Cellular Control

1. Cellular Control Unit 1 Communication, Homeostasis and Energy

2. Meiosis Module 1: Cellular Control

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);

4. Reproduction and variation • Asexual reproduction • Single organism divides by mitosis • New organism is genetically identical to the parent • 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 46 Mitosis Haploid Sperm 23 Meiosis Adult 46 Haploid Egg 23 fertilisation

6. Self assessment questions • The fruit fly Drosophila melangaster has eight chromosomes in its body cells. How many chromosomes will there be in a Drosophila sperm? • The symbol n is used to indicate the number of chromosomes in one set – the haploid number of chromosomes. For example in humans n = 23, in a horse n = 32. • How many chromosomes are there in a gamete of a horse? • What is the diploid number of chromosomes (2n) of a horse?

7. Meiosis • Meiosis is a reduction division • 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

8. Meiosis • You can view an animation of Meiosis at http://www.cellsalive.com/meiosis.htm

9. Meiosis I

10. Early Prophase 1

11. Late Prophase 1

12. Metaphase 1

13. Anaphase 1

14. Telophase 1

15. Cytokinesis 1

16. Meiosis II

17. Prophase II

18. Metaphase II

19. Anaphase II

20. Telophase II

21. Cytokinesis II

22. Learning outcomes • explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles

23. Key words • Allele • Locus • Crossing over • Maternal chromosome • Paternal chromosome

24. Alleles, locus and homologous chromosomes

25. Meiosis and variation • Meiosis enables sexual reproduction to occur by the production of haploid gametes. • Sexual reproduction increases genetic variation • Genetic variation increases the chances of evolution through natural selection

26. Meiosis and Variation • Crossing over – prophase I • Independent assortment of chromosomes – metaphase I • Random assortment of chromatids – metaphase II • Random fertilisation • Chromosome mutations • Number of chromosomes • Non-disjunction - polysomy or polyploidy • Structure of chromosomes • Inversion, deletion, translocation

27. Crossing over

28. During metaphase I

29. During metaphase I

30. No crossing over

31. Crossing over – new combinations of alleles

32. Independent Assortment

34. Learning Outcomes • explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive; • explain the terms linkage and crossing-over;

35. Gene Locus Allele Genotype Phenotype Heterozygous Homozygous Monohybrid cross Dominant allele Recessive allele Glossary

36. Genetics • Genetics is the study of inheritance • Allele • different varieties of the same gene • Locus • position of a gene on a chromosome

37. Genetics • Dominant • An allele whose effect is expressed in the phenotype if one copy present • Recessive • An allele which only expresses as a homozygote • Co-dominant • Both alleles have an effect on the phenotype

38. Genotype • genetic constitution of the organism • Phenotype • appearance of character resulting from inherited information

39. Homozygous • Individual is true breeding • Possesses two alleles of a gene e.g. RR or rr • Heterozygous • Two different alleles for a gene e.g. Rr

40. Monohybrid inheritance • Mendel’s First Law • principle of segregation “The alleles of a gene exist in pairs but when gametes are formed, the members of each pair pass into different gametes, thus each gamete contains only one of each allele.”

41. Inheritance of height in pea plants • Follow out the following cross to the F2 generation • Homozygous tall pea plant with a homozygous dwarf pea plant • Write out the genotypic and phenotypic ratios from the F2 generation

42. Inheritance of height in pea plants • Laying out the cross • P phenotype • P genotype • Gametes • F1 genotype • F1 phenotype • F1 self-fertilised • Gametes • Random fertilisation • F2 genotypic ratio • F2 phenotypic ratio

43. Pupil Activity • Answer the questions on monohybrid inheritance • Remember to write out each cross in full.

44. Cystic Fibrosis • Cystic Fibrosis is caused by a mutation to a gene on one of the autosomes. • Mutation • Changes the shape of the transmembrane chloride ion channels (CFTR protein) • The CFTR gene is found on Chromosome 7 • The faulty gene is recessive

45. Genetic Cross conventions • Use symbols to represent two alleles • Alleles of the same gene should be given the same letter • Capital letter represents the dominant allele • Small letter represents the recessive allele • Choose letters where the capital and small letter look different • The examiner needs to be in no doubt about what you have written

46. Inheritance of cystic fibrosis • Three possible genotypes • FF unaffected • Ff unaffected • ff cystic fibrosis • Remember gametes can only contain one allele for the CFTR gene • At fertilisation, any gamete from the father can fertilise any gamete from the mother • This can be shown in a genetic diagram

47. Genetic diagram showing the chances of a heterozygous man and a heterozygous woman having a child with cystic fibrosis.

48. Phenotype ratio of offspring • Genotype ratio 1FF:2Ff:1ff • Phenotype ratio 3 unaffected:1cystic fibrosis • Can also be expressed as • 25% chance of the child having cystic fibrosis • Probability of 0.25 that a child will inherit the disease • Probability that 1 in 4 that a child from these parents will have this disease.

49. Learning Outcome • Use genetic diagrams to solve problems involving sex-linkage and codominance.

50. Sex-Linkage • Sex-linked genes are genes whose loci are on the X or Y chromosomes • The sex chromosomes are not homologous, as many genes present on the X are not present on the Y. • Examples • Haemophilia • Fragile X syndrome • Red green colour blindness