Although I have studied a series of specimens of this species, it is still poorly known, especially with regard to characters of stature and colour of the various floral
Plants 2.5-5 m tall. Rhizomes (measured from alcohol material) ca 20 mm thick
50. Renealmia thyrsoidea (Ruiz & Pav6n) Poeppig & Endlicher
6 Nucleic Acids and Their Functions
. intRoDuction
This.chapter.is.about.the.molecular.biology.of.the.nucleic.acids,.DNA.and.RNA..As.we.have.stressed.
in.earlier.chapters,.all.biological.macromolecules.have.natural,.complex.3-D.structures.in.which.
the.interatomic.binding.energy.is.minimized;.DNA,.RNA,.and.proteins.are.no.exception..Their.3-D.molecular.structures,.including.chirality,.are.necessary.for.intermolecular.specificity.and.their.
biochemical.and.physiological.activity..Deoxyribonucleic.acid.(DNA).is.unique.in.that.it.appears.in.
all.cells.today.(Prokaryotes,.Archaeans,.Eukaryotes).and.is.used.as.a.linear.programming.code.for.
protein.and.RNA.synthesis.and.control.of.gene.expression..Protein-coding.segments.of.DNA.are.
called.genes;.they.are.also.called.exons.
Portions.of.non–protein-coding.DNA.Nt.sequences,.called.introns,.are.also.implicated.in.con- trol.of.gene.expression,.and.hence,.regulation.of.embryological.development.and.cellular.metab-olism.. Some. introns. also. contain. the. information. for. the. synthesis. of. specific. ribonucleic. acid.
molecules..RNAs.generally.have.the.role.of.implementers.of.DNA.instructions,.and.some.serve.as.
enzymes.or.catalysts.
6.1.1 dna: the coMMand and control nucleic acid
The.basic.organizational.unit.of.the.DNA.polymer.is.the.2′-deoxyribose.(a.sugar).molecule.cova-lently. bonded. to. one. of. two. pyrimidine bases. (cytosine. or.thymine). or. one. of. two. purine bases (adenine.or.guanine)..The.bases.cytosine.(C),.thymine.(T),.adenine.(A),.and.guanine.(G).are.the.
four.coding symbols.in.a.DNA.molecule’s.linear.genetic.code..The.bonding.between.the.base.and.
the.deoxyribose.sugar.involves.the.C1′.carbon.of.the.sugar.and.the.N9.nitrogen.of.a.purine base.(or.
the.N1.nitrogen.of.a.pyrimidine base.in.an.N-β-glycosidic.linkage)..The.base-deoxyribose.subunits.
are.connected.to.one.another.by.3′-5′.phosphodiester.linkages,.as.illustrated.in.the.schematic.of.a.
dsDNA,.four-base.oligo,.shown.in.Figure.6.1..Note.that.the.5′.methyl.group.on.the.deoxyribose.
binds.to.a.phosphate’s.oxygen..Another.phosphate.oxygen.binds.to.the.3′.carbon.in.the.deoxyribose.
ring..Figure 6.1.also.illustrates.the.cross-strand.hydrogen.bonds.between.the.bases.as.dotted.lines..
These.are.the.“glue”.that.holds.the.“rungs.of.the.DNA.ladder”.together.
A.nucleoside subunit.of.DNA.is.defined.as.one.of.the.four.bases.attached.to.the.C1′.position.of.
2′-deoxyribose,.where.the.sugar.lacks.a.phosphate.group.
A.nucleotide (Nt) is.a.nucleoside.having.one.or.two.phosphate.groups.covalently.bonded.to.
the.deoxyribose’s.3′-.or.5′-hydroxyl.group’s.oxygen..A.single.strand.of.DNA.is.thus.composed.of.a.
polymer.formed.from.phosphodiester links.between.alternating.nucleotide.and.nucleoside.sugars..
These.links.are.also.illustrated.in.Figure 6.1.
In.its.simplest.3-D.form,.the.B-DNA helix.contains.two.complementary.polymer.chains.with.
the.same.number.of.bases.organized.as.an.alpha helix. The.two.DNA.strands.wrap.around.the.
helix.axis.with.a.right-hand.spiral;.the.two.polynucleotide.chains.run.in.opposite.(complementary).
directions..The.radius.of.the.alpha.helix.is.about.1.nm,.and.the.repeat.length.(pitch,.or.axial.period).
is.about.3.4.nm..The.bases.are.oriented.perpendicular.to.the.helix.axis..The.axial.displacement.
