Translation of RNA to Proteins

Read this article to learn about the process of translation of RNA to proteins.

Translation:

The genetic information stored in DNA is passed on to RNA (through transcription), and ultimately expressed in the language of proteins. The biosynthesis of a protein or a polypeptide in a living cell is referred to as translation.

The term translation is used to represent the biochemical translation of four-letter language information from nucleic acids (DNA and then RNA) to 20 letter language of proteins.

The sequence of amino acids in the protein synthesized is determined by the nucleotide base sequence of mRNA.

Variability of Cells in Translation:

There are wide variations in the cells with respect to the quality and quantity of proteins synthesized. This largely depends on the need and ability of the cells. Erythrocytes (red blood cells) lack the machinery for translation, and therefore cannot synthesize proteins.

In general, the growing and dividing cells produce larger quantities of proteins. Some of the cells continuously synthesize proteins for export. For instance, liver cells produce albumin and blood clotting factors for export into the blood for circulation. The normal liver cells are very rich in the protein biosynthetic machinery, and thus the liver may be regarded as the protein factory in the human body.

Genetic Code:

The three nucleotide (triplet) base sequences in mRNA that act as code words for amino acids in protein constitute the genetic code or simply codons. The genetic code may be regarded as a dictionary of nucleotide bases (A, G, C and U) that determines the sequence of amino acids in proteins.

The codons are composed of the four nucleotide bases, namely the purines—adenine (A) and guanine (G), and the pyrimidine’s—cytosine (C) and uracil (U). These four bases produce 64 different combinations (4³) of three base codons, as depicted in Table 4.1. The nucleotide sequence of the codon on mRNA is written from the 5′-end to 3′ end. Sixty one codons code for the 20 amino acids found in protein.

The three codons UAA, UAG and UGA do not code for amino acids. They act as stop signals in protein synthesis. These three codons are collectively known as termination codons or non­sense codons. The codons UAG, UAA and UGA are often referred to, respectively, as amber, ochre and opal codons.

The codons AUG—and, sometimes, GUG—are the chain initiating codons.

Other Characteristics of Genetic Code:

The genetic code is universal, specific, non- overlapping and degenerate.

1. Universality:

The same codons are used to code for the same amino acids in all the living organisms. Thus, the genetic code has been conserved during the course of evolution. Hence genetic code is appropriately regarded as universal. There are, however, a few exceptions. For instance, AUA is the codon for methionine in mitochondria. The same codon (AUA) codes for isoleucine in cytoplasm. With some exceptions noted, the genetic code is universal.

2. Specificity:

A particular codon always codes for the same amino acid, hence the genetic code is highly specific or unambiguous e.g. UGG is the codon for tryptophan.

3. Non-overlapping:

The genetic code is read from a fixed point as a continuous base sequence. It is non-overlapping, comma less and without any punctuations. For instance, UUUCUUACAGGG is read as UUU/CUU/AGA/GGG. Addition or deletion of one or two bases will radically change the message sequence in mRNA. And the protein synthesized from such mRNA will be totally different. This is encountered in frame shift mutations which cause an alteration in the reading frame of mRNA.

4. Degenerate:

Most of the amino acids have more than one codon. The codon is degenerate or redundant, since there are 61 codons available to code for only 20 amino acids. For instance, glycine has four codons. The codons that designate the same amino acid are called synonyms. Most of the synonyms differ only in the third (3′ end) base of the codon. The Wobble hypothesis explains codon degeneracy.

Codon-Anticodon Recognition:

The codon of the mRNA is recognized by the anticodon of tRNA (Fig. 4.14). They pair with each other in antiparallel direction (5′ → 3′ of mRNA with 3′ → 5′ of tRNA). The usual conventional complementary base pairing (A = U, C = G) occurs between the first two bases of codon and the last two bases of anticodon. The third base of the codon is rather lenient or flexible with regard to the complementary base. The anticodon region of tRNA consists of seven nucleotides and it recognizes the three letter codon in mRNA.

Wobble Hypothesis:

Wobble hypothesis, put forth by Crick, is the phenomenon in which a single tRNA can recognize more than one codon. This is due to the fact that the third base (3′-base) in the codon often fails to recognize the specific complementary base in the anticodon (5′-base). Wobbling is attributed to the difference in the spatial arrangement of the 5′-end of the anticodon. The possible pairing of 5′-end base of anticodon (of tRNA) with the 3′-end base of codon (mRNA) is given

Wobble hypothesis explains the degeneracy of the genetic code, i.e. existence of multiple codons for a single amino acid. Although there are 61 codons for amino acids, the number of tRNAs is far less (around 40) which is due to wobbling.

