26766888 15 RNA Jack of All Trades Master of None

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D R A F T15: RNA

Q - Jack of all trades, master of none?



D R A F TChapter Map

RNA

Catalysis

Replication

Information storage

G

GGG

G

G

GGGGG

G

GG

GG

Y

C

C

C CC

CCC

C

C

CC

C

C C

C

UU

U

TUU

U

U

U

U

UUU A

AA A

A

A

A

AA

A

A

AA

A A

A

DD

Ψ

Ψ

G

GG

G

G

G

A

A

CC

anticodon

5’

3’

Assists with



D R A F T

15.1: The Question 301

15.1 The Question

Figure 15.1: Modern cells are highlycomplex structures. Source: NHGRI

Modern cells, be they a bacterium or

part of a large animal or plant, are

by any standards very sophisticated as

illustrated in Fig.15.1. They consist

of many highly adapted interdependent

molecules.

For example proteins are manufac-

tured with the help of highly special-

ized molecular complexes called ribo-

somes but ribosomes themselves con-

tain proteins. How is this possible? Is

this another one of life’s chicken and

egg problems? To find out, we need

to have a closer look at how protein

production actually works and take it

from there. It turns out that special

molecules of ribonucleic acid (RNA)

bring information about the structure of the protein from DNA to the ribosome

and that different RNA molecules bring the building blocks (amino acids) from

which the ribosome produces the protein. Initially, therefore, RNA appears in a

somewhat auxiliary role. Important but not glorious.

Some further inspection shows that RNA is quite flexible, and involved in a

rather large number of processes in the cell. Considering the hard work done by

proteins and the center-stage role of DNA in storing genes, does this make RNA a

jack of all trades? Perhaps an evolutionary relic soon to be replaced?To probe further, let us contemplate some essential aspects which one would

expect any cell, even the earliest ones to have.

Catalysis

Replication

Information storage

Fig. 15.2: Key requirements for

life.

If we look at early cells, we know that there must have been some kind of

information storage to make heredity possible, some kind of (self-) replication to

allow for offspring and some kind of catalysis to speed up otherwise tardily slow

chemical reactions. Due to its complicated nature, interdependency between dif-

ferent types of molecular groups is almost certainly something that has evolved

as a specialization to specific requirements like being the most efficient catalyst.

Consequently, we may surmise a single class of molecules that is at least adequate

in any of the three necessary functions to have originated first.

If we take a hint from modern cells we find that there are three types of poly-mers to consider: DNA, RNA and proteins. As argued in chapter 14, only polymers

are considered to be suitable for functions like heredity in living systems. Now

clearly, one has to be careful when trying to draw conclusions about pre-modern

cells by starting from moderns cells. However, since the modern genetic appara-

tus in essence dates back some 3 billion years and since evolutionary processes do

have a clear path even when their details are unknown, it is reasonable to take the

modern cell as a starting point and work back from there.

Amongst DNA, RNA and proteins, RNA is the only type of molecule that



D R A F T

302 15: RNA

is known to be good albeit perhaps not always excellent with regards to all three

must-haves of early cells: information storage, self-replication and catalysis. DNA,

while excelling in information storage is at best marginal in catalysis (not enough

flexibility in its 3D structure) and self-replication (as a consequence of not being

able to catalyze the replication). Similarly, proteins excel in catalysis but are poorchoices for information storage (no base-pairing and hence hard to copy) and self-

replication (most 3D structures cannot be reversibly unfolded). Hence amongst

the three modern types of molecules, RNA seems to be good at many things but

outstanding at nothing. Hence the chapter question Is RNA a jack of all trades

and a master of none?

In order to answer this question, we need to understand what RNA can do.

Therefore let us now look at RNA in the context of each of the three functions that

must have been available to some degree in early cells: replication, catalysis and

information storage.

15.2 RNA and replication

In moderns cells, genetic information is stored on DNA and much of the catalytic

work carried out by proteins. Although both DNA and proteins are polymers built

up of monomers that can be in any order, they are otherwise rather dissimilar. DNA

has only four distinct monomers while proteins have twenty in humans. Further-

more, the structure of the nucleotide monomers that form DNA is very different

from that of the amino acid monomers that form proteins: the nucleotides consist

of a phosphate groups, a five carbon sugar and a base while an amino acid consists

of an amino group, a carbon atom with a side chain and a carboxyl group.

