6.1 Transfer of Information from DNA. SBI4U Ms. Ho-Lau

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1 6.1 Transfer of Information from DNA SBI4U Ms. Ho-Lau

2 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

3 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

4 One Gene-One Enzyme Hypothesis George Beadle and Edward Tatum Experimental evidence used the bread mold Neurospora Crassa

5 One Gene-One Enzyme Hypothesis Minimal Medium: culture contains sugar and inorganic salts 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

6 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

7 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

8 One Gene-One Enzyme Hypothesis

9 One Gene-One Enzyme Hypothesis Conclusion: one gene codes for one enzyme. This relationship was updated to the onegene/one-polypeptide hypothesis, since not all proteins are enzymes.

10 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

11 What is the link between DNA and Proteins? Conclusion: RNA could be synthesized in the nucleus and transported to the cytoplasm to synthesize protein

12 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.

13 Gene Expression transfer of genetic information from DNA to RNA to protein. This theory is called the central dogma of genetics.

14 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

15 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?

16 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.

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18 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.

19 Determining the Genetic Code AUG: an initiator codon codes for methionine UAA, UAG, UGA: terminator codons

20 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

21 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.

22 Characteristics of Codons 3. Genetic code is universal almost all organisms build proteins with the same genetic code