Retrotransposons resemble retrovirus in their structure. Retrovirus encodes at least three different proteins essential for their replication. The three proteins are the products of the genes gag - which have a role in maturation of the RNA (Ribonucleic Acid) genome; pol which encodes the enzyme reverse transcriptase; and env which encodes the structural proteins that make up the protein capsid.
Retrotransposons contain sequences similar to gag and pol, but not env. Due to the absence of genes that are related to env, the retrotransposons cannot leave the cell. Instead, the dsDNA (double stranded Deoxyribo nucleic acid) copies produced by them are inserted back into the genome of the same cell where the transposons are present.
Lack of Env protein restricts retrotransposon mobility within the cell where they are present. Hence, the correct answer is option (a).
• Retrotransposons are autonomous elements as they do not need the presence of another transposon for their transposition. Hence, option (b) is not correct
• The enzyme reverse transcriptase is present in retrotransposons. Hence, option (c) is not correct
• Option (b) is not true for retrotransposons. Hence, option (d) a and b are both true is not correct