It is therefore necessary to employ some sort of a mechanism which translates

the information about a protein that is stored in DNA as a nucleotide sequenceto the correct amino acid sequence found in a protein. It turns out that this is

achieved with the so-called genetic code discussed in section 19.10 in the context

of information processing where groups of three successive nucleotides encode for

a specific amino acid.

A - T (DNA)U (RNA)

G - C

Fig. 15.3: Nucleotide base-pairs

are A-T, G-C for DNA and A-U,

G-C for RNA.

Contrary to the huge differences between the building blocks of proteins and

DNA, DNA and RNA consist of very similar, if not identical monomers. Both DNA

and RNA are built up of nucleotides containing the bases adenine (A), guanine

(G), and cytosine (C), while DNA further uses thymine (T) and RNA uracil (U),

the unmethylated form of thymine. Thus both DNA and RNA use four distinct

nucleotides.

The sugar in the DNA nucleotides lack an oxygen atom in the five carbonsugar ribose hence called deoxyribose that the RNA nucleotides do have. A special

property essential for the process of information storage and processing is that

these nucleotides can pair up in predetermined and generally fixed ways. The base

adenine (A) can pair up with the base thymine (T) or uracil (U) while the base

guanine (G) can pair up with cytosine (C). Since the pairs are formed between the

bases of these nucleotides, such pairs are generally referred to as base pairs.

The bonds between the base pairs are hydrogen bonds and as a consequence

quite weak. That means that they can fairly easily be broken if so desired.



D R A F T

15.3: RNA and catalysis 303

Let us now have a bit a closer look at how RNA assists in the process of ob-

taining a protein from a sequence of nucleotides in DNA. When a gene encoding a

protein needs to be expressed, first the relevant sequence of nucleotides is copied

onto a strand of so-called messenger RNA (mRNA) with the help of RNA poly-

merase such that the mRNA strand is exactly complimentary to the DNA beingcopied. This process is called transcription. Now DNA has two complementary

strands so how how does RNA polymerase know which one to copy? RNA poly-

merase only works in one direction and thus which of the two strands is copied

is predetermined. If this weren’t so, then all copyable regions would need to be

symmetric, clearly an undesirable situation.

Direction of Movement

RNA Polymerase

Active site

3’5’

D N A

DNA

unwinding

DNA

rewinding

5’

R N A

Figure 15.4: Schematic representation of

transcription.

At first, in what usually is referred

to as initiation, RNA polymerase binds

to a specific region of the DNA iden-

tified by a certain sequence of nu-

cleotides (e.g. CAAT in eukaryotes).

After stabilization with the help of asmall protein called factor sigma, the

RNA polymerase separates the double

stranded DNA to form a bubble so that

it can pair the first nucleotide monomer

with the beginning of the DNA se-

quence to be copied. It then moves

down the DNA pairing one nucleotide

after another to the DNA while attach-

ing that nucleotide covalently to the

growing RNA sequence (this process is generally called elongation) as illustrated

in Figure 15.4. Rather than leaving the growing RNA sequence paired with the

DNA, RNA polymerase detaches the newly formed RNA trailing a few nucleotides

behind the attachment site such that the DNA can rewind. Finally when the RNA

polymerase reaches a stop signal in the DNA, termination occurs. In eukaryotes,

the resulting RNA strand is then post-processed before being translated into a pro-

tein by a ribosome while in prokaryotes, the mRNA can be translated immediately.

It should come as little surprise that quite some energy is necessary to carry out

the process described in the previous paragraph. How is this energy supplied?

Rather than having a separate energy and nucleotide sources, RNA polymerase

uses energy-rich triphosphate versions of the nucleotides, i.e. ATP, UTP, CTP, GTP.

When these triphosphate nucleotides arrive at the RNA polymerase, the energy

stored in the phosphate-phosphate bonds is released by splitting the triphosphate

nucleotides into phospate groups and the needed RNA nucleotide monomer.

15.3 RNA and catalysis

The catalytic activity of a molecule is strongly dependent on its three-dimensional

structure since the reaction rate of substrates only increases significantly if the

substrates are brought together in exactly the right way. This then requires specific

chemical properties from the catalyst to first bind the substrates and secondly the



D R A F T

304 15: RNA

correct spatial arrangement so that the substrates are spatially suitably oriented for

the reaction to proceed at a fast pace. Furthermore, in many cases, an enzyme needs

to be able to (partially) change shape to e.g. first capture the substrates, then with a

slightly different conformation catalyze the reaction, and lastly change shape one

more time to release the product - quite akin to the workings of a tool like pair of plyers.

