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