This early morning, when I took the Morning Daily in my hand, I was awestruck and pained by the most haunted news about the pandemic – the steeply rising global tally of Covid-19 cases and death toll. This aching health issue anguished me and urged me to take a call, and share with readers, the biology of this one entity- the VIRUS,which is playing havoc in the lives of a great chunk of world’s populations. In my last blog, I had given a fair idea about the different types of WBCs which fight against the foreign invaders – bacteria and viruses, which are the causative agents of so many diseases/ infections in our body. As we all know that these foreign intruders viz. virus and bacteria cannot be seen with naked eye. Viruses are ultramicroscopic, even smaller than bacteria, ranging in size from 30 nm to 300 nm.
To know the structure & functioning of virus, we must first broadly understand the set of rules (Genetic code) by which information encoded in the genetic material viz. Nucleic acid (DNA or RNA) is translated into proteins (amino acid sequences). Biological decoding is accomplished by the ribosome using tRNA to read mRNA three nucleotides at a time. Most viruses have either RNA or DNA as genetic material (or genome). The nucleic acid may be single or double stranded. The simplest viruses contain only enough RNA or DNA to encode 4 proteins. Most complex can encode 100- 200 proteins. The portion of the genome that codes for a protein or an RNA is referred to as a GENE. (Significance of Proteins – Autism is linked to egg cells’ difficulty creating large proteins.) The Genetic code is highly similar among all organisms with the exception of some RNA viruses. They all use the same 4 bases ie. Guanine(G), Adenine(A), Cytosine(C) & Thymine(T) – or Uracil(U) in case of RNA and make the same amino acids.Because of Genetic code, many different species have same genes.
What is Genetic code ?
RNA or DNA is a long polymer made from repeating units called nucleotides. A Nucleotide has three components viz. (1) a nitrogenous base, which are two types- Purines (A,G) and Pyrimidines (C,T). Purines pair with Pyrimidines. So, Adenine(A) pairs with Thymine (T) (in RNA, Thymine is replaced with Uracil (U) ) & Guanine (G) pairs with Cytosine (C). This is called Complementary Base Pairing. (2) a pentose sugar (5-carbon sugar)– ribose in case of RNA and 2-deoxyribose for DNA. (3) a phosphate group The Genetic code is the sequence of the nucleotide bases in nucleic acids (RNA & DNA), that code for 20 different amino acids, by triplets formed from combinations of the above 4 nucleotide bases. A triplet is called a CODON. The Genetic code is the set of rules by which information encoded within DNA or (mRNA sequences) is translated into Proteins by living cells. The Genetic code can be expressed in 64 entries. Of the 64 codons, 61 codons specify 20 different amino acids and 3 codons (UAA, UAG, UGA) serve as Stop Codons to end Protein Synthesis. The codon AUG (codes for amino acid – Methionine) serve as Start Codon for beginning of Protein Synthesis (Translation).
Multiple codons may specify the same amino acid. For example – the codons UCU, UCC, UCA, UCG, AGU, and AGC all specify the amino acid – Serine. See the table below :-
The Genetic code has redundancy but no ambiguity.
Types of RNA – 3 types:-
(1) messenger RNA (mRNA):
mRNA plays important roles in Transcription process. Transcription is the first step of Gene Expression. It is the process that involves copying the genetic information contained in DNA into an RNA message. During transcription, the DNA strand unwinds and allows the enzyme RNA polymerase to transcribe only a single strand of DNA. When RNA polymerase transcribes the DNA into a mRNA molecule, Adenine pairs with Uracil and Cytosine pairs with Guanine.
(2) transfer RNA (tRNA):
The job of tRNA is to translate the nucleotide sequences of mRNA into specific amino acid sequences. The amino acid sequences are joined together to form a Protein.
(3) ribosomal RNA (rRNA):
The cell organelle called Ribosome consists of ribosomal proteins and rRNA. A ribosome consists of 2 subunits – a large subunit and a small subunit.
Process of Protein Synthesis (Translation):
During translation, a small ribosomal subunit attaches to a mRNA molecule. At the same time an initiator tRNA molecule recognises and binds to a specific codon sequence on the same mRNA. A large ribosomal subunit then joins the newly formed complex. Both ribosomal subunits travel along the mRNA translating the codons on mRNA into a polypeptide chain as they go. When a termination codon is reached on mRNA, the translation process ends. The polypeptide chain is released from the tRNA and the ribosome splits back into large and small sub units. The new formed polypeptide chain undergoes several modifications before becoming a fully functional protein. These proteins are used in the membrane of the cell, others remain in cytoplasm or transported out of the cell.
Tips for following’Flow Chart’ methodology:
(1) Divide the matter into brief, small, all- inclusive key points .
(2) Step-wise/ sequential jotting down of these points.
(3) Separating each step by a downward arrow.
Quick Recap of Transcription & Translation through FLOW CHART teaching methodology:
TRANSCRIPTION PROCESS :
Enzyme RNA polymerase binds to ⬇️ DNA transcription unit at PROMOTOR Site ⬇️ Sigma unit of RNA polymerase target START site to align polymerase onto Promotor ⬇️ Unwinding of one helix turn of DNA ⬇️ Tight binding of Polymerase with initiation of RNA Synthesis (first base added is usually a Purine) ⬇️ RNA polymerases use nucleoside triphosphate as substrate & polymerises in a template dependent fashion following the rule of Base Complementarity ⬇️ Elongation continues at a rate between 30 & 50 nucleotides per second ⬇️ Cessation of Elongation ⬇️ Release of Transcript ⬇️ Dissociation of Polymerase & mRNA chain from Template ⬇️ Post- transcriptional modifications of mRNAs (removal of INTRONS etc) ⬇️ Poly-A tail (Adenine bases) added to one end & a Guanosine triphosphate Cap added to the other end of mRNA for its protection
TRANSLATION PROCESS :
mRNA binds to smaller subunit of Ribosome ⬇️ Anticodon arm of tRNA binds to a specific codon sequence on same mRNA ⬇️ A large ribosomal subunit joins this complex ⬇️ Initiator tRNA in the P site of ribosome is released & tRNA in A site of ribosome is translocated to P site ⬇️ ‘A’ binding site becomes vacant ⬇️ Another tRNA recognising new mRNA codon attaches to this site ⬇️ Pattern continues ie. old tRNAs are released from the complex, new tRNAs attach and amino acid chain grows ⬇️ Ribosome reaches a termination codon on mRNA ⬇️ Polypeptide chain formed and released from tRNA ⬇️ Ribosome splits back into large and small subunits ⬇️ Newly formed polypeptide chain undergoes modifications ⬇️ Fully functional PROTEIN is formed
About next blog:
In the wake of the knowledge on Genetic code and Protein Synthesis shared in this blog, we’ll apply this information to know the ‘Genetic change in Viruses’ causing serious diseases, Structure of Virus, with a mention of Coronavirus also, in my next blog.