Reading Essentials for Biology Glencoe (794133), страница 30
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A second type of RNA is called the ribosome, orribosomal RNA (rRNA). It binds to the mRNA and uses theinstructions to assemble the amino acids in the right order. Thethird type of RNA is transfer RNA (tRNA). It delivers aminoacids to the ribosome to be made into a protein.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Identify MainIdeas Highlight the mainpoint under each head. Stateeach main point in your ownwords.DNA contains information used to make proteins. Proteinshave many uses. Some proteins become structures and some control cell functions.
Since DNA has the information for makingproteins, DNA controls cells. Remember that all this informationis based on the sequence of nucleotides in the DNA molecule.NameDateClassSection11.2From DNA to Protein, continuedWhat does RNA look like?RNA, like DNA, is a nucleicacid. But the structure of RNA isquite different. RNA is a singlestrand. It looks like one half of azipper. DNA is a double strand.The sugar in RNA is differentthan the sugar in DNA. Finally,both RNA and DNA have fournitrogenous bases, but instead ofthymine, RNA has uracil.
Remember that in DNA, guanine bindswith cytosine, and thymine bindswith adenine. In RNA uracil (U)binds with adenine. The structureof RNA helps it do all the work ofbuilding proteins.An RNA molecule usually consists of asingle strand of nucleotides, not a doublestrand.
This single-stranded structure isclosely related to its function.PGRRUCCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.TranscriptionIn order to get the information to the cytoplasm, first messenger RNA has to be made. In this process, called transcription(trans KRIHP shun), RNA is made from part of a DNA strand.Use the illustration on page 120 to help you understand theprocess after you read the following description.First, a portion of the DNA molecule unzips. Free RNAnucleotides pair with the nucleotides on the DNA strand. ThemRNA strand is complete when the RNA nucleotides form astrand by bonding together.
The mRNA strand breaks away andthe DNA strands rejoin. The mRNA strand leaves the nucleusand enters the cytoplasm. You can see that transcription is similarto replication with one important difference—a single strand RNAmolecule is created rather than a double strand DNA molecule.You can also see from the illustration that mRNA pairs guaninewith cytosine, but pairs uracil with adenine.Identify DetailsCircle the parts of the diagramon page 120 that illustrateeach part of the transcriptionprocess.RNA ProcessingNot all of a DNA strand carries information to make proteins.There are long sequences of noncoding nucleotides on DNAstrands.
Enzymes cut out any noncoding sequences that may havebeen transcribed. In this way, the mRNA carries only informationit needs to make protein.READING ESSENTIALSChapter 11119NameDateClassSection11.2From DNA to Protein, continuedA The process of transcriptionbegins as enzymes unzip themolecule of DNA in the regionof the gene to be transcribed.GCCGDNAstrandAGUURNAstrandTRNAstrandGGCACbase pairs with theircomplementarynucleotides on the DNAstrand. The mRNA strandis complete when theRNA nucleotidesbond together.TAbreaks away, andthe DNA strandsrejoin. The mRNAstrand leaves thenucleus and entersthe cytoplasm.DNAstrandThe Genetic CodeThe nucleotide sequence transcribed from DNA to a strand ofmessenger RNA is a genetic message that has all the informationneeded to build a protein.
The message is in a special languagethat uses nitrogenous bases as the alphabet. Remember thatproteins are made up of amino acid chains. There are 20 different amino acids. These amino acids are made from only four120Chapter 11READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.C The mRNA strandB Free RNA nucleotidesNameDateClassSection11.2From DNA to Protein, continuednitrogenous bases.
Scientists wondered how four nitrogenousbases could make a code for 20 amino acids.Scientists were able to crack the genetic code when they discovered that it takes a group of three nitrogenous bases in mRNAto code for one amino acid. Each group of three nitrogenousbases is known as a codon. For example, the codon for the aminoacid alanine is G-C-U. The codon for lysine is A-A-A. Everyamino acid has a three-letter codon, each letter representing oneof the four nitrogenous bases. That is how four nitrogenous basescan code for 20 amino acids.
There is even a codon that tells themRNA that this is the start of the amino acid chain and anothercodon that says this is the end. To simplify, those codons arecalled start and stop.2. What is a codon?________________________________________________________________________Translation: From mRNA to ProteinCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Remember the factory example? Messenger RNA is the workerthat brings the instructions for making protein to the cytoplasm.It takes two more kinds of RNA to actually make the protein. Theprocess of changing the information in mRNA into an amino acidchain in protein is called translation.Here is how it works.1.
