Module 1 Cellular Control Variation Glossary Gene Codes

Module 1 Cellular Control & Variation

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

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

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 • m. RNA formed • Leaves through nuclear pore

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

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

Cyclic AMP • Activates proteins by altering their 3 D structure

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

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

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

Meiosis I 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 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 II 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 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 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

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

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

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 • Aa. Bb or Aabb = purple/pink • BUT aa. Bb 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)

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 • X 2 = 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

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

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

Phenotypic Variation • Genotype and environment contribute – VP = V G + V E • 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 27 C for last day of pupal life • Colouring of rabits- pigment only expressed at cooler temperatures found at extremities e. g nose, feet, ears

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

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

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

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

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

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

Natural Selection and Artificial Selection • Compare and Contrast Natural Artificial Selective agent is the total environment Selective agent is the breeder Adaptations to prevailing conditions selected for Traits of use to breeder (not necessary advantage) selected for Many different traits contributing to fitness Single trait

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

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