Reading Essentials for Biology Glencoe (794133), страница 29
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How isthis related to DNA? Within DNA is the information for life.DNA contains the instructions to make all the different proteinsan organism needs.How can DNA hold all that information?DNA can contain amazing amounts of information because ofits structure. It is a very long molecule made up of repeating unitscalled nucleotides. Each nucleotide is made up of a simple sugar, aphosphate group, and a nitrogenous base.
A nitrogenous base isa carbon ring structure that contains one or more atoms of nitrogen. In DNA, there are four of these bases: adenine (A), guanine(G), cytosine (C), and thymine (T). Because each nucleotidecontains just one of these nitrogenous bases, there are only fourdifferent nucleotides in DNA.Nucleotides join together to form long chains, or strands. Thephosphate groups and the sugar form the backbone of the strandand the nitrogenous bases stick out like the teeth of a zipper.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is DNA?NameDateClassSection11.1DNA: The Molecule of Heredity, continuedWhat does the DNA molecule look like?In 1953, James Watson and Francis Crick published a papercorrectly describing the shape of a DNA molecule for the firsttime. They said that DNA is made of two strands of nucleotidesheld together by the nitrogenous bases, the parts that stick outlike zipper teeth.
Adenine on one strand always joins withthymine on the other strand. Likewise, guanine always joins withcytosine. They also noted that DNA was shaped like a long zipperthat is twisted into a coil like a spring. Something twisted into acoil is called a helix. DNA is made of two strands that are twistedinto a coil, so it is called a double helix.
The figure at the rightshows an example of a double helix.PPhosphategroupsPSPSSCSC GPP SPPSS A TGPSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.If every organism is made up of the same four nucleotides, howcan organisms be so different from one another? The key to variety in organisms is the sequence, or order, of the four nucleotides.For example, a nucleotide sequence of A-T-T-G-A-C carries different information than the sequence T-C-C-A-A-A. It is thesequence of nucleotides that forms the unique genetic information for every organism.
In a similar way, words that have thesame letters but in different order have different meanings. TEAis not the same as EAT or ATE.SPSPSGCPPSPCSHow can DNA do so much with so little?SGGSSATCPSTPSAPSugarSSphosphatePbackboneS T A SPSSATPPSGCSPSST APP SPPSS C GPGCSPReplication of DNAFor most organisms to reproduce, a sperm cell and an egg cell,both produced through meiosis, must unite to form a fertilizedegg.
From one fertilized egg, an organism with millions of cellsis produced through mitosis. Each cell in that organism has acopy of the DNA that was in the original fertilized egg. As youmay recall, before a cell can divide, it must first make a copy ofits chromosomes. The DNA in the chromosomes is copied in aprocess called DNA replication. If the DNA did not make acopy of itself, new cells would have only half the amount of DNAof their parents and could not grow and reproduce successfully.Species could not survive.
All organisms replicate DNA.2. What is the purpose ofDNA replication?________________________________________________________________________How does DNA replicate?Remember that a DNA molecule is made of two strands ofnucleotides joined together like a zipper at the nitrogenous bases.As you know, adenine on one strand always pairs with thymine onthe other strand. In the same way, guanine on one strand alwaysREADING ESSENTIALSChapter 11115NameDateClassSection11.1DNA: The Molecule of Heredity, continuedC Bonding of basesThe sugar and phosphate partsof adjacent nucleotidesbond together to form thebackbone of the new strand.New DNAmoleculeA Separation of strandsWhen a cell begins to copy its DNA,the two nucleotide strands of a DNAmolecule separate. As the DNAmolecule unzips, the bases are exposed.PFree nucleotidesGDPPCPOriginal DNA strandDNew DNAmoleculeNew DNA strandTDGPDTPDADPDGAGACTOriginal DNA strandTTAOriginal DNACD Results of replicationThe process of replication producestwo molecules of DNA.
Each newmolecule has one strand from theoriginal molecule and one strandthat has been newly synthesizedfrom the free nucleotides in the cell.B Base pairingThe bases in free nucleotides pair with exposed bases in the DNA strand.If one nucleotide on a strand has thymine as a base, the free nucleotidethat pairs with it would be adenine. If the strand contains cytosine,a free guanine nucleotide will pair with it. Thus, each strand builds itscomplement by base pairing—forming hydrogen bonds—with free nucleotides.3.
Analyze When one DNAmolecule replicates, howmany new DNA moleculesare formed? (Circle yourchoice.)a. oneb. twoc. four116Chapter 11pairs with cytosine on the other strand. During replication, theDNA molecule unzips and separates. As the DNA unzips,nucleotides that are floating free in the cell attach to the unzippedchains. On the unzipped strand where there is guanine, cytosinewill attach.
The original DNA molecule continues to unzip untilevery base has become a pair and a new DNA molecule is formed.In this way the original DNA strand serves as a pattern or template to make a new DNA molecule. Each new strand formed is acomplement of one of the original strands. The result is twoDNA molecules, each is the same as the parent molecule.
Thefigure above shows how DNA replicates.When all of the DNA in all the chromosomes has been replicated, the cell can divide, passing on the genetic information tothe new cell.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.CANameDateClassSection11.1DNA: The Molecule of Heredity, continued◗ After You ReadMini GlossaryDNA replication: process in which the DNA in thechromosome is copieddouble helix: shape of a DNA molecule consisting of two strands of nucleotides that aretwisted into a coil and held together by thenitrogenous basesnitrogenous base: carbon ring structure found inDNA molecules that contains one or moreatoms of nitrogen1. Read the key terms and definitions in the Mini Glossary above.
Use each term in a sentencethat shows your understanding of the term.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________phosphateTAATCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.CGGGsugarTCAATnitrogen base2.
Moving from left to right, write the letters (A, T, C, or G) in the empty circles of the basesthat will pair with the bases on the top strand. The first three pairs have been completedfor you.3. Circle the letter of the correct statement below.a. The illustration above is of a DNA molecule unzipping.b. The illustration above is of a DNA strand replicating.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about DNA: the molecule of heredity.READING ESSENTIALSChapter 11117NameDateClassSection11.2 From DNA to Protein◗ Before You ReadSC.F.1.4.1 The student knows that the body processes involve specific biochemicalreactions governed by biochemical principles. SC.F.2.4.2 The student knows thatevery cell contains a “blueprint” coded in DNA molecules that specify how proteinsare assembled to regulate cells.Have you ever seen a computer code? Computers understand a language called binary language.
Itis made up of only two options—zero and one. 00001100100110010101000 is an example of a computer code. From simple games to Web browsers, complex software is built by stringing togetherzeros and ones into long computer codes. Think about this as you read this section.◗ Read to LearnGenes and Proteins1. What are the three typesof RNA?________________________________________________________________________118Chapter 11RNAWhat is the role of RNA in a cell? Think of a car factory andan assembly line. A car is a complicated piece of machinery. Butcars are built by following simple steps. Engineers tell workershow to make the cars, and workers follow the directions to buildthe cars on the assembly line.
Suppliers bring parts to the assembly line so they can be used to build the cars. This is similar tothe role of DNA and RNA in a cell. DNA gives the instructionsto make proteins. Workers build proteins. Other workers bringthe parts, in this case, amino acids, to the assembly line. In ourexample the workers are the RNA.Just as in a typical factory, workers have specific tasks. So doesRNA. There are three different kinds of RNA. One type ismessenger RNA (mRNA). It brings instructions from DNA tothe cytoplasm.
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