In the same way, in order for RNA to be able to catalyze a large range of

different reactions it therefore needs to be able to assume many different shapes.

Catalytic RNA molecules are often referred to as ribozymes by combination of the

words ribonucleic acid and enzyme.

Although RNA as such is single stranded, the fact that it is made up of nu-

cleotides that allow for complimentary base pairing means that an RNA sequence

can have some of its sections pair up with other of its sections thus creating intricate

three dimensional structures. Furthermore, it turns out that some short structural

elements are used quite frequently as illustrated in Figure 15.5

5’ 3’

single strand

5’ 3’

3’ 5’

double strand

5’ 3’

5’3’single nulceotide

buldge

three nulceotide

buldge

5’ 3’

5 ’ 3 ’

5’ 3’hairpin loop

5’ 3’

5’

3’

3’

5’

three stem

junction

5’3’

5’ 3’

3’5’ 3’

5’

four stem

junction

base pair

unpaired nucleotide

Figure 15.5: Some common elementary RNA secondary structures.

In modern cells ribozymes are quite rare with the exception of the for life on

earth essential ribosome which contains relatively large ribosomal RNA molecules

generally abbreviated as rRNA.

In ribosomes, the messenger RNA is translated into proteins according to the

genetic code which matches sequences of three nucleotides to certain amino acidsby stringing these amino acids together with covalent bonds.

Although ribosomes also contain about 35% proteins besides rRNA (3 rRNA

molecules in prokaryotes and 4 rRNA molecules in eukaryotes), the catalytic ac-

tivity needed for joining amino acids into a protein is carried out by the rRNA.

In bacteria, only one type of RNA polymerase is used but in eukaryotes, there

are three types: RNA polymerase I is responsible for three of the four rRNA

molecules by transcribing a precuror rRNA which is then modified into the three

types of rRNA while RNA polymerase III directly transcribes the fourth rRNA



D R A F T

15.4: RNA and information storage 305

molecule. On the other hand, mRNA molecules are transcribed by RNA poly-

merase II which is similar to the RNA polymerase of bacteria.

Just like in the case of enzymes, ribozymes often have metal atoms to assist

them in their function. Due to the relative rarity of rybozymes versus enzymes

in modern cells, one could suspect that ribozymes are not as versatile. However,experiments have shown that ribozymes can catalyze a great number of reactions.

The key difference with enzymes is that ribozymes appear to have in general lower

maximum reaction speeds. It is therefore quite conceivable that many reactions

which are now catalyzed by enzymes once were catalyzed by ribozymes during

the earlier stages of evolution.

What we therefore see as with regards to the catalytic activity of RNA is that

while perhaps often simply adequate, in a case where it really counts, namely pro-

tein production, it is very capable.

15.4 RNA and information storage

In order to be an efficient information storage medium, a molecule made of a vari-

ety of monomers (minimally two) that can be strung together in arbitrary sequences

is required. As mentioned above, out of the main molecules found in biological

systems, this points towards DNA, RNA and proteins.

However, the ability to store information is of little use unless it can also ef-

ficiently be accessed or in other words read out. A key advantage of DNA and

RNA over proteins in this regard is the mechanism of base-pairing. Due to base

pairs, exact complementary copies of sections or entire RNA or DNA molecules

can be made. This is not readily possible with proteins. A second issue is that of

conformation. In general, the three-dimensional structure of a complicated RNA

or protein molecule does not allow access to all (or most) of its molecules sincethe overall structure cannot reversibly be unwound. Of course only small sections

could be used for information storage but hat would be inefficient.

Consequently, proteins are not particulary suitable for information storage.

RNA, on the other hand, is suitable due to it allowing for base-pairing but its fold-

ing would imply some limitation. In this sense it is perhaps not unexpected that all

information in modern life is stored on DNA.

There is, however, good reason to believe that RNA may have fulfilled the role

of information storage medium earlier on during evolution.

Fig. 15.6: Formaldehyde can formribose in an environment that may

have been present on the young

earth.

