Module 1

1. Module 1 Cellular Control & Variation

2. Glossary • Gene • Codes for polypeptides, including enzymes • Genetic code • The sequence of nucleotide bases on a gene, codes for the construction of a proteins • Allele • Alternative version of a gene • Locus • Specific position on a chromosome, occupied by a specific gene • Phenotype • Observable characteristics of an organism • Genotype • Alleles present within cells of a individual for a characteristic

3. Glossary • Dominant • Characteristic in which the allele responsible is expressed in the phenotype even in heterozygous • Codominant • Characteristic where both alleles contributes to the phenotype • Recessive • Characteristic in which allele responsible is only expressed in phenotype if there is no dominant allele present • Linkage • Genes for different characteristics that are present at different loci on the same chromosome are linked • Crossing over • Where non sister chromatids exchange alleles during prophase I of meiosis

4. Transcription • DNA unzips • Exposing genes • RNA nucleotide align with DNA • U with A, A with T, C with G and G with C • RNA polymerase • mRNA formed • Leaves through nuclear pore

5. Translation • mRNA attaches to ribosome • tRNA brings amino acid • Each tRNA attached to a specific amino acid • tRNA binds to mRNA with complementary anticodon • Peptide bond forms between amino acids • DNA sequence determines amino acid sequence

6. Mutations • Can cause changes to sequences of nucleotides in DNA molecules • Mutations can effect the way a proteins functions • Beneficial • E.g. change in skin pigment better when humans migrated North • Harmful • Insertions/deletions can cause a frameshift (change all the subsequent amino acids) • Neutral • Point mutation: one base changes, may not change the amino acid coded for

7. Cyclic AMP • Activates proteins by altering their 3D structure

8. Lac operon • Normally repressor substance bound to operator • Prevent RNA polymerase from binding (to promoter) • Lactose binds to repressor • Changes shape of protein molecule • Unable to bind (operator) • RNA polymerase binds (at promoter) and gene is switched on

9. Body Plan • Genes that control development of body plans are similar in plants, animals and fungi • Homeobox sequence • Clusters known as hox genes • Where body segments go • Which way round they are • Drosophilia experimented on • Two hox clusters one for head and thorax and one for thorax and abdomen

10. Apoptosis • Programmed cell death • Undergo 50 mitotic divisions • Biochemical events cause tidy cell death • Enzymes break down cytoskeleton • Cytoplasm becomes dense • Cell surface membrane change • DNA fragments • Cell breaks into vesicles and taken up by phagocytosis • Acts as a mechanism to change body plans

11. Prophase I Chromosomes Chromatin condenses and shortens Homologous pairs, pair up (forms bivalents) Chiasmata: crossing over Nuclear envelope Disintegrates Centrioles Spindle forms Metaphase I Chromosomes Bivalents line up on equator Centrioles Spindle fibres bind to centromeres Meiosis I

12. Anaphase I Chromosomes Homologous chromosomes separated Centrioles Spindle fibres pull chromosomes to opposite poles Telophase I Chromosomes Chromosomes uncoil Nuclear envelope 2 nuclear envelopes form around each set of chromosomes Cell membrane Cells divide by cytokinesis Meiosis I

13. Prophase II Chromosomes Chromosomes condense Nuclear envelope Disintegrates Centrioles Spindles form Metaphase II Chromosomes Line up on equator Centrioles Spindle fibres attach to centromere Meiosis II

14. Anaphase II Chromosomes Centromeres divide Chromatids go to opposite poles Centrioles Spindle fibres contract Telophase II Chromosomes uncoil Nuclear envelope Reforms around haploid daughter nuclei Cell membrane Cytokinesis to make 4 haploid cells Meiosis II

15. Meiosis and Fertilisation • Lead to variation through the independent assortment of alleles • Meiosis • Crossing over in prophase I shuffles alleles • Random distribution of chromosomes in homologous pairs in meiosis I • Random distribution of sister chromatids in meiosis II • Random mutation • Fertilisation • Random which sperm fertilises the egg