Introduction to Molecular Biology, Genomics and Proteomic for Biomedical Engineers
(“steps”).between.bases.is.about.0.34.nm,.so.about.10.base.pairs.are.found.in.an.axial.period..Fig-ure.6.2 illustrates.the.α-helix.of.B-DNA.in.a.3-D.side.view..Note.that.from.the.side,.the.B-DNA.
α-helix.has.a.large.and.small.“groove.”.Most.physiological.DNA.is.of.the.B-form.
figuRe . 2-D.schematic.of.a.four-base,.double-stranded.DNA.oligo..The.dotted.lines.in.the.center.rep-resent.the.hydrogen.bonds.holding.the.strands.together..Also,.the.5′.carbon.of.the.end.ribose.on.each.strand.
is.linked.to.a.triphosphate.group..(Adapted.from.Northrop,.R.B..2003..Signals and Systems in Biomedical Engineering..CRC.Press,.Boca.Raton,.FL..Used.with.permission.from.Taylor.&.Francis.)
The.sugar-phosphate.links.of.the.two.DNA.strands.wrap.around.the.helical.molecular.axis.like.
the.railing.of.a.spiral.staircase,.the.“steps”.of.which.are.the.hydrogen-bonded.bases.in.the.center..The adenine.on.one.chain.always.pairs.with.thymine.on.the.opposite.strand;.they.are.held.together.by.two.
weak.hydrogen bonds. A.guanine.on.one.side.always.pairs.with.a.cytosine.on.the.other.strand;.in.this.
case,.the.pair.is.held.loosely.by.three.hydrogen.bonds..Figure 6.3 illustrates.2-D.schematics.of.pairs.of.
purine.and.pyrimidine.bases.and.the.hydrogen bonding.between.them..Approximate.dimensions.and.
angles.are.given..Thus,.because.of.the.complementary.base.structure,.if.we.know.the.base.sequences.
on.one.strand.of.the.DNA.helix,.we.know.it.in.on.the.opposing.complementary.strand..This.redun-dancy.permits.enzymatic.repair.of.damaged.DNA.molecules.in.eukaryotic.cells.
The.backbone.of.polynucleotides.is.highly.charged:.One.unit.(electron).negative.charge.for.each.
phosphate.group.and.two.negative.charges.per.base.pair..If.there.are.no.salt.ions.in.the.surrounding.
aqueous.medium,.there.is.a.strong.electrostatic.repulsion.between.the.two.DNA.strands.and.they.
separate..Therefore,.counterions.are.essential.to.maintain.the.double.helix.structure.in.an.aqueous.
medium..Counterions.effectively.shield.the.charges.on.the.deoxyribose-phosphate.backbone..They.
may.also.contribute.to.an.attractive.interaction.between.the.DNA.strands.
Resting.DNA.in.the.nucleus.(DNA.that.is.not.being.“read”.for.its.codes).is.found.in.association.
with.many.specialized.proteins.called.histones..This.DNA.is.further.wrapped.and.folded.at.five.dif-ferent.length.scales.in.order.to.achieve.a.high-density.packing.factor.for.storage.in.a.cell’s.nucleus..If.
fully.unwrapped.and.laid.end.to.end,.all.DNA.in.a.human.cell would be about 2.m.long [Masum.et.al..
2001]!.Clearly,.from.nuclear.volume.restrictions,.for.a.gene.to.be.read,.the.two.DNA.helix.strands.must.
be.locally.separated..The.single-strand.bases.must.be.read.sequentially.and.then.rejoined.and.refolded,.
both.in.DNA.replication.(in.mitosis.and.meiosis).and.in.protein.synthesis..Little.is.known.about.the.in.
vivo.molecular.mechanisms.that.perform.these.complex.3-D.molecular.manipulations.
6.1.2 rna: the executiVe nucleic acid
If.DNA.is.the.command.and.control.nucleic.acid,.then.by.analogy,.RNA.is.the.executive.nucleic.
acid..It.carries.out.the.instructions.encoded.in.the.DNA.genome.
