1. 5. Meiosis – 2nd source of Variation in a Gene Pool
A. Recombination during meiosis
B. Independent assortment
C. Segregation

2. Meiosis – Card sort race
Remember:
Meiosis is the cell division that produced gametes (ova and sperm)
Meiosis involves 2 divisions
1st: homologous chromosomes pair up (synapsis). One chromosome from each pair of homologous chromosomes separates into the two separate cells produced at the end of the 1st division
2nd: this is like mitosis. Chromatids separate into separate cells
Each cell at the end of the first division produces two daughter cells to give a total of 4 cells at the end of meiosis

3. Modelling variation… Using a white board as your cell outline,
Create two sets of replicated homologous chromosomes in different colours to show prophase 1.
Crossing over occurs during this phase as the tetrads form.
Cross over one of the arms of each of the two centre chromatids to form a chiasma.

4. Modelling variation continued.
Break the chromatids at the chiasma and rejoin so each has a section of new (different coloured DNA).
Now separate each homologous pair to the poles of your “cell”
You have achieved Variation #1 – crossing over.
This source of variation can separate linked genes.

5. Recombination – crossing over
During meiosis, pieces of homologous chromosomes are exchanged or swapped over
This results in new combinations of genes on each of the pair of homologous chromosomes
This is called recombination

6. Recall- Crossing Over
Crossing over occurs during the first division of meiosis when the two homologous chromosomes line up together
The ends of the non-sister chromatids lie over each other, break and then rejoin onto the other chromatid
The site where the chromatids cross over each other is called chiasma

7. Video

8. Pretty Picture of Crossing Over

9. Independent Assortment
Animation
This occurs, when homologous chromosomes line up independently at the centre of the cell
In a cell with two pairs of homologous chromosomes there are two ways the chromosomes can line up
The way they line up is random and this leads to a random assortment of c/s in each daughter cell

10. Pretty Picture of Independent Assortment

11. Modelling continued
Now use your crossed over chromosomes to model independent assortment showing both possible options…

12. Segregation
During meiosis, alleles of different genes on different chromosomes segregate (separate) independently of each other
This means that in a heterozygous individual AaBb:
A separating from a does not depend on B separating from b
There will be equal numbers of AB, Ab, aB, and ab gametes
So segregation during meiosis produces gametes with different allele combinations / gametes all being different.

13. Mutations Usually proteins are made in a very reliable process
Many copies of identical proteins are produced from the same base sequence of DNA
BUT mistakes can occur in the transcription of the DNA which can alter the DNA base sequence
This altered DNA sequence is called a mutation

14. Mutations
Many mutations have no noticeable effect, while other mutations will alter the protein formed because a new amino acid will be coded for
Mutations lead to evolution because the protein formed from a mutation may be useful, producing an organism more suited to its environment
However most protein changes have no effect, are bad or are lethal

15. Mutations Page 89 of POL workbook make highlight notes.
Complete self check.