The mcb operon encodes the following four core proteins: mcb250, mcb251, mcb252 and mcb253. mcb354 is encoded by a separate gene. a mutation occurs in the third gene of the operon (i. e., mcb252) resulting in a stop codon in the middle of its coding sequence. as described previously, this might result in a polar effect on the downstream gene (i. e., mcb253) involving rho dependent termination. what are two possible ways that this nonsense mutation might result in a polar effect post-transcriptionally?

In one way, the existence of a nonsense mutation would lead to the generation of a premature termination codon that will be identified by the RNA polymerase as a termination sequence encouraged by the activity of Rho factor to dissociate ribosome, thus, discharging RNA polymerase and preventing further transcription mechanism making the transcription of the downstream sequence impossible.  

For the second way, there is a need to consider that the mechanism is taking place post-transcriptionally. Thus, the effects should be devised after transcription has taken place and the only fate lies in the mechanism encouraged by the RNA dependent RNA-polymerase. However, for this to take place, the event of genetic recombination can also be taken into account leading to the appearance of the faulty gene in the sequence. Apart from this, the open reading frame is required to be co-expressive that would be the most suitable factor, which determines whether the downstream sequences will be transcribed or not post nonsense mutation.  

However, the total change relies upon the fact that the mutation is taking place artificially or is induced naturally. One more thing to consider is that there is an existence of another gene known as MCB 354, which is encrypted by another gene and is probably monitored by another promoter sequence. Thus, co-expression would probably be the mechanism in terms of the rho-dependent termination.  

b is the best answer for this question.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?