protein synthesis

introduction

• the central dogma states that genetic information is encoded in the DNA and transferred to the mRNA during transcription

  • tRNA and rRNA are also transcribed

• eukaryotic transcription: pre-mRNA synthesised is then processed to form mature mRNA

• during translation: information on mRNA is used to synthesise polypeptides, which are folded into functional proteins

• DNA contains the codes for synthesis of polypeptide chains ⇒ sequence of bases in DNA determines sequence of amino acids in a polypeptide chain (primary structure)

the central dogma of molecular biology

refers to the unidirectional flow of genetic information from DNA (gene) through mRNA to the protein

• involves 2 processes

  • transcription

  • translation

• however, the discovery of reverse transcription challenged the central dogma

  • reverse transcription: RNA to DNA (e.g. HIV virus)

a gene

a gene is a unit of inheritance:

• which contains the nucleotide sequence for synthesis of a functional gene product → e.g., a polypeptide chain, tRNA, rRNA [not always a protein]

• it includes BOTH coding sequences (exons) and non-coding sequences (introns)

• if the gene product is a polypeptide chain, the sequence of nucleotides in the DNA codes for the sequence of amino acids in a polypeptide chain (primary structure)

coding sequences (exons):

• the sequence of nucleotides in the DNA exon that codes for sequence of amino acids (primary structure) in a polypeptide chain

non-coding sequences (introns):

• these nucleotides sequences that do not code for any sequence of amino acids

• within the gene are introns

• other examples of such non-coding sequences are the promoter, enhancers, silencers, important for controlling the transcription of a gene

genes in eukaryotes

• an exon is coding sequence of DNA that codes the sequence of amino acids in a polypeptide chain

• an intron is a non-coding sequence of DNA interspersed between exons

• exons and introns are transcribed to the pre-mRNA which undergo processing so that only exons are joined together to form a continuous coding sequence and all introns are removed

genetic code

• the genetic code determines the amino acid sequence of a polypeptide chain

  • during transcription, the specific sequence of DNA nucleotide (or base sequence) of the gene is copied into a specific sequence of nucleotides in the mRNA

  • during translation, the mRNA sequence is decoded, giving rise to the amino acid sequence of the polypeptide chain

• DNA → proteins

  • 4 types of nucleotides in a DNA strand ⇒ adenine, guanine, thymine and cytosine

    • different genes have different nucleotide sequences

  • 20 amino acids commonly found in nature

    • different proteins have different sequences of amino acids

    • the sequence of nucleotides in DNA defines sequence of amino acids

[FAQ] how many nucleotides codes for one amino acid → need to know how to calculate

• 1 nucleotide codes for 1 amino acid → single code

  • only 4 amino acids can be coded < 20 amino acids found in nature

• if 2 nucleotides code for 1 amino acid → doublet code

  • 4² = 16 amino acids can be coded < 20 amino acids found in nature

• if 3 nucleotides code for 1 amino acid → triplet code

  • 4³ = 64 amino acids can be coded

  • with only about 20 amino acids to code, some triplets can be redundant

features of the genetic code

1. it is a triplet code made up of 3 nucleotides

   • 3 nucleotides of a gene exon code for one amino acid in a protein

   • the DNA code is first transcribed into mRNA

   • 3 bases in the mRNA are called codons

   • each triplet of bases / codon is complementary to the DNA template strand from which it is transcribed

   • each codon codes for one amino acid

     template strand of DNA	number of hydrogen bonds	mRNA
     A	2	U
     T	2	A
     C	3	G
     G	3	C

2. the code is degenerate

   • degenerate: more than one codon can code for the same amino acid

     • some amino acids are coded for by several codons

       • e.g. the amino acid glycine is coded by 4 different codons

       • note that the first 2 of the 3 nucleotides must be the same, but the third nucleotide can be different

3. the code is non-overlapping

   • each nucleotide in a triplet code is used only once

   • when the mRNA is read, the codons in the genetic code do not overlap

4. the code is punctuated

   • presence of a start codon: AUG → signals the initiation of translation of the mRNA into a sequence of amino acids

   • presence of stop codons: UGA, UAG, UAA → act as “stop signals” for the termination of translation

