1. Meiosis forms variable gametes CfE Advanced Higher Biology Unit 2: Organisms and Evolution

2. 2.3 Variation and sexual reproduction
Costs and benefits of sexual and asexual reproduction
Meiosis forms viable gametes
Sex determination

3. Starter questions
Name the two types of asexual reproduction in eukaryotic cells.
List the 2 disadvantages and the benefit of sexual reproduction.
What conditions would asexual reproduction be a successful reproductive strategy?
What is the ‘Red Queen Hypothesis’?

4. Key areas Describe what the term homologous chromosomes means.
Describe the process of meiosis 1 and meiosis 2.
Describe the purpose and the effect of meiosis.
Define the terms chiasmata, linked genes, chromatids, gametes and independent assortment.
Describe the differences in process and outcome of meiosis in different organisms.

5. Homologous chromosomes
A human body cell has 46 chromosomes in its nucleus.
There are two copies of each of 23 types of chromosomes.
The two chromosomes in a pair are called homologous chromosomes and they have:
the same length
the same centromere position
genes for the same characteristics at the same loci (Latin: ‘places’).

6. A closer look at the genes on human chromosome 12
A homologous pair
A closer look at the genes on human chromosome 12
Phenylalanine hydroxylase
One chain of collagen
Myosin
Potassium channel
Centromere
Chromosome 12 has between 1200 and 1400 genes.
Genes that are found on the same chromosome are called linked genes.

7. Homologous pairs can be different
From female parent
From male parent
Homologous pairs are found because one copy of each chromosome has come from the female parent and the other copy has come from the male parent.
Because they are inherited from different parents, the alleles of the genes on each homologous chromosome may be different.

8. Sets of chromosomes
A single set of chromosomes comes from the female parent in her gametes.
Another single set of chromosomes comes from the male parent in his gametes.
Each gamete cell has a single set of chromosomes and is haploid.
A cell with a full double set of homologous chromosomes is diploid.
Haploid female gamete
(1 set)
Haploid male gamete
Diploid cell
(2 sets)
YouTube: Diploid and haploid (1:55 min)

9. Meiosis forms variable gametes -Key Concepts
Homologous chromosomes are pairs of chromosomes of the same size, same centromere position and with the same genes at the same loci.
Meiosis is the mechanism by which variation is increased through the production of haploid gametes by meiosis in gamete mother cells.

10. Meiosis Meiosis reduces the number of chromosomes.
Meiosis is not a cycle.
Diploid
gamete mother cell
Chromosomes duplicate
Meiosis I
Homologous chromosomes separate
Meiosis II
Sister chromatids separate
Haploid cells
Sister chromatids
Pair of
homologous chromosomes

11. Meiosis I Pairing of homologous chromosomes
Sister chromatids
Bivalent
During interphase, the homologous chromosomes duplicate so each is now made up of two sister chromatids.
This is still a diploid cell, with two sets of homologous chromosomes (though it has four sets of genetic information).
At the start of meiosis I, homologous chromosomes pair up so that they are aligned gene by gene.
Protein strands form a complex to link the sister chromatids and the homologous pairs to form a bivalent.

12. Meiosis I Crossing over
A chiasma (Greek: ‘cross mark’) forms at a random position between the homologous pairs.
Human chromosomes usually have two or three chiasmata.
(Chiasma = singular. Chiasmata = plural.)
Chiasmata never form between sister chromatids.
Chiasma
Chiasmata allow the shuffling of sections of DNA between homologous chromosomes, a process called crossing over.
Crossing over leads to the recombination of alleles, and so helps to increase variation in the gametes.
YouTube: Recombination (3:40 min)

13. Meiosis I Alignment on the metaphase plate
Centrosome
Spindle fibres
The protein complex between all the chromatids breaks down.
The centromeres still hold the sister chromatids together.
Chiasmata still hold the homologous pair together so they can be aligned.
The nuclear membrane breaks down.
Centrosomes send out microtubules to connect with kinetochores which lie beside each centromere.
The microtubules form spindle fibres linking across the cell.
The homologous chromosomes align in the centre of the cell.

