Transcript document
Genetic Control Chapter 29 Control of gene action • Genes code for the synthesis of proteins • Some genes can be ‘switched off’ or ‘on’ • E.g. B-galactosidase lactose glucose + galactose • In E.coli, enzyme only produced if lactose is present • Gene for B-galactosidase is switched on if lactose is present • Switched off in absence of lactose • This is enzyme induction • Glucose can then be used to aid respiration Jacob-Monod hypothesis • • • • • • • • • • Operon area of gene activity on a DNA strand contains structural genes - these code for the protein (e.g. enzyme) & an operator - controls the structural gene Operator is affected by a repressor molecule If the repressor is joined to the operator, the operon is switched off The repressor is coded for by a regulator gene Repressor will not bind to the operator gene if it is already bound to an inducer • This control prevents waste of cell resources • E.g. ATP & amino acids Role of genes in cell metabolism • Metabolism – all the chemical processes that occur in an • • • • • • • organism A metabolic pathway (e.g. respiration) – several stage process Each stage controlled by an enzyme One metabolite converted to another However, if mutation occurs, enzymes not produced Pathway not complete, intermediate metabolites can accumulate This is an inborn error of metabolism Sufferers are usually homozygous recessive Inborn errors of metabolism • Phenylalanine & tyrosine – amino acids obtained from protein in the • • • • • • • • • • • • • diet An enzyme converts phenylalanine to tyrosine Phenylketonuria (PKU) Affected person lacks allele of gene to make enzyme 1 Phenylalanine cant be converted to tyrosine Undergoes alternative metabolic pathway – produces toxins (affect brain cells) Occurs in 1 in 10,000 All babies now screened – blood tested for excess phenylalanine If disorder is present, can be managed by diet control Possible genetic cure in the future Albinism Lack of enzyme 3 No melanin produced Pink skin, pale iris, white hair Genetic control of differentiation • Every cell has all the genes for constructing the whole • • • • • • • • • • • organism After differentiation, groups of cells specialise Only certain types of genes continue to operate Reversibility Amphibian: -Fully differentiated nucleus transplanted into a fertilised egg (nucleus removed) -Egg can grow into a fertile adult Dolly the sheep: - Nucleus from udder cell transferred into an egg (no nucleus) - Egg transplanted into surrogate mother - ‘Dolly’ identical to the ewe where the original udder cell came from Illustrate that cell genes can be switched on again Two categories of genes • • • • • • • • First group – switched on in all cells - code for vital metabolites Second group - only switched on in a particular type of cell - code for proteins characteristic of that cell type E.g. insulin from pancreas cells - many proteins may cause specific modifications to occur • All other genes permanently switched off Genetic control of cell differentiation • Blood • - red and white cells come from an original • • • • • undifferentiated ‘stem’ cell Parenchyma - large, thin walled, vacuolated cells - contain plastids (formed from proplastids) - many types – features depend on which genes are switched on - relatively unspecialised (potentially meristematic) Genetic control of leaf shape • Leaf starts as a primordium • Produced by an apical meristem • Leaf blade develops from meristematic cells at edge of primoridum • Equal activity of these cells produces regular shaped leaves e.g. privet • Unequal activity – irregular shape e.g oak