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RNA & Protein Synthesis 12-3 Intro • Genes code DNA instructions that control the production of proteins within the cell. • The first step in decoding these genetic messages is to copy DNA into RNA • These RNA molecules contain coded information for making proteins. The Structure of RNA • RNA consists of a long chain of nucleotides. • Each nucleotide is made up of 1. 5-carbon sugar 2. a phosphate group 3. a nitrogenous base. 3 Differences of RNA & DNA • There are three main differences between RNA and DNA: 1. The sugar in RNA is ribose instead of deoxyribose 2. RNA is generally single-stranded 3. RNA contains uracil in place of thymine. RNA • You can think of an RNA molecule as a disposable copy of a segment of DNA. • RNA molecule is a working copy of a single gene. Types of RNA • RNA molecules have many functions, but in the majority of cells most RNA molecules are involved in just one job— protein synthesis. • The assembly of amino acids into proteins is controlled by RNA. • There are three main types of RNA: 1. messenger RNA 2. ribosomal RNA 3. transfer RNA. Messenger RNA • The RNA molecules that carry copies of protein making instructions are known as messenger RNA (mRNA) • They serve as “messengers” from DNA to the rest of the cell Ribosomal RNA • Proteins are assembled on ribosomes • Ribosomes are made up of several dozen proteins and RNAribosomal RNA (rRNA) Transfer RNA • During the construction of a protein RNA transfers each amino acid to the ribosome based on the messages in mRNA. • These RNA molecules are known as transfer RNA (tRNA). Checkpoint • Ribosomes are made of ribosomal RNA and A.) Transfer RNA B.) Protein C.) Messenger RNA Transcription • RNA molecules are produced by copying part of a sequence of DNA into a complimentary sequence in RNA • • called transcription. Transcription requires an enzyme known as RNA polymerase. Transcription Continued • RNA polymerase binds to DNA separates the DNA strands. • RNA polymerase uses one strand of DNA as a template and nucleotides are assembled into a strand of RNA. Promotors • The enzyme will bind only to regions of DNA known as promoters They have specific base sequences. • Promoters are signals that “tell” enzymes where to bind to make RNA. RNA Editing • Many RNA molecules require editing before they are ready to go into action. • Remember that an RNA molecule is produced by copying DNA. Introns and Exons • Eukaryotic genes contains sequences of nucleotides called introns • Introns are not involved in protein synthesis Introns and Exons • The DNA sequences that code for proteins are called exons • Exons are“expressed” in protein synthesis • When RNA molecules are formed, both the introns and the exons are copied from the DNA. Introns and Exons • The introns are cut out of RNA molecules • The remaining exons are then spliced back Checkpoint • If the introns are not removed from the pre-mRNA, the resulting protein would be A.) Made correctly B.) Made incorrectly C.) Not made at all Why Throw Away RNA? • Some RNA molecules may be cut and spliced in different ways so a single gene can produce several different forms of RNA. • It is thought that introns and exons may play a role in evolution Small changes in DNA sequences could have dramatic effects in gene expression. Checkpoint • Intervening sequences of DNA are known as A.) introns B.) Exons C.) Codons The Genetic Code • Proteins are made by joining long chains called polypeptides combination of any or all the 20 different amino acid Properties of proteins depend on order of AA • The “language” of mRNA instructions is called the genetic code. The Genetic Code • As you know, RNA contains four different bases: Adenine Uracil Cytosine Guanine. • The genetic code is read three letters at a time • Each “word” of the coded message is three bases long. Codon • Each three-letter “word” in mRNA is known as a codon • A codon consists of three consecutive nucleotides that specify a single amino acid • The amino acid will be added to the polypeptide. partnerwork mRNA code UCGCACGGU mRNA codon UCG-CAC-GGU write the amino acids that correspond to the codons on page 303 Answer • Serine • Histidine • Glycine Codon Numbers • Because there are four different bases, there are 64 possible, three-base codons (4 *4 *4 = 64) • Six different codons specify the amino acid leucine • Six other codons specify arginine. AUG • AUG can either specify: 1. methionine 2. “start” • There are three “stop” codons that do not code for any amino acid. UAG, UAA, UGA AUG • Stop codons act like the period at the end of a sentence • They signify the end of a polypeptide, which consists of many amino acids Translation • The decoding of an mRNA message into a polypeptide chain (protein) is known as translation. • Translation takes place on ribosomes. • During translation, the cell uses information from messenger RNA to produce proteins. Overall Summary • Messenger RNA is transcribed from DNA in the nucleus and released into the cytoplasm • Translation begins when an mRNA molecule attaches to a ribosome. • In the ribosome, the amino acid is transferred to the growing polypeptide chain. Anti-codon • Each tRNA molecule carries only one kind of amino acid. • In addition to an amino acid, each tRNA molecule has three unpaired bases. • These bases, called the anticodon, are complementary to one mRNA codon. Steps • 1. mRNA attaches to a ribosome, where codons move thru ribosome, and are read 1 at a time • 2. tRNA enters ribosome, carrying an amino acid and anticodon Translation always starts with the start codon AUG (methionine) • 3. Ribosomes form peptide bonds between each amino acid Also breaks the bond between amino acid and tRNA • tRNA is released back into cytoplasm • Continues until stop codon is reached • Protein is released and translation is complete The Roles of RNA and DNA • The cell uses the vital DNA “master plan” to prepare RNA “blueprints.” • The DNA molecule remains within the safety of the nucleus, while RNA molecules go to the protein-building sites in the cytoplasm—the ribosomes. Genes and proteins • Many proteins are enzymes which catalyze and regulate chemical reactions • A gene that codes for an enzyme can produce skin color or blood type • Genes for certain proteins can regulate growth, controlling size and shape • Proteins are the keys to almost everything that living cells do • • • • 1. List the three main types of RNA. 2. What happens during transcription? 3. What happens during translation? 4.Describe the three main differences between RNA and DNA.