Question

Difference between monocistronic and polycistronic transcription unit

Answer 1

Basically, the term CISTRON came from experimentation on complementarity test done on Drosophila by Lewis.A gene is actually a unit of function calledCISTRON. The concept of cistron is based on cis-trans effect.To understand, let us take an example of gene ∆ having two mutant alleles ∆1 and∆2. When ∆1 is present on chromosome 1 and ∆2 is present on chromosome 2 of thepair of homologous chromosomes they are said to be in transarrangement (in opposite positions ontwo chromosomes) and are non-complementing as they produce a visible mutant phenotype. When, both the alleles are present on same chromosome (∆1∆2/++), it is aclledto be in cis arrangement and produces a wild phenotype. Now, consider a third allele ∆3 present in trans arrangement with ∆1 on the paired chromosome and produces wild phenotype and thus called to be complement of ∆1.Hence, when the two mutations are presentin the trans position produce a mutant phenotype, they are said to be members of same functional unit called CISTRON. But, if the two mutations are presentin trans position and complement each other, they are said tobe belonging to different CISTRON.So the crux of it all is that, a cistron is a genetic unit (of DNA) within which two mutations cannot complement, it can be equated with the region of DNA that encodes a single polypeptide.Monocistronic as the name suggests is the one that codes for single polypeptide. Whereas, polycistronic codes for multiple polypeptides (from within a single mRNA molecules having multiple ORF).
I hope it helps u and sorry it's long

Answer 2

Polycistronic mRNA is that messenger RNA which encodes for two or more proteins, whereas monocistronic mRNA is that messenger RNA which encodes for only one or specific protein or polypeptide. Polycistronic mRNA contains many codons of cistrons whereas, monocistronic mRNA contains single codon of a cistron.