So what would the reasons be to assume that RNA preceded DNA? As per

its name, RNA contains the five carbon sugar ribose that can relatively easily be

formed from formaldehyde (H2CO). The deoxyribose in DNA on the other hand isharder to obtain. In modern cells, it is manufactured from ribose with the help of

an enzyme. However, chemically, the deoxyribose-phosphate backbone of DNA

is significantly more stable than RNA and hence it is understandable that DNA

would be the preferred infomation storage molecules. What this does not imply is

that RNA is unsuitable - rather that it is not as suitable.

Well then, perhaps RNA is not so good in information storage and not a mas-

ter at this. Although that may be true to some degree, one should nevertheless be

somewhat careful with such a statement. The genome of many viruses is made



D R A F T

306 15: RNA

up of RNA only. Indeed, viruses with both RNA and DNA are rather rare. Even

though it is arguable whether viruses are life forms or not, it is clear that informa-

tion is being transported. Therefore, at least under certain circumstances, RNA can

be a good information carrier.

In the context of viruses, it is notable that viral genomes can be single strandedor double stranded, be they made up of DNA or RNA. Hence in viruses we find an

example of single stranded DNA.

15.5 RNA and computation

Computational processes in general contain instructions that need to be translated

and control statements that influence the sequence of how these instructions are

executed. For example, in a simple program we may want to count from 1 to 10

and display the result on a monitor on the condition that the result is larger than 20.

In order to do this in for example the programming language C++, we could write

the program

Fig. 15.7: The keyboard, a

common means to enter a

program.

#include<iostream>

int main()

{

int i;

int sum = 0;

for(i=1;i<=10;i++){ sum = sum + i; }

if(sum > 20){ std::cout << sum << endl; }

return 0;

}

The first line indicates that a set of instructions defining input and output needs

to be used. The line int main() indicates the start of the code that needs

to be processed while the following two lines, int i; and int sum = 0,

indicate that we have an integer variable with the name i and also an in-

teger variable with the name sum that is initialized to 0. Much happens

in the next line for(i=1;i<=10;i++) { sum = sum + i; }. Firstly,

for(i=1;i<=10;i++) means that our integer variable i starts at 1, after

which the command { sum = sum + i; } is executed, and then i is increased

by 1 (this is indicated by i++). The part i<=10 in for(i=1;i<=10;i++)

means that the program should proceed to the next instruction (if(sum >20) std::cout << sum << endl; ) when the integer i is larger than

10. The line if(sum > 20) std::cout << sum << endl; is a condi-

tional statement. The part if(sum>20) checks whether the value of the variable

sum is larger than 20. If so, the part std::cout << sum << endl; is exe-

cuted printing the value of sum on the monitor. If sum is smaller than or equal to

20, nothing happens. Finally, return 0; marks the end of the program.

In computers, a large collection of similar constructs eventually amounts to a

in a certain way unrecognizable outcomes such as a word processor.



D R A F T

15.5: RNA and computation 307

Translation

One of the key processes in a cell, the translation of the information stored in

messenger RNA into a protein, has a distinctly computational flavor to it. After

a messenger RNA strand has been transcribed, and if necessary processed, it pro-

ceeds to the ribosome where it is translated into a protein. This procedure is called

translation since specific sequences of nucleotides need to be ’translated’ into a

corresponding amino acid. After all, in humans there are five times as many differ-

ent amino acids than there are nucleotides. To be more specific, three successive

nucleotides, referred to a a codon, represent a certain amino acid.

G

GGG

G

G

GGG

GG

G

GG

GG

Y

C

C

C CC

CCC

C

C

CC

C

C C

C

UU

U

TUU

U

U

U

U

UUU A

AA A

A

A

A

AA

A

A

AA

A A

A

DD

Ψ

Ψ

G

GG

G

G

G

A

A

C

C

anticodon

anticodon

loop

T loop

D loop

5’

3’

attached amino acid

(phenylalanine)

base-pair

Figure 15.8: The tRNA molecule for the

amino acid phenylalanine.

Since there are four different types

of nucleotides, there are in total 43 =

64 different codons. The table which

matches these codons with amino acids

as well as instructions start and stop is

called the genetic code and shown in

figure 19.12.

The question of course is, how

would the genetic code be processed?

