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Download Transcription Protein Synthesis So what does it mean? Transcription
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10/16/2014 Protein Synthesis Central Dogma of Biology: Transcription Date ________ Central Dogma • Model that describes how DNA serves as a genetic code for protein synthesis • Proteins are the structural building blocks for cells, and they act as enzymes • Geneticists accept that the basic mechanism of reading and expressing genes is DNA RNA protein. • This chain of events occurs in all living things, from bacteria to humans. • Scientists refer to this mechanism as the central dogma of biology: DNA codes for RNA, which guides the synthesis of proteins. So what does it mean? • When a protein is needed, the cell must go through protein synthesis. • DNA cannot leave the nucleus of the cell, but proteins are made in the cytoplasm – How do we get proteins?! • An intermediate must be made – RNA is used to carry the message from the nucleus to the cytoplasm RNA Transcription • RNA is a nucleic acid that is similar to DNA, however RNA contains the sugar ribose, the base uracil replaces thymine, and usually is single stranded. • Three types: • Purpose: make a copy of mRNA using DNA as a template. This allows the recipe in DNA to make the protein in the cytoplasm. • mRNA base-pairs with DNA based on specific rules – A—U, G—C • The specific type of RNA formed during this process is called mRNA (messenger) • Only one gene is copied during transcription • After transcription, the mRNA leaves the nucleus and enters the cytoplasm; the two strands of DNA join back together – Messenger RNA (mRNA): long strands of RNA nucleotides formed complementary to one strand of DNA. They travel from the nucleus to the ribosome to direct synthesis of a specific protein. – Ribosomal RNA (rRNA): RNA that associates with proteins to form ribosomes in the cytoplasm. – Transfer RNA (tRNA): smaller segments of RNA nucleotides that transport amino acids to the ribosome. 1 10/16/2014 How does it happen? • Chemical signals trigger a gene for a specific protein to turn ON • RNA polymerase attaches to the exact location on the DNA molecule where the gene is • Two strands separate at that location • RNA strand is made – this is called mRNA, specifically 4. Only a small part of the DNA double helix is unwound/unzipped at a time – RNA polymerase travels along the gene, bringing in RNA nucleotides to base-pair with the existing DNA nucleotides along the 3’ 5’ leading strand, called the template strand here. - Uracil is incorporated instead of thymine, because there is no thymine in RNA. 5. When RNA polymerase reaches the termination signal at the end of the gene, it lets go of the DNA and mRNA and leaves to transcribe elsewhere. The mRNA travels out of the nucleus and into the cytoplasm where it may be translated. The Steps 1. A protein needs to be made from a gene (DNA). 2. Proteins called transcription factors are located at the beginning of the needed gene. These tell RNA polymerase where to begin transcribing. 3. As soon as RNA polymerase has positioned itself at the beginning of the gene, it begins transcription. What happens once mRNA is made? • It must go through processing before it can be used to make a protein. • The strand of mRNA is much shorter than the gene it is copied from in DNA. Why? – DNA sequences are made up of segments called introns and segments called exons. • Exons code for the gene. After transcription, the introns are removed from the mRNA, leaving only the exons to code for the protein. • Other methods of processing include: – 5’ cap at the beginning of mRNA – Poly-A tail at the end of mRNA (sequence of repeating As) 2