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RNA and Protein Synthesis
Section 12-3
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Objectives for this section
Compare and contrast DNA and RNA Name the 3 main types of RNA Describe transcription and the editing of RNA Identify the genetic code Summarize translation Explain the relationship between genes and proteins
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Introduction The structure of DNA explains how it can be copied, but it does not tell how a gene works. Genes—coded DNA instructions that control the production of proteins within the cell
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Introduction We now know that the central dogma of biology is DNA to RNA to protein The first step in decoding the genetic message is to copy part of the DNA nucleotide sequence into RNA (ribonucleic acid) These RNA molecules contain the coded instructions for making proteins
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The Central Dogma of Bilogy
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The Structure of RNA RNA, like DNA, is a long chain of nucleotides
RNA consists of the same components as DNA A 5-carbon sugar A phosphate group A nitrogenous base
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The Structure of RNA There are 3 main differences between DNA and RNA
The sugar in RNA is ribose instead of the deoxyribose in DNA RNA is generally single-stranded RNA contains uracil instead of thymine
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The Structure of RNA RNA is like a disposable copy of a segment of DNA
In many cases, RNA is a copy of a single gene—the ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules
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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 There are 3 main types of RNA
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Types of RNA Messenger RNA (mRNA)—carry copies of instructions for assembling amino acids into proteins; serve as “messengers” from DNA to the rest of the cell
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Types of RNA Ribosomal RNA (rRNA)—a component of ribosomes
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Types of RNA Transfer RNA (tRNA)—works during the construction of a protein; transfers each amino acid to the ribosome as it is specified by the coded messages in the mRNA
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Types of RNA
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Transcription Transcription—when RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA Transcription requires the enzyme RNA polymerase—binds to DNA and separates the DNA strands. Then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA
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Transcription How does the RNA polymerase know where to start and stop making the RNA copy? The enzyme will only bind to regions of DNA known as promoters, which have specific base sequences Similar signals tell the RNA polymerase where to stop
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Transcription
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RNA Editing The first molecule of mRNA (known as the pre-mRNA) produced by copying the DNA sequence is like a rough draft and it requires editing DNA contains sequences of nucleotides called introns, which are not involved in coding for proteins
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RNA Editing The DNA sequences that code for proteins are called exons, because they are expressed in the synthesis of proteins When an RNA molecule is formed, it contains both introns and exons The introns are cut out of the RNA molecule while it is still in the nucleus The remaining exons are spliced back together and form the final pre-mRNA molecule
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RNA Editing
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The Genetic Code Remember that proteins are made by joining amino acids into long chains called polypeptides Each polypeptide contains a combination of any or all of the 20 different amino acids The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides
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The Genetic Code The “language” of mRNA instructions is called the genetic code RNA contains 4 different nitrogenous bases (U, C, G, A) How can a code with only 4 letters translate into 20 different amino acids? The genetic code is read 3 letters at a time, so each “word” of the coded message is 3 bases long
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The Genetic Code Each 3-letter “word” in mRNA is known as a codon
A codon consists of 3 consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide Example: UCGCACGGU would be read as UCG-CAC-GGU. These codons represent 3 amino acids: serine-histidine-glycine
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The Genetic Code Because there are 4 different bases, there are 64 possible 3-base codons. Note that some amino acids can be specified by more than one codon There are also “start” and “stop” codons Start codons (AUG) tell where protein synthesis is to begin Stop condons (3 different ones) tell where the end of the polypeptide is
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The Genetic Code
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Translation The mRNA molecule has been transcribed and serves as instructions, but we need something to read the instructions and put them to use In the cell, the ribosome takes care of this Translation—the decoding of an mRNA message into a polypeptide chain (protein)
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Translation Steps in translation
Begins when an mRNA molecule in the cytoplasm attaches to a ribosome Each codon of the mRNA moves through the ribosome and the proper amino acid is brought to the ribosome by the tRNA
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Translation Each tRNA carries only one kind of amino acid and picks it up based on the anitcodon it is carrying Example: if the anticodon is UUU, the tRNA would pick up the amino acid with the codon AAA (they are opposites)
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What codon would these anticodons pick up??
ACG UGG CAG GGG GCC CAA
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Translation The ribosome forms a peptide bond between the first and second amino acids At the same time, it breaks the bond with the tRNA molecule and releases it The ribosome moves on the third amino codon, where a tRNA molecule brings it the amino acid specified by the third codon
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Translation The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule When the stop codon is reached, it releases the newly formed polypepetide and mRNA molecule, completing the process of translation
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Genes and Proteins What does protein synthesis have to do with the color of a flower, eye color, or height? Remember that many proteins are enzymes, which catalyze and regulate chemical reactions. A gene that codes for an enzyme to produces pigment controls flower color. Proteins are specific tools that build or operate components of living cells.
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