14. Meiosis I Separating homologous chromosomes
The microtubules of the spindle fibres begin to shorten.
The microtubules pull on the kinetochores so the homologous chromosomes separate to opposite ends of the cell.
The chromosomes group in each end of the cell and a nuclear membrane forms around them.
Cytokinesis separates the two cells.
The sister chromatids are no longer identical due to the crossing over.

15. Meiosis II Alignment on the metaphase plate
Each cell is haploid, with one copy of each homologous chromosome (though it has two sets of genetic information).
The nuclear membrane breaks down again.
Centrosomes again send out microtubules and bind to the kinetochores of each sister chromatid.
The chromosomes align in the centre of the cells.

16. Meiosis II Separating sister chromatids
The protein complex between the centromeres breaks down.
The microtubules of the spindle fibres begin to shorten.
The microtubules pull on the kinetochores so the sister chromatids separate to opposite ends of the cell.
After being separated, sister chromatids are called chromosomes.
The new chromosomes group in each end of the cell and a nuclear membrane forms around them.
Cytokinesis separates the two cells.

17. After meiosis Haploid cells become gametes
Meiosis produces four genetically different haploid cells.
Each cell has one copy of every homologous chromosome.
In human males, each cell develops to form a sperm cell.
In human females, it is more complex:
meiosis I occurs in the last 3 months before birth
only one of the cells develops further
after an egg cell is released from the ovary it will not undergo meiosis II until a sperm nucleus has entered
the nucleus of only one of the new cells will fuse with the sperm nucleus.

18. Stages of Meiosis Key Concepts
Stages of meiosis I include;
pairing of homologous chromosomes,
random crossing over at chiasmata resulting in exchange of DNA between homologous pairs and recombination of alleles of linked genes,
independent assortment and separation of parental chromosomes irrespective of their maternal and paternal origin.
Stages of meiosis II include;
separation of sister chromatids/chromosomes,
gamete formation.

19. Independent assortment
All diploid organisms have more than one homologous pair of chromosomes.
Homologous pairs align in the centre of the cell.
The orientation of the homologous chromosomes is irrespective of their maternal or paternal origin.
Even with just three pairs of homologous chromosomes, there are four possible alignments.

20. Chromosome combinations in gametes
At meiosis I, homologous pairs are separated irrespective of the maternal or paternal origin of the chromosome.
This leads to variation in the combinations of chromosomes found in the haploid cells at the end of meiosis II.
With three pairs of chromosomes, there are 23 = 8 combinations.
In humans, with 23 pairs, there are 223 = combinations … and crossing over shuffles pieces between chromosomes!

21. Meiosis and sexual reproduction
Meiosis produces haploid cells that are genetically variable.
Sexual reproduction uses two haploid gamete cells to make a new diploid organism.
Allows shuffling of sections of DNA between homologous chromosomes
Crossing over
Allows many combinations of chromosomes of maternal and paternal origin in the gametes
Independent assortment
Brings genetic information from two different parents together in one organism
Sexual reproduction
Two human parents can produce offspring with more than 70 million million combinations of chromosomes … without considering the effects of crossing over. We are all unique!

22. Linkage maps Linked genes stay together
Purple eye (r) and black body (g) are two alleles found on chromosome 2 of Drosophila melanogaster.
Red eyes (R) and grey body (G) are the dominant alleles.
Crossing RRGG with rrgg.
The genes are linked so all the offspring inherit one chromosome with R G and the other with r g .
What is the phenotype of the offspring?
RRGG
rrgg
Gametes
R G
Gametes
r g
OffspringR G
r g

23. Linkage maps Linked genes can recombine
The offspring have red eyes and grey bodies (see left).
Crossing these flies with rrgg (see right) produces four offspring phenotypes (shown below).
Red eye
Grey body
Purple eye
Black body
Almost all the offspring
look like the parents.
A few of the offspring
show recombinant phenotypes.
Red eye
Grey body
Purple eye
Black body
Red eye
Black body
Purple eye
Grey body