One option would be for the ribosome

to recognize the codons and then grab

a suitable amino acid. This is, how-

ever, not quite how nature does it. In

nature, amino acids destined for pro-

tein production are attached to special-

ized RNA molecules called transfer

RNA (tRNA). The tRNA molecules,an example of which is shown in Fig-

ure 15.7, have a complementary se-

quence of bases called an anticodon

on one side. This anticodon exactly

matches a codon and since a specific

tRNA molecule can only have one kind

of amino acid attached to it, if the ribosome finds a tRNA whose anticodon matches

the codon of the mRNA currently being processed, then the ribosome has the cor-

rect amino acid for the protein to be manufactured.

Since there are significantly more codons than different types of amino acids,

even taking account start and stop signals, many amino acids can be represented bymore than one codon. Of course, it would be possible that nature simply doesn’t use

all the possible 64 codons but this is not the case. All codons are used. Since base-

pairing needs an exact match between codon and anticodon, some amino acids are

therefore carried by more than one type of tRNA.

A notable feature of tRNA is that after its synthesis, some bases are chemically

modified. For example the base ’D’ in the thereafter named D-loop is a modifica-

tion of uracil.

Since being read as such does not destroy an mRNA strand, it can in principle



D R A F T

308 15: RNA

be read over and over again. When that happens, enormous amplification can occur

and a single gene can yield a huge quantity of protein.

In contrast, rRNA (after suitable processing) is the final product when being

transcribed and consequently in order to manufacture the 10 million or so ribo-

somes a mammalian cell needs, multiple copies of the necessary rRNA genes existin the genome. For example humas have about 200 rRNA genes copies per haploid

genome!

Control

In computational processes, control is essential, especially if constant adaptation

to changing environments is necessary. Due to their versatility, RNA molecules are

involved in a number of regulatory activities such as RNA processing, modification

and editing. Let us now have a look at some interesting classes of RNA molecules:

snRNA

The so-called small nuclear RNA (snRNA) is a class of RNA molecules in-

volved in RNA splicing and the regulation of transcription factors.

They are also involved in the regulation of telomeres. Telomeres are highly

repetitive sequences of DNA that can be found at the terminal ends of chromo-

somes. In general telomeres become shorter as a cell divides since the DNA poly-

merase responsible for replication cannot proceed all the way to the end of a strand.

By having telomeres, none of the essential genetic material will be missed at the

end of a chromosome when a cell divides. However, the telomeres do become

shorter and are therefore thought to play a role in ageing. A subclass of snRNA,

the small nuleolar RNAs (snoRNA) plays an important role in the chemical modi-fication of certain types of RNA molecules like rRNA and tRNA.

snRNA are generally about 150 nucleotides long and form complexes with

proteins called small nuclear ribonucleoproteins (snRNP) to fulfill their function.

eRNA

In general the uses of promoters and inhibitors leads to a kind of on-off reg-

ulation that resembles a digital process. It appears that by interfering with the

transcription apparatus, efference RNA (eRNA), allows for a somewhat analog

fine grain control.

tmRNA

Common in all bacteria but thus far not found in eukaryotes is a class of RNA

molecules that have both tRNA as well as mRNA regions. Their main purpose is

to deal with ribosomes where the production of a protein has become stuck. Unfin-

ished proteins could be entirely useless but could also be damaging to a cell as their

function is unpredictable. It is therefore important to identify stuck ribosomes and

tag the incomplete protein for destruction. This is done by the transfer-messenger



D R A F T

15.6: RNA genes 309

RNA (tmRNA).

siRNA

One of the mechanisms of regulating gene expression is through so-calledRNA interference (RNAi). In this process, small 20 to 25 nucleotide long, double-

stranded, RNA molecules called small interfering RNA (siRNA) play a key role.

For example if a certain type of mRNA strand has already been transcribed but

needs to be stopped from being translated into a protein by a ribosome, then the

so-called RNA induced silencing complex (RISC) can be used as follows: one of

the strands of siRNA called the guide strand is incorporated into the RISC complex

which subsequently binds to the complementary regions on an mRNA strand. The

RISC complex then destroys the mRNA thus silencing it.

miRNA

Similarly to siRNA, micro RNA (miRNA), plays an important role in gene

regulation by base-pairing to mRNA strands. However, they are single stranded

rather than the double stranded siRNAs. miRNAs are encoded in RNA genes that

are significantly longer than the actual miRNA they code for. When transcribed, an

miRNA coding RNA gene first yields a primary transcript (pri-miRNA) which is

processed in the nucleus to a roughly 70 nucleotide long pre-miRNA. Finally, this

pre-miRNA is then processed into miRNA in the cytoplasm.