All. living. cells. we. know. today. (Prokaryotes, Archaea, and Eukaryotes). use. DNA. for. their.
genetic.code..Cells.use.ribonucleic acid.(RNA).to.implement.the.command.and.control.codes.con-tained.in.DNA..RNA.differs.from.DNA.in.that.it.uses.the.pyrimidine.uracil.instead.of.thymine.as.
one.of.its.four.bases..RNA.also.has.a.hydroxyl.(−OH).group.instead.of.hydrogen.on.the.2′.carbon.
of.the.ribose.sugar..Because.of.the.−OH.group.on.the.sugar,.RNA.is.too.bulky.to.form.a.long,.
stable.double.helix..However,.it.does.form.short.regions.of.double.helix.where.there.is.some.base.
complementation.(U.to.A,.G.to.C)..When.base.complementation.is.not.possible,.the.RNA.forms.
Minor groove
Major groove 34 Å
figuRe . A.3-D.rendering.of.a.dsDNA.molecule.showing.its.helical.structure.and.its.major.and.minor.
grooves..To.the.right.is.an.axial.(end-on).view..(Adapted.from.Northrop,.R.B..2003..Signals and Systems in Biomedical Engineering..CRC.Press,.Boca.Raton,.FL..Used.with.permission.from.Taylor.&.Francis.)
0 Introduction to Molecular Biology, Genomics and Proteomic for Biomedical Engineers
single-stranded.hairpin loops, giving.it.a.tertiary.structure..Because.RNA.is.not.restricted.to.the.
more.stable.double.helix,.it.can.form.various.stable.3-D.structures,.as.do.proteins.
The.first.step.in.gene expression.(protein.manufacture).is.called.transcription..In.transcription,.
the.DNA.code.for.a.protein.is.converted.to.messenger.RNA..mRNA,.in.association.with.ribosomes.
premessenger RNA.(pmRNA),.transfer RNA.(tRNA),.ribosomal RNA.(rRNA),.small.nuclear RNAs.
(snRNAs),.and.others..pmRNA.is.a.complementary.RNA.copy.of.a.DNA.gene,.including.any.intron.
Nt.sequences.embedded.in.the.gene..Spliceosome enzymes (see Section.6.2.1).“edit.out”.the.nonpro-tein-coding.intron.Nt.sequences.to.form.a.pure.copy.of.the.protein-coding.gene.as.mRNA,.which.
carries.the.gene.code.from.the.nucleus.to.the.ribosome..tRNAs.read.the.mRNA.codons;.there.are.
tRNAs.for.each.codon.in.the.protein.code..A.tRNA binds.only.to.the.amino.acid (AA).it codes.for..
Thus,.three.RNA.bases.from.the.DNA.gene.form.a.codon.that.specifies.the.linear.placement.of.an.
AA.in.the.primary.structure.of.the.protein.being.synthesized..Figure 6.4 illustrates.a.“typical,”.trilobe.
tRNA.molecule.with.the.anticodon.for.the.AA.methionine.(Met)..Methionine.is.the.“start”.tRNA.
Note.that.a.tRNA.molecule.has.three.major.hairpin.loops,.including.the.anticodon loop.at.one.
end..The.anticodon.loop.has.three.particular.bases.that.match.the.particular.codon.bases.in.the.
To deoxyribose 5'-phosphate To deoxyribose
5'-phosphate C1
C1
H
To deoxyribose 5'-phosphate
To deoxyribose 5'-phosphate
Adenine Thymine
1.11 nm c. 0.3 nm
H-bonds
Guanine Cytosine
H
figuRe . Details. of. the. hydrogen. bonds. linking. opposing. purine. and. pyrimidine. bases. in. dsDNA..
Dimensions.are.given.in.nanometers..(Adapted.from.Northrop,.R.B..2003..Signals and Systems in Biomedical Engineering..CRC.Press,.Boca.Raton,.FL..Used.with.permission.from.Taylor.&.Francis.)
mRNA.which.code.for.a.certain.AA..The.distal.end.of.the.tRNA.molecule.has.bases.that.cause.it.to.
have.a.highly.specific.affinity.to.the.particular.AA.the.codon.codes.for..The.AAs.are.found.in.the.
cell’s.cytosol..The.tRNA.structure.is.highly.conserved.throughout.all.life-forms;.only.the.“variable.
arm”.exhibits.variability.in.its.length.and.structure;.3.to.21.Nts.can.be.in.it..The.3-D.structure.of.
tRNA.has.an.approximate.“L”.shape,.with.each.leg.about.6.nm.long;.the.anticodon.site.and.the.AA.
acceptor.site.are.about.7.6.nm.apart..Unusual.bases.are.used.in.the.construction.of.tRNA:.The.TψC.
arm.contains.pseudouridine,.and.the.D.arm.contains.dihydrouridine.
