Therefore due to the structure of the pre-mRNA strand adenine can pair with uracil and cytosine can pair with guanine. Also the complementary base pairings allow hydrogen bonds to be formed between the base pairs. The mRNA strand that is produced is called an mRNA transcript.
The mRNA transcript in eukaryotic cells is also altered further so that it can be used accordingly. A cap is placed at one end of the strand and a poly-A tail (150 to 200 Adenines) is added to the codon end of the molecule. This allows the mRNA to carry out its function.
There are regions of DNA that are not used for translation, these regions are called introns. The regions of DNA that are translated into protein are known as exons. The intron-regions are removed from mRNA and through a process called splicing the exon regions are spliced together. This process of splicing allows a mature mRNA transcript to be formed and the addition and deletion of the introns and exons in the structure allows protein synthesis to take place. The various, diverse combinations used to join the exon regions together, has an effect on the sequence of bases and also in the production of the polypeptide.
In translation, the sequence of bases on the mRNA strand and amino acids are used to manufacture a protein. On the mRNA strand a triplet of bases which are three consecutive nucleotides are present, these are known as a codon. The codon is a structural property present on the mRNA strand which codes for one amino acid. Each codon on the mRNA strand codes for a specific amino acid.
Furthermore, another type of RNA called transfer RNA (tRNA) also has structural properties suited to perform a role in translation. There are a collection of 3 exposed bases called anti-codons which is a single-stranded loop on each type of tRNA. These anti-codons on the tRNA strand allow complementary base pairings to occur between themselves and the codons on an mRNA strand.
The tRNA molecule also has another important structural property which is a binding site on its opposite side. This binding site allows amino acids to bind to it. A distinctive feature of tRNA is that each tRNA has its own individual anticodon therefore only a specific amino acid can bind to its binding site. For example, a tRNA possessing the anticodon GTC will have the amino acid glutamine loaded onto its amino acid binding site. The anticodon and the amino acid acceptor stem form the double helix of the tRNA molecule.
The mRNA provides 6 exposed bases (2 codons) when it travels into the ribosome. This allows complementary base pairings to occur between the codons on the mRNA strand and the anticodons on the tRNA strand which has its amino acid attached to its binding site.
In addition, at the beginning of an mRNA strand there is a start codon which has the nucleotides AUG and it pairs with the anticodon UAC on a tRNA molecule. The next tRNA molecule binds with the next mRNA codon according to the value of each codon on the mRNA strand. For example if the mRNA codon is UAA then the anticodon will be AUU.
Furthermore due to the complementary base pairings a peptide bond is formed between the two amino acids from the two tRNA molecules when they are joined together with the aid of a condensation reaction and energy provided from ATP. The mRNA travels through the ribosome, and exposes a third codon. This is done so that a third anticodon from a tRNA molecule can bind to it and consequently this allows another amino acid to become attached to the chain of amino acids.
Nevertheless, the first tRNA molecule, which had the first amino acid, detaches itself from its complementary mRNA codon and leaves the amino acid behind. The tRNA molecule does this to its structure so that it can now be released and it can merge itself with another amino acid. This cycle carries on in this manner along the mRNA strand due to the structural properties of tRNA and mRNA until a polypeptide chain is formed. At the end of the mRNA strand, the stop codon (UAA) is present which is an indicator to show that the polypeptide is released at this point.
