A ribosome is an organelle found in the cytosol of all cells. It is composed of RNA and ribosomal proteins which translate mRNA into protein (i.e a polypeptide chain).

Ribosomes consist of two subunits that fit together and work as one to translate the mRNA into a protein. The size of the ribosome and its subunits are expressed in terms of Svedberg units (S) which is a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle and its 3D shape. Note : Svedberg units are not additive ! !

The three-nucleotide sequence at the end of a transfer RNA molecule that is complementary to, an amino acid specifying codon in messenger RNA is called an anticodon.

The leading codon of the messenger RNA molecule is the sequence AUG which encodes the amino acid, methionine. The initial methionine unit is usually split off the finished polypeptide. In bacteria, the first aminoacyl-tRNA to initiate translation is always a formyl derivative of methionine called FMet-tRNA.

The Wobble Hypothesis says that a single tRNA can decode more than one codon because there is a relaxation of the complementation rule of base pairing between the anticodon and codon in the third position (of the codon sequence).

AUG is a start codon and encodes the protein methionine. In some organisms, GUG is also used as a start codon for some proteins.

Codons are triplets of four possible bases (A, U, G and C). Basic math tells us that we can have 4³ or 64 triplets in all. Since there are only 20 amino acids, each of them is encoded by more than one codon except tryptophan and methionine.

The Shine Dalgarno sequence is a section of nucleotides on a prokaryotic mRNA molecule upstream of the translational start site, that serves to bind to ribosomal RNA and thereby bring the ribosome to the start codon on the mRNA. It is complementary to the 3' end of 16S rRNA.

If the genetic code were a four letter code, then the number of amino acids that could be encoded would be equal to 4⁴ = 256.

A polycistronic prokaryotic mRNA molecule must possess a series of start and stop codons since it encodes more than one protein. A spacer simply seperates two successive cistrons (genes).

A tRNA molecule enters the large ribosomal subunit through the A site. Follwing this, a peptide bond forms between its amino acid and the one being held in the P site and the tRNA molecule in the P site releases its hold on its amino acid, while the tRNA molecule in the A site moves to the P site. At the same time, the mRNA molecule moves a distance of three bases in order to position the next codon at the A site. This whole process is called translocation.

In prokaryotes, both transcription and translation take place in the cytoplasm.

Peptidyl transferase catalyses peptide bond formation between adjacent amino acids in the ribosome during translation.

A cluster of several ribosomes simultaneously translating the same mRNA is called a polysome (short form for polyribosome !).

In eukaryotes, reinitiation of polypeptide synthesis following an encounter of a ribosome with a stop codon does not occur. Eukaryotic mRNA is always polycistronic.

The base inosine is found in the anticodons of several tRNA molecules.

UAA, UAG are stop codons and do not encode any proteins. AUG is the only codon that encodes methionine. This eliminates all options except GUA and GUG.

The tRNA anticodon corresponding to the codon in question is AUU. Hence, the base sequence of the corresponding codon is UAA (a stop codon). The DNA sequence that corresponds to the RNA sequence UAA, is ATT.

The last codon UGA is a stop codon. All the other codons in the sequence encode specific amino acids. Since there are five codons other than the stop codon, the correct answer is 5.