6.1-Transfer of Information from DNA

SBI4U1

BIG QUESTION

How does a gene determine a trait?

Gene Expression

 • Info from a gene’s DNA sequence is used to

synthesize (make) a protein. • The information flows from the gene’s DNA

sequence to RNA to protein during the process. • An altered or deleted nucleotide pair will change

the DNA sequence of a gene and affect its product

The Transfer of Information from DNA

Early studies in genetics showed a relationship between the inheritance of a gene and the expression of a trait. A question that resulted from this discovery was “How does a gene determine a trait?”

 In the early 1900s, researchers began to investigate the relationship between genes and proteins. The researchers suggested that proteins were the molecules involved in inheritance, since they carry out several key cellular functions.

One Gene/One Enzyme HypothesisIn 1941, George Beadle and

Edward Tatum looked for experimental evidence of the relationship between genes and proteins. They used the bread mold Neurospora crassa to investigate whether one gene controlled the production of one enzyme or multiple enzymes.  

• Mutant strains of N. crassa were created by exposing wild-type strains to X rays to cause changes in genes. The mutant strains that required minimal medium supplemented with the amino acid arginine were isolated. These mutants were known as arg mutants, since they required arginine to grow.

• Created mutant strains (exposed them to x-rays)– Some strains could not longer grow and were

given additional nutrients

• Isolated some mutant strains that only grew in the presence of arginine (an amino acid)– Then supplemented growth media with different

intermediates in the arginine synthesis pathway

Beadle and Tatum’s Experiment

Beadle and Tatum wanted to identify which arg mutants affected particular steps in the arginine synthesis pathway.

 They hypothesized that a defective gene would produce a defective enzyme. A defective enzyme in one of the steps of the pathway would mean that the intermediate compound it produced would not be synthesized.

 The arg mutants were grown in media supplemented with intermediates of the arginine synthesis pathway. Using this method, Beadle and Tatum isolated mutant strains that were defective at a specific step.

CONCLUSIONS?They won a Nobel Price!!!

One gene codes for one enzyme. This relationship was updated to the one-gene/one-polypeptide hypothesis, since not all proteins are enzymes

Finding a 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

THE GENETIC CODE

-Genetic code -a set of rules for determining how genetic info (in the form of a nucleotide sequence) is converted to an amino acid sequence of a protein. Recall: 4 nucleotides in RNA (A, U, G, and C) and 20 amino acids. -Triplet hypothesis- the genetic code consists of a combination of 3 nucleotides, called a codon.( ie. AUG)

Each codon codes for an amino acid.

*AUG is an initiator codon. It also codes for the amino acid methionine.** UAA, UAG, and UGA are terminator codons

Summarizing the Genetic CodeThe genetic code has three important characteristics. It is:

1) redundant: more than one codon can code for the same amino acid2) continuous: the code is read as a series of three-letter codons3) universal: almost all organisms build proteins with the same genetic code. For example, a codon in a fruit fly codes for the same amino acid as in a human.

The Central Dogma

• Gene expression refers to the transfer of genetic information from DNA to RNA to protein. This theory is called the central dogma of genetics

• The two main steps in gene expression are transcription & translation

Transcription: mRNAis synthesized based on DNA template (TAC->AUG)

Translation: ribosome assembles amino acids in sequence to synthesize protein coded by mRNA

Transcription of a DNA template produces an RNA molecule that is a copy of the genetic information. The nucleotide sequence of this RNA molecule is then translated using the genetic code so that the protein coded for by the gene is produced.

Learning Expectations...

• One-gene/One-enzyme hypothesis• Function of mRNA• Central Dogma• Genetic Code & triplet hypothessi• Know how to use the chart (do not memorize the

chart)• Transcription and Translation( more to come…)• Application Problems: converting from

DNAmRNAAmino Acids