Question

a. (20 point) Explain the transcription steps in order. Which molecules take roles? (you can draw and explain)

b. (20 points) How a ribosome work for protein synthesis? (you can draw and explain)

c. (20 points) What is the molecular mechanisms of Coronavirus entry into cell, explain.

Answer 1

(a)

Protein synthesis occurs in two stages- Transcription and translation. The process in which DNA information is copied on mRNA is known as transcription and the process by which the information on the mRNA is transferred to the protein and the mRNA is read by ribosomes for the synthesis of proteins is called translation. The flow of genetic information is shown below:

Transcription is a process of formation of RNA (ribonucleic acid) which takes place in the nucleus and is catalyzed by the enzyme RNA polymerase. It occurs unidirectionally in which transcript (RNA) is synthesized in direction. The various steps involved in transcription are:-

• Formation of transcription unit- The transcribed units are called a transcription unit. It starts with the transcription start site and terminates at the transcription termination site. At transcription start site, first transcription factors bind and then RNA polymerase binds to laid down the process of transcription.
• Initiation- DNA strands unwind and polymerase initiates RNA synthesis at the start point on the template strand.
• Elongation- Polymerase starts moving downstream unwinds the double-strand and elongates transcript in direction.
• Termination- RNA transcript is released and the polymerase detaches from DNA leads to the termination of transcription.

Transcription factors are the trans-acting proteins that control the transcription by either activating or repressing the particular genes. Transcription factors can be activated and inactivated by various processes like phosphorylation, methylation, etc. The transcription factors get activated when they are required and inactivated when they are not required.

Activators are the DNA-binding proteins that stimulate the transcription. Activators mainly bind to enhancers which facilitate the recruitment of the enzyme RNA polymerase II.

(b)

The translation is a process of protein synthesis which is directly mediated by RNA. A cell translates the information contained in the nucleotide sequence of RNA into a sequence of amino acids which combines and form polypeptide by the use of tRNA. The flow of genetic information occurs as:

A cell translates an mRNA message into polypeptide using tRNA. Transfer RNA bound to a specific amino acid by an aminoacyl tRNA transferase. The tRNA lines up by anticodon at the complementary codon present on mRNA. This coupling is facilitated by the ribosome which is made up of ribosomal RNA (rRNA) and proteins. Thus mRNA, tRNA, and ribosomes involve directly in translation.

In eubacteria such as E.coli, protein synthesis involves three steps:-

• Initiation- It involves the binding of the small ribosomal subunit to mRNA and then is joined by the 50S ribosomal subunit. For the initiation to start, mRNA, small ribosomal subunit and initiator tRNA come together. Initiator tRNA carries a formylated methionine or methionine residue that initiates protein synthesis. It recognizes the initiation codons on mRNA. It involves the reactions that laid down the formation of a peptide bond between the first two amino acids of the protein.
• Elongation- It involves the cyclic process on the ribosome in which one amino acid is added at a time to the nascent polypeptide chain. It is aided by the elongation factors and remains continue until the addition of the last amino acid.
• Termination- It includes the steps that are involved in releasing the complete polypeptide chain. It is aided by the release factors and stop codons. Dissociation of the ribosome from mRNA also involves in the termination process.

Translocation is a process that occurs during the elongation step of translation. It occurs by the elongation factors EF-G which is also called translocase and is driven by the splitting by the GTP (guanine triphosphate). Translocation is the step that occurs when all the translation factors, tRNA and ribosomes attached, and polynucleotide synthesis is initiated.

Codons or Genetic code is the set of three nucleotides present on mRNA, which is recognized by tRNA during translation. The translation of genetic information into amino acid sequences takes place on the ribosome. Codons are present on the mRNA molecule. tRNA has anticodons that recognize the codons on mRNA. During translation, the genetic information present on DNA is transcribed and then translated to form a polynucleotide chain. The translation of genetic information into amino acid sequences takes place on the ribosome and is mediated by tRNA.

(c)

Coronaviruses replicate and make its copies in the cytoplasm and are enveloped positive-stranded RNA viruses. They are dependant on the fusion between their envelope and the host cell to transport or deliver their nucleocapsid into the host cell.

The virus entry is mediated by spike glycoprotein (S) and is a primary determinant of pathogenesis and cell tropism. It is listed as a class I fusion protein that is responsible for binding to the host cell's receptor and mediate the fusion of viral membranes and the host.

It is a process that is directed by significant conformational variations of the S protein- low pH exposure, proteolytic activation, and receptor binding.