     • they are codons which do not code for any amino acid

5. the code is universal

   • the same 3 bases code for the same amino acids in almost all organisms

differences between prokaryotes and eukaryotes → for protein synthesis

• differences between prokaryotes and eukaryotes

  • absence of nucleus in prokaryotes

  • prokaryotic genes do not have introns

• process of protein synthesis is similar in prokaryotes and eukaryotes, but differences are also present

  	prokaryotes	eukaryotes
  location of transcription and translation	both in cytosol (no nucleus and nuclear envelope)	• transcription in nucleus • translation in cytoplasm
  occurrence of transcription and translation	both occurs simultaneously	transcription occurs first in the nucleus followed by translation
  post-transcriptional modification	does not occur	takes place between transcription and translation in the nucleus
  RNA splicing	does not occur because there are no introns present	occurs

protein synthesis in eukaryotes

• in eukaryotes, DNA molecules are too large to fit through nuclear pores in the nuclear envelope

  • part of the genetic information carried by the DNA is copied into smaller messenger RNA (mRNA) molecules which pass through the nuclear envelope

  • ribosomes bind to mRNA and transfer RNA (tRNA) to translate information in the mRNA molecule into a polypeptide with the correct amino acid sequence

• for a gene to be expressed, the genetic code stored in the DNA directs the synthesis of a polypeptide chain

  • this is done via two key mechanisms: transcription and translation

transcription → definition

transcription is the process by which the base sequence in the DNA template of a gene is copied into the complementary base sequence of RNA (mRNA, tRNA, rRNA)

requirements for transcription

1. RNA polymerase

   • the enzyme which catalyses the formation of phosphodiester bonds between free ribonucleotides to form an RNA molecule

   • synthesizes a polyribonucleotide chain in the 5’ → 3’ direction

   • does not require a primer to start synthesizing the RNA molecule

   • lack 3’ to 5’ exonuclease proof-reading ability (unlike DNA polymerase)

   • to speed up transcription of certain genes, many molecules of RNA polymerase may simultaneously transcribe the same gene

2. free ribonucleotides

   • monomers of an RNA molecule

   • they are matched by complementary base pairing to nucleotides on the DNA template during transcription

     DNA	number of hydrogen bonds	RNA
     A	2	U
     T	2	A
     C	3	G
     G	3	C

3. DNA template

   • the DNA strand (only one) in a double helix used for transcription to form the RNA molecule

   • the mRNA has a complementary nucleotide sequence to the template DNA strand

before the start of transcription in eukaryotes

• chromatin must uncoil (i.e., loosen the histone complex) so that Transcription Factors (TF) and RNA polymerase can access promoter of the gene to ensure assembly of the Transcription
  Initiation Complex (TIC)

• transcription starts at the promoter and ends at the terminator sequence

stage 1: initiation → transcription

1. transcription starts at the promoter near the beginning of the gene

   • the promoter contains the TATA box (a short sequence of T and A nucleotides)

2. proteins called general / basal transcription factors recognise and bind to TATA box and other sequences in the promoter

   • [further explanation] the TATA-Binding Protein (TBP) is a general transcription factor which recognises and binds to the TATA box, and it distorts the DNA, causing the helix to partially unwind and placing strain on the two DNA strands

3. general transcription factors recruit RNA polymerase (an enzyme) to bind to the promoter, to form the transcription initiation complex (TIC)

4. the formation of TIC causes DNA double helix to completely unwind

5. only one exposed strand of DNA (template strand), is used as template for mRNA synthesis

   • the other strand which is not transcribed is known as the non-template strand

stage 2: elongation → transcription

4. RNA polymerase continues to unwind the DNA helix by breaking hydrogen bonds between complementary bases

5. RNA polymerase reads the template strand from 3’ to 5’

6. free ribonucleotides base pairs with template strand via complementary base pairing

   • adenine in DNA pairs with uracil

   • thymine in DNA pairs with adenine

   • guanine in DNA pairs with cytosine

   • cytosine in DNA pairs with guanine

7. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent ribonucleotides

8. the new RNA strand is synthesized in the 5’ to 3’ direction

9. as the RNA polymerase continues to move along the template strand, the DNA double helix behind it will re-wind/reform

10. three main types of RNA can be produced from this process: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA)

stage 3: termination → transcription

13. in eukaryotes, RNA polymerase transcribes the sequence at the end of the gene which codes for the termination signal/polyadenylation signal AAUAAA on the pre-mRNA

