Chapter 17 From Gene to Protein 1 Chapter 17 From Gene to Protein The Flow of Genetic Information Inherited traits are determined by genes The trait of albinism is caused by a recessive allele of a pigmentation gene The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins and of RNA molecules involved in the synthesis proteins are the link between genotype and phenotype Gene expression is the process by which DNA directs the synthesis of proteins or in some cases just RNAs The expression of genes that code for proteins includes two stages transcription and translation Concept 17 1 Genes specify proteins via transcription and translation Evidence from the Study of Metabolic Defects o o o o o Beadle and Tatum shared a Nobel Prize in 1958 for Their discovery that genes act by regulating definite chemical events Not all proteins are enzymes Many proteins are constructed from two or more different polypeptide chains and each polypeptide is specified by its own gene Beadle and Tatum s idea was therefore restated as the one gene one polypeptide hypothesis o Basic Principles of Transcription and Translation the nucleic acid RNA Genes provide the instructions for making specific proteins The bridge between DNA and protein synthesis is RNA is chemically similar to DNA except that it contains ribose instead of deoxyribsose as its sugar and has the nitrogenous base uracil rather than thymine Each nucleotide along a DNA strand has A G C or T as its base Each nucleotide along an RNA strand has A G C or U as its base An RNA molecule usually consists of a single The monomers in DNA and RNA are the four types of nucleotides which differ in their nitrogen bases Each polypeptide of a protein also has monomers arranged in a particular linear order but its monomers are Thus nucleic acids and proteins contain information written in two different chemical languages Getting from DNA to protein requires two major stages transcription and translation Transcription is the synthesis of RNA using information in the DNA The two nucleic acids are written in different forms of the same language and the information is simply transcribed or rewritten from DNA to RNA Transcription is the general term for the synthesis of any kind of RNA on a DNA template Messenger RNA mRNA carries a genetic message from the DNA to the protein synthesizing machinery of the strand amino acids cell Translation is the synthesis of a polypeptide using the information in the mRNA During translation the cell must translate the nucleotide sequence of an mRNA molecule into the amino acid sequence of a polypeptide Ribosomes are the side of translation They are complex particles that facilitate the orderly linking of amino acids into polypeptide chains Transcription and translation occur in all organisms both those that lack a membrane bound nucleus Because bacteria do not have nuclei their DNA is not separated by nuclear membranes from ribosomes and other protein synthesizing equipment In eukaryotic cells the nuclear envelope separates transcription from translation in space and time Transcription occurs in the nucleus and mRNA is then transported to the cytoplasm where translation occurs The transcription of a protein coding eukaryotic gene results in pre mRNA and further processing yields the Chapter 17 From Gene to Protein 2 finished mRNA translated into protein o The Genetic Code Primary transcript is the initial RNA transcript from any gene including those specifying RNA that is not Summary genes program protein synthesis via genetic messages in the form of messenger RNA Cells are governed by a molecular chain of command with a directional flow of genetic information DNA RNA Protein There are only four nucleotide bases to specify 20 amino acids Triplet code is the genetic instructions for a polypeptide chain are written in at the DNA as a series of non overlapping three nucleotide words in mRNA which is then translated into a chain of amino acids Template strand provides the pattern or template for the sequence of nucleotides in an RNA transcript The gene determines the sequence of nucleotide bases along the length of the RNA molecule that is being synthesized functions as the template For any given gene the same strand is used as the template every time the gene is transcribed For other genes on the same DNA molecule however the opposite strand may be the one that always An mRNA molecule is complementary rather than identical to its DNA template because RNA nucleotides are assembled on the template according to base pairing rules In DNA base pairing A pairs with T and C pairs with G and vice versa During translation U replaces T and pairs with A therefore U pairs with A or G A pairs with only U C pairs with only G and G pairs with C or U U Uracil T Thymine A Adenine G Guanine C Cytosine Like a new strand of DNA the RNA molecule is synthesized in an antiparallel direction to the template strand of DNA The template DAN strand runs 3 5 The second strand that binds to the template runs 5 3 The transcription strand replaces the second strand and also runs 5 3 Codons are the mRNA nucleotide triplets that are customarily written in the 5 3 direction The term codon is also used for the DNA nucleotide triplets along the non template strand Each codon specifies which one of the 20 amino acids will be incorporated at the corresponding position along a polypeptide The number of nucleotides divided by 3 gives the number of amino acids in the chain o o 3 of the 64 possible combinations of codons do not have designated amino acids They are called stop signals or termination codons marking the end of translation AUG has a dual function it codes for the amino acid methionine Met and also functions as a Start signal or initiation codon Genetic messages usually begin with the mRNA codon AUG which signals the protein synthesizing machinery to begin translating the mRNA at that location DNA controls metabolism by directing cells to make specific enzymes and other proteins via the process of gene expression Beadle and Tatum s studies of mutant strains of Neurospora led to the one gene one polypeptide hypothesis Genes code for polypeptide chains or specify RNA molecules Transcription is the synthesis of RNA complementary to a template strand of DNA providing a nucleotide to nucleotide transfer of information Translation is the synthesis of a polypeptide whose amino acid sequence is specified by the nucleotide