1. Cell Division - Meiosis

2. Why is Meiosis so Important?
Meiosis is responsible for all of us inheriting genes from our parents
It is the process by which gametes (sex cells – eggs & sperm) are formed
It is the process by which we get a mix of characteristics from mom and from dad

3. Principles of Genetics
Each organism must inherit a single copy of every gene from its “parents”
When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains only one set of genes
Remember, genes are located on chromosomes inside the cell’s nucleus

4. Chromosome Number
Let’s consider the fruit fly… A somatic cell in an adult fruit fly has 8 chromosomes
4 of the chromosomes came from the fruit fly’s male parent, and 4 came from the female parent
These two sets of chromosomes are homologous – meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome (and therefore corresponding genes) from the female parent

5. Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid, which means “two sets”
The number of chromosomes in a diploid cell can be represented by the symbol 2N **(think 2, like 2 sets)
So for the fruit fly, the diploid number is 8, which can be written as 2N = 8
But the gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes – these are called haploid, which means “one set”
The haploid number can symbolized as just N **(think 1 N, like 1 set)
So for the fruit fly, the haploid number is 4, which can be written as N = 4

6. Meiosis
How are haploid (N) gamete cells produced from diploid (2N) cells? That’s where meiosis comes in!
Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell
Meiosis involves two distinct divisions, called Meiosis I and Meiosis II
By the end of Meiosis II, the diploid cell that entered meiosis has become four haploid cells

7. Gamete Formation
In males, the four haploid cells produced at the end of Meiosis are called sperm
In females, only one of the four cells produced is involved in reproduction, and it is called an egg

8. Phases of Meiosis – Meiosis I
Description
Interphase I
Cells undergo a round of DNA replication, forming duplicate chromosomes
Prophase I
Each chromosome pairs with its corresponding homologous chromosome and crossing over occurs (Crossing Over: homologous chromosomes exchange portions of themselves to produce new combinations of genes)
Metaphase I
Chromosomes line up down the middle and spindle fibers attach to them

9. Crossing Over

10. Phases of Meiosis – Meiosis I
Description
Anaphase I
Homologous pairs separated and are pulled to the poles of the cell
Telophase I
Nuclear membrane reforms, cytoplasm is still connected
Cytokinesis I
A cleavage furrow separates it into two haploid (N) daughter cells

11. Phases of Meiosis – Meiosis II
Description
Prophase II
Similar to other prophases – nuclear membrane disintegrates and spindle fibers begin to form
Metaphase II
The haploid number of chromosomes line up down the middle and the spindle fibers attach
Anaphase II
Sister chromatids separate and move towards the poles of the cell

12. Phases of Meiosis – Meiosis II
Description
Telophase II
Similar to other telophases – nuclear membrane begins to reform and cytoplasm is still connected
Cytokinesis II
Meiosis II results in 4 haploid daughter cells

13. Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes pair up
Crossing Over occurs
One cell division
Two cell divisions
Two daughter cells
Four daughter cells
Daughter cells are diploid (2N)
Daughter cells are haploid (N)