14. this signals the end of transcription to release the newly formed pre-mRNA (in eukaryotes)

15. proteins bind to the pre-mRNA 10–35 nucleotides past the AAUAAA sequence to cut and release the pre-mRNA from the polymerase

    • this cut site is also the site of addition of the poly(A) tail

16. finally, the entire DNA rewinds

relationship between gene products and products of transcription

genes coding for	products of transcription	note
ribosomal RNA (rRNA)	rRNA	rRNA is the final products of the genes that code for them
transfer RNA (tRNA)	tRNA	tRNA is the final products of the genes that code for them
polypeptide	mRNA	mRNA is not the final product of the gene → mRNA needs to be translated to give rise to polypeptide which is the final product of the gene

post-transcriptional modification (RNA processing)

• in eukaryotes, transcription gives rise to pre-mRNA

  • pre-mRNA is processed (in nucleus) to form the mature mRNA (transported to cytoplasm) before translation takes place

  • mRNA processing ⇒ post transcriptional modification

• takes place in the nucleus of the eukaryotic cell

• enzymes modify pre-mRNA to form the mature mRNA which exits the nucleus via nuclear pore to the cytoplasm

• steps: addition of 5’ cap, polyadenylation at 3’ end, RNA splicing

addition of 5’ cap

a modified (methylated) guanine nucleotide is added to 5’ end of the newly synthesized pre-mRNA

addition of 3’ poly(A) tail/polyadenylation

a poly(A) tail consisting of 50-250 adenine molecule is added to the cut site at the 3’ end of the pre-mRNA by an enzyme

RNA splicing

• introns are removed (excised) and exons are joined (spliced) together to form a mature mRNA with a continuous coding sequence

1. splice sites (short nucleotide sequences) are located at ends of introns where they act as signals for RNA splicing

2. small nuclear RNA (snRNA) in the small nuclear ribonucleoproteins (snRNPs) recognize and binds to splice sites at each end of an intron via complementary base pairing

3. snRNPs join to form a spliceosome (enzyme)

4. spliceosome cuts at specific points to release intron (hydrolyse phosphodiester bonds) and exons are spliced (joined) together (catalyse the formation of phosphodiester bonds)

5. spliceosome dissociates and mature mRNA (containing only exons) is released

6. mature RNA exits the nucleus via nuclear pore to the cytoplasm
   for translation to take place

alternative RNA splicing

• during RNA splicing, alternative splicing may occur whereby

  • different exons are removed together with introns

  • certain exons are treated as introns and so are excluded from the mature mRNA

• [how] different cell types/organisms at different stages of development contain different snRNPs

  • different splice sites on the mRNA are recognised, leading to different parts of the gene recognised as introns

• outcomes of alternative splicing

  • results in different mature mRNA, each coding for a slightly different polypeptide to be formed the same pre-mRNA

  • one gene can give rise to different proteins ⇒ explains why total number of different proteins a eukaryote cell can synthesise is more than total number of genes in the DNA

significance of post-transcriptional modification

both 5’ cap and 3’ poly(A) tail

• facilitate export of mature mRNA from nucleus into cytoplasm via nuclear pore

• protect mature mRNA from degradation by RNase (nuclease) in the cytoplasm

• facilitate binding of certain proteins (i.e. translation initiation factors) and small ribosomal subunit to 5’ end of the mature mRNA for translation to occur

the length of 3’ poly(A) tail

• determines the half-life / stability of the mRNA, hence the duration of translation before the mRNA is degraded by enzymes

  • shorter → less stable ⇒ shorter duration

RNA splicing

• the main function is to form an mRNA molecule with a continuous coding sequence

• alternative splicing ensures different mature mRNAs formed from the same pre-mRNA ⇒ different types of proteins are synthesized from the same gene (e.g., antibodies)

  • alternative splicing does not occur all the time in all cells

translation → definition

translation is the process by which sequence of nucleotides in a mRNA molecule directs the incorporation of amino acids into a polypeptide at the ribosome

RNA → requirements for translation

• 3 main types of RNA molecules (mRNA, rRNA & tRNA) can be produced when DNA is transcribed ⇒ all involved in protein synthesis

  • all single-stranded

messenger RNA (mRNA)

