Question

A 3000 bp region of the human genome encodes two genes. One of the genes encodes a protein of 700 amino acids and the other gene encodes a protein of 310 amino acids. The mRNA sequences of the two genes do not contain any of the same nucleotide sequences. How is this possible? Fully explain your answer.

Answer 1

mRNA sequences are transcribed from the DNA and synthesise protein . Nucleotide sequences on mRNA are arranged as codon. Codons are the group of three nucleotide sequences on mRNA. These codons specifies particular amino acids. The mRNA sequences of the two genes do not contain any of the same nucleotide sequences. This can be possible as DNA nucleotide sequences are varried in human genome. Two genes may undergo mutation which is a random chanegs in the nucleotide sequences or it may undergo genetic recombination in which genetic material is rearranged. Insertion or deletion of one or many nucleotides during the DNA replication process can lead to frameshift mutation. This insertion or deletion of any base can alter the complete sequence and thus the reading frame of mRNA will be completely changed. suppose the RNA seqence that codes foraminoacids are as follows:

AAA AUG UUC CGA CGC AGG GUG

now if mutation occur and if deletion occurs at fifth nucleotide in the sequence the complete set is changed and read as:

AAA AGU UCC GAC GCA AAA UG

Like that any addition of base in the sequence also results in sequence alteration.

Rate of recombination determines the base composition.

Besides mutation and recombination GC content also show high heterogenity in human genome. GC rich regions have less number of introns and shows compact genes but GC poor regions have large number of introns.

RNA editing at the time of post transcriptional modification may also causes change in nucleotide sequence. In RNA editing mRNA is modifiedas cytosine undergoes deamination and converted into uracil. Or adenine converted into inosine which behaves like guanosine. This process is site specific base mutation.