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Bowie- Biology 12 Understanding Protein Synthesis Page 1 of 8 Protein Synthesis Notes Name: ___________________________________________________ Date: ____________________________ To begin protein synthesis (formation), genes (the DNA structures of the cell) are first transcribed into an RNA strand. The RNA is then translated by ribosomes into protein. The genetic material is made of nucleotides. The sequence of nucleotides in DNA determines the order in which amino acids are assembled. Each gene codes for a specific protein. Before DNA can be expressed as protein, it must be transcribed into RNA. Activity #1: Complete Translating the Code on page 233 of Nelson Bio 12 now. Bowie- Biology 12 Understanding Protein Synthesis Page 2 of 8 Protein Synthesis DNA is contained inside the nucleus and can never leave the nucleus. Proteins are synthesized outside the nucleus on the ribosomes. Since DNA is too valuable to leave the nucleus, there must be another method of getting the "recipe" out to the ribosomes. DNA must 1st be transcribed into a messenger RNA. Many copies of the needed DNA sequence can be created (as mRNA) and sent out to multiple ribosomes to increase the speed at which the protein can be synthesized. The ribosomes then translate the sequence into polypeptide chains of amino acids. These, in turn, are processed into proteins. Transcription is when DNA is copied into the form of messenger RNA (mRNA). Transcribe mean "to copy". Translation is when ribosomes use the mRNA as a blueprint to synthesize a protein made of amino acids. Translate means to change it into another language. RNA refresher: RNA is a blueprint of genetic information. It is made of ribose sugar, rather than deoxyribose sugar found in DNA. This means that it has a hydroxyl group on the 2' carbon (while DNA has lost one oxygen from this location). RNA has 4 nitrogenous bases, just like DNA, but instead of thymine, RNA uses Uracil (U). RNA is only single stranded, whereas DNA is double stranded. When transcribing DNA into messenger RNA there is no need to waste energy creating a double strand, only a single complementary strand is needed with U replacing T in the sequences. There are 3 major classes of RNA molecules: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) Bowie- Biology 12 Understanding Protein Synthesis Page 3 of 8 Types of RNA Messenger RNA (mRNA): is the copy of the needed section of the DNA. It varies in length. The longer the gene, the longer the mRNA will be. Transfer RNA (tRNA) : it transfers the appropriate amino acid to the ribosomes to build the protein (as coded for in the mRNA). tRNA's are relatively short (about 70-90 ribonucleotides in length). Ribosomal RNA (rRNA): are structural components of a ribosomes itself. It binds with proteins to provide a construction site for the assembly of the polypeptides. Transcription & Translation Transcription has 3 main steps: initiation, elongation and termination. Initiation: RNA polymerase binds to the DNA at a specific site known as a promoter, near the beginning of the gene. Promoter regions are often sequences of A's and T's. Elongation: the RNA polymerase adds ribonucleotides (A/U, G/C) to complement the DNA sequence. Elongation occurs along only one of the DNA strands, called the template strand. The other strand is referred to as the coding strand. DNA contains coding regions (called exons) and noncoding regions (called introns) Termination: Shortly after passing the end of the gene, the polymerase recognizes a "stop" sequence, known as a terminator sequence. The mRNA transcript is then completely release from the DNA and it will eventually leave the nucleus. Some modifications to the RNA strand must be made before it leaves the nucleus. A 5'cap is added to the start of the mRNA to protect it from digestion in the cytoplasm and to serve as a start location in translation. An enzyme is also added as a tail to the end of the mRNA called poly-A-polymerase (poly-A tail, for short). Finally, the noncoding, introns, must be removed so that the protein will fold correctly. Spliceosomes do the removal of the introns. Bowie- Biology 12 Understanding Protein Synthesis Page 4 of 8 Translation Translation also has 3 main steps: initiation, elongation and termination. Initiation: The ribosome recognizes a specific sequence on the mRNA and binds to that site. Elongation: The ribosome moves along the mRNA three nucleotides at a time. Each triplet is the code for a specific amino acid. This is called the reading frame. The tRNA delivers the appropriate amino acid and