How does an organism tell the difference between the two strands of DNA?

Question

The two stands of DNA in a double helix have two different codes only one is useful so, why isn't the other one coded to protein what prevents this?

Answer 1

There is a recognition sequence before each gene (protein code) to tell the RNA transcription machinery where to bind (like a sign post). As for why only one strand is readable... think of the complementary strand as the protective cover on the readable strand. The code on the complementary strand is determined by the reading strand, it is the property of DNA that allows it to be copied.

Answer 2

Both DNA strands can be used for protein coding. And they are, although not at the same time. At each point in time, the RNA polymerase will decide which one of the 2 strands to transcribe. Mostly the decision depends on epigenetic changes which are modifications of the DNA structure independently of its sequence. The most common epigenetic change is methylation, i.e. the addition of a CH3 group to one of the OH-residues of Guanines. Methylation normally silences the portion of DNA immediatly adjacent and is an energetically "cheap" way of controlling transcription and gene expression.
Other epigenetic changes include nucleotide acetylation and the repositioning of DNA remodeling complexes (such as SWI/SNF complexes) which will "open" or "close" the DNA structure to transcription (and replication) machineries.

Answer 3

DNA replication or RNA transcription always begins at 5' end and progresses to 3' end

BUT You are missing a step in the central Dogma

DNA -> RNA -> Protein

The following is for Eukaryotes which include humans

in RNA transcription the RNA polymerase recognizes and specifically binds to the promoter region on DNA. The DNA is unwound and becomes single-stranded in the vicinity of the initiation site. Only one strand is used and it progresses from 5' to 3'

In DNA replication the initiator is a protein complex called origin of recognition complex recognizes a specific DNA sequence(s) called the origin of replication and binds to it. DNA replication occurs with both strands simultaneously in opposite directions 5' to 3' because the strands are mirrors

Answer 4

Actually there are genes on both strands of DNA. Which strand is used as a template and where the gene transcription begins is determined by a promoter sequence. A promoter is a specific sequence of bases that the RNA polymerase machinery recognizes.

Answer 5

Structurally, nucleic acids are giant polymers composed of repeating units called nucleotides, each of which consists of a sugar, a phosphate, and a base. In the case of DNA, the structure is double-stranded: that is, two nucleotide polymers pair up, matching nucleotide to nucleotide along their length. The sugar in DNA is deoxyribose. The four bases are adenine(A), thymine(T), cytosine(C), and guanine(G). Each can readily form hydrogen bonds to one of the others--A with T, C with G--so that two matching strands of DNA will group together in a double helix. The beginning of the strand is called 3' and the end 5'; these numbers refer to carbons in deoxyribose. The 3' end of one strand is next to the 5' end of the other; such an arrangement is called a crab canon. This form is called B-DNA; there are a few others