Unit 10
Genes and Gene Expression
Archibald Garrod
Garrod concluded that inherited disorders can involve specific enzymes. Certain diseases seemed to be more
prevalent in certain families. After studying the disorder, alkaptonuria, he suspected that disease was a result of
enzyme deficiencies.
Beadle and Tatum
Beadle and Tatum were able to change the genes of Neurospora
crassa using X-rays, making them non-functional. The specific defect
in each mutant was identified by growing on medium supplemented
with a carbon source (glucose), a vitamin (biotin) plus inorganic salt.
Any cell able to grow on this minimal medium must be able to
synthesize all necessary biological molecules to survive.
After exposing spores to X-rays they suspected that nutritional
mutants would result due to damaged genes which encoded functions
needed to make important biological molecules.
They first grew cultures on a rich medium, then placed subcultures of
individual cells onto minimal medium. This identified any cell that
had lost the ability to make compounds necessary for growth. They
concentrated on the ability to synthesise the amino acid arginine.
They were able to identify the biosynthetic pathway for arginine by
supplementing minimal medium with intermediates. They defined 4
genes they named argE, argF, argG and argH,
If the mutation affects the enzyme earlier in the pathway than the
supplement, then growth should be supported. They were able to
isolate a mutant strain defective for each enzyme.
They concluded that genes specify the structures of enzymes that
encodes the structure of one enzyme. One-gene/one-enzyme
hypothesis. Today many enzymes contain multiple polypeptide
subunits each encoded by a separate gene, the relationship is
generalized to the one-gene/one-polypeptide hypothesis.
Central Dogma
The conversion of genotype to phenotype requires info stored in DNA to be converted to protein. The central
dogma describes the info flow in cells from DNA to RNA to protein. DNA is transcribed to make mRNA and
then translated to make protein.
A class of viruses called retroviruses can convert RNA genome into a DNA copy
using viral enzymes reverse transcriptase. This conversion defies flow of the
, central dogma and therefore a “reverse” flow from RNA to DNA had to be incorporated into the possible flow
of information.
Transcription
Only one of the 2 DNA strands are copied, we call this the template strand or antisense strand. The strand that is
not copied is called the coding strand or sense strand.
The RNA transcribed is called messenger (mRNA) as it
carries the DNA message from the nucleus to the
ribosomes in the cytoplasm for processing.
RNA polymerase has no proofreading capacity, but RNA
is short lived, and therefore mistakes are not permanent
as in DNA replication.
Translation
RNA cannot be used as a direct template for a protein because there is no molecular basis for recognition
between amino acids and nucleotides. A transfer (tRNA) interacts with RNA by base-pairing and is covalently
bonded to an amino acid. Translation takes place on the ribosome and involved multiple kinds of RNA and
proteins:
Types of RNA
• mRNA to transport info from nucleus to cytoplasm.
• Ribosomal (rRNAs) are found in ribosomes and critical for their function.
• tRNA molecules have amino acids covalently attached to one end and an anticodon that can base-pair
with an mRNA codon at the other. The tRNA acts to interpret info in mRNA and help position the
amino acids on the ribosome.
• Small nuclear RNAs (snRNAs) are part of the machinery that is involved in nuclear processing of
eukaryotic “pre-mRNA”.
• Signal recognition particle (SRP RNA) contains both RNA and proteins. snRNAs mediate the process
whereby proteins are synthesized by ribosomes on the rough ER.
• Small RNAs include both micro-RNA (miRNA) and small interfering RNA (siRNA). These are
involved in gene expression control.
The Genetic Code:
Genetic code is read in groups of three called codons, each corresponding to an amino acid. With 4 nucleotides
(G,C,T and A) and 2 in each codon, this produces only 16 different codons which is not enough for 20 amino
acids. In the same way, using 4 nucleotides per codon would produce 256. Using 3 nucleotides results in 64
different codons which is enough.
Do genetic messages include spaces between codons or not?
