1. GCSE comparison of mitosis and meiosis
Forms body cells
Growth and repair
Cells produced are genetically identical
One division
Homologous pairs do not meet
No crossing over
Produces 2 cells from each division
Diploid number of chromosomes
Forms sex cells/gametes
For sexual reproduction
Cells produced are genetically different
Two divisions
Homologous pairs meet
Crossing over
Produces 4 cells from each division
Haploid number of chromosomes
In both the chromosomes replicate before division begins

2. Meiosis key words Gametes Haploid Meiosis I Meiosis II Prophase
Metaphase
Anaphase
Telophase
DNA replication
Sister chromatids
Centromere
Chromatin
Homologous pairs
Bivalent
Loci
Maternal/paternal
Chiasmata
Crossing over
Spindle
Random assortment
Fertilisation
Mutation

3. Learning outcomes Describe, with the aid of diagrams and photographs,
the behaviour of chromosomes during meiosis,
the associated behaviour of the nuclear envelope, cell membrane and centrioles.
(Names of the main stages are expected, but not the subdivisions of prophase)
Pages 118-9

4. Reproduction and variation
All living organisms can reproduce, this can be:
Asexual reproduction
Single organism divides by mitosis
New organism is genetically identical to the parent
Genetic variation can only be by mutation
Mitosis (in eukaryotes) or binary fission (in prokaryotes).
Sexual reproduction
Meiosis produces haploid gametes
Which fuse at fertilisation to form a diploid zygote
This produces genetic variation amongst offspring

5. Human Life Cycle Diploid Zygote Mitosis 46 Haploid Sperm 23 Meiosis
Adult 46
Haploid
Egg 23
fertilisation

6. Eukaryotic chromosome structure
Chromosomes in a non-dividing cell (unduplicated) exist as single-armed structures (each is equivalent to one of the chromatids in a metaphase chromosome prepared for cell division)
It would not be visible as a coiled structure but would be unwound
Metaphase (duplicated) chromosome has 2 sister chromatids. Each chromatid contains an identical copy of the genetic material. (DNA molecule)

7. Chromosome structure
Chromatid vs. Chromosome: Remember that when two DNA molecules are joined together, each molecule is called a chromatid. When a DNA molecule (and proteins) is not attached to another one then that single molecule of DNA is not a chromatid but an unduplicated chromosome
Chromatin: During certain times of the cell's life cycle the chromosomes are not visible. This is because the chromosomes are stretched out very thin to allow surfaces for the various chemical reactions that involve chromosomes to take place. When the nucleus is stained and examined, it appears uniformly coloured and the chromosomes collectively are termed chromatin.

8. Read the worksheet and complete the definitions
Task 1: 6 minutes
Read the worksheet and complete the definitions

9. Homologues
Sexually reproducing organisms in nearly all cases have paired sets of chromosomes, one set coming from each parent (maternal/paternal)
The equivalent chromosomes that form the pair are called Homologous pairs/homologues
Each homologue carries an identical assortment of genes, but the version of the gene, the allele, from each parent may differ

10. Alleles A pair of homologous chromosomes centromere
Some basic points… showing that different versions (alleles) occupy the same position (locus) on a chromosome.
A pair of homologous chromosomes
centromere
Paternal chromosome, originating from the sperm T
Locus of gene for height (dominant allele T = tall)
Maternal chromosome, originating from the egg t
Locus of gene for height (recessive allele t = short)

11. Meiosis Meiosis is a reduction division Meiosis has two divisions
Resulting daughter cells have half the original number of chromosomes
Daughter cells are haploid
Can be used for sexual reproduction
Source of genetic variation
Meiosis has two divisions
Meiosis I and Meiosis II
Each division has 4 stages
Prophase, metaphase, anaphase, telophase
(PMAT x2)

12. An animation of Meiosis at http://www.cellsalive.com/meiosis.htm
Just watch first
Then again….Let’s go!!!!

13. Meiosis –
During interphase the DNA replicates forming two sister chromatids joined at the centromere
PROPHASE 1:
The chromatin condenses, supercoils and become visible.
The homologues pair up in a process called synapsis to form bivalents
One of the chromosomes is maternal and one is paternal.
The chromosomes in a bivalent may cross over at points called chiasmata where they may swap sections of DNA/alleles.
The nucleolus disappears and the nuclear envelope breaks down.

14. Prophase 1
This swapping of genetic information is called crossing over.
The centriole divides (animal cells only) and starts migrating to opposite poles of the cell.
A spindle forms (protein microtubules) produced by centrioles lining up at the equator.
Prophase 1 can last days, months, years!!

15. Meiosis I
METAPHASE 1:
The bivalents randomly line up on the equator. (random assortment)
The spindle fibres attach to them at their centromeres.
Each member of the homologous pair facing opposite poles
This allows for independent assortment

16. Meiosis I
ANAPHASE 1:
The homologous chromosomes of each bivalent are pulled apart by the spindle fibres contracting, towards the poles. (independent assortment)
The chiasmata separate as each homologous pair moves towards it separate pole.

17. Meiosis I
TELOPHASE 1:
In animal cells the chromosomes will de condense a little and the nuclear envelope will reform.
Cytokinesis will occur and the cell will split.
However most plant cells progress straight to Meiosis II 

18. Meiosis I Prophase I Metaphase I Anaphase I Telophase I
Chromatin condenses
Homologous pairs form a bivalent. Crossing over occurs
Nucleolus disappears
Spindle forms
Metaphase I
Bivalents line up on equator of cell
Attached by centromere
Random
Anaphase I
Homologous chromosome in each bivalent are pulled to opposite poles by spindle contracting
Telophase I
Two new nuclear envelopes form
Cell divides by cytokinesis
Each cell has???

19. Meiosis II: mitotic division
PROPHASE 2:
Everything from now on happens in two cells.
The chromosomes condense.
The centrioles replicate and the spindle starts to develop perpendicular to the previous division.
The nuclear envelope disintegrates.

20. Meiosis II
METAPHASE 2:
The duplicated chromosomes randomly line up on the equator.
They are attached to the spindle fibres at their centromeres.

21. Meiosis II
ANAPHASE 2:
The centromeres divide to allow the chromosomes to split.
The chromatids (now called chromosomes, just to confuse things) are pulled to the poles by the contracting spindle fibres
The chromatids randomly segregate

22. Meiosis II
TELOPHASE 2:
The chromosomes reach the poles and de condense.
The nuclear envelope reforms, forming four separate nuclei.
Cytokinesis may now occur and the cells divide.
Each cell has half the number of chromosomes as the original parent cell, the cells are haploid.

23. Meiosis II Metaphase II Prophase II Anaphase II Telophase II
Duplicated chromosomes arrange themselves on equator
Attach by centromere to spindle fibres
Random arrangement of chromatids
Prophase II
Nucleolus disappears
Chromosomes condense
Spindle forms
Anaphase II
Centromeres divide
Chromatids pulled apart to opposite poles by contracting spindle
Telophase II
Nuclear envelopes reform around haploid nuclei
Cell divides by cytokinesis
4 daughter cells formed

24. Let’s watch??: https://www.youtube.com/watch?v=16enC385R0w
Learning outcome states diagrams and pictures: SORT in pairs! (Homework!)
Recap on some definitions
Homologous pairs
Bivalent
Loci
Maternal chromosomes
Paternal chromosomes

25. Mitosis
Meiosis
No. of Divisions
One
Two
Products
Two genetically identical daughter cells
Four cells, genetically different from each other & parent cells
Chromosome number
Maintained
Halved
Bivalents formed? No
Yes
Crossing over?