• constitutes 3 – 5% of total RNA of the cell

• in eukaryotes, pre-mRNA synthesized in the nucleus, undergoes RNA processing to form mature mRNA before transported to cytoplasm for translation

• structure of mRNA

  • single-stranded molecule with a base sequence complementary to sequence on the DNA it was transcribed from

• role of mRNA in protein synthesis

  • acts as a carrier molecule, to carry genetic information from nucleus to ribosomes (either free or bound) in the cytoplasm for translation to occur

  • the sequence of nucleotides is complementary to the DNA template

  • the codons on the mRNA specify the order in which amino acids are joined to form a polypeptide chain

  • each codon (triplet of bases) codes for one amino acid

    structural features of mRNA	significance and contribution to function of molecule
    smaller size than DNA	• allows mRNA to move out of nucleus via nuclear pores • acts as carrier molecule, carrying genetic information from nucleus to ribosomes for translation to occur in the cytoplasm
    sequence of codons complementary to bases on the DNA template strand from which it was transcribed	• each codon (triplet of bases) codes for one amino acid • the codon sequence on mRNA specifies amino acid sequence (i.e. primary structure) of the polypeptide chain
    single-stranded	• allows amino acyl-tRNA complex with complementary anticodon to base-pair with codon on the mRNA during translation
    presence of start codon, AUG	• recognition site for binding of large ribosomal subunit • site where translation is initiated → codes for 1st amino acid, methionine
    presence of stop codon, UAA, UAG, UGA	• site of recognition for binding of release factors • signals termination of translation for the polypeptide is released from the ribosome

transfer RNA (tRNA)

• constitutes 15% of total RNA in the cell

• there are at least 20 different tRNA molecules in a cell, with at least one (or more) for each of the 20 amino acids required for protein synthesis

• structure of tRNA

  • single-stranded RNA about 80 ribonucleotides long

  • folds back on itself (form L-shaped structure) and is held in shape by hydrogen bonding between complementary base pairs at certain regions

    • 5’ end of the tRNA ends in a guanine (G)

    • 3’ end of the tRNA ends in the sequence CCA → binds to specific amino acid to tRNA molecule

  • the anticodon is a specific three bases complementary to a codon on the mRNA

  • each tRNA molecule is specific because it only carries a specific corresponding amino acid

• role of tRNA in protein synthesis

  • tRNA carries a specific amino acid to the ribosome during translation (at least 20 specific types of tRNA)

  • the anti-codon forms complementary base pairs with codons on the mRNA which allows for correct sequencing of amino acids on the polypeptide chain

    structural features of tRNA	significance and contribution to function of molecule
    single-stranded	• hydrogen bonds between complementary base pairs at different regions cause tRNA to fold back on itself • this forms a 3D L-shaped structure to fit into the E, P, A sites on the large ribosomal subunit
    CCA site at 3’ end	• for attachment of specific (activated) amino acid to form an amino acyl-tRNA complex • allows tRNA molecule to carry a specific amino acid to the ribosome
    anticodon of triplet base sequence (3 bases) found at the extending loop	• anticodon forms complementary base pairing with codon of mRNA • this ensures the correct amino acyl-tRNA complexes occupy the ‘P’ and ‘A’ site of the ribosome during translation • this allows for correct sequencing of amino acids on the polypeptide chain coded for by the gene

attachment of amino acid to tRNA [amino acid activation]

• the attachment of a specific amino acid to its tRNA forms aminoacyl-tRNA complex

• amino acid is attached to the 3’ end of tRNA and is activated by reacting with one ATP molecule

• catalysed by a group of enzymes known as aminoacyl-tRNA synthetases

  • aminoacyl tRNA synthetase has an active site with two binding sites, each with shape complementary to:

    1. the shape of a specific amino acid, and

    2. the shape of the anticodon of a specific tRNA molecule

• this makes the enzyme highly specific

  • there are at least 20 aminoacyl-tRNA synthetases, each bind to a specific amino acid ⇒ ensures the correct amino acid is joined to a tRNA

ribosomal RNA (rRNA)

• constitutes 80% of total RNA of the cell

• rRNA genes in the nucleolus code for rRNA

• synthesised in nucleus

• structure:

  • single-stranded molecule

  • folded into a highly compact, precise 3-D structure → different rRNA have different 3D shapes

