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2.1
2
Translation
By the end of this spread, you should be able to …
1 Describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to
construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes.
1 State that cyclic AMP activates proteins by altering their three-dimensional structure.
Key definition
Translation is the assembly of
polypeptides (proteins) at ribosomes.
Translation is the second stage of protein synthesis, when the amino acids are
assembled into a polypeptide. They are assembled into the sequence dictated by the
sequence of codons (triplets of nucleotide bases) on the mRNA. The genetic code,
copied from DNA into mRNA, is now translated into a sequence of amino acids. This
chain of amino acids is a polypeptide. It happens at ribosomes, which may be free in the
cytoplasm but many are bound to the rough endoplasmic reticulum.
Ribosomes
Ribosomes are assembled in the nucleolus of eukaryote cells, from ribosomal RNA
R2.!ANDPROTEIN%ACHISMADEUPOFTWOSUBUNITSANDTHEREISAGROOVEINTOWHICHTHE
length of mRNA, with the code for the sequence of amino acids, can fit. The ribosome
CANTHENMOVEALONGTHEM2.!WHICHCANSLIDETHROUGHTHERIBOSOMALGROOVEREADING
the code and assembling the amino acids in the correct order to make a functioning
protein.
Figure 1 !RTISTSIMPRESSIONOFA
ribosome with mRNA (purple)
synthesising a protein chain (yellow)
The sequence of amino acids in a protein is critical because:
s ITFORMSTHEPRIMARYSTRUCTUREOFAPROTEIN
s THEPRIMARYSTRUCTUREDETERMINESTHETERTIARYSTRUCTUREnHOWTHEPROTEINFOLDSUPINTO
its three-dimensional shape and is held in that 3D shape by hydrogen or ionic bonds
and hydrophobic interactions forming between the R groups of amino acids
s THETERTIARYSTRUCTURESHAPEISWHATALLOWSAPROTEINTOFUNCTION4HETERTIARYSTRUCTURE
of a protein is dependent upon its primary structure, and this primary sequence of
AMINOACIDSINAPOLYPEPTIDEISDETERMINEDBYTHEGENETICCODEEVENTUALLYLEADINGTO
THEPROTEINHAVINGTHECORRECTSHAPESOTHATITCANFUNCTION
s IFTHETERTIARYSTRUCTUREISALTEREDTHEPROTEINCANNOLONGERFUNCTIONSOEFFECTIVELYIFAT
ALLFOREXAMPLETHEACTIVESITEOFANENZYMEMAYHAVEANALTEREDSHAPEANDTHE
substrate molecules will no longer fit, or if a chloride ion channel protein in cell surface
MEMBRANESHASADIFFERENTSHAPEITWONTALLOWTHEIONSTOPASSTHROUGHIT
mRNA
Transfer RNA
The anticodon,
which binds to its
complementary
codon on the mRNA
Another form of RNA, transfer RNA (tRNA), is made in the nucleus and
passes into the cytoplasm. These are lengths of RNA that fold into hairpin
SHAPESANDHAVETHREEEXPOSEDBASESATONEENDWHEREAPARTICULARAMINO
acid can bind. At the other end of the molecule are three unpaired
nucleotide bases, known as an anticodon. Each anticodon can bind
temporarily with its complementary codon.
How the polypeptide is assembled
Figure 2
Transfer RNA
The three unpaired bases
where an amino acid joins
1 !MOLECULEOFM2.!BINDSTOARIBOSOME4WOCODONSSIXBASESARE
ATTACHEDTOTHESMALLSUBUNITOFTHERIBOSOMEANDEXPOSEDTOTHELARGE
SUBUNIT4HElRSTEXPOSEDM2.!CODONISALWAYS!5'5SING!40
ENERGYANDANENZYMEAT2.!WITHMETHIONINEANDTHEANTICODON5!#
forms hydrogen bonds with this codon.
2 A second tRNA, bearing a different amino acid, binds to the second
EXPOSEDCODONWITHITSCOMPLEMENTARYANTICODON
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Module 1
Cellular control
Translation
3 !PEPTIDEBONDFORMSBETWEENTHETWOADJACENTAMINOACIDS!NENZYMEPRESENTIN
the small ribosomal subunit, catalyses the reaction.
4 4HERIBOSOMENOWMOVESALONGTHEM2.!READINGTHENEXTCODON
A third tRNA brings another amino acid, and a peptide bond
1.
Large subunit
Met
FORMSBETWEENITANDTHEDIPEPTIDE4HElRSTT2.!LEAVESANDIS
of ribosome
able to collect and bring another of its amino acids.
5 The polypeptide chain grows until a stop codon is reached. There
Small subunit
of ribosome
ARENOCORRESPONDINGT2.!SFORTHESETHREECODONS5!!5!#
OR5'!SOTHEPOLYPEPTIDECHAINISNOWCOMPLETE
3OMEPROTEINSHAVETOBEACTIVATEDBYACHEMICALCYCLIC!-0
CYCLICADENOSINEMONOPHOSPHATEORC!-0THATLIKE!40ISA
NUCLEOTIDEDERIVATIVE)TACTIVATESPROTEINSBYCHANGINGTHEIR$
shape so that their shape is a better fit to their complementary
molecules.
2.
Protein synthesis in prokaryotes
3.
Met
Tyr
Met
Tyr
mRNA
Phe
Met
Tyr
Met
Phe
Lys
Phe
Peptide bond
In prokaryotes the DNA is not inside a nucleus; translation begins
as soon as some mRNA has been made.
4.
Phe
Tyr
Val
Lys
tRNA
5.
Met
Tyr
Phe
Lys
Val
Ile
Leu
Glu
Amino acids
Figure 4 Scanning electron
micrograph of translation:
RIBOSOMESBLUEMOVEALONGTHE
mRNA strand (pink) assembling
proteins (green)
Figure 5 0ROTEINSYNTHESISINTHE
bacterium Escherichia coli: DNA
(pink) transcription yields mRNA
strands (green), which are
immediately translated by
ribosomes (blue)
Figure 3 Translation of a length of mRNA at a ribosome and
assembly of a polypeptide
STRETCH and CHALLENGE
Glycogen in muscle cells can be broken down by an enzyme, glycogen phosphorylase.
Glycogen can be synthesised by the enzyme, glycogen synthase. If both were to be
happening at the same time, it would waste the cell’s energy, so there has to be a
control mechanism to ‘make or break’ glycogen according to the cell’s needs.
Glycogen phosphorylase is activated by cAMP but inhibited by ATP and by glucose 6-P.
cAMP binds to an allosteric site (not the active site) of the enzyme and causes it to
change its shape and bring its previously hidden active site to a more exposed position.
If ATP or G-6-P bind, then the shape changes back and the active site becomes buried
into the molecule.
Questions
A What do you think the effect of cAMP is on the activity of the enzyme glycogen
synthase and how do you think the effect is brought about?
B What do you think the effect of G-6-P is on the activity of glycogen synthase?
Examiner tip
Remember that transcription is copying
and then you will remember which
stage is which in protein synthesis.
Transcription is copying the DNA code
onto a piece of mRNA. At ribosomes this
code is translated into a protein.
Questions
1 In which type of cell, prokaryote
OREUKARYOTEWOULDYOUEXPECT
protein synthesis to be faster?
'IVEREASONSFORYOURANSWER
2 What is the minimum number of
different tRNA molecules that are
needed for protein synthesis?
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