Protein synthesis consists of two stages: transcription and translation. Transcription involves the production of mRNA whereas translation involves reading mRNA and putting together the correct amino acids to make a specific protein.
The role of DNA
A gene is formed from the sequence of bases in a DNA molecule. The sequence of bases determines the aminoacid sequence. Amino acids will be joined together to form a polypeptide (this is a protein).
To determine the correct amino acid, the base sequence is read in triplets. This means that a sequence of three bases (which is called a codon) will code for one amino acid, the next sequence of three bases along the gene will code for another amino acid and so on.
The sequence of amino acids in a protein directly relates to the sequence of base triplets within a gene. Therefore, the base sequence of a gene determines the protein it makes.
Transcription
Transcription occurs in the nucleus and involves DNA, free nucleotides and some enzymes. The product of transcription is messenger RNA (mRNA) which is a single-stranded RNA molecule that is complementary to the original DNA.
1.
DNA strands unwinding
2.
RNA polymerase binding site
3.
Free bases joining with the DNA strand
4.
Free bases
5.
The direction of the RNA polymerase enzyme action
A.
In the double helix, the bases are held together with weak hydrogen bonds. These weak bonds are broken by a DNA helicase enzyme and the DNA molecule is unzipped (unwinds).
B.
RNA polymerase attaches to the DNA in a non-coding region before the start of the gene.
C.
Free bases will form hydrogen bonds with their complementary bases on the original DNA strand. RNA polymerase is an enzyme that will move along the DNA strand and join the complementary free bases. This forms a strand of mRNA.
Note: The free RNA bases are C, G, A and U.
D.
The mRNA strand leaves the nucleus and travels to the ribosome in the cytoplasm of the cell.
Translation
Once transcription has occurred and mRNA has been produced, the protein can now be made via the process of translation at ribosomes.
1.
Ribosome
2.
mRNA
3.
Anticodon
4.
Amino acid
5.
Polypeptide
6.
tRNA released
1.
The mRNA strand, that was formed in transcription, attaches to a ribosome. The ribosome will move along the mRNA strand and read one codon at a time.
2.
tRNA molecules have an anticodon (this is a triplet of bases complementary to the codon) and a specific amino acid. The amino acid present on the tRNA will depend on the sequence of bases on the anticodon.
3.
The complementary bases on the tRNA will bind to the bases on the mRNA strand.
4.
Adjacent amino acids will bind to one another to form a chain of amino acids. This chain is called a polypeptide.
5.
The tRNA molecules are released. This means they can collect another amino acid.
Each protein will have a different mRNA code which means the tRNA and amino acids will also be different. Therefore, the resultant proteins will be different.
Protein specificity
Different amino acid chains are folded in different ways which produces proteins with different shapes. These unique shapes are what make proteins very specific. This is very important for proteins such as enzymes and antibodies, which can only bind to one type of molecule.
If a mutation causes a change in the base sequence, the chain of amino acids may also be different, which will result in a protein with a different shape. This might give the protein a different function, or may simply make the protein faulty. This is why the base sequence of genes is crucial in protein synthesis.
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FAQs - Frequently Asked Questions
What is translation?
Translation involves reading mRNA and putting together the correct amino acids to make a specific protein.
What determines the amino acid sequence?
The sequence of bases determines the amino acid sequence.
