Protein synthesis


DNA is like a code, which explains which order amino acids go in, when making a protein. However, like any code, it needs a translator if it is going to make sense.

What happens then if the translator is somewhere else?

A copy of the original code will be written down and taken to whenever the translator is. mRNA is the messenger. The original code has to be transcribed from the DNA’s template. But since the DNA template has the code for every single protein the body is going to make, it is far too big to move around. The mRNA that is made, however, can slip happily out of the nucleus, and find its way to where the code can be translated.

The first stage of protein synthesis is transcription. At this stage, the DNA helix is untwisted (uncoiled) by the enzyme DNA HELICASE. Then the DNA is split into two as the hydrogen bonds are broken between the complementary pairs, much like with DNA replication.  However, this mRNA strand leaves the nucleus via the nuclear pores to the cytoplasm.

RNA is much smaller than DNA as the latter contains the code for making lots and lots of different proteins, whereas mRNA, for instance, contains the information to make just one single poly-nucleotide chain (just one protein or part of a protein). The process of transcription means that the RNA molecule uses the sequence of nucleotides in the DNA molecule to determine its own structures.

1. RNA polymerase transcribes DNA to make messenger RNA (mRNA). 2. The mRNA sequence (dark red strand) is complementary to the DNA sequence (blue strand). 3. On ribosomes, transfer RNA (tRNA) helps convert mRNA into protein. 4. Amino acids link up to make a protein.

The second step of protein synthesis is translation, which occurs in the cytoplasm and requires ribosomes.  It is under the control of the enzyme RNA polymerase, which also plays a vital role in finding the start of the gene on the coding strand. When the DNA is translated, amino acids have to be brought together so they can bind together and form a protein. This is so because if the amino acids just happily floated about, the chances of them coming together in the right order would be very slim.  Fortunately, the DNA system has been developed to make sure they come together appropriately. Each amino acid will have to bind to a tRNA molecule.

Amino acids in the cytoplasm do not interact directly with the triplet code along the mRNA, but is brought into action by another RNA molecule called tRNA, which brings an amino acid to the end of the growing protein chain.

Start codon for protein synthesis

In the language of DNA, groups of three bases, known as triplets, code for the 20 amino acids, from which all proteins are made. Because there are no empty spaces between the triplets in the DNA, it is difficult to recognize the three bases that belong to one triplet and, particularly, to identify the starting point for protein synthesis on the nucleic acid strand.

Out of the 64 possible combinations of the four bases that make up triplets, three different codons either specify a stop codon or do not code for any amino acid. Hence, there are 61 codons that can be used to specify 20 amino acids, with considerable redundancy proving that the genetic code is degenerate.

Hence, before proteins can be manufactured, the DNA is transcribed into its transport form, the mRNA, and introduced into the cell plasma. Small protein factories, the ribosomes, bind to the mRNA here and commence with their work. They “read” the series of bases and translate them into amino acids. They begin their task neither directly at the beginning of the mRNA nor at a random point, but always at an AUG base triplet, the start codon. This triplet codes for the amino acid Methionine, which thus constitutes the first amino acid in every protein. However, Methionine can also appear at other locations in the protein.

For it to start, the first codon (base triplets on mRNA) must be METHIONINE (AUG). Then the anti-codons on a tRNA molecule with an amino acid come into the large ribosomal unit and match with a codon. Hence, the first anti-codon must be UAC, because it is complementary to AUG. Then the next anti-codon comes along.

Methionine is one of eight amino acids adults cannot synthesize from other nutrients. This means that you need to obtain your daily methionine needs from your diet. Consuming enough methionine in your diet is crucial to your metabolism, and may potentially aid your mental, bone and liver health.

Ribosome shave three binding sites: one for mRNA and two for tRNA. Now that mRNA has left the nucleus and reached the cytoplasm, a small ribosomal sub-unit attaches to the bottom of the mRNA strand, and a large ribosomal sub-unit to the top of the mRNA so that the synthesis can begin. Each tRNA molecule picks up the appropriate amino acid from the cytoplasm at its site of attachment.

tRNA are strands of DNA that exist within the cell cytoplasm, where one end on its clover-leafed shape is bound to a specific free amino acid, and the other end possesses an anti-codon  (consisting of base triplets) complementary to a set  of three bases on the mRNA strand (codon).  tRNA (transcribed in the nucleus by RNA Polymerase III) is made much in the same way to the mRNA molecule (transcribed by RNA Polymerase II). RNA Polymerase I only transcribes ribosomal RNA, which forms the structural component in protein synthesis.

During translation, an mRNA molecule is read by a ribosome in triplicates. Every three bases on the mRNA molecule effectively tell the ribosome to bind a specific tRNA molecule to this three-base site. With the tRNA molecule comes the associated amino acid which is attached to it. In short, tRNA carries amino acids to mRNA in order to build the polypeptides.

A mRNA molecule could be as big as 3000 nucleotides long, as there may be many amino acids to code for. However, tRNA molecule is not coding for loads of amino acids – only one amino acid is going to be bound to it – hence understandably short. tRNA is a small RNA chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation.

Before the tRNA leaves the ribosome, RNA POLYMERASE creates a peptide bond between the amino acids.  Then the TRNA leaves the ribosome, leaving the amino acid behind. This process repeats itself until it reaches a stop codon.  Then the amino acid chain (polypeptide) is released into the cytoplasm as a protein when the ribosome and mRNA separate.

This protein is in its primary stricture, and it can then assume its secondary structure (α-helix or β-pleated sheet), then tertiary 3D precise shape (globular or fibrous).

No eukaryotic mRNA has more than one protein coding region on it as opposed to some prokaryotic mRNAs, which have several coding regions (with several start and stop signals for translation) of proteins related in their action.

In a nutshell, for translation of mRNA into a protein, a ribosome becomes attached to the end of mRNA molecule to be translated. The anti-codon of tRNA forms a weak hydrogen bond with codon of mRNA. Amino acids are aligned in the correct order and are joined by strong peptide bonds into a polypeptide  chain (bond), then once completed will be released into the cytoplasm, modified by golgi bodies, and then released out of the cell .

Other things to get acquainted with regarding this process are that genes are not necessarily continuous segments of the DNA chain.

Often a gene will begin in one small section of DNA called an EXON, then be interrupted by a non-coding section called an INTRON, and then take up again further down the chain in another exon. Hence, the final mRNA molecule results only after the non-coded sections (introns) are cut out and the remaining pieces are spliced together.

Approximately, 90% of human DNA seems to be made up of INTRONS, and only about 10% of DNA contains coding instructions. Only 3-5% of genes are active in any one cell at any time. The reason for such a large proportion of introns are not yet known, but it is clear that there are reasons and that INTRONS do not represent junk DNA.

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One Response to Protein synthesis

  1. Dag says:

    Lucid and concise account of protein synthesis. The role of the different enzymes in the process and its immense proof-reading ability is incredible.

    Like

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