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Chapter 17 - Chapter 17 From Gene to Protein The...

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Chapter 17- From Gene to Protein The information content of DNA, the genetic material, is in the form of specific sequences of nucleotides along the DNA strands. How does this determine an organism’s traits? Proteins are the link between genotype and phenotype. Gene expression is the process by which DNA directs protein synthesis. It includes 2 stages called transcription and translation . One gene one polypeptide. One gene encodes for one polypeptide. A couple of polypeptides create one protein. Some genes also encode for RNA molecules that are never translated into proteins. Basic principles of Transcription and Translation: The bridge between DNA and protein synthesis is the nucleic acid RNA. RNA is chemically similar to DNA except that it contains ribose instead of deoxyribose and has the nitrogenous base uracil rather than thymine. RNA molecules usually consist of a single strand. DNA and RNA have monomers of four different types of nucleotides which differ in their nitrogenous bases. Genes are typically hundreds or thousands of nucleotides long. Each gene having a specific sequence of bases. Each polypeptide of a protein (the amino acids) has monomers arranged in a linear order (the protein’s primary structure. Getting from gene to protein, means getting from nucleotides to amino acid structure. This requires two major stages, transcription and translation. Transcription is the synthesis of RNA under the direction of DNA. The DNA strand provides a template for the assembling the sequence of RNA nucleotides. This type of RNA is called mRNA or messenger RNA because it carries a genetic message from the DNA to the protein-synthesizing machinery of the cell. Translation is the actual synthesis of a polypeptide, which occurs under the direction of mRNA. During this process, the cells must translate the base sequence of an mRNA molecule to the amino acid sequence of a polypeptide. The site for translation is the ribosome, where complex particles facilitate the ordered linking of amino acid into polypeptide chains. Why use RNA? o It provides protection for the DNA and its genetic information. o RNA allows more copies of the protein to be made simultaneously. o Each RNA transcript can be translated repeatedly.
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In Prokaryotic cells (have no nucleus and their DNA is not segregated from ribosomes and other protein synthesizing equipment) translation of mRNA begins while transcription is still in progress. In Eukaryotic cells the nuclear envelope separate transcription and translation. Transcription occurs in the nucleus. The mRNA is then transported to the cytoplasm where Translation occurs. RNA transcripts (pre mRNA) need to modified before they become mRNA.
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