"A portion of a DNA molecule that serves as the basic unit of heredity"
Gene (pre-genomics)
"The portion of a DNA molecule which produces one enzyme" later "one polypeptide"
Gene (post-genomics)
"A union of genomic sequences encoding a coherent set of potentially overlapping functional products"
One-gene one-enzyme hypothesis
Based on Crick's central dogma of molecular biology
The central dogma states that once information has passed into proteins it cannot get out again
Archibald Garrod studied alkaptonuria
1902
Alkaptonuria
Urine turns black on exposure to air
Tendency to develop arthritis later in life
Accumulation of alkapton (homogentisic acid) in the body is an error of metabolism</b>
Alkaptonuria is normally present in several members of a family
More common in children of 1st cousin marriages
Alkaptonuria
The trait is recessive
Garrod and William Bateson concluded alkaptonuria is genetically controlled
The pathway that is affected in alkaptonuria is the phenylalanine-tyrosine metabolic pathway
Phenylalanine is an amino acid that we can't make ourselves and so have to get it from our diet
The mutation for alkaptonuria is recessive and it's on chromosome three
One-gene one-enzyme hypothesis
1942 - experiments on red bread mould showed direct relationship between genes and enzymes
Neurospora crassa
Mycelial fungus
Haploid "n" (-> can see effects of mutations directly)
Short life cycle
Easy to propagate in lab
Wild type (WT) Neurospora crassa
Can grow both on complete medium and on minimal medium
Called "prototrophic" because it can synthesize most of its own components needed for growth even from minimal medium
Mutating Neurospora crassa spores
1. Expose spores to x-rays
2. Let spores grow up
3. Transfer offspring of x-ray spores to complete medium
4. Transfer to minimal medium
5. Select spores that can't grow on minimal medium (= nutritional mutants)
Nutritional mutants
Called "auxotrophic" as they need "help" to grow
Identifying mutants
1. Put mutant in medium with vitamins but it still can't grow
2. Put mutant in minimal medium with each of the 20 amino acids and the mutant is rescued
3. Find the mutant is rescued by arginine
Genetic dissection of a biochemical pathway
Each step of biochemical pathway is catalysed by an enzyme
Grow mutant strains on media supplemented with various nutrients
Use growth response to work out the biochemical pathway
The further along the pathway the mutant strain is "blocked", the fewer intermediate compounds it needs to grow
Many genetically based enzyme deficiencies in humans provide further evidence that many genes code for enzymes
Phenylketonuria (PKU)
1/12000 Caucasians
Mutations in gene for phenylalanine hydroxylase
Patients cannot breakdown phenylalanine
Accumulation of phenylpyruvic acid affects CNS
Newborns screened for PKU using Guthrie test, can be completely managed by avoiding intake of phenylalanine
Sickle cell anaemia
Affects haemoglobin
Sickle red blood cells in low oxygen
Sickle-cell trait (SCT) is a milder form of the disease, genotype is one normal haemoglobin and one sickle cell haemoglobin variant
The problem in sickle cell anaemia is a mutation where there's a substitution of glutamic acid with valine, which changes the shape of the red blood cells
Transcription by RNA polymerases requires access to the DNA
RNA genes never become proteins
Transposable jumping elements and tandemly repeated DNA are examples of non-protein coding genes
The mediator complex binds to activator and promotor sequences which can be thousands of bases away from the mRNA-encoding transcript, questioning the nature of what a gene actually is