Thus, once again, we find that RNA does an excellent job in yet another cate-

gory of function.

15.6 RNA genes

In moderns cells, information is stored in DNA. Those parts of the DNA that en-

code the information for the production of proteins are called genes. However, be-

sides proteins, as we have seen in the sections above, there are many essential RNA

molecules. Not surprisingly, the information for these molecules is also stored on

DNA in regions that are called RNA genes. The resulting RNA molecules, e.g. the

catalyst rRNA or the transport molecule tRNA, are then called non-coding RNA

(ncRNA) since they do not code for proteins.

15.7 OverviewThe main RNA functions as with regards to protein production are summarized in

table 15.1 Some of the other non-coding RNA not involved in protein synthesis are

listed in table 15.2

Given names such a small and micro one might be tempted to assume that that

all non-coding RNA is relatively pretty small. This is not the case however. For

example, in female mammals, one of the two X chromosomes is inactivated (so as

to have the same number of active X chromosomes as males - namely one) by an



D R A F T

310 15: RNA

Molecule Abbreviation Function

transfer RNA tRNA brings amino acid monomers to ribosome

messanger RNA mRNA brings instruction for protein to ribosome

ribosomal RNA rRNA catalyzes the joining of amino acids

Table 15.1: Key non-coding RNA types in protein manufacture

Molecule Abbreviation Function

small nuclear RNA snRNA regulatory functions in eukaryotic nuclei

efference RNA eRNA gene regulation

transfer-messanger RNA tmRNA identifies faulty ribosomal activity in bacteria

small interfering RNA siRNA regulates gene expression and combats viruse

micro RNA miRNA control gene expression

Table 15.2: Key non-coding RNA types in protein manufacture

RNA gene name Xist which is 18,000 base pairs long. Nevertheless, the majority

of ncRNA is rather small.

Another issue is the number of different ncRNAs. Humans have slightly over

20,000 genes so it is interesting to consider how many RNA genes there are. Al-

though this is currently not known, some expectations go as high as 100,000.

15.8 The Answer

We started this chapter by wondering whether RNA is a jack of all trades. In

its modern meaning, jack of all trades has a rather negative connotation implying

mediocrity. When we looked at RNA molecules and their many functions, it be-

comes clear that RNA is far from mediocre in the many things it does. Indeed, it

does appear to be very efficient in most if not all of the functions it performs, be this

replication, catalysis, information storage or control. RNA is far from mediocre

and in many senses a master. Then if so, why DNA and proteins? The answer to

that is likely not so much the inadequacy of RNA but the excellence of DNA and

proteins in the current use - they are the grand-masters so to speak.

Of course, it is very well possible that RNA, DNA and proteins emerged simul-

taneously as specializations of a currently unknown class of polymers. Or perhapsit is known but not recognized as relevant. Be that as it is, with what we know

today, RNA is an good candidate for the precursor molecule of the modern cell.

It has been said that the original jack of all trades expression was somewhat

different: ’Jack of all trades, master of none, though ofttimes better than master of

one’. And in this form, the figure of speech may very well be true. With what we

know today, it is quite feasible to imagine life with only RNA out of the trio RNA,

DNA and proteins. Neither proteins nor DNA by themselves could fulfill the three

essential functions of replication, catalysis and information storage. So we now



D R A F T

15.9: Exercises 311

can answer our chapter question somewhat tongue in cheek by RNA, master of all

trades, much better than grand-master of one!

15.9 Exercises

1. Give two reasons why RNA might have preceded DNA in the evolution of

life on earth.

2. How much energy does it take to break a bond between two nucleotides in

RNA?

3. If different parts of RNA can form base-pairs leading to intricate structures,

why is RNA generally not called double stranded?

4. Can RNA polymerase read DNA in both directions?

5. How is the energy supplied for protein production?

6. How is the energy for mRNA prodcution supplied?

7. What is the difference between a ribosome and a ribozyme?

8. The catalytic activity of the ribosome is carried out by which type of biopoly-

mer?

9. Why is is so that RNA can be a reasonably efficient catalyst?

10. What is ’non-coding RNA’?



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26766888 15 RNA Jack of All Trades Master of None