Molecular Basis of Inheritance

In the early 1980s, Thomas Cech and Sidney Altman discovered that some RNA molecules can act as catalysts (ribozymes), e.g. self-splicing introns and RNase P.
This showed that RNA can both store genetic information and catalyse reactions.
So it was proposed that early life was based on RNA alone (the 'RNA world'), before DNA and protein enzymes evolved.
So the term RNA world came from the discovery that RNA has enzymatic properties.

Eukaryotic nuclei have three types of RNA polymerase:
RNA polymerase I transcribes rRNAs (28S, 18S and 5.8S).
RNA polymerase II transcribes the precursor of mRNA (hnRNA).
RNA polymerase III transcribes tRNA, 5S rRNA and snRNAs.
So 3 types of RNA polymerase are required in the nucleus. (Prokaryotes have only one.)

RNA polymerase can add nucleotides only to the free 3'-OH end of the growing chain.
So the new mRNA is always synthesised in the 5' $\rightarrow$ 3' direction (the template strand is read 3' $\rightarrow$ 5').
So mRNA is synthesised in the 5' $\rightarrow$ 3' direction.

In negative control (e.g. the lac operon), a repressor protein binds to the operator and switches the genes off.
In an inducible operon, the inducer (e.g. lactose or allolactose) binds to the repressor, changes its shape and makes it unable to bind to the operator. So the genes are switched on.
In a repressible operon (e.g. trp operon), the co-repressor (tryptophan) binds to the repressor (not to the inducer) and activates it.
cAMP, with CAP, has a positive effect on the lac operon.
So in a negative operon, the inducer binds with the repressor.

DNA replication is semiconservative: each daughter DNA molecule has one old (parental) strand and one newly synthesised strand.
During the five minutes in radioactive thymidine, the new strands being made take up the label, so the DNA formed has one radioactive (new) strand and one non-radioactive (old) strand.
When this cell then replicates in a normal medium, each labelled strand acts as a template for a new unlabelled strand. So DNA molecules carrying the label have only one radioactive strand.
So, as per the answer key, one strand of the DNA will be radioactive.

Benzer defined the cistron as the unit of function: the segment of DNA that codes for one polypeptide chain.
A gene is also usually defined as the DNA segment that codes for one polypeptide (one gene–one polypeptide).
Since one gene corresponds to one cistron, the two words are often used synonymously.
So they are used synonymously because one gene contains one cistron.

During development (organogenesis), different genes are switched on and off in different cells and at different times.
In eukaryotes, the coding sequences (exons) of a gene can be joined in different combinations by alternative splicing, so one gene can give different proteins in different tissues and at different stages.
As per the answer key, this regulation at different levels and times is attributed to exons.
(This answer is debatable: control of when and where genes are expressed is usually attributed to promoters and regulatory sequences.)
So, as per the answer key, the answer is exon.

Seymour Benzer studied rII mutants of T4 phage, which cannot grow on E. coli strain K.
When two different rII mutants infect the same cell together, they can complement each other only if the mutations are in different functional units (cistrons A and B). Each phage supplies the product that the other lacks, so the bacterium is lysed.
If both mutations were in the same cistron, neither could supply that function and there would be no lysis.
So the two strains lyse E. coli together because they have mutations in different cistrons.