Biology - An Illustrated Guide to Science (794127), страница 7
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It is less efficient than aerobicrespiration, producing less energy pergram of glucose, so it is usually onlyused when aerobic respiration is notpossible.● In animals the supply of oxygen toactively respiring cells may not be ableto keep up with the demand. The cellswill already be respiring as rapidly aspossible aerobically but need toproduce more energy—perhaps dueto excessive stress or physical activity.At this point anaerobic respirationbegins, so both forms of respirationare operating at the same time.● The process converts glucose intopyruvic acid and makes energy in theform of ATP.●The “oxygen debt”In mammals anaerobic respirationgives a useful extra energy boost instressful situations.
However, itproduces toxic lactic acid (see topdiagram).● Once the stress is over and oxygensupplies are plentiful again, the lacticacid must be destroyed. The amountof oxygen needed to do this is calledthe “oxygen debt.”●12ATP2ADPNADglucoseNADNADH2Alcoholic fermentation4ADPAnaerobic respiration producesalcohol (ethanol) in yeasts and manyother fungi (see bottom diagram). Thisis the basis of the brewing and bakingindustries.● Alcohol is toxic, and yeasts will poisonthemselves if the alcohol they produceas a waste product of respirationexceeds about seven percent of theirenvironment.NADH254ATP436C6H12O6(glucose)2C2H5OH + 2CO2 + 2ATP(alcohol)(energy)123456GlucoseFructose diphosphatePyruvic acidAcetaldehyde (ethanal)EthanolCarbon dioxide© Diagram Visual Information Ltd.●38UNITYKey wordscentromerechromatidchromatinchromosomeDNAgeneChromosome structureChromosomechromatidcentromereVisible during division●Chromosomes are large structuresfound in the nucleus of cells.
They areonly visible during cell division. Theytake up certain dyes very well, and soare often treated with these beforethey are observed with a lightmicroscope. “Chromosome” is Greekfor a “colored body.”Chromatids andcentromeresChromosomes have three clear parts:two pairs of chromatids, which extendfrom either side of a cetromere. Thechromatids on one side of thecentromere are always the samelength, but this can be different fromthe length of the two chromatids onthe other side.● During cell division, the centromeresplits to create chromosomes withsingle chromatids.
These thenduplicate to return to pairs ofchromatids.●SupercoilsA chromatid is a coiled springof protein anddeoxyribonucleic acid (DNA)called chromatin. The proteinand DNA are, in turn, coiledinto a spiral.● The spiral coil is called asupercoil.● The supercoil unravels whenthe cell is not dividing so thatthe enzymes of the nucleus canget easy access to the genes inthe DNA strand.● The supercoil condenses duringcell division to make it easier toensure that each daughter cellgets a copy of all of the genes fromthe parent.© Diagram Visual Information Ltd.●protein and DNA chromatin fiberProtein and DNA supercoilchromatin is highlycondensed in a supercoilDNA double helix39Summary of proteinsynthesisUNITYKey wordsamino acidenzymemessenger RNAribosometranscriptionProtein synthesisTranscriptionmovement of mRNAfrom nucleus tocytoplasmnuclearmembranecirculation of tRNAfrom cytoplasmicpool to ribosometo cytoplasmic poolfree amino acidDNAnuclearporeProtein synthesis●Protein synthesis requires two majorprocesses: translation andtranscription.TranscriptionTranscription takes place in thenucleus and involves the creation of amolecule of mRNA with a basesequence that mirrors the sequence ofthe relevant portion of the DNAmolecule.
This means that a singlelength of DNA can give rise to manycopies of mRNA.● The mRNA molecules leave thenucleus through a nuclear pore andmoves to the ribosomes.●tRNAtRNA carrying aminoacidmRNAtransfer RNAtranslationpolypeptide chainTranslationTranslation is the process ofconverting information on themessenger RNA (mRNA) moleculesinto a sequence of amino acids. It iscatalyzed by ribosomes.● Ribosomes depend on another nucleicacid, called transfer RNA (tRNA).● tRNA has a clover leaf shape with anamino acid attached at one end and atriplet of bases revealed at the otherend.● Each tRNA molecule carries aparticular amino acid and has aparticular triplet revealed.● When the mRNA molecule threadsthrough the ribosome, tRNAmolecules with corresponding tripletcodes fall into place. Enzymes join theamino acids at the other end togetherto build the new protein chain.TranslationribosomemRNA© Diagram Visual Information Ltd.●40UNITYKey wordsdeoxyribonucleicacidnucleotideBase pairingPortion of DNA moleculehydrogen bonddeoxyribose (five-carbon sugar)Nucleotide unitsA nucleotide consists of threeparts: a phosphate group, afive-carbon sugar, and anorganic base.● The phosphate–sugar part ofthe nucleotide joins withother nucleotide molecules toform a strong backbone.● The complete nucleotidechain is called a nucleic acid.Deoxyribonucleic acid orDNA is a nucleic acid.phosphate●Organic basesThere are four possible basesin DNA: adenine, thymine,guanine, and cytosine.● The nucleotides in a moleculeare arranged in a long chainjoined by thesugar–phosphate groups.
