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 oneenzyme
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
Garrod studied alkaptonuria, a human disease
1902
Alkaptonuria is characterised by urine that turns black on exposure to air and the tendency to develop arthritis later in life
Accumulation of alkapton in the body is homogentisic acid
Alkaptonuria is normally present in several members of a family and is more common in children of 1st cousin marriages
Alkaptonuria is a recessive trait
Garrod and William Bateson concluded alkaptonuria is genetically controlled
The pathway that is affected in alkaptonuria is the phenylalanine-tyrosinemetabolic 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
The position of a block in a metabolic pathway can be determined by the accumulation of the chemical compound that precedes the blocked step
One-gene one-enzyme hypothesis
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
Unmatured spores that can grow on both complete medium and minimal medium
Mutate 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. Some spores can't grow on minimal medium due to mutations
Mutant spores
Can grow on complete medium but not on minimal medium (= nutritional mutants)
Auxotrophic
Mutants that need "help" to grow
Identify mutants
1. Put mutants in medium with vitamins but they still can't grow
2. Put them in minimal medium with each of the 20 amino acids and the mutant is rescued
3. Divide up again to 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 are screened for PKU using Guthrie test and it 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 which is a negative electric charge, changing it to valine which has no electric charge
This causes the beta pleated sheets to fold in a different way and this caused the odd shape of the red blood cell in sickle cell anaemia
Non-protein coding genes
RNA genes never become proteins
Transposable jumping elements
Tandemly repeated DNA
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
Transcription by RNA polymerases requires access to the DNA
Every cell cycle the entire genome must be replicated precisely, and replication and cell division are not independent
Watson and Crick's model of DNA structure
Two polynucleotide chains wound around each other in a right-handed double helix
Two chains have opposite polarity
The sugar-phosphate backbone is on the outside, with bases in the centre
Bases are bonded together with hydrogen bonds, A with T and G with C
Bases are 0.34 nm
There is a major groove and a minor groove
The structure of DNA double helix was the key to how it replicates
Parent and daughter DNA
The original DNA duplex is the parent
The newly formed duplexes are daughters
Models of DNA replication
Semiconservative
Conservative
Dispersive
Semiconservative replication
Parental duplex unwinds, each strand acts as a new template for new strand synthesis
1st generation is a mix of an old strand and a new strand
2nd generation is 50% new and 50% old/new mixed duplex
Conservative replication
Parental duplex replicates to give 1 new and 1 parental duplex in generation 1
No mixing of old and new DNA strands together
2nd generation has 3 new and 1 parental duplex
Dispersive replication
Parental duplex replicates to give duplexes consisting of new parts and parental parts
No strand is entirely new or parental
Each strand is mixed up of new/old pieces – fragmented copy of DNA