Review How many chromosomes do you have in each cell? How many sets of your chromosomes do you have (1, 2, 3, 4, more?) How many of each type of chromosome is inherited from your mom? How many of each type of chromosome is inherited from your dad? How many sex chromosomes do you have.

Meiosis Remember: Mitosis “happens in my toesies” Meiosis happens in my ovaries

If parents have two copies of each chromosome, why and how do they pass on only one to their offspring? MEIOSIS is the process of cell division that decreases the number of chromosomes to make sperm or egg cells Original cell Sperm or egg cells

The purposes of Meiosis: Reduce the total number of chromosomes by half so that only ONE chromosome from each pair is inherited from each parent. Produce variation in offspring by: randomly “sorting” (pairing up) the homologous chromosomes when the gametes meet. exchanging (crossing over) homologous chromosome arms

Meiosis Diploid: pairs of homologous chromosomes Original cell Diploid: pairs of homologous chromosomes Crossing over occurs here Sperm or egg cells Haploid single chromosomes

Fertilization Gametes: Zygote: (haploid sex cells) (diploid embryo) Sperm Egg Gametes: (haploid sex cells) Zygote: (diploid embryo) Fertilized egg

Meiosis Overview Each pair of chromosomes first replicates to form sister chromatids. The cell then divides twice without any further replication of DNA. The first division separates the homologues (Meiosis 1). The second division separates the 2 sister chromatids (Meiosis 2).

Comparison of meiosis and mitosis Draw and label each step on your worksheet. Fill in the concept map. Meiosis animation Mitosis animation