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6.1 Transfer of Information from DNA
SBI4U
Ms. Ho-Lau
Link between Genes and Proteins
Early 1900s, scientists suggested proteins were
involved in inheritance
Gregor Mendel:
Certain “factors” were responsible for the patterns
of inheritance
In 1902, Garrod suggested a link between genes
and proteins
Garrod’s Hypothesis
Alcaptonuria:
urine turns black when exposed to air
an inherited disease
a defective inheritance factor
defective enzyme in the catabolism of
amino acid phenylalanine
This conclusion demonstrated a link
between genes and proteins
One Gene-One Enzyme Hypothesis
George Beadle and Edward Tatum
Experimental evidence
used the bread mold ‘Neurospora Crassa’
One Gene-One Enzyme Hypothesis
Normal (wild-type) N.
crassa can grow on
minimal medium
They have the enzymes
necessary to produce other
molecules needed in their
diet
N. crassa treated with X
rays could not grow on
minimal medium
They do not have the
enzymes necessary to
produce other molecules
needed in their diet
Minimal Medium: culture contains sugar and inorganic salts
One Gene-One Enzyme Hypothesis
Mutant N. crassa that could grow on medium supplemented with the
amino acid arginine were isolated.
arg mutants - they required arginine to grow
One Gene-One Enzyme Hypothesis
To identify which arg mutants affected particular steps in the
arginine synthesis pathway
Hypothesis: a defective gene would produce a defective enzyme
and a defective enzyme in one of the steps of the pathway would
stop the production of the intermediate compound
Grew arg mutants in media supplemented with intermediates of
the arginine synthesis pathway
isolated mutant strains that were defective at a specific step
One Gene-One Enzyme Hypothesis
Conclusion: one gene codes for one enzyme.
This relationship was updated to the one-
gene/one-polypeptide hypothesis, since not all
proteins are enzymes.
One Gene-One Enzyme Hypothesis
What is the link between DNA and Proteins?
Genes were located on the chromosome within the DNA in the nucleus
Proteins were synthesized in the cytoplasm
Scientists found RNA both in the nucleus and the cytoplasm
Amount of RNA found in cytoplasm often correlates with the amount of protein in the cytoplasm
Conclusion: RNA could be synthesized in the nucleus and
transported to the cytoplasm to synthesize protein
What is the link between DNA and Proteins?
Messenger between DNA and Proteins
In 1953, Frederick Sanger showed that each protein had a specific amino acid sequence.
In 1961, François Jacob and Jacques Monod hypothesized that a special type of RNA, called messenger RNA (mRNA), is synthesized from DNA.
Messenger RNA is complementary to DNA and provides the amino acid sequence information for protein synthesis.
Gene Expression
transfer of genetic information from DNA to RNA to
protein.
This theory is called the central dogma of genetics.
Gene Expression
1. Transcription:
• In the nucleus, using DNA as a template, mRNA is synthesized
• mRNA is transported into the cytoplasm
2. Translation:
• in the cytoplasm, using mRNA as a template, protein is synthesized
The Genetic Code
a set of rules for determining how genetic information (nucleotide
sequence) is converted to an amino acid sequence of a protein.
four nucleotides in RNA (A, U, G, and C)
20 amino acids
Mathematically, there could not be a one-to-one relationship
between nucleotides and amino acids, nor could there be just two
nucleotides per amino acids.
How many bases correspond to one amino acid?
The Triplet Hypothesis
the genetic code consists of a combination of three nucleotides, called a codon.
Each codon codes for an amino acid.
Francis Crick and Sydney Brenner showed that the code is read in triplets.
Determining the Genetic Code
To determine the mRNA codons and their corresponding amino acids, researchers compared artificially synthesized RNA of known nucleotide sequences with the amino acid sequences of polypeptides
REMEMBER:
• The codes for protein synthesis are always interpreted through the mRNA strand.
• the thymine nitrogenous base is replaced with uracil in the mRNA sequence.
Determining the Genetic Code
AUG:
• an initiator codon
• codes for
methionine
UAA, UAG, UGA:
• terminator codons
Characteristics of Codons
1. Genetic code is redundant
More than one codon can code for a particular amino acid
eg. UUU and UUC both code for the amino acid phenylalanine.
UUA and UUG both code for the amino acid leucine
Characteristics of Codons
2. Genetic code is continuous
the code is read as a series of
three-letter codons
the mRNA is read continuously
without any spaces or pauses
There is always a start and an end
to the translation process.
Characteristics of Codons
3. Genetic code is universal
almost all organisms build proteins with the same genetic code