Crick + demonstrated that there are not spaces. They created mutations causing a single base insertion or
deletion. They then demonstrated that combing a deletion with an insertion restored function. By selecting a
Genes and Gene Expression
Archibald Garrod
Garrod concluded that inherited disorders can involve specific enzymes. Certain diseases seemed to be more
prevalent in certain families. After studying the disorder, alkaptonuria, he suspected that disease was a result of
enzyme deficiencies.
Beadle and Tatum
Beadle and Tatum were able to change the genes of Neurospora
crassa using X-rays, making them non-functional. The specific defect
in each mutant was identified by growing on medium supplemented
with a carbon source (glucose), a vitamin (biotin) plus inorganic salt.
Any cell able to grow on this minimal medium must be able to
synthesize all necessary biological molecules to survive.
After exposing spores to X-rays they suspected that nutritional
mutants would result due to damaged genes which encoded functions
needed to make important biological molecules.
They first grew cultures on a rich medium, then placed subcultures of
individual cells onto minimal medium. This identified any cell that
had lost the ability to make compounds necessary for growth. They
concentrated on the ability to synthesise the amino acid arginine.
They were able to identify the biosynthetic pathway for arginine by
supplementing minimal medium with intermediates. They defined 4
genes they named argE, argF, argG and argH,
If the mutation affects the enzyme earlier in the pathway than the
supplement, then growth should be supported. They were able to
isolate a mutant strain defective for each enzyme.
They concluded that genes specify the structures of enzymes that
encodes the structure of one enzyme. One-gene/one-enzyme
hypothesis. Today many enzymes contain multiple polypeptide
subunits each encoded by a separate gene, the relationship is
generalized to the one-gene/one-polypeptide hypothesis.
Central Dogma
The conversion of genotype to phenotype requires info stored in DNA to be converted to protein. The central
dogma describes the info flow in cells from DNA to RNA to protein. DNA is transcribed to make mRNA and
then translated to make protein.
A class of viruses called retroviruses can convert RNA genome into a DNA copy
using viral enzymes reverse transcriptase. This conversion defies flow of the
, central dogma and therefore a “reverse” flow from RNA to DNA had to be incorporated into the possible flow
of information.
Transcription
Only one of the 2 DNA strands are copied, we call this the template strand or antisense strand. The strand that is
not copied is called the coding strand or sense strand.
The RNA transcribed is called messenger (mRNA) as it
carries the DNA message from the nucleus to the
ribosomes in the cytoplasm for processing.
RNA polymerase has no proofreading capacity, but RNA
is short lived, and therefore mistakes are not permanent
as in DNA replication.
Translation
RNA cannot be used as a direct template for a protein because there is no molecular basis for recognition
between amino acids and nucleotides. A transfer (tRNA) interacts with RNA by base-pairing and is covalently
bonded to an amino acid. Translation takes place on the ribosome and involved multiple kinds of RNA and
proteins:
Types of RNA
• mRNA to transport info from nucleus to cytoplasm.
• Ribosomal (rRNAs) are found in ribosomes and critical for their function.
• tRNA molecules have amino acids covalently attached to one end and an anticodon that can base-pair
with an mRNA codon at the other. The tRNA acts to interpret info in mRNA and help position the
amino acids on the ribosome.
• Small nuclear RNAs (snRNAs) are part of the machinery that is involved in nuclear processing of
eukaryotic “pre-mRNA”.
• Signal recognition particle (SRP RNA) contains both RNA and proteins. snRNAs mediate the process
whereby proteins are synthesized by ribosomes on the rough ER.
• Small RNAs include both micro-RNA (miRNA) and small interfering RNA (siRNA). These are
involved in gene expression control.
The Genetic Code:
Genetic code is read in groups of three called codons, each corresponding to an amino acid. With 4 nucleotides
(G,C,T and A) and 2 in each codon, this produces only 16 different codons which is not enough for 20 amino
acids. In the same way, using 4 nucleotides per codon would produce 256. Using 3 nucleotides results in 64
different codons which is enough.
Do genetic messages include spaces between codons or not?
Crick + demonstrated that there are not spaces. They created mutations causing a single base insertion or
deletion. They then demonstrated that combing a deletion with an insertion restored function. By selecting a
