17.1 Genes specify proteins via transcription and translation * George Beadle and Edward Tatum worked together with mutated (Neurospora crass) bread mold to figure out that they were missing a specific enzyme (gene) that catalyzed and synthesized a pathway required. They concluded that they were missing that enzyme because it was lacking the amino acid that coded for the enzyme, thus was mutated and incapable of growing. Led to the one enzyme-one gene hypothesis. The Products of Gene Expression: A Developing Story
* Revisions made: not all proteins are enzymes
* Because proteins that are not enzymes are nevertheless gene products, molecular biologists began to think in terms of one gene-one protein. * Beadle and Tatum’s idea was restated as “one gene-one polypeptide hypothesis.” * BUT: many eukaryotic genes can each code for a set of closely related polypeptides via process call alternative splicing BUT: quite a few genes code for RNA molecules that have important functions in cells even though they are not translated into protein Basic Principles of Transcription and Translation
* Genes provide the instruction for making proteins. But a gene does not a build a protein directly. * Bridge between DNA and protein synthesis is the nucleic acid RNA. * Genes are typically hundreds or thousands of nucleotides long, each gene having a specific sequence of nucleotides. * Each polypeptide of a protein also has monomers arranged in a particular linear order (the protein’s primary structure), but its monomers are amino acids. * TRANSCRIPTION: the synthesis of RNA using information in the DNA (General term for the synthesis of any kind of RNA on a DNA template. * For a protein-coding gene, the resulting RNA molecule is a faithful transcript of the gene’s protein-building instructions. * (mRNA) messenger RNA carries a genetic message from the DNA to the protein-synthesizing machinery of the cell. * Translation: synthesis of polypeptide using the information in the mRNA; during this stage there is a change in language: cell must translate the nucleotide sequence of mRNA molecule into the amino acid sequence of a polypeptide; site is done at ribosomes: complex particle that facilitate the orderly linking of amino acids into polypeptide chains * Difference in Bacteria & Eukaryotes:
* Bacteria don’t have nuclear membranes and thus its DNA is not separated by other compartments such as ribosomes and other protein-synthesizing equipment: so for bacteria that allows translation of mRNA to begin while transcription is still in progress * In Eukaryotes, the nuclear envelope separates transcription and translation in space and time: transcription occurs in the nucleus and mRNA is transported to the cytoplasm, but before mRNA is sent out it is modified to produce function mRNA. Transcription of a protein-coding eukaryotic gene results in pre-mRNA, further processing yields the finished mRNA. Those that are not translated into proteins are called primary transcript. * DNA RNA Protein
The Genetic Code
* Flow of information from gene to protein is based on a triplet code: genetic instructions for a polypeptide chain are written in the DNA as series of non-overlapping, three-nucleotide words. * During transcription, the gene determine the sequence of nucleotide bases along the length of the RNA molecule that is being synthesized * For each gene, only one of the two DNA strands is transcribed, template strand, which provides the pattern for the sequence of nucleotides in an RNA transcript * The Triplet Code: A strand of DNA is blueprint for a particular gene, which is transcribed to mRNA, which is translated by an outside compartment to particular amino acids that make up polypeptide chain (protein): mRNA is read 5’ 3’ * mRNA is complementary rather than identical to DNA because it follows the base pairing rules, similar the strand...
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