1. White boards
What is the fundamental difference between gametes and normal body cells?
Why is this significant?
What are the differences between mitosis and meiosis?

2. Gamete formation by meiosis
Gametes are specialised reproductive cells.
Fusion of male and female gametes produces the diploid zygote cells from which new individuals will develop.
In order to maintain the chromosome number from one generation to the next each gamete can only carry half the number of chromosomes i.e. gametes are haploid.

3. Significance of meiosis
Produces haploid gametes (n) by halving the number of chromosomes
Ensures the maintenance of the diploid chromosome number from generation to generation when the haploid gametes fuse at fertilisation i.e. n + n = 2n

4. Task: Look at the worksheet on Meiosis
Describe Meiosis in pairs to each other : main points/ refer to learning outcomes. (do not need to know individual stages)
At which points in Meiosis is variation caused? HIGHLIGHT on the sheet.
Extension: start thinking about: ‘What are the differences you can now describe between Mitosis and Meiosis’

5. Outline of meiosis in humans
Male:46
meiosis
Female: meiosis 46 23
sperm
haploid
fertilisation
egg
Zygote
diploid
diploid
haploid

6. Key concepts about chromosome numbers
The total number of chromosomes in a cell is called the diploid number = 2n.
Half the number found in the gametes is called the haploid number = n
There are two copies of each chromosome, so chromosomes are found in PAIRS called Homologous pairs and a pair of sex chromosomes

7. Homologous pairs: Chromosomes are found in homologous pairs because:
one of each pair was carried in the male gamete
and the other in the female gamete,
and the pairs were re-established by fertilisation
The formation of Homologous pairs is specific to Meiosis: 2 together are called bivalents

8. MEIOSIS: Meiosis 1 Reduction division
Separates the homologous pairs into 2 intermediate cells
2N =46
N=23
N= 23
Meiosis 2
N=23
N=23
N=23
N=23
N=23
2N =46
FERTILISATION

9.
Meiosis animation WATCH: whiteboards.
3.8 Explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles and genes including independent assortment and crossing over (details of the stages of meiosis are not required).

10. Genome of the new individuals produced at fertilisation from the random fusion of two haploid gametes is unique i.e. the product of genetic information from two individuals and so is different to either parent  variation.

11. How meiosis increases variation
Crossing over
Independent assortment
What do you understand at this point??

12. Crossing over: prophase 1
Homologous chromosomes carry the same genes but the maternal and paternal chromosomes may carry different alleles of these genes e.g. both carry the CFTR gene but one may carry the normal allele and the other the CF allele
Crossing over: prophase 1
Each chromosome now carries different combinations of the alleles of its genes along its length so more gamete genotypes have been produced
Board diagram

13. How meiosis increases variation
Crossing over:
Each chromosome carries different combinations of the alleles of its gene
In addition errors can occur: mutations which can introduce new combinations

14. Independent assortment in Meiosis 1 prophase
The arrangement of each pair of homologous chromosomes during the first division is completely random; each pair lines up independently of any other pair.
In a cell with 3 pairs of chromosomes
(2n = 6) the arrangements shown here are all equally possible.
The daughter cells contain different assortments of the chromosomes (some have all the paternal ones, some have all the maternal ones and some different combinations of both).
The significance of this is that, while all these different gametes carry the same genes, they will be carrying different combinations of the alleles of those genes.
Gametes are not genetically identical  genetic variation

15. Summary
Crossing over
increases the combinations of alleles of the genes on the individual chromosomes only during meiosis
Independent assortment
increases the total combinations of alleles of all the genes on all the chromosomes on the gametes during meiosis
Each gamete receives 23 chromosomes: could be ALL or NONE could come from paternal/maternal chromosomes!
This increases the number of potential genotypes in the gametes to increase the genetic diversity

16. Tasks Make a table comparing Mitosis and Meiosis: Pairs 10 minutes
Produce a concept map which summarises how genetic variation is generated: pairs 10 minutes

17. Comparison and meiosis and mitosis
Number of chromosomes halved
Involves 2 divisions of the nucleus
Produces 4 cells
Homologous chromosomes pair up
Random /crossing over
produces gametes that are not
genetically identical
Produces gametes
Mitosis
Number of chromosomes remains the same
Involves one division of the nucleus
Produces 2 cells
Homologous chromosomes remain separate
Daughter cells are genetically identical
Produces new cells for growth or replacement

18. random fertilisation resulting in heterozygous crosses
genetic diversity
greater variety of genotypes in a population
shuffling genetic material into new combinations
creating new alleles
point mutations during replication
mutations during meiosis – rearrangement of DNA
independent assortment
crossing over during meiosis
inversion
duplication
deletion
insertion
produced by
as a result of by
means

19. Whiteboards

20. Meiosis;
Halved / ½ ;
Random(ly) / independently;
(Digestive) enzymes / lysozyme / eq;
Acrosome;
Zygote;
Mitosis;
Total 7marks

21. Exam question: 2 marks
Mammalian gametes are formed by meiosis and have the ability to fuse to form a zygote. Explain why it is important that gametes are produced by meiosis.

22. 1. halves the chromosome number / one of each pair in gamete / haploid cells ; 2. randomly assorts the chromosomes / genes ; 3. provides genetic variation ; 4. the diploid number is maintained (in the zygote) ;

23. Independent assortment and crossing over both result in genetic variation
1. Explain how independent assortment leads to genetic variation (2)
Describe how crossing over further increases genetic variation (2)

24. 1. Award one mark for each of the following points in context.
1. ref. to random aligning of chromosomes/eq;
2. idea of new combinations of {(parental) chromosomes /alleles};
2. Award one mark for each of the following points in context to a maximum of two marks.
1. breaking and rejoining of {chromatids / DNA /eq}; at chiasmata
2. on same chromosome pair;
3. recombines {genes / alleles} / produces recombinants/eq;