(i) (a) :The two subunits of ribosomes come together at the time of protein formation. This phenomenon is called association. Mg²⁺ is essential for it.
(ii) (d): Eukaryotes have nine initiation factors- elf2, elf3, elfl, elf4A, elf4B, elf4C, elf4D, elf5,elf6.
(iii) (d): An mRNA molecule has some additional sequences that are not translated and are called untranslated regions (UTR). The UTRs are present at both 5' end (before start codon) and at 3'-end (after stop codon). They are required for efficient translation process.
(iv) (d): Amino-acyl tRNA-synthetase is also called aa-activating enzyme.
(v) (a): Enzyme peptidyl transferase is component of larger subunit of ribosome. It catalyses two principal chemical reaction - peptide bond formation and peptide release. Enzyme pyrophosphatase hydrolyses pyrophosphate and provides energy for driving the initial reaction of activating amino acids.

(i) (b): In DNA the nitrogenous base and a pentose sugar joins to form nucleoside with the help of bond called glycosidic bond or N-glycosidic linkage.
(ii) (a): The correct structure of DNA was first worked out by James Watson and Francis Crick in 1953. Their double-helix model of DNA structure was based on two major investigations, i.e., Chargaff's rules of base i'airing and study of X-ray diffraction pattern of DNA produced by Maurice Wilkins and Rosalind Franklin which helped Watson and Crick to design the 3-dimensional structure of DNA.
(iii) (b): Due to spiral twisting, the B-DNA duplex comes to have two types of alternate grooves, i.e., majo: (length 22 Â) and minor (length 12 Â).
(iv) (a) 
(v) (b): The double helical chains of B-DNA are bound to each other via hydrogen bonds in an antiparallel fashion, i.e., 5'-3' in one and 3'-5' in other. The pitch of helix per turn is 3.4 nm with 10 base pairs in each turn.

(a) Initiation step in transcription.
(b) A - RNA polymerase.
B - Sigma (\(\sigma \)) factor.
(c) Yes, this process can be coupled to translation because
(i) the RNA synthesised does not require any processing to become active.
(ii) both transcription and translation occur in the cytosol, as there is no distinct nucleus and cytosol in prokaryotes.

(a) Francis Crick proposed this.
(b) A - Replication (of D A)
B - Transcription
C - Translation
(c) In some viruses (retroviruses), the flow of information is in reverse direction, i.e., from RNA to DNA catalysed by enzyme, reverse transcriptase; hence, it is not applicable to these viruses.

(i) a - Histone octamer
b - DNA
c - H₁ histone
d - Core of histone molecules.
(ii) (a) Positively-charged component-Histone.
(b) Negatively-charged component-DNA.