SBI4U 6.1 Transfer of Information from DNA PDF

 TRANSFER OF INFORMATION FROM DNA RECALL: Inheritance of a certain gene à Expression of a particular trait HOW does a gene determine a trait? Since proteins carry out most functions in a cell, research began by looking for a relationship between genes and enzymes  TRANSFER OF INFORMATION FROM...

 TRANSFER OF INFORMATION FROM DNA RECALL: Inheritance of a certain gene à Expression of a particular trait HOW does a gene determine a trait? Since proteins carry out most functions in a cell, research began by looking for a relationship between genes and enzymes  TRANSFER OF INFORMATION FROM DNA GENES - A sequence of nucleotides in DNA that performs a specific function such as coding for a particular protein PROTEINS – Complex molecules composed of one or more polypeptide chains made of amino acids and folded into specific 3D shapes ž Determine cellular processes and physical characteristics ž Manifest genetic disorders by their absence or presence in an altered form  ESTABLISHING A LINK BETWEEN GENES AND PROTEINS ARCHIBALD GARROD (1857-1936) ž 1902 ž Proposed: Inherited human diseases were linked to biochemical pathways ž Analyzed blood and urine of alcaptonuria patients — Urine turns black when exposed to air — Colour change is due to increased levels of homogentisic acid in urine  ESTABLISHING A LINK BETWEEN GENES AND PROTEINS ARCHIBALD GARROD (1857-1936) ž Hypothesis: disease is caused by a series of biochemical reactions — Alkaptonuria is caused by a defective enzyme in the metabolic pathway that breaks down phenylalanine — Enzymes are under the control of hereditary material and an error in the hereditary material results in an error in an enzyme ○ Does one gene control all enzymes involved or does one gene control one enzyme? — A recessive inheritance factor caused the black urine phenotype ESTABLISHING A LINK BETWEEN GENES AND PROTEINS ARCHIBALD GARROD (1857-1936) Garrod hypothesized that a defective enzyme causes an “inborn error of metabolism.” If there is an accumulation of substance B, then enzyme b must be defective; if there is an accumulation of substance C, enzyme c is defective. The hereditary material directs the production of enzymes. ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM ž 1941 ž Conducted experiments to prove that ONE gene directs the production of only ONE enzyme – “One gene-One enzyme hypothesis” ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM ž Worked with the bread mould Neurospora crassa ž Normal/Wild type Neurospora crassa: CAN synthesize all required complex amino acids and vitamins required for growth if given minimal nutrient medium ž Mutant strains of Neurospora crassa - UNABLE to grow on minimal nutrient medium because they do not manufacture one or more complex compounds required for growth. They can however, grow on medium supplemented with additional nutrients — Produced by exposing Neurospora crassa to x-rays — Beadle and Tatum isolated strains that could grow when exposed to arginine ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM ž Experiment: To identify mutants that affected particular steps in the biochemical pathway ž If there is a one-to-one relationship between a gene and and enzyme, a defective gene will produce a defective enzyme ž The intermediate compound it produces will not be synthesized ž There would be no growth unless the missing intermediate was added to the medium ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM ž Placed mutant strains in various viles containing minimal medium plus one intermediate from the arginine synthesis pathway — Inferred which enzymes were defective based on whether the strain grew with each addition — Growth was only possible on media provided with an intermediate produced AFTER the step that involves the defective enzyme — They identified 4 genes (E, F, G and H) that produced a certain enzyme that catalyzed the production of an intermediate ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM ONE GENE/ONE ENZYME HYPOTHESIS GEORGE BEADLE and EDWARD TATUM - Conclusion: one gene specifies one enzyme - “one-gene/one-enzyme hypothesis” - Today referred to as the one-gene/one- polypeptide hypothesis - Not all proteins are enzymes - Enzymes can be composed of more than one polypeptide chain FINDING A MESSENGER BETWEEN DNA AND PROTEINS FREDERICK SANGER -1953 - Showed proteins consist of amino acids covalently linked together - Each protein is always made of the same sequence of amino acids - Ex: Insulin always has the same sequence of amino acids ** How is the information in DNA converted to the amino acid sequence of a protein?  