Mutations and Genetic Code:

Mutations result in the change of nucleotide sequences in the DNA, and consequently in the RNA. The ultimate effect of mutations is on the translation through the alterations in codons. Some of the mutations are harmful.

The occurrence of the disease sickle-cell anemia due to a single base alteration (CTC → CAC in DNA, and GAG → GUG in RNA) is a classical example of the seriousness of mutations. The result is that glutamate at the 6th position of β-chain of hemoglobin is replaced by valine. This happens since the altered codon GUG of mRNA codes for valine instead of glutamate (coded by GAG in normal people).

Frame shift mutations are caused by deletion or insertion of nucleotides in the DNA that generates altered mRNAs. As the reading frame of mRNA is continuous, the codons are read in continuation, and amino acids are added. This results in proteins that may contain several altered amino acids, or sometimes the protein synthesis may be terminated prematurely.

Protein Targeting:

The eukaryotic proteins (tens of thousands) are distributed between the cytosol, plasma membrane and a number of cellular organelles (nucleus, mitochondria, endoplasmic reticulum etc.). At the appropriate places, they perform their functions.

The proteins, synthesized in translation, have to reach their destination to exhibit their biological activity. This is carried out by a process called protein targeting or protein sorting or protein localization. The proteins move from one compartment to another by multiple mechanisms.

The protein transport from the endoplasmic reticulum through the Golgi apparatus, and beyond uses carrier vesicles. It may be, however, noted that only the correctly folded proteins are recognized as the cargo for transport. Protein targeting and post-translational modifications occur in a coordinated manner.

Certain glycoproteins are targeted to reach lysosomes, as the lysosomal proteins can recognize the glycosidic compounds e.g. N-acetyl- glucosamine phosphate. For the transport of secretory proteins, a special mechanism is operative. A signal peptide containing 15-35 amino acids, located at the amino terminal end of the secretory proteins facilitates the transport.

Protein Targeting to Mitochondria:

Most of the proteins of mitochondria are synthesized in the cytosol, and their transport to mitochondria is a complex process. Majority of the proteins are synthesized as larger pre-proteins with N-terminal pre-sequences for the entry of these proteins into mitochondria. The transport of unfolded proteins is often facilitated by chaperones.

One protein namely mitochondrial matrix targeting signal, involved in protein targeting has been identified. This protein can recognize mitochondrial receptor and transport certain proteins from cytosol to mitochondria. This is an energy-dependent process.

Protein Targeting to Other Organelles:

Specific signals for the transport of proteins to organelles such as nuclei and peroxisomes have been identified. The smaller proteins can easily pass through nuclear pores. However, for larger proteins, nuclear localization signals are needed to facilitate their entry into nucleus.

Mitochondrial DNA, Transcription and Translation:

The mitochondrial DNA (mtDNA) has structural and functional resemblances with prokaryotic DNA. This fact supports the’ view that mito­chondria are derivatives of prokaryotes. mtDNA is circular in nature and contains about 16,000 nucleotide bases.

A vast majority of structural and functional proteins of the mitochondria are synthesized in the cytosol, under the influence of nuclear DNA. However, certain proteins (around 13), most of them being the components of electron transport chain, are synthesized in the mitochondria. Transcription takes place in the mitochondria leading to the synthesis of mRNAs, tRNAs and rRNAs. Two types of rRNA and about 22 species of tRNA have been so far identified. This is followed by translation resulting in protein synthesis.

The mitochondria of the sperm cell do not enter the ovum during fertilization; therefore, mtDNA is inherited from the mother. Mitochondrial DNA is subjected to high rate of mutations (about 10 times more than nuclear DNA) that causes inherited defects in oxidative phosphorylation. The best known among them are certain mitochondrial myopathies and Leber’s hereditary optic neuropathy.

The latter is mostly found in males and is characterized by blindness due to loss of central vision as a result of neuroretinal degeneration. Leber’s hereditary optic neuropathy is a consequence of single base mutation in mtDNA. Due to this, the amino acid histidine, in place of arginine, is incorporated into the enzyme NADH coenzyme Q reductase.