. Rnas coDeD by Dna 6.2.1 introduction
We.have.seen.in.Chapter.6.that.various.RNAs.are.the.“executive.molecules”.that.carry.out.the.
instructions.of.the.DNA.genome.(the.“planner”);.they.are.essential.for.protein.synthesis.as.well.
as.communication.between,.and.control.of,.cellular.processes..The.RNA polymerase enzymes.are.
necessary.to.assemble.various.RNA.molecules.from.DNA.codon.“templates”.in.a.cell’s.genome..
There. is. a. large. number. of. RNA. classes:. We. have. already. encountered. pmRNA,. mRNA,. and.
tRNA..In.addition,.there.are.ribosomal RNA.(rRNA),.small nuclear RNA.(snRNA),.small nucleo-lar RNA.(snoRNA),.micro-RNA.(miRNA),.and.XIST RNA..In.each.class.of.RNA,.there.can.be.
several.subclasses.
All.three.classes.of.RNA polymerase.(RNAP).are.dependent.upon.a.DNA.template.in.order.to.
synthesize.RNA..The.RNA.synthesized.is.thus.complementary.to.the.dsDNA.strand.used.and.iden-tical.to.the.nontemplate.dsDNA.strand..The.nontemplate.strand.is.called.the.coding strand.because.
its.Nt.sequences.are.identical.to.those.of.the.RNA,.except,.as.noted.earlier,.the.pyrimidine.uracil is.
used.instead.of.thymine.in.RNA..RNAP-I.synthesizes.three.of.four.rRNAs..RNAP-.II.synthesizes.
the.mRNAs.and.some.of.the.snRNAs.involved.in.RNA.splicing.(editing.out.introns.from.pmRNAs)..
RNAP-III.synthesizes.the.tRNAs,.one.rRNA,.and.some.snRNAs.[King.2005a].
In. spite. of. the. molecular. complexity. of. eukaryotic. cells,. we. should. take. solace. in. the. fact.
that.there.is.a.finite.number.of.molecular.species.in.each.cell,.and.generally.only.a.few.genes.are.
expressed.in.a.given.cell.at.a.given.time.
6.2.2 Messenger rnas
As.we.have.seen.earlier.and.in.Chapter.6,.this.class.of.RNAs.is.used.as.the.active.genetic.coding.
template.by.the.translational.machinery.to.determine.the.primary.AA.sequence.in.a.protein.being.
synthesized..Premessenger.RNA.(pmRNA).is.first.synthesized.from.an.exposed.DNA.strand.in.the.
nucleus.by.the.enzyme.RNAP-II..In.eukaryotes,.the.pmRNA.can.contain.the.Nt.sequences.from.one.
or.more.introns.embedded.in.the.gene’s.DNA..Intron.Nt.sequences.do.not.code.protein.and.must.be.
edited.out.of.pmRNA.before.precise.protein.synthesis.can.commence..The.molecular.algorithms.
that.accomplish.splicing.out.the.introns.in.pmRNA.are.complex.and.will.not.be.covered.here..(The.
interested.reader.should.consult.King.2005c.and.Kimball.2005c.).Splicing.basically.cuts.out.the.
string.of.intron.Nts.from.the.pmRNA.and.then.rejoins.the.cut.ends.to.form.a.purely.protein-coding.
mRNA.molecule,.which.then.leaves.the.nucleus.and.joins.a.ribosome.in.the.cytoplasm.where.the.
actual.protein.assembly.occurs.
6.2.3 transfer rnas
There.are.more.types.of.tRNAs.in.cells.than.there.are.codons..The.human.DNA.genome.uses.59.
codons.to.code.the.20.amino.acids.used.to.build.proteins.(counting.ATG.for.Start Coding.and.the.
AA.methionine)..However,.it.has.been.found.that.the.human.genome.has.497.genes.coding.tRNAs!.
About.a.quarter.of.these.genes.are.found.together.on.chromosome.6;.the.others.are.found.on.all.
other.chromosomes.except.22.and.Y.
Introduction to Molecular Biology, Genomics and Proteomic for Biomedical Engineers
The.structure.of.each.tRNA.is.coded.by.a.separate.gene..tRNAs.typically.contain.only.73.to.
93.Nts..As.we.have.seen.earlier,.they.have.a.curious.three-loop.structure:.the.loops.are.formed.by.
unpaired.Nts;.the.paired.sections.are.dsRNA.
