RNA and Protein Synthesis PDF

12-3 RNA and Protein Synthesis Bl 1.d. Students know the central dogma of molecular biology outlines the flow of information from transcription of ribonucleic acid (RNA) in the nucleus to translation of proteins on ribosomes in the cytoplasm. Bl 4.a. Students know the general pathway by which riboso...

12-3 RNA and Protein Synthesis Bl 1.d. Students know the central dogma of molecular biology outlines the flow of information from transcription of ribonucleic acid (RNA) in the nucleus to translation of proteins on ribosomes in the cytoplasm. Bl 4.a. Students know the general pathway by which ribosomes synthesize proteins, using tRNAs to translate genetic information in mANA. Bl 4.b. Students know how to apply the genetic coding rules to predict the sequence of amino acids from a sequence of codons in RNA. Bl 5.a. Students know the general structures and functions of DNA, RNA, and protein. ·------Guide for Reading Key Concepts What are the three main types of RNA? What is transcription? What is translation? he double helix structure explains how DNA can be copied, but it does not explain how a gene works. In molecular terms, genes are coded DNA instructions that control the production of proteins within the cell. The first step in decoding these genetic messages is to copy part of the nucleotide sequence from DNA into RNA, or ribonucleic acid. These RNA molecules contain coded information for making proteins. T Vocabulary gene messenger RNA ribosomal RNA transfer RNA transcription RNA polymerase promoter intron ex on codon translation anticodon Reading Strategy: Using Visuals Before you read, preview Figure 12-18. As you read, notice what happens in each step of translation, or protein synthesis. The Structure of RNA RNA, like DNA, consists of a long chain of nucleotides. As you may recall, each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base. There are three main differences between RNA and DNA: The sugar in RNA is ribose instead of deoxyribose, RNA is generally single-stranded, and RNA contains uracil in place of thymine. You can think of an RNA molecule as a disposable copy of a segment of DNA. In many cases, an RNA molecule is a working 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. 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: messenger RNA, ribosomal RNA, and transfer RNA. The structures of these molecules are shown in Figure 12-12. T Figure 12-12 ~ The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA. Ribosomal RNA is combined with proteins to form ribosomes. Messenger RNA 300 Chapter 12 Amino acid Ribosome Ribosomal RNA Transfer RNA Most genes contain instructions for assembling amino acids into proteins. The RNA molecules that carry copies of these instructions are known as messenger RNA (mRNA) because they serve as "messengers" from DNA to the rest of the cell. Proteins are assembled on ribosomes, shown in Figure 12-13. Ribosomes are made up of several dozen proteins, as well as a form of RNA known as ribosomal RNA (rRNA). During the construction of a protein, a third type of RNA molecule transfers each amino acid to the ribosome as it is specified by coded messages in mRNA. These RNA molecules are known as transfer RNA (tRNA). a sp Transcription RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA, a process called transcription. Transcription requires an enzyme known as RNA polymerase that is similar to DNA polymerase. ~ During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA. The process of transcription is shown in Figure 12-14. How does RNA polymerase "know" where to start and stop making an RNA copy of DNA? The answer to this question begins with the observation that RNA polymerase doesn't bind to DNA just anywhere. The enzyme will bind only to regions of DNA known as promoters, which have specific base sequences. In effect, promoters are signals in DNA that indicate to the enzyme where to bind to make RNA. Similar signals in DNA cause transcription to stop when the new RNA molecule is completed. ~ Figure 12-13 In this detailed model of a ribosome, the two subunits of the ribosome are shown in yellow and blue. The model was produced using cryo-electron microscopy. Data from more than 73,000 electron micrographs, taken at ultra-cold temperatures to preserve ribosome structure, were analyzed to produce the model. 'Y Figure 12-14 ~ During transcription, RNA polymerase uses one strand of DNA as a template to assemble nucleotides into a strand of RNA... Adenine (DNA and RNA).. Cytosine (DNA and RNA).. Guanine (DNA and RNA) Thymine (DNA only) Uracil (RNA only) DNA DNA and RNA 301 RNA Editing Ex on ~ ~ pre-mANA mANA Tail Cap.._ Figure 12-15 Many RNA molecules have sections, called introns, ed ited out of them before they become functional. The remaining pieces, called exons, are spliced together. Then, a cap and tail are added to form the final RNA molecule. Predicting What do you think would happen if the introns were not removed from the pre-mRNA? Like a writer's first draft, many RNA molecules require a bit of editing before they are ready to go into action. Remember that an RNA molecule is produced by copying DNA. Surprisingly, the DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins. The DNA sequences that code for proteins are called exons because they are "expressed" in the synthesis of proteins. When RNA molecules are formed, both the introns and the exons are copied from the DNA. However, the introns are cut out of RNA molecules while they are still in the nucleus. The remaining exons are then spliced back together to form the final mRNA as shown in Figure 12-15. Why do cells use energy to make a large RNA molecule and then throw parts of it away? That's a good question, and biologists still do not have a complete answer to it. Some RNA molecules may be cut and spliced in different ways in different tissues, making it possible for a single gene to produce several different forms of RNA. Introns and exons may also play a role in evolution. This would make it possible for very small changes in DNA sequences to have dramatic effects in gene expression. The Genetic Code 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. How, you might wonder, can a particular order of nitrogenous bases in DNA and RNA molecules be translated into a particular order of amino acids in a polypeptide? The "language" of mRNA instructions is called the genetic code. As you know, RNA contains four different bases: A, U, C, and G. In effect, the code is written in a language that has only four "letters." How can a code with just four letters carry instructions for 20 different amino acids? The genetic code is read three letters at a time, so that each "word" of the coded message is three bases long. Each three-letter "word" in mRNA is known as a codon, as shown in Figure 12-16. A codon consists of three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide. For example, consider the following RNA sequence: r Codon r Codon UCGCACGGU This sequence would be read three bases at a time as:.._ Figure 12-16 A codon is a group of three nucleotides on messenger RNA that specify a particular amino acid. Observing What are the three-letter groups of the two codons shown here? 302 Chapter 12 UCG-CAC-GGU The codons represent the different amino acids: UCG-CAC-GGU Serine-Histidine-Glycine A/~1). · G 1'1)