  • held by hydrogen bonds between complementary bases at different parts of the molecule

• role of rRNA in protein synthesis

  • combines with ribosomal proteins to form the large subunit and small subunit of ribosomes (site of protein synthesis)

  • within small subunit of ribosome:

    • a rRNA binds mRNA to ensure translation starts at the correct location on the mRNA

  • within large subunit of ribosome, rRNA forms:

    • the binding sites for tRNA, and

    • the catalytic site for peptide bond formation because the enzyme, peptidyl transferase is made up of rRNA

formation of the ribosome in the eukaryotic cell

1. rRNA is transcribed from rRNA genes in the nucleolus

2. in the nucleolus, rRNA combine with ribosomal proteins (from cytoplasm) to form immature large and small subunits (i.e. partially assembled in the nucleolus)

3. they exit the nucleus via nuclear pores, and combines with more ribosomal proteins to form the mature large and small subunits

4. large and small subunits are only assembled during translation

structure of ribosomes

• 20nm – 30 nm in size

• made up of ribosomal proteins and rRNA

• 2 subunits are present in ribosomes

  • eukaryotes (80S): 60S (large subunit) and 40S (small subunit)

  • prokaryotes (70S): 50S (large subunit) and 30S (small subunit)

• the ribosome has 4 sites

  1. mRNA binding site

     • in small ribosomal subunit

     • binds to mRNA

  2. peptidyl-tRNA site (P site)

     • in large ribosomal subunit

     • binds to aminoacyl-tRNA complex

  3. aminoacyl-tRNA site (A site)

     • in large ribosomal subunit

     • binds to aminoacyl-tRNA complex

  4. exit site (E site)

     • in large ribosomal subunit

     • release of tRNA

role of ribosome in protein synthesis

• ribosomes are the sites of protein synthesis

• it holds the tRNA and mRNA in close proximity so that anticodons of amino acyl tRNA can bind to complementary codons on the mRNA

  structural features of ribosomes	significance and contribution to function of molecule
  small subunit of ribosome with mRNA binding site	• to recognise and bind to 5’ cap of mRNA and moves along mRNA to identify the start codon • so that binding of large subunit can take place to form translation initiation complex
  large subunit of ribosome	• to hold tRNA and mRNA in proximity • allow anticodons of amino acyl tRNA complexes to bind to complementary codons on mRNA • to ensure correct matching of codon and anti-codon pairs for accurate protein synthesis • for the translation of mRNA codons into amino acid sequence
  “P” site of large subunit	• the site where initiator tRNA binds to the start codon, which results in binding of the large ribosomal subunit • site of peptide bond formation catalysed by peptidyl transferase on the large ribosomal subunit
  “A” site of large subunit	• the site of binding of next amino acyl-tRNA complex to allow the next amino acid to be joined to the existing polypeptide chain
  “E” site of large subunit	• the site for release of tRNA to be recycled in the cytoplasm

formation of polyribosome

• an mRNA molecule translated simultaneously by several ribosomes in cluster is called polyribosomes

• each ribosome assemble at the start codon then moves along mRNA till it reaches the stop codon at the 3’ end

• as soon as each ribosome moves a sufficient distance from the start codon, the next ribosome attaches to the mRNA and begins its translation activity

• significance: many of the same polypeptide chain is formed at the same time ⇒ increases the rate of translation and hence rate of polypeptide synthesis

free amino acids

• form the basic units of a polypeptide chain

• must first be attached to a specific tRNA via a process called amino acid activation before they can take part in protein synthesis

stage 1: initiation of translation

1. small ribosomal subunit binds to 5’ cap of mRNA and moves along mRNA in the 5’ to 3’ direction until it reaches the start codon

2. initiator amino acyl-tRNA complex with anticodon UAC binds (forms hydrogen bonds) to start codon (AUG) on mRNA (by complementary base pairing)

3. tRNA with anticodon UAC always carries the amino acid methionine → usually the first amino acid in a polypeptide chain

4. now the large subunit of the ribosome binds to the small subunit, and the initiator amino acyl-tRNA complex is positioned at the “P” site of ribosome (translation initiation complex)

stage 2: elongation of polypeptide chain

6. the second amino acyl-tRNA complex with anticodon complementary to second codon on mRNA binds to mRNA at ”A” site of ribosome

