1. The great gene shuffling machine (From the Greek: meioun to diminish)
MEIOSIS
The great gene shuffling machine
(From the Greek: meioun to diminish)
Ohio State University Horticulture & Crop Science
© 2016 Paul Billiet ODWS

2. The sexual reproduction life cycle
Meiosis
Fertilisation
Haploid (n)
Diploid (2n)
© 2016 Paul Billiet ODWS

3. Haploid and Diploid
Karyograms of somatic cells show paired sets of chromosomes
The origin of the pairs are the maternal and paternal chromosomes of the egg and the sperm
The number of types of chromosomes of a species is constant = n
So the diploid (paired set) = 2n
© 2016 Paul Billiet ODWS

4. Meiosis performs two functions
It halves the number of chromosomes to make haploid sets
So the chromosome numbers do not double every generation
It shuffles the genes to produce new combinations (recombinations)
© 2016 Paul Billiet ODWS

5. Meiosis and sexual reproduction
Meiosis is needed to produce sex cells (gametes) with unpaired sets of chromosomes (haploid)
Sex cells are used in fertilisation
At fertilisation two sets of genes come together to form a hybrid with a set of paired chromosomes (diploid)
The hybrid, while similar to the parents, is unique
© 2016 Paul Billiet ODWS

6. We are all hybrids
The life cycle of sexual reproduction seems like a lot of effort for no return
The result of fertilisation is the combination of genes from different individuals
So the zygote is a hybrid of the two parents
It is genetically different
It has a new combination of genes.
© 2016 Paul Billiet ODWS

7. Meiosis a two step process
Meiosis 1 is the reduction division
Meiosis 2 resembles mitosis.
© 2016 Paul Billiet ODWS

8. Meiosis 1: Early Prophase 1
Chromosomes condense
Homologous pairs linked by chiasmata (chiasma sing.).
© 2016 Paul Billiet ODWS

9. Meiosis 1: Late Prophase 1
Spindle fibres form and spread out between the centrioles.
© 2016 Paul Billiet ODWS

10. Meiosis 1: Metaphase 1 The pairs of chromosomes line up on the equator
The orientation of the maternal and the paternal chromosomes is random.
© 2016 Paul Billiet ODWS

11. Meiosis 1: Anaphase 1
Maternal and paternal chromosomes segregate (pulled separate on the spindle)
They move to opposite poles.
© 2016 Paul Billiet ODWS

12. Meiosis 1: Metaphase 1 revisited
The pairs of chromosomes could orientate in different ways.
© 2016 Paul Billiet ODWS

13. Meiosis 1: Anaphase 1 revisited
Resulting in different combinations of chromosomes
This means there are 2n combinations
In humans this means 223 or over 8 million combinations.
© 2016 Paul Billiet ODWS

14. Meiosis 2: Prophase 2 Now the cells are haploid
The chromosomes do not de-condense at the end of meiosis 1
Each chromosome has still two chromatids.
© 2016 Paul Billiet ODWS

15. Meiosis 2: Prophase 2
Spindles form again.
© 2016 Paul Billiet ODWS

16. Meiosis 2: Metaphase 2
The chromosomes line up on the spindle equator independently.
© 2016 Paul Billiet ODWS

17. Meiosis 2: Anaphase 2 The sister chromatids separate on the spindle
Each cell will receive a copy of each chromosome type (i.e. it will receive n chromatids all different)
The genes on the different chromosomes are recombined (shuffled).
© 2016 Paul Billiet ODWS

18. Meiosis 2: Telophase 2 4 haploid sex cells are produced.
© 2016 Paul Billiet ODWS

19. Crossing over During prophase 1 not only do the homologous pairs link…
…they exchange genetic material
The genes on each chromosome are not identical they may be alleles
Alleles are different versions of a gene
e.g. Ear shape gene has two alleles the ear lobe allele and the no ear lobe allele.
© 2016 Paul Billiet ODWS

20. Chiasmata
© 2016 Paul Billiet ODWS

21. Crossing over Chiasmata generate recombinants through crossing over
Crossing over also permits repairs to chromosomes using the undamaged copy as a template
Chiasmata pair up homologous chromosomes so that they segregate properly in Meiosis 1
Crossovers start where the chromosome is cut by an endonuclease.
© 2016 Paul Billiet ODWS

22. Meiosis: Anaphase 1
When the pairs are separated, the alleles of the genes on the same chromosome are recombined (reshuffled)
Genes on the same chromosome = linked genes.
© 2016 Paul Billiet ODWS

23. Meiosis: Prophase 2 Each cell is haploid (n)
The sister chromatids are no longer identical copies.
© 2016 Paul Billiet ODWS

24. Meiosis: Anaphase 2
At anaphase 2 the chromatids segregate (separate) randomly
Even greater variation is achieved in the sex cells.
© 2016 Paul Billiet ODWS

25. Meiosis: Telophase 2 Thus an infinite variety of sex cells is possible
Combined with random mating between males and females an infinite variety of individuals is conceived at fertilisation.
© 2016 Paul Billiet ODWS