Each.species.of.tRNA.has.an.amino.acid.attached.to.its.3′ end.one.of.the.20.types.of.AAs.used.
in.all.proteins..Note.that.not.all.59.codons.in.the.protein.code.have.a.specific.tRNA,.because.of.
wobble.in.the.third.Nt.position.of.anticodons..On.the.distal.loop.of.tRNA.are.three.exposed.Nts.(the.
anticodon),.which.are.complementary.to.the.mRNA.codon..With.the.help.of.ribosomal.enzymes,.
the.anticodon.binds.to.the.active.codon,.and.the.tRNA.delivers.its.AA.to.be.added.to.the.peptide.
being.assembled.by.the.ribosomal.enzyme.complex.
The wobble hypothesis, proposed.by Francis.Crick.in.1966,.suggested.that.although.the.speci-ficity.between.the.first.two.Nts.of.the.mRNA.codon.and.the.tRNA.anticodon.had.to.be.exact,.it.did.
not.need.to.be.exact.in.the.third.Nt.position..Crick.proposed.that.nonstandard.base.pairing.might.
occur.between.the.Nt.base.in.the.5′.position.of.the.tRNA’s.anticodon.and.the.base.in.the.3′.position.
of.the.codon.in.question..Crick.postulated.that.the.base.pairing.schemes.shown.in.Table.6.1.were.
operative.between.tRNA.anticodons.and.mRNA.codons.
It.appears.that.the.so-called.wobble rules.are.not.followed.exactly..If.they.were,.only.31.tRNAs.
would.be.needed.to.code.20.AAs..However,.it.has.been.found.that.well.over.31.tRNAs.exist.in.cells..
For.example,.in.E. coli.there.are.five.distinctly.different.tRNA.molecules.with.which.the.amino-acyl-tRNA synthetase.(AARS).enzymes.bind.to.the.AA.leucine.
The.rogue.tRNA.anticodon.base.inosine.appears.to.be.generated.from.adenosine.by.a.gene.that.
produces.a.protein.enzyme,.adenosine deaminase. Even.with.this.inosine.ambiguity,.the.genetic.
code.in.a.species.is.stable.and.almost.error.free..Biochemical.systems.do.not.tolerate.even.small.
errors.in.protein.composition..The.error.rate.in.attaching.the.correct.AA.to.the.correct.tRNA.is.esti-mated.to.be.10−4..The.AARS.enzymes.use.two.means.to.ensure.this.level.of.coding.accuracy:.(1).the.
AA.specificity.“pocket”.in.a.specific.AARS.will.only.bond.to.AAs.similar.in.size.and.charge,.and.
(2).the.AARS.has.a.unique.“proofreading”.capability.with.which.it.detects.its.binding.to.a.wrong.
AA.and.then.hydrolyzes.it.(destroys.it).before.it.can.be.attached.to.the.end.of.the.tRNA.
6.2.4 riBosoMal rnas
Before.being.incorporated.into.ribosomes,.rRNA.is.processed.in.the.subnuclear.organelle,.the.nucle-olus..The.eukaryote.28S,.18S.and.5.8S.rRNAs.are.processed.from.a.long.primary.rRNA.transcript.
(a.“pre-rRNA”).containing.long.noncoding.spacer.and.flanker.sequences.of.Nts..Psuedouridylation.
and.2′-O-ribose.methylation.of.the.rRNAs.described.earlier.occur.before.nucleolytic.processing.of.
the.pre-rRNA.[Kiss.2005].
Ribosomes.are.ubiquitous.cellular.organelles,.found.throughout.all.domains.of.life,.that.cata-lyze.the.sequential.polymerization.of.amino.acids.into.proteins.following.the.linear.genetic.code.
contained.in.messenger.RNA..They.are.about.20.nm.in.diameter.and.can.only.be.visualized.with.
electron.microscopes..Ribosomes.are.built.of.two.complex.molecular.subunits.that.act.together.to.
table .
Wobble ambiguities in mRna–tRna coding
′ tRna, anticodon base ′ mRna, codon base
A.(adenosine) U
C.(cytosine) G
G.(guanosine) C.or.U
U.(uridine) A.or.G
I.(inosine) A.or.C.or.U
effect.protein.synthesis..Both.subunits.are.complexes.of.mRNA.and.many.proteins;.they.surround.
and.travel.along.a.mRNA.molecule.as.the.protein.is.made..The.larger.subunit.creates.the.peptide.
bonds. between. adjacent. AAs. and. provides. the. path. through. which. the. linear. polypeptide. chain.
emerges. from. the. ribosome.. The. smaller. unit. acts. more. in. an. executive. manner;. it. decodes. the.
genetic.message,.discriminates.between.correct.and.incorrect.aminoacylated.tRNA.molecules,.and.
controls.the.fidelity.of.codon.and.anticodon.interactions.and.in.mRNA-tRNA.translocations.[Yon-ath.2001]..A.major.difficulty.in.discovering.the.structures.and.functions.of.rRNAs.is.the.fact.that.
they.are.intimately.surrounded.by.and.bonded.to.many.protein.molecules.