7. formation of a peptide bond between first and second amino acid, using energy from hydrolysis of GTP (a molecule similar to ATP), catalysed by peptidyl transferase (RNA enzyme) on large subunit of ribosome

8. ribosome moves along mRNA to next codon in the 5’ to 3’ direction (once the peptide bond is formed)

   • the ribosome “reads” mRNA in the 5’ to 3’ direction

9. the first tRNA, now at ”E” site, is released into cytoplasm

   • recycled to attach to respective amino acid

10. the second amino acyl-tRNA complex moves from “A” site to “P” site, and the “A” site is available for next amino acyl-tRNA complex with anticodon complementary to third codon on mRNA [polypeptide chain is synthesized from the amino to carboxyl end]

11. the process is repeated until the ribosome reaches the “stop” codon on the mRNA

stage 3: termination of translation

12. termination occurs when a stop codon (UAA, UAG or UGA) occupies A site on the ribosome

    • there is no tRNAs with anticodons complementary to the stop codons

13. instead, release factors (proteins) recognise and binds to stop codon at A site

14. release factors cause addition of a water molecule that hydrolyses the bond between last amino acid residue and tRNA

15. polypeptide synthesis stops

16. the polypeptide chain is released from ribosome, and it folds into its secondary and tertiary structure and may undergo modification at the Golgi apparatus

17. the ribosome dissociates from mRNA and separates into its subunits

comparison between dna and rna

similarities

1. both are made up of polynucleotides

2. both can act as genetic/hereditary material

differences

feature	DNA	RNA
monomer	deoxyribonucleotide	ribonucleotide
sugar	deoxyribose sugar	ribose sugar
nitrogenous bases	adenine, thymine, cytosine, guanine	adenine, uracil, cytosine, guanine
number of strands	usually double stranded	usually single stranded
stability	it is chemically stable due to: • complementary base pairing between the purines and pyrimidines • hydrophobic interactions between stacked bases • presence of deoxyribose sugar	it is chemically less stable due to: • presence of 2’-OH of in the ribose sugar that is susceptible to hydrolysis
types	exists as 1 form as a double helix	exists at least in 3 forms: mRNA, tRNA, rRNA
location	located in the • nucleus (including nucleolus) • mitochondria • chloroplasts	located in the • nucleus (including nucleolus) • cytoplasm
size	it is a large molecule	it is a smaller molecule than DNA
amount	the amount is the same in ALL cells of an organism, except gametes, which have half the amount	the amount is variable but abundant in cells actively synthesizing proteins

comparison between transcription and replication

similarities

• they both occur within the nucleus

• complementary base-paring occurs

• unwinding and rewinding of DNA strands occurs

• separation of parental strands occurs progressively in short segments

differences

features	DNA replication	transcription
enzyme involved	DNA polymerase	RNA polymerase
raw materials	deoxyribonucleotides	ribonucleotides
template	both strands of DNA molecule	only template strand of the DNA
base pairing	• adenine with thymine and vice versa • cytosine with guanine and vice versa	• adenine on DNA with uracil on RNA • thymine on DNA with adenine on RNA • cytosine with guanine and vice versa
proofreading property on enzyme involved	DNA polymerases carry out 3’ to 5’ exonuclease proofreading on daughter strand, ensuring precise complementary base pairing.	RNA polymerase does not carry out 3’ to 5’ exonuclease proofreading of RNA transcript
product(s)	2 DNA molecules	mRNA, tRNA or rRNA
products destination	products remain in the nucleus	products leave nucleus via nuclear pore

comparison between transcription and translation

feature	transcription	translation
location of process	in the nucleus	on ribosomes in the cytoplasm
template	DNA template strand	mRNA
reading of template	DNA template read in the 3’ to 5’ direction	mRNA read in the 5’ to 3’ direction
complementary base pairing	between ribonucleotides and deoxyribonucleotides on DNA	between codons on mRNA and anticodons on tRNA
raw material	ribonucleotides	amino acids
enzyme	RNA polymerase catalyses formation of phosphodiester bonds between ribonucleotides	peptidyl transferase in large subunit of ribosome catalyses formation of peptide bonds between amino acids
products	mRNA, tRNA, rRNA	polypeptide chain
involvement of ribosomes	no	yes
involvement of tRNA	no	yes