The base sequence of the mRNA molecule is used as a template for the sequence of amino acids that will form a polypeptide. This process is known as translation. The genetic code converts the base sequence on mRNA to an amino acid sequence. This can be applied to languages. The genetic code represents the dictionary that converts Japanese (mRNA) to English (amino acids). A sequence of three nucleotide bases on the mRNA is a codon. Each codon codes for a specific amino acid that is added to the polypeptide. Each amino acid is carried by a specific tRNA, which has a three-base anti-codon that is complementary to the codon on mRNA. If they are complementary, they can attach. In terms of a table, the first position is 5’ end, next is the second position, and the third position is the 3’ end. There is one stop codon and three stop codons. Because some codons can code for the same amino acid, the code is ‘degenrate’. The genetic code is universal and operates in the same way for all like forms on Earth. However there are some expcetions such as some stop codons being used to code non-standard amino acids.
After mRNA is transcribed, it leaves the nucleus and enters the cytoplasm and translation occurs. Translation takes place on ribosomes. Each ribosome has a small and large subunit. First, mRNA binds to the small subunit of a ribosome. tRNA molecules are present and each one carries the specific amino acid that corresponds to the anti-codon. The tRNA binds to the ribosomes at the site where the anti-codon and codon math. Two tRNAs binds at the same time and the first one transfers the growing polypeptide chain to the next one. The ribosome moves along the mRNA and this process continues until they reach a stop codon and the polypeptide is released. Basically, tRNA enters, binds to the ribosomes, moves, then exits.
Details of translation
Each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP. There are 20 tRNA-activating enzymes, one for each amino acid. The amino acids attaches at the 3’ end of the tRNA, and this terminates with the sequence CCA.
Ribosomes are made of ribosomal RNA, or rRNA and many large individual proteins. Each ribosome is made up two subunits, one large one and a small one. IT has three tRNA binding sites, the ‘E’ site which is the exit site, the ‘P’ or peptidyl site, and the ‘A’ or aminoacyl site. Someitmes, more than one ribsome can translate the same mRNA molecule at the same time. The resulting complex of ribosomes on a single mRNA is called a polyribosome or polysome. Translation always starts in the cytoplasm. IF the proteis are destined vetually for lysosome or export then the ribosomes bind to the endoplasmic reticulum for translation to be completed.
The Stages of Translation
- An mRNA molecule binds to the small ribosomal subunit at the binding site. An initital tRNA molecules binds at the start codon.
- The large ribosomal subunit arrives
- The next codon signals another tRNA to bind.
- A peptide bodn is formed between the two amino acids, one in the ‘A’ site and the other amino acid in the ‘P’ site.
- The ribsome moves the first tRNA into the ‘E’ site so that the next codon can attach to the ‘A’ site and add to the growing amino acid chain on the tRNA in the ‘P’ site.
- The process continues until a stop codon is reached when the polypeptide is rfree. The direction of the movement of mRNA is from the 5’ end to the 3’ end of mRNA.
Exam questions on Topic 7:
1ai. The temperature for x was higher than 40 degrees Celcius. This is because at the beginning, the substrate rate was faster however over time the concentration remained constant due to denaturation of the enzyme.
1aii. The temperature could be around 30 degrees because the rate is very slow but it is not remaining constant showing that the enzyme has not denatured but the rate is just very slow.
2a. The 3’ terminal is the sugar (deoxyribose for DNA and ribose for RNA) where nucleotides are linked. The 5’ terminal is the phosphate group where nucleotides are linked.
2b. Firstly, purines are double ringed whereas pyrimidines are singled ringed. Also, purines contain adenine while pyrimidines contain cytosine, thymine (only for DNA), and uracil (only for RNA).
2c. Firstly in transcription, DNA is transcribed whereas in translation, mRNA is translated. Secondly, RNA polymerase does transcription whereas translation is done by ribosomes. Lastly, RNA is produced in transcription whereas amino acids are joined to form polypeptides in translation.
3a. This is a globular protein.
3b. The primary structure of the protein is the sequence and number of its amino acids.
3ci. X= beta pleating, Y= alpha helix
3cii. To stabilize these structures, hydrogen bonds are used.
3d. The tertiary structure is very important because it enables the protein to have its shape such as the beta pleating and also determines the shape of the enzymes active site.