Question

how a single reading frame can contain multiple open reading frames? Differentiate an open reading frame from a reading frame.

Answer 1

Difference between reading frame and open reading frame.

Reading frame is a stretch of mRNA contains triplet codon which are non overlapping and codes for particular amino acid sequence. Reading frame contains stop codon. In contrast, open reading frame (ORF) contains start codon but do not have stop codon. It is a portion of double stranded DNA which can code amino acid in either direction. Here the stop codon is located after the end of ORF (in reading frame) which stops the amino acid synthesis.

Reading frames do not overlap however in case of mitochondrial DNA and in some prokaryotes (like bacteria and virus), reading frame contains multiple ORF. In case of bacteria and viruses the genome size is very restricted hence genes tends to overlap for synthesis of different amino acid chains. So in one reading frame, multiple ORF produces multiple protein by overlapping . This will provide complexity in bacterial and viral genome. Three nucleotides are required for single amino acid hence in a single reading frame, there is 6 possible ORF is present in double stranded DNA, three in forward direction and three in reverse direction. It is also possible that multiple start codon is present in long sequences which can serve as start codon for different proteins, depending upon the position of start codon. However the stop codon is single (present in reading frame not in ORF).