- 96 -

The Law

Belief - VIII

Our Cellular Biology - VII

Once an mRNA has been produced by transcription, the information present in its nucleotide sequence is used to synthesize a protein.  Transcription is simple to understand as a means of information transfer: since DNA and RNA are chemically and structurally similar, the DNA can act as a direct template for the synthesis of RNA by complementary base-pairing. As the term transcription signifies, it is as if a message written out by hand is being converted, say, into a typewritten text. The language itself and the form of the message do not change, and the symbols used are closely related.

In contrast, the conversion of the information in mRNA into protein represents a translation of the information into another language that uses quite different symbols. Moreover, since there are only four different nucleotides in mRNA and twenty different types of amino acids in a protein, this translation cannot be accounted for by a direct one-to-one correspondence between a nucleotide in RNA and an amino acid in protein. The nucleotide sequence of a gene, through the medium of mRNA, is translated into the amino acid sequence of a protein by rules that are known as the genetic code. This code was deciphered in the early 1960s. Every amino acid is represented by a three-nucleotide sequence or codon along the mRNA molecule.(See the diagram below). For example, AGC is the mRNA codon for amino acid serine; UAG is the codon which when encountered stops translation - stop codon. 

Note: The genetic code is a set of rules defining how the four-letter code of DNA is translated into the 20-letter code of amino acids, which are the building blocks of proteins.Three-letter combinations from the four-letter alphabet of bases form the genetic code. These base triplets act as codons to specify one of the letters of the 20 letter alphabet of amino acids. With three letters out of four possible, there are 4³ = 64 possibilities.

To recall, the sequence of nucleotides in the mRNA molecules is read consecutively in groups of three. RNA is a linear polymer of four different nucleotides, so there are 4 × 4 × 4 = 64 possible combinations of three nucleotides: the triplets AAA, AUA, AUG, and so on. However, only 20 different amino acids are commonly found in proteins. Either some nucleotide triplets are never used, or the code is redundant and some amino acids are specified by more than one triplet. The last possibility is, in fact, the correct one - some amino acids are specified by multiple codons.

In principle, an RNA sequence can be translated in any one of three different reading frames, depending on where the decoding process begins. However, only one of the three possible reading frames in an mRNA encodes the required protein. We see later how a special punctuation signal at the beginning of each RNA message sets the correct reading frame at the start of protein synthesis.

Note: In the process of translating a nucleotide sequence (blue) into an amino acid sequence (green), the sequence of nucleotides in an mRNA molecule is read from the 5′ to the 3′ end in sequential sets of three nucleotides. In principle, therefore, the same RNA sequence can specify three completely different amino acid sequences, depending on the reading frame In reality, however, only one of these reading frames contains the actual message.

The codons in an mRNA molecule do not directly recognize the amino acids they specify: the group of three nucleotides does not, for example, bind directly to the amino acid. Rather, the translation of mRNA into protein depends on adaptor molecules that can recognize and bind both to the codon and, at another site on their surface, to the amino acid. These adaptors consist of a set of small RNA molecules known as transfer RNAs (tRNAs), each about 80 nucleotides in length.

After transcription occurs, the transcribed mRNA moves out from the nucleus through nuclear pores into the cytoplasm and binds to the ribosome. mRNA binds to the small subunit of the ribosome with its first two codons contained within the ribosome. The first codon is called the start codon (AUG) which codes for the amino acid methionine. The corresponding tRNA attaches to the mRNA bringing the amino acid methionine to the ribosome to start the polypeptide chain. While still attached, a second tRNA attaches to the mRNA second binding site on the ribosome, carrying the amino acid that corresponds to the codon on the mRNA. 

The two amino acid molecules form a bond between them by a condensation reaction. The bond between the first amino acid molecule - methionine - and the tRNA is severed by an enzyme. The ribosome slides along the mRNA - a ratchet like movement - moving down one codon at a time, releasing the tRNA into cytoplasm so it can pick up another amino acid - in this case methionine. Another tRNA moves into the empty site, carrying another amino acid corresponding to the codon on the mRNA. Again the amino acid is attached to the polypeptide and the previous tRNA is released back into the cytoplasm as the ribosome moves along the mRNA. The process continues until 1 of the 3 stop codons (UAA, UGA and UAG) is encountered. These tRNAs have no amino acid attached to them. Finally, when the ribosome along the mRNA, the polypeptide chain falls off and is released into the cytoplasm. 

Check the video below for easy understanding of the translation process.

Note: A condensation reaction, is a chemical reaction in which two molecules or moieties (functional groups) combine to form a larger molecule, together with the loss of a small molecule.

Namaste

See you soon.

Prabir


In this truly inspirational memoir, Anita Moorjani relates how, after fighting cancer for almost four years, her body-overwhelmed by the malignant cells spreading throughout her system-began shutting down. As her organs failed, she entered into an extraordinary near-death experience where she realized her inherent worth . . . and the actual cause of her disease. Upon regaining consciousness, Anita found that her condition had improved so rapidly that she was able to be released from the hospital within weeks .without a trace of cancer in her body! Within these pages, Anita recounts stories of her childhood in Hong Kong, her challenge to establish her career and find true love, as well as how she eventually ended up in that hospital bed where she defied all medical knowledge. As part of a traditional Hindu family residing in a largely Chinese and British society, she had been pushed and pulled by cultural and religious customs since she had been a little girl. After years of struggling to forge her own path while trying to meet everyone else's expectations, she had the realization, as a result of her epiphany on the other side, that she had the power to heal herself . . . and that there are miracles in the Universe that she had never even imagined. In Dying to Be Me, Anita freely shares all she has learned about illness, healing, fear, "being love," and the true magnificence of each and every human being! This is a book that definitely makes the case that we are spiritual beings having a human experience . . . and that we are all One!