About Open Reading Frame ( ORF)

The genetic code is the correspondence between triplets in DNA (or RNA) and amino acids in protein.

A codon is a triplet of nucleotides that represents an amino acid or a termination signal.



The sequence of a coding strand of DNA, read in the direction from 5'>to 3' , consists of nucleotide triplets (codons) corresponding to the amino acid sequence of a protein read from N-terminus to C-terminus.

The coding strand (Sense strand) of DNA has the same sequence as the mRNA(except for possessing T instead of U) and is related by the genetic code to the protein sequence that it represents.

The antisense strand (Template strand) of DNA is complementary to the sense strand, and is the one that acts as the template for synthesis of mRNA.



Gene expression occurs by a two-stage process:

Transcription generates a single-stranded RNA identical in sequence with one of the strands of the duplex DNA.

Translation converts the nucleotide sequence of mRNA into the sequence of amino acids comprising a protein. The entire length of an mRNA is not translated, but each mRNA contains at least one coding region that is related to a protein sequence by the genetic code: each nucleotide triplet (codon) of the coding region represents one amino acid.



The genetic code is read in nonoverlapping triplets from a fixed starting point:

Nonoverlapping implies that each codon consists of three nucleotides and that successive codons are represented by successive trinucleotides.

The use of a fixed starting point means that assembly of a protein must start at one end and work to the other, so that different parts of the coding sequence cannot be read independently.

There are six potential reading frames in every DNA sequence, with three possible frames on one strand, and another three on the complementary strand.
All ORFs read in the 5' to 3' direction on a given sequence.

A reading frame is one of the three possible ways of reading a nucleotide sequence. Each reading frame divides the sequence into a series of successive triplets. There are three possible reading frames in any sequence, depending on the starting point.

If the first frame starts at position 1, the second frame starts at position 2, and the third frame starts at position 3.

An open reading frame (ORF) is a sequence of DNA consisting of triplets that can be translated into amino acids starting with an initiation codon and ending with a termination codon.

The initiation codon is a special codon (usually AUG) used to start synthesis of a protein.

A stop codon (Termination codon) is one of three triplets (UAG, UAA, UGA) that causes protein synthesis to terminate. They are also known historically as nonsense codons. The UAA codon is called ochre, and the UAG codon is called amber, after the names of the nonsense mutations by which they were originally identified.

A blocked reading frame cannot be translated into protein because of the occurrence of termination codons.

If the genetic code is read in nonoverlapping triplets, there are three possible ways of translating any nucleotide sequence into protein, depending on the starting point. These called reading frames.

For the sequence:

A C G A C G A C G A C G A C G A C G

the three possible reading frames are:

ACG ACG ACG ACG ACG ACG ACG
-CGA CGA CGA CGA CGA CGA CGA
--GAC GAC GAC GAC GAC GAC GAC

A reading frame that consists exclusively of triplets representing amino acids is called an open reading frame or ORF. A sequence that is translated into protein has a reading frame that starts with a special initiation codon (AUG) and that extends through a series of triplets representing amino acids until it ends at one of three types of termination codon.

A reading frame that cannot be read into protein because termination codons occur frequently is said to be blocked.

If a sequence is blocked in all three reading frames, it cannot have the function of coding for protein.

When the sequence of a DNA region of unknown function is obtained, each possible reading frame ( 6 for each double strand) is analyzed to determine whether it is open or blocked. Usually no more than one of the three possible frames of reading is open in any single stretch of DNA.