RNA, protein synthesis and the genetic code
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Explainer Video
Summary
RNA, protein synthesis and the genetic code
In a nutshell
Protein synthesis is the process of making proteins in both eukaryotic and prokaryotic organisms. It has three key stages which are transcription, splicing in eukaryotes and translation.
Types of RNA
RNA is a type of single-stranded nucleic acid that contains the base uracil instead of thymine. There are two types.
RNA | Description | Graphic |
| Messenger RNA (mRNA) | mRNA is made from DNA during transcription. It carries the genetic code to the ribosomes for translation. Three adjacent mRNA nucleotides are called codons. |  |
| Transfer RNA (tRNA) | tRNA is required for translation, it carries amino acids to the ribosomes. It is made up of a single polynucleotide strand that forms a clover shape due to hydrogen bonds. Each tRNA molecule has an anticodon at one end which is a specific sequence of three bases. On the other end they have an amino acid binding site. |  |
Transcription
| 1. | An enzyme called RNA polymerase binds to DNA at the start of a gene. |
| 2. | In eukaryotes, another enzyme called DNA helicase breaks hydrogen bonds between the two strands of DNA. The strands separate and the DNA uncoils. One strand is the template strand that is used to make an mRNA copy. |
| 3. | Free nucleotides line up alongside exposed, complementary bases. RNA polymerase then forms phosphodiester bonds between these RNA bases to form a molecule of mRNA. This mRNA strand is complementary to the DNA template strand. |
| 4. | RNA polymerase moves along the DNA molecule to assemble mRNA and when it reaches a stop signal, which is a specific codon, it detaches from the DNA. |
| 5. | Hydrogen bonds reform between the DNA strands and the double-helix is reformed. |
Note: Prokaryotes do not have a nucleus so transcription occurs in the cytoplasm.
Splicing
In eukaryotes, transcription produces pre-mRNA, this is because it contains both coding and non-coding sections. These are known as exons and introns respectively. The introns must therefore be removed and this process is known as splicing. This is carried out by large ribonucleoprotein complexes called spliceosomes.
Once introns are removed and exons have been joined together, the strand is now known as mRNA. This mRNA molecule then exits the nucleus through the nuclear pore and attaches to a ribosome for translation.
Translation
Translation is the process where amino acids are joined together to form polypeptides using codons carried by the mRNA strand, it happens on ribosomes.
| A | mRNA |
| B | Ribosome |
| C | Polypeptide |
| D | Direction the ribosome is moving in |
| 1. | A tRNA molecule that is carrying an amino acid attaches itself to the mRNA via specific base pairing. The tRNA molecule is able to attach as it carries an anticodon that is complementary to the first codon on the mRNA molecule. |
| 2. | A second tRNA molecule attaches to the adjacent codon in the same way. A peptide bond forms between the two amino acids using energy provided by ATP. |
| 3. | A third tRNA molecule attaches to the next codon and the amino acid binds to the first two amino acids. |
| 4. | The tRNA molecules move away leaving just the amino acid chain behind. |
| 5. | The process continues until there is a stop signal on the mRNA. This produces a polypeptide chain, or protein. |
Features of the genetic code
The genetic code is the sequence of codons in mRNA that code for specific amino acids and lead to protein formation. There are three key features of the genetic code and these are non-overlapping, universal and degenerate.
Feature | Description |
| Non-overlapping | This means that each codon is read in sequence and separately from the previous codon. There is no overlapping and triplets do not share bases. |
| Universal | This means the same codons code for the same amino acids in all living organisms. ExampleGUG codes for the amino acid valine in all organisms. |
| Degenerate | This means that some amino acids are coded for by more than one codon. ExampleUCA and UCU both code for the amino acid serine. |
cAMP
Some proteins are not functional after they've been synthesised, this means they need to be activated at the post-translational level.
Cyclic AMP (cAMP) can activate proteins by altering their three-dimensional structure.
| 1. | PKA is an enzyme with four subunits, when cAMP isn't bound, it is inactive. |
| 2. | When cAMP binds the enzyme's structure changes. This activates the subunits so PKA is now active. |
FAQs - Frequently Asked Questions
What is RNA?
RNA is a type of single-stranded nucleic acid that contains the base uracil instead of thymine.
What is translation?
Translation is the process where amino acids are joined together to form polypeptides using codons carried by the mRNA strand.
What does non-overlapping mean?
Non-overlapping means that each codon is read in sequence and separately from the previous codon. There is no overlapping and triplets do not share bases.