16. Codominant • CRCW have both red and white hair • Blood groups also codominant e.g. IAIB

17. Sex Linkage • Gene that codes for it is found on a sex chromosome • E.g. haemophilia, duchenne muscular dystrophy, red-green colour blindness

18. Epistasis • Interactions between loci so that one gene masks the expression of another or complement each other • Recessive epistasis • Homozygous recessive will mask the phenotype produced at another loci • AaBb or Aabb = purple/pink • BUT aaBb or aabb = white • Ratio 9:3:4 (purple, pink, white) • Dominant epistasis • Heterozygous or homozygous dominant will mask the phenotype produced at another loci • Ratio 12:3:1 (white, yellow, green)

19. Chi squared test • Significance of the difference between observed and expected results • Start by looking at ratio of offspring expected • Not always going to happen exactly in nature • X2 = sum (observed number –expected number) expected number • Probability= differences are due to chance, reject or accept hypothesis • Degrees of freedom = number of phenotypes (classes of data) – 1 • Differences in observed and expected can be explained by linkage and epistasis

20. Variation • Difference between continuous and discontinuous variation • Discontinous • Qualitative differences, distinguishable categories • Continuous • Quantitative traits, no distinguishable categories, range of values between 2 extremes

21. Basis of continuous & discontinuous variation • Reference to the number of genes which influence the variation • Discontinuous • Different alleles at single gene locus have large effect • Different gene loci have different effects on a trait • Continuous • Different alleles at single gene locus have small effects • Different gene loci have the same effect on trait • A large number of gene loci (polygenes) may have a combined effect on the trait

22. Phenotypic Variation • Genotype and environment contribute • VP = VG + VE • Environment-may allow full genotypic potential to be reached or restrict it in some way • Cannot be inherited • E.g curled wing Drosophila must be kept at 27C for last day of pupal life • Colouring of rabits- pigment only expressed at cooler temperatures found at extremities e.g nose, feet, ears

23. Variation is essential to selection • Explain why • Gives them an advantage over other individuals with different phenotypes • More likely to survive, reproduce and pass on this gene

24. Hardy-Weinberg Principle • Calculate allele frequencies in populations • p+q=1 • p = dominant allele • q = recessive allele • p2 + 2pq + q2 = 1 (whole population) • q2 = recessive genotype • 2pq = heterozygotic genotype • p2 = dominant genotype • Does not account for: • selection pressure against one genetype • Non-random mating • Migration of individuals

25. Environmental factors can act as stabilising or evolutionary forces of natural selection • Explain with examples • Stabilising • Maintains existing adaptations so maintains existing allele frequencies • Usually seen when heterozygotes have selection advantage over homozygous e.g. sickle cell anaemia in malaria areas • Directional • Alters allele frequencies e.g. peppered moths • New allele is an advantage and is selected for

26. Genetic Drift • Can cause large changes in small populations • Explain • Change in allele frequency that occurs by chance • Seen more when small number of individuals are isolated from the rest of the population • Genetic drift in small population is known as founder effect

27. Isolating Mechanisms • Role in evolution of new species • Ecological (geographical) e.g river mountain range • Seasonal (temporal) e.g climate change • Reproductive e.g. members no longer able to physically mate, genitals incompatible or courtship ritual has changed

28. Species • Biological Species Concept • A group of similar organisms that can interbreed to produce fertile offspring • Becomes problematic when biologists want to classify • Phylogenetic (cladistic/evolutionary) species concept • Group of organisms that have similar morphology, physiology, embryology and behaviour • Based on DNA, RNA and proteins • Can include extinct species

29. Natural Selection and Artificial Selection • Compare and Contrast

30. Artificial Selection • Modern Dairy Cow • Fertility, milk yield and milk quality • Females- performance testing for milk yield • Males- progeny testing to see which bulls produce daughter with high milk yield • Can increase offspring with desired features through artificial insemination and embryo transfer

31. Artificial Selection • Bread Wheat (Triticum aestivum) • Number of different species contributed • Able to undergo polyploidy (contain more than one diploid set of chromosome) • 14  28  21  42 (no. of chromosomes after each selection) • Traits selected for increased yield, shorter stalks and disease resistance