the polypeptide elongates. The 1st codon is the START codon AUG. AUG is the code for the amino acid, methionine. Elongation continues until a 3-base nucleotide sequence is reached that does not code for a specific amino acid. This is a "stop" sequence. Termination: Elongation continues until a 3-base nucleotide sequence is reached that does not code for a specific amino acid. This is a "stop" sequence. The ribosome then falls off the mRNA and the polypeptide chain is released. Bowie- Biology 12 Understanding Protein Synthesis The Genetic Code There are 20 amino acids, but only 4 nitrogenous bases in mRNA. To code for all 20 amino acids, a sequence of 3 nucleotides must be used. Each triplet is called a codon. More than one codon can code for an amino acid. We use a "Genetic Code" to determine which codons code for each amino acid. Page 5 of 8 Bowie- Biology 12 Understanding Protein Synthesis Page 6 of 8 Control Mechanisms There are about 42 000 genes that code for proteins in humans. Housekeeping genes are genes that code for proteins that are continuously in need in the cells. Not all proteins are needed all the time, however. For example, insulin is only needed when glucose levels in the blood become high. It would be inefficient if genes were constantly being transcribed to produce insulin, even when blood glucose levels were low. Gene regulation is the process of turning genes on or off as needed. Cells have developed ways to control when specific proteins are transcribed. Transcription factors are proteins that turn genes on when they are needed. Mutations Mutations are errors in the DNA sequence that are inherited. Errors may have a negative impact, a positive impact or no impact detected. A mutation could cause a disease, as it does in cystic fibrosis or it could be good for the evolution of a species, as it was in the increasing size and complexity of the human brain. Silent mutations have no effect on the operation of the cell. Usually silent mutations occur in the noncoding regions (introns) of DNA. Missense mutations occur when a change in the base sequence of DNA alters a codon (a code for a specific amino acid), resulting in the wrong amino acid being placed in the protein sequence. This can lead to disease, like sickle cell anemia. Nonsense mutations occur when a change in the DNA sequence causes a stop codon to replace a codon specifying an amino acid. During translation, only the part of the protein that precedes the stop codon is produced, and the fragment may be digested by cell proteases. These mutations are often lethal to the cell. Missense and nonsense mutations arise from substitutions of one base pair for another. Deletions are mutations that occur when one or more nucleotides are removed from the DNA sequence. This shifts the reading frame leading to the wrong amino acids being put together. This results in the production of a defective protein. Insertions are mutations that occur when a nucleotide has been added to the sequence. Since amino acids are read in triplets of code sequences, adding or removing one baser pair results in frameshift mutations. Bowie- Biology 12 Understanding Protein Synthesis Page 7 of 8 Point mutations occur when there is a mutation at a specific base pair in the genome. Translocation is a mutation that occurs when a group of base pairs from one part of the genome relocate or switch places with a group of base pairs from another chromosome. The result will be a fusion protein that may have an altered function. Some forms of leukemia are associated with translocations and their respective fusion proteins. Transposable elements are fragments that are always on the move. They are sometimes referred to as "jumping genes". Transposable elements can jump into a coding region and leave it inactive. Inversions are reversals of segments of DNA within the chromosome. Bowie- Biology 12 Understanding Protein Synthesis Page 8 of 8 Online Simulations: Interactive Concepts in Biochemistry - Interactive Animations http://www.wiley.com/college/boyer/0470003790/animations/translation/translation.htm Protein Synthesis http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/ micro06.swf::Protein+Synthesis Protein Synthesis http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1302 Drag and Drop Protein Synthesis http://www.zerobio.com/drag_oa/protein/overview.htm Practice Building a Protein http://learn.genetics.utah.edu/content/molecules/transcribe/ Protein Synthesis Online Building Activity http://learn.genetics.utah.edu/content/molecules/transcribe/ Video http://www.teachertube.com/viewVideo.php?video_id=60707