1. How Cells Divide for Sexual Reproduction
Meiosis
How Cells Divide for Sexual Reproduction

2. Learning Goals
Describe the process of meiosis, including independent assortment and crossing over. Explain how reduction division results in the formation of haploid gametes or spores.
Over the next few days we will contrast the number of chromosomes in body cells and in gametes, summarize the events of meiosis, contrast mitosis and meiosis, and describe how alleles from different genes can be inherited together.

3. What is Meiosis?
Meiosis is a type of cell division, known as reduction division, that reduces the number of chromosomes by half, creating four haploid cells.
Each cell is genetically distinct from the parent cell that gave rise to them.
This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi.

4. Meiosis Vocabulary
Somatic cells – cells that make up all the internal organs, skin, bones, blood and connective tissue, etc. in an organism.
Germ cell - any biological cell that gives rise to the gametes of an organism that reproduces sexually.
Gametes – sex cells (sperm & egg)
Zygote – a fertilized egg – develops into an embryo

5. Meiosis Vocabulary
Chromosome – thread-like structure of DNA and proteins that store genetic information
Homologous – set of chromosomes (one maternal chromosome and one paternal chromosome) that pair up with each other inside a cell during meiosis
Humans have 23 pairs of homologous chromosomes (46 chromosomes all together)

7. Meiosis Vocabulary
Diploid – cell that contains both sets of homologous chromosomes.
Diploid number = 2N
Haploid - cell that contains only one set of chromosomes, therefore only has one set of genes.
Haploid number = N
N = number of chromosomes in one set
Human somatic cells – diploid (2x23 = 46 chromosomes)
Human sex cells – haploid (23 chromosomes)

8. Diploid v. Haploid

9. Meiosis Vocabulary
Tetrad – a set of four chromatids. This occurs after each chromosome duplicates itself and pairs with its corresponding homologous chromosome.
Crossing over – process in which chromosomes exchange portions of their chromatids during meiosis.
Duplication of each homologous chromosome
Tetrad – pair of duplicated homologous chromosomes

10. Phases Meiosis First comes Interphase (A)
Meiosis involves two distinct divisions:
Meiosis I (B)& Meiosis II (C)
At the end of Meiosis II, the original diploid cell has divided into four haploid cells (gametes)

11. (Cell Growth & Preparation)
Phases Meiosis
INTERPHASE
(Cell Growth & Preparation)
MEIOSIS I
(First Division)
MEIOSIS II
(Second Division)

12. Interphase
Prior to Meiosis, Interphase occurs and DNA is duplicated.

13. Homologous chromosomes pair up, forming tetrads
Crossing-over occurs, and new combinations of alleles are formed
Prophase I
Spindles form and attach to each tetrad.
Paired homologous chromosomes line up across the middle of the cell
Metaphase I
Homologous chromosomes move apart
Anaphase I
The nuclear envelope forms around each cluster of chromosomes.
Cytokinesis forms two new cells
Telophase I & Cytokinesis
MEIOSIS I
Just like MITOSIS, Meiosis I goes through the four PMAT phases, except they all have a “I” after them.
The BIG difference is that the chromosomes have crossed-over and created new allele combinations.

15. Between Meiosis I & Meiosis II
Meiosis I results in 2 new cells that are genetically different from each other AND from the original cell.
After Meiosis I, the cell DOES NOT go into interphase again, therefore, DNA does not replicate again.

16. MEIOSIS II Prophase II Metaphase II
Chromosomes condense as nuclear envelope breaks down
Prophase II
Spindles form and attach to each chromatid
Chromosomes line up across the middle of the cell
Metaphase II
Chromosomes are pulled apart
Anaphase II
Nuclear envelope forms around chromatids as they relax
Cytokinesis occurs, forming FOUR haploid daughter cells
Telophase II & Cytokinesis
MEIOSIS II
Just like Mitosis and Meiosis I, Meiosis II goes through the four PMAT phases, except they all have a “II” after them.
Meiosis II results in FOUR haploid daughter cells, these are the gametes (eggs or sperm).

17. (Cell Growth & Preparation)
Phases Meiosis
INTERPHASE
(Cell Growth & Preparation)
MEIOSIS I
(First Division)
MEIOSIS II
(Second Division)

18. Mitosis vs. Meiosis Mitosis One cell divides into two cells
Diploid cell  Diploid cells
DNA in daughter cells are identical to parent cell
Each daughter cell has the same amount of chromosomes as the parent cell
Meiosis
One cell divides into four cells
Diploid cell  Haploid cells
DNA in daughter cells are not identical to parent cell, alleles have been “shuffled” around
Each daughter cell has half the amount of chromosomes

20. VIDEO TIME Pull out a blank piece of paper and draw the following:

22. S phase – DNA Replication
Interphase
G1 - Growth
Interphase
S phase – DNA Replication 2 2 1 1 2 2 1 1

23. S phase – DNA Replication
Interphase
S phase – DNA Replication
Prophase I
(part one) 2 1 2 1