24. Recombinants are the result of crossing over
Red eye
Grey body
Purple eye
Black body
R G
r g
r g
Gametes
r g
Phenotype
Number of offspring
R G
Red eye
Grey body
113
Purple eye
Black body
122
R g 9
r G 6
Most gametes
R G
r g
r g
r g
R g
r g
Only 15 of the 250
offspring are
recombinants.
So recombination frequency is 6%.
r G
r g
A few gametes show recombination
Total = 250

25. Data for other linked genes
Repeating these types of crosses for other genes on chromosome 2 gives different recombination frequencies.
Genes used in the crosses
Recombination frequency (%)
Purple eye v. Black body 6
Purple eye v. Lobe eye 17
Vestigial wing v. Lobe eye 5
Black body v. Lobe eye 23
Chiamsata formation occurs at random positions along the chromosomes.
What does a small recombination frequency suggest about the position of the two genes?

26. Recombination frequency (%)
How to map the genes
The recombination frequency for linked genes correlates with the distance between the loci of the genes on the chromosome.
Genes used in the crosses
Recombination frequency (%)
Purple eye v. Black body 6
Purple eye v. Lobe eye 17
Vestigial wing v. Lobe eye 5
Black body v. Lobe eye 23
Black body
Purple eye
Vestigial wing
Lobe eye 6 5 17 23
Animation: Discovery of linkage maps

27. Sexual life cycle of animals
Most of the life cycle is spent as a diploid multicellular organism.
Meiosis produces genetically variable haploid cells which develop into gametes.
During fertilisation, gametes fuse their haploid nuclei to produce a diploid cell.
Mitosis then produces genetically identical diploid cells to make a multicellular organism.

28. Other sexual life cycles: Comparing the three types
Animals
Plants
Fungi & protists

29. Other sexual life cycles Plants
In mosses and ferns, mitosis occurs after meiosis and so produces a large multicellular haploid organism with differentiated cells.
Gametes are formed later by differentiation of haploid cells.
In higher plants, a tiny male haploid organism is held within a pollen grain and a tiny female haploid organism is in the ovule.
Fern image: Olegivvit / Wikimedia

30. Other sexual life cycles Fungi and most protists
Again, mitosis occurs after meiosis and produces a unicellular or multicellular haploid organism.
Gametes form later by the differentiation of the haploid cells.
The diploid zygote goes straight into meiosis to form gametes.
The malarial parasite (Plasmodium spp.) has this type of lifecycle.

31. Sexual life cycles Key Concepts
In many organisms, gametes are formed directly from the cells produced by meiosis.
In other groups, mitosis may occur after meiosis to form a haploid organism; gametes form later by differentiation.

32. Past paper questions

38. KNJML

47. Discuss meiosis under the following headings:
(i) the sequence of events;
(ii) the origin of genetically variable gametes

48. 1. occurs in gamete mother cells + in sex organs 2
1. occurs in gamete mother cells + in sex organs 2. 2 cycles of nuclear division OR state meiosis I following by II Meiosis I 3. nuclear membrane breaks down and spindle forms 4. homologous chromosomes associate in pairs 5. homologous pairs line up along equator 6. homologous chromosomes segregate/pulled apart 7. new nuclear membrane forms and cytoplasm divides 8. 2 cells with half the number of chromosomes/one set of chromosomes/one copy of genome

49. Meiosis II
9. two new spindles form, one in each cell
10. chromosomes line up singly on equator
11. chromatids separate/are pulled apart
12. new nuclear membranes form and cytoplasm divides
13. to give 4 haploid cells/gametes
(Any 7)

50. 14. crossing over occurs at chiasmata
15. chromatids break and rejoin
16. this process shuffles sections of DNA between the homologous pairs and recombining alleles
17. crossing over separate linked genes
18. chromosomes move apart irrespective of their maternal or paternal origin OR
chromosomes show Independent Assortment
(Any 3)

51. Describe how the events that occur during crossing over contribute to the production of variable gametes

52. homologous chromosomes pair (during meiosis I)
breakage and re-joining of DNA strands
at chiasmata
shuffles sections of DNA between homologous chromosomes
allows the recombination of alleles
(as) linked genes are separated