Thebases protrude from thisbackbone.● If two chains are broughtclose together, the bases canlink up by hydrogen bonds toform a “ladder” where thebases’ links act as the rungs.However, the bases can onlylink up in particular patterns:adenine links with thymineand guanine links withcytosine (see page 41).● The hydrogen bonds betweenthe base pairs are weaker thanthe sugar–phosphate links, sothat pulling on a DNAmolecule splits it down themiddle between these bonds.● If you have one half of anucleic acid molecule, you cancreate the other half byjoining the correct bases andthen bonding them withsugar–phosphate groups.
Thisis how DNA molecules arecopied.thymineHNHNC●© Diagram Visual Information Ltd.basesadenineCNOHCH3CCNCNHCNCHCCNOHHNHOCCNNHCCNCNHHNCCCNHCNOHcytosineguaninebasesH41DNA structureUNITYKey wordsSchematized double helixamino acidDNAnucleotideArrangement ofnucleotides in DNAPhosphate-sugar backbonedeoxyriboseDNA consists of two, intertwinedchains of nucleotides that form adouble helix.● The backbone of these chains, foundon the outside of the helix, is a longsequence of sugar–phosphate groups.These linkages hold the moleculetogether strongly to make DNA a veryresilient molecule. DNA from ancientsources can still be identified longafter many other chemicals havedecayed beyond recognition.●phosphateGuanineCytosineAdenineThyminepaired basesAInformation carrierTCGTA●ATGCThe complex structure of DNA allowsit to carry and duplicate information inthe form of a code.
The sequence ofbases in a chain can be used tosequence amino acids in a protein.This allows the cell to store theblueprint for any protein as a “triplet”code of bases.View from aboveDNA strandbase pairsDNAstrand© Diagram Visual Information Ltd.helixaxis42UNITYKey wordsDNAenzymenucleotideDNA replicationDNA replicationdeoxyribose(five-carbon sugar)phosphateGuanineTwo possiblemethodsDNA consists of twointertwined chains ofnucleotides that form adouble helix. Two modelshave been proposed toexplain how DNA is copied:the “new build” or the“semi-conservative” model.● In the new build model acompletely new DNAmolecule would be createdfrom scratch. The old DNAmolecule would beuntouched, but perhapsused as a model to copy.● The semi-conservativemodel (see diagram)assumes that the DNAmolecule unzips to createtwo separate but completenucleotide chains.
Newbases are added to each ofthese chains, and these arethen linked together by asugar–phosphatebackbone.● Evidence from radiotracershas shown that the semiconservative model iscorrect and that each newDNA molecule containshalf of the originalmolecule.●Enzyme controlled© Diagram Visual Information Ltd.●The replication, orduplication, of DNA isclosely controlled byenzymes. This reaction canbe quite rapid. In bacteriacomplete DNA moleculescan be copied in fewerthan 20 minutes underoptimum conditions.free nucleotidesin nucleoplasmCytosineAdenineThymineDNA moleculeEnzymes and ATP breakhydrogen bonds andDNA chains separate.Free nucleotides findtheir complementarybases.Two new identical DNAmolecules result.43DNA transcriptionUNITYKey wordsDNA transcriptiondeoxyribose(five-carbon sugar)GuanineAdeninephosphateCytosineThymineRUricil (substituted forthymine in RNA)DNAenzymemessenger RNAnucleotideribosomeTypes of nucleic acidRfree RNAnucleotidesinnucleoplasmThere are two major groupsof nucleic acids:deoxyribonucleic acid (DNA)and ribonucleic acid (RNA).● DNA is always found in thenucleus and has thecharacteristic double-helicalstructure.● RNA has a more variablestructure than DNA and hastwo major forms: messengerRNA (mRNA) and transferRNA (tRNA).
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