The mRNA moves to the cytoplasm.2. A ribosome (rRNA) attaches itself to the start codon, A-U-G,on the mRNA.3. Transfer RNA (tRNA) molecules, carrying amino acids,approach the ribosome. The nitrogenous base sequencethat is the complement to the mRNA sequence is the anticodon. If the mRNA codon is G-C-C, the tRNA anticodonis C-G-G. For every codon on mRNA there is an anticodonon tRNA.3. Analyze What is the difference between transcription and translation?________________________________________________________________________4. The ribosome attaches the anticodon to the codon and theamino acids bond. The ribosome then slides to the next codon.5.
Again the ribosome attaches the anticodon to the codon,amino acids bond, and the ribosome slides over.6. This translation process continues until the stop codon isreached. At this point the amino acids have formed a chainand when the stop codon is reached, the chain is released.You can see from the illustration of the translation process onpage 122 that the tRNA does not stay attached during the wholeprocess. As soon as the amino acid bonds to the amino acid nextto it, the tRNA that brought it moves away to bring anotheramino acid.READING ESSENTIALSChapter 11121NameDateClassSection11.2From DNA to Protein, continuedRibosomeA U G G C A U A CA AC G AG U UmRNA codonAlanineMethioninetRNAanticodonMethioninePeptidebondU A CA AC G AG U UA U G G C A U A CU ACC G UG CA U GA U A CA AC G AG U UPeptide bondStopcodonU ACC G UG C A U A CA U GA AC G AG U UA U G G CA U AA A CC A A U U G U A AAmino acid chains become proteins when they are freed fromthe ribosome.
The amino acid chains twist and curl into complexthree-dimensional shapes. Each protein chain forms the sameshape every time it is produced. These proteins become enzymesand cell structures.4. Sequence The pathwayof information flowsfrom (Circle your choice.)a. DNA to mRNA toprotein.b. mRNA to DNA toprotein.c. protein to mRNA toDNA.122Chapter 11What is the central dogma?If you were to summarize the process of replication, transcription, translation, and protein formation you might say simply thatthe pathway of information flows from DNA to mRNA to protein.This process is called the central dogma of biology.
This meansthat the same process occurs in every living thing, from the simplest bacteria to the most complex animal.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.AlanineNameDateClassSection11.2From DNA to Protein, continued◗ After You ReadMini Glossarycodon: group of three nitrogenous bases inmRNA that code for one amino acidmessenger RNA: RNA that carries informationfrom DNA in the nucleus to the cell’scytoplasmribosomal RNA: RNA that makes up the ribosomes; binds to mRNA and uses its information to assemble amino acids in theright ordertranscription (trans KRIHP shun): process in thecell nucleus where a copy of RNA is madefrom part of a DNA strandtransfer RNA: RNA that delivers amino acids tothe ribosomes to be assembled into proteinstranslation: process of changing the information in mRNA into an amino acid chain ina protein1.
Read the key terms and definitions in the Mini Glossary above. Then on the lines, write adefinition of transcription and translation using your own words.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________2.
Under each type of RNA, write the words or phrases that tell something about it.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.mRNAcodonmoves along mRNAanticodontranscriptiontranslation→rRNA→tRNAconnects codon to anticodonbrings instructionsbrings amino aciduses instructions to assemble amino acidsVisit the Glencoe Science Web site at science.glencoe.comto find your biology book and learn more about DNA to protein.READING ESSENTIALSChapter 11123NameDateClassSection11.3 Genetic Changes◗ Before You ReadSC.F.2.4.2 The student knows that every cell contains a “blueprint” coded in DNAmolecules that specify how proteins are assembled to regulate cells. SC.F.2.4.3 Thestudent understands the mechanisms of change (e.g., mutation and natural selection)that lead to adaptations in a species and their ability to survive naturally in changingconditions and to increase species diversity. Also covers SC.F.1.4.1If you have had an X ray taken, you may remember that before they took it they covered the part ofyour body not being X rayed with a heavy lead shield.
Think about why the technician protects youfrom X rays and write the reason on the lines below. After you read this section, check your answer.You may want to add new information that you learned.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnIdentify MainIdeas As you read this section, highlight the main ideain each paragraph.
Stop afterevery paragraph and put whatyou just read into your ownwords.1. When is a mutationhelpful?________________________________________________________________________124Chapter 11Every day millions of cells correctly transfer DNA informationto proteins. Occasionally, however, there is a change in the DNAsequence. Any change in the DNA sequence is called a mutation.Mutations can be caused by errors in replication, transcription, orcell division. Forces outside of the cell can cause changes to DNA.You will learn about each of these mutations in this section.Can a mutation be a good thing?Imagine that the nucleotide sequence of a sperm or egg cellchanges. If that sperm or egg cell results in fertilization, then thechange in sequence would become part of the offspring.
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