Question

1. Give four specific counter examples for why each of the following definitions for a gene are incorrect.  For each definition, there may be several possible counter examples.  For full credit, each counter example should be distinct.  For example, if a friend of yours claimed that “a gene is all the DNA sequences that determine an inherited trait”, you could reply: “No, that is incorrect, because multiple genes will often determine an inherited trait.”

A. “A gene is a sequence of nucleotides from a start codon to a stop codon.”

B. “A gene is a sequence of nucleotides that codes for a functional protein.”

C. “A gene is a sequence of nucleotides that codes for an RNA molecule (which may be translated).”

D. “A gene is the entire DNA sequence that affects the production of a protein.”

Answer 1

A. Genes do not only include the region of nucleotides between the start and stop codons, but genes also includes regions before the start codon (5' untranslated region) and regions after the stop codon (3' untranslated region).

B. Genes do not only code for proteins, they also code for tRNA and rRNA molecules, which are never translated or converted into proteins.

C. Some genes are silenced in a genome, they are paternally or maternally imprinted and are never transcribed into RNA.

D. There are promoters, enhancers and silencer regions on DNA that affect the production of proteins. These regions are not considered as 'genes'.