FINDING A MESSENGER BETWEEN DNA AND PROTEINS Observation: In eukaryotes, genes are located on chromosomes that can only be found in the nucleus. However, protein synthesis occurs in the cytoplasm Conclusion: Proteins CANNOT be directly synthesized from DNA Supporting Evidence: RNA can be found in the nucleus and cytoplasm of eukaryotes and the concentration of RNA correlated with the level of protein production FINDING A MESSENGER BETWEEN DNA AND PROTEINS FRANCOIS JACOB AND JACQUES MONOD - 1961 - Hypothesis: a special type of RNA acts as a genetic messenger (mRNA) - Proposal: - mRNA is synthesized from the DNA of genes - mRNA base sequence is complementary to gene DNA sequence - mRNA sequence provides amino acid sequence information needed for protein synthesis FINDING A MESSENGER BETWEEN DNA AND PROTEINS FRANCOIS JACOB, SYDNEY BRENNER AND MATTHEW MESELSON - 1961 - Results: - When bacteria were infected by a virus, a virus-specific RNA molecule was synthesized and became associated with pre-existing bacterial ribosomes - New RNA molecules had a base sequence complementary to the DNA and carried the genetic information to produce the viral protein - This viral RNA molecule was newly synthesized and was not a permanent part of the bacterial ribosomes THE GENETIC CODE GENETIC CODE: A code specifying the relationship between a nucleotide codon and amino acid TRIPLET HYPOTHESIS: the genetic code consists of a combination of three nucleotides called a codon THE GENETIC CODE FRANCIS CRICK and SYDNEY BRENNER - 1961 - Generated a series of T4 bacteriophages in which the DNA sequence for a specific viral protein needed for E. coli infection was altered (nucleotides added or deleted) - Results: - When a nucleotide was added or deleted, the viral protein was not produced - Results could be reversed if the original reading frame was reinstated - Additions or deletions in groups of 3 codons resulted in bacterial infection  DETERMINING THE GENETIC CODE The genetic code is read in triplets, with no spaces in the code. It is read continuously 1. The code is redundant – more than one codon can code for the same amino acid 2. The code is continuous – the code is read in groups of 3 without stopping 3. The code is nearly universal – most organisms use the same codons to code for the same amino acids The code is read on the mRNA GENE EXPRESSION The synthesis of a protein based on the DNA sequence of a gene CENTRAL DOGMA OF GENETICS: Genetic information flows from DNA to RNA to protein TRANSCRIPTION AND TRANSLATION Protein Synthesis occurs in 2 steps: 1. Transcription 2. Translation TRANSCRIPTION AND TRANSLATION 1) TRANSCRIPTION - Copying information in DNA into messenger RNA (mRNA) Transcription occurs in 3 steps: 1. INITIATION: RNA polymerase binds to the DNA at the promoter (near the beginning of the gene) 2. ELONGATION: Using the 3’ → 5’ parental strand as a template, the RNA polymerase puts together the appropriate ribonucleotides and builds the mRNA transcript 3. TERMINATION: After the RNA polymerase passes the end of the gene, it recognizes a signal to stop transcribing (i.e. TATAAT or TTGACA). The mRNA is then released from the DNA and exits the nucleus. TRANSCRIPTION AND TRANSLATION 2) TRANSLATION - Ribosomes use the mRNA as a blueprint to synthesize a protein composed of amino acids Translation occurs in 3 steps: 1. INITIATION: ribosome recognizes a specific sequence on the mRNA (i.e. AUG) and binds to that site 2. ELONGATION: the ribosome moves along the mRNA reading 3 nucleotides at a time. Each set of 3 nucleotides (a codon) codes for an amino acid. Transfer RNA (tRNA) delivers the appropriate amino acid and the polypeptide chain is elongated 3. TERMINATION: elongation continues until a codon is reached that signals the ribosome to stop (i.e. UAG, UAA, UGA). The ribosome then falls off the mRNA and the polypeptide chain is released