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Translation

Topic 7
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3

Biology HL 

This area is free of proteins, so binding of mRNA and tRNA is carried out the 
rRNA 

➢ tRNA moves sequentially through the 3 binding sites 
○

From A to P and finally to E  

➢ The growing polypeptide chain exits the ribosome through a tunnel in the large 
subunit 
 
RNA ⇒ proteins 
➢ Codons on mRNA are translated at the ribosomes with the help of tRNA carrying 
amino acids 
➢ Codons are made up of 3 bases 
○

64 different combinations 

○

3 of them do not code for any amino acids = no complementary tRNA 
anticodon 
■
■

These are s
​ top codons 

The ​start codons​ (AUG-methionine) signals the beginning of a 
polypeptide chain 

➢ Many codons code for the same amino acid 
➢ The genetic code is the same for almost all known organisms 
○

A few minor exceptions 

○

Allows the exchange of genes from one species to another with the use of 
genetic engineering 
■

Example: human insulin coding gene into bacteria, so that the 
bacteria can produce the protein for human use 

➢ The translation process involves several phases 
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Initiation  

○

Elongation  

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Translocation  

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Termination  

 
Initiation phase  
➢ The start codon (AUG) is on the 5’ end of all mRNAs 
➢ Each codon, other than the 3 stop codons, attaches to a particular tRNA 
➢ The tRNA has a 5’ end and a 3’ end and is single stranded  
➢ On the 3’ end it has the base sequence CCA where the amino acids are attached  
○

Attached by a specific enzyme  
2 

Translation

Topic 7
...
Now it is called ​activated amino 

acid 

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A specific enzyme joins a specific amino acid and a specific tRNA molecule 

➢ Hydrogen bonds​ will be formed between complementary bases in tRNA  
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This causes tRNA to fold and form loops that include unpaired bases 

➢ One of the loops contains an exposed anticodon 
○

This anticodon is unique to each type of tRNA  

○

This anticodon will pair with a specific codon of mRNA 

➢ Initiation of translation involves assembly of the components that carry out the 
process 
○

The activated amino acid methionine attaches to a tRNA with the anticodon 
UAC and then combines with the codon AUG that exists on the 5’ of the 
mRNA 
■

○

○

The small ribosomal subunit joins in 

The small subunit moves down the mRNA until it contacts the start codon 
■

This contact starts the translation process 

■

Hydrogen bonds form between the initiator tRNA and the start codon 

The large ribosomal subunit combines with these parts to form the 

translation initiation complex  
■
■

The complex is joined by proteins called ​initiation factors 
They require energy from g
​ uanosine triphosphate (GTP) 
●

Similar to ATP 

 
Elongation phase  
➢ tRNAs bring amino acids to mRNA  
○

The triplet bases of the mRNA codon form complementary base pairs with 
the triplet anticodon of the tRNA 

➢ Process:  
○

tRNAs bind to mRNA codons at the A site with the help of proteins called 

elongation factors  
○

The initiator tRNA moves to the P site 

3 

Translation

Topic 7
...
3

Biology HL 

○

In prokaryotic cells we can see the DNA 

○

In eukaryotic cells we can see the rough endoplasmic reticulum  

 
Levels of protein organisation  
➢ There are proteins that have a structural role, proteins that store amino acids and 
proteins that have receptor functions, so that cells can respond to chemical signals 
○

The function of the protein depends on its structure/shape 

➢ There are 4 levels of organisation 
○

Primary 
■

Refers to the unique sequence of amino acids that build a specific 
protein 
●

■

Simply a chain of amino acids attached by peptide bonds 

It is determined by the nucleotide base sequence in the DNA 
●

Every organism has its own proteome, as it has its own 
genome 

■

Polypeptide chains can include hundreds of amino acids 

■

It determines the next 3 levels of organisation 

■

Changing one amino acid can alter completely the structure and 
function of a protein 
●

Example: sickle cell anaemia  
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One amino acid has been changed in the normal 
protein (haemoglobin) of red blood cells 

○

The red blood cells are unable to carry oxygen which is 
their normal function 

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Secondary  
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Created by the formation of hydrogen bonds between the oxygen 
from the carboxyl group of one amino acid and the hydrogen from 
the amino group of another 

○

■

Does not involve the side chains, R-groups 

■

Most common structures: 
●

α-helix (alpha helix) 

●

β-pleated sheets (beta pleated sheets) 

Tertiary 
■

The α-helices and the β-pleated sheets influence each other  
●

The polypeptide chain bonds and folds over itself because of 
interactions among the R-groups and the peptide backbone 
5 

Translation

Topic 7
...
3

Biology HL 

Fibrous and globular proteins  

➢ Fibrous proteins​ are long and narrow and usually insoluble in water 
○

Example: collagen, actin 

➢ Globular proteins​ have a three-dimensional shape and are mostly water soluble 
○

Example: haemoglobin, insulin 

 
Amino acids  
➢ Are grouped based on the properties of their side chains 
➢ Amino acids with non-polar side chains are hydrophobic 
○

Non-polar amino acids are found in the regions of proteins that are linked 
to the hydrophobic area of the cell membrane 

➢ Polar amino acids have hydrophilic properties 
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Found in regions of proteins that are exposed to water 

○

Amino acids in membrane proteins create hydrophilic channels through 
which polar substances can move 

➢ Polar and non-polar amino acids are important in determining the s
​ pecificity of an 

enzyme 
○

Only specific substrates can fit in the active site of an enzyme 
■

The fitting involves the general shape and polar properties of the 
substrate and of the amino acids exposed at the active site 

7 



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