicellular organism (like us) wants to reproduce? Joining of egg & sperm Are egg & sperm cells made by mitosis? If they were, then… 46 46 92 Egg Sperm Zygote It doesn’t work!

So, how do we make sperm & egg cells? Must reduce the number of chromosomes in the cell from 46 chromosomes  23 chromosomes! Must reduce the number of chromosomes by half Egg Zygote Sperm Fertilization

Gametogenesis – Gamete Formation In male animals, the haploid gametes are called sperm. In the female animals, the haploid gametes are called eggs. The process in which male & female gametes join is called fertilization. This produces a fertilized egg called a zygote!

Meiosis & Sexual Reproduction

Meiosis The process that divides 1 diploid (2n) cell to form 4 haploid (n) cells How gametes (sex cells) are formed Meiosis has 2 divisions: Meiosis I and Meiosis II

Meiosis – Production of Gametes (Sperm & Egg) Chromosome number must be reduced Diploid  Haploid 2n  n In humans, 46  23 Fertilization restores the chromosome number Haploid  Diploid n  2n

Sexual Reproduction Life Cycle 2 copies Diploid (2n) Fertilization Meiosis 1 copy Haploid (n) 1 copy Haploid (n) Gametes Gametes

Meiosis Before meiosis begins, cells go through interphase! DNA Replication occurs in interphase, forming a duplicate copy of each chromosome.

Meiosis After DNA Replication, each chromosome is made of 2 sister chromatids!

Meiosis – 2 Cell Divisions Meiosis I Reduces Diploid (2n)  Haploid (n) Separates homologous pairs Meiosis II Sperm & egg formed Separates sister chromatids Warning! Meiosis evolved from mitosis, so the stages & “machinery” are similar, but the processes are different! Do NOT confuse them!

Overview of Meiosis (I.P.M.A.T.P.M.A.T.)

Overview of Meiosis I 2n = 4 1 copy of each homologous pair 2n = 4 Now sister chromatids Reduction 1n = 2 Homologous pairs separated, sister chromatids remain

Meiosis I – similar to mitosis! Prophase I Centrioles move to opposite sides of the nucleus Nuclear envelope breaks down.

Prophase I 3. Homologous chromosomes pair up. This is different than mitosis! Each pair of chromosomes align & a tetrad is formed.

Prophase I Chromosomes exchange portions of their chromatids in a process called crossing over. Crossing over produces new combinations of DNA!

Here is a tetrad formed from 2 sets of sister chromatids. Gene A – Height (“A” is tall & “a” is short) Gene B – Hair Color (“B” is brown hair & “b” is blonde) The gametes produced can carry several varieties of genes; they’re not identical to the parent cell! This person can pass on tall/brown traits, tall/blonde traits, short/brown traits, or short/blonde traits!

Metaphase I Spindle fibers attach to the chromosomes at the centromere.

Anaphase I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. Chromosomes move & separate into 2 groups near the spindle. Anaphase ends when the chromosomes stop moving.

Remember! Cytokinesis is when the cytoplasm divides! Telophase I Nuclear membranes form around chromosomes. Remember! Cytokinesis is when the cytoplasm divides!

Meiosis I End Results Produces 2 haploid (n) daughter cells that have only 1 set of each chromosome Each set is made of 2 sister chromatids.

Meiosis I

Meiosis II After Meiosis I, cells enter Meiosis II: No DNA Replication occurs between Meiosis I & Meiosis II. Meiosis II separates the sister chromatids.

Prophase II Spindle fibers appear.

Metaphase II Chromosomes align like they do in mitosis & attach to the spindle at the centromere.

Anaphase II Sister chromatids separate & move towards opposite ends of the cell.

Telophase II Nuclear membranes form around chromosomes. Then, cytokinesis!

Meiosis II End Results Produces 4 haploid (n) daughter cells

What are the advantages to sexual reproduction? Increased diversity Independent assortment of chromosomes: random alignment of homologous chromosomes in Metaphase 1 Crossing over: mixing of alleles across homologous chromosomes Random fertilization – which sperm fertilizes which egg? Driving evolution Providing variation for natural selection

Variation from Independent Assortment of Chromosomes Random assortment in humans produces 223 (8,388,608) different combinations in gametes!

Variation from Crossing Over Creates completely new combinations of traits on each chromosome

Variation from Random Fertilization Sperm + Egg = ? Any 2 parents will produce a zygote with over 70 trillion possible diploid combinations!

Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers