6. Introducing genes Flashcards
Overview
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What is a gene?
PRE - GENOMICS
- > portion of DNA molecule serving as basic unit of heredity
- > portion producing one enzyme -> later one polypeptide
POST - GENOMICS
-> Union of
One - gene one - enzyme hypothesis
Based on Crick’s Central Dogma of molecule Biology
Proposed by Garrod
The central Dogma (Crick 1957)
DNA -> transcription into RNA -> Translation into Protein
=> Protein coding genes -> once information passed into proteins it can’t get back out
Assumption from other set of experiments done at time about finding genetic material -> proteins don’t make genes
Garrod’s “inborn errors of metabolism” enzyme deficiencies can be inherited
In 1902 Archibald Garrid, english physician, studied alkaptonuria
First evidence of relationship between genes and enzymes
=> Enzyme deficiencies can be inherited
- > normally present in members of a family
- > most common in children of 1st cousin marriages
- > Garrod and William Bateson concluded alkaptonuria genetically controlled
- > Family tree of disease
First evidence of relationship between genes and enzymes
People with Alkaptonuria excrete HA in urine (don’t have functional enzyme needed to metabolise) -> “inborn error of metabolism”
- > Alkaptonuria caused by absence of enzyme needed to metabolize HA
- > First evidence of specific relationship
The one - gene one - enzyme hypothesis
- > 1942 - experiments on red bread mould showed direct relationship between genes + enzymes
- > Neurospora crassa:
- > Mycelial fungus
- > Haploid “n” (can see effects of mutations directly)
- > short life cycle
1.) Wild type = unmutated spores
mutate spores
Spores -> X-rays resulting in some spores having random mutations in their DNA -> Offspring of x-rayed spored
- > each spore is transferred to its own tube of complete medium
- > part of each colony transferred to minimal medium
- > Nutritional mutant
- ) Mutant spores
- > select those who can still grow on complete medium but not on minimal medium (= NUTRITIONAL MUTATNT)
ID mutants
- > nutritonal mutant (can’t grow on minimal medium)
- > exclusion principle to figure out which one it is
- ) minimal medium + full set of vitamins
- ) minimal medium + 20 amino acids -> mutant rescued by amino acid mix -> mutation must block synthesis of one or more amino acids
- > mutant rescued by arganine -> mutation must disrupt arginine biosynthesis
Another example …….
Conidia (asexual spores) from each culture then tested on minimal medium
Genetic dissection of a biochemical pathway
- > each step of biochemical pathway is catalysed by an enzyme
- > Basic idea: grow mutant strains on media supplemented with various nutrients ….
- > use growth response working out biochemical pathway
- > further along - pathway mutant strain is BLOCKED - fewer intermediate compounds needed to grow
e. g. Arginine pathway
Many genetically based enzyme deficiencies in humans
-> further evidence that many genes code for enzymes
PHENYLKETONURIA (PKU)
-> 1 / 12 000 caucasians
Gene control of protein structure
-> Haemoglobin + sickle cell anaemia
PAULIN + ITANO (1949)
-> electrophoresis -> genes control protein structure
SCA affects haemoglobin
-> sickled RBC in low O2
Sickle cell trait (SCT) milder form of disease, genotype one normal haemoglobin + one sickle cell haemoglobin gene variant
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Gene control of protein structure
-> Haemoglobin + sickle cell anaemia
Structure of haemoglobin (beta polypeptide, Haema group, alpha polypeptide) -> involved in O2 binding