Most.of.what.we.know.about.rRNA.and.ribosomal.structure.comes.from.the.study.of.bacte-rial.ribosomes..Bacteria.grow.rapidly.and.their.ribosomes.are.easy.to.harvest..The.bacterial.large.
ribosomal. subunit. (50S). contains. 33. proteins. and. two. rRNA. molecules;. one. of. 23S. (2904. Nts),.
the.other.of.5S.(120.Nts)..(The.S-number.in.50S.refers.to.the.Svedberg constant,.which.relates.to.
the.amount.of.necessary.g-force.created.by.a.centrifuge.to.sediment.the.molecule.from.a.standard.
solution.).The.30S.smaller.ribosomal.subunit.contains.21.proteins.associated.with.a.16S.(1542.Nts).
rRNA.molecule.
The. eukaryotic. cell. mitochondrial. large. (60S). subunit. contains. about. 49. proteins. and. three.
rRNA.molecules;.one.28S.(4,700.Nts),.one.5S.(120.Nts),.and.one.5.8S.(160.Nts)..The.eukaryotic.
smaller.subunit.has.about.33.proteins.and.an.18S.(1,900.Nts).rRNA.molecule.(these.numbers.vary.
slightly.with.species).
All. mitochondrial. ribosomes. also. have. two. subunits:. The. larger. has. 48. proteins. and. a. 16S.
(1,650.Nts).rRNA.molecule..The.smaller.has.29.proteins.and.a.12S.(950.Nts).rRNA.molecule.[Kim-ball.2003a].
Much.of.what.we.know.about.the.3-D.structures.of.ribosomal.subunits.comes.from.x-ray.crys-tallography.on.protein.and.RNA.crystals.prepared.from.the.purified.molecular.components.(e.g.,.
70S,.50S,.and.30S.concentrates.from.bacteria)..The.entire.process.is.very.specialized.and.requires.
nearly. monochromatic. x-rays. produced. from. a. linear. accelerator. such. as. at. CERN. or. the. U.S..
Argonne.National.Lab.in.Chicago..The.studies.reveal.molecules.of.considerable.3-D.complexity..
Published.pictures.of.a.bacterial.ribosomal.subunits.(rRNA.together.with.proteins).appear.like.a.
dish.of.pasta.(noodles,.spaghetti,.rotini,.etc.).all.joined.together.and.cross-linked.at.various.points.
[Harms.&.Schlünzen.2005].
Moore.and.Steitz.[2002] stated.the.obvious:.No.enzyme.system.as.complex.as.the.modern.mam-malian.ribosome.could.possibly.have.emerged.from.the.“primordial.soup”.in.a.single.step..They.
argued.that.the.ribosomes.in.the.first.cells.were.very.likely.composed.entirely.of.RNA..Moore.and.
Steitz.went.on.to.point.out.that.the.functional.core.of.the.modern.eukaryotic.ribosome,.its.decod-ing.site.and.its.peptidyl transferase.center,.consist.primarily.of.RNA..Also,.the.bulk.of.ribosomal.
proteins.are.found.on.its.surface,.well.removed.from.its.RNA-based.functional.centers..Probably.
the.earliest.ribosomes.were.almost.certainly.smaller,.simpler.structures.than.the.modern.molecular.
complex,.and.had.more.limited.functions..These.“protoribosomes”.might.have.been.a.one-subunit.
complex.that.catalyzed.peptide.bond.formation.in.an.uncoded.way,.and.thus.made.small.oligopep-tides.of.random.sequence.that.were.not.proteins.in.the.modern.sense,.and.may.have.found.some.
other.purposes.in.the.protocell.
Note.that.the.protoribosome.was.in.fact.a.ribozyme,.that.is,.an.RNA.molecule.capable.of.cataly-
Note.that.the.protoribosome.was.in.fact.a.ribozyme,.that.is,.an.RNA.molecule.capable.of.cataly-