RNA and Protein Synthesis

During transcription, the enzyme RNA polymerase synthesizes a strand of messenger RNA (mRNA) using one strand of the gene as a template.

mRNA is synthesized using nucleotides that contain the bases A, C, G, and U (instead of T), which pair with the complementary bases of the DNA template strand.  You've got the same base pairing rules we saw with DNA, that is, C hydrogen bonds with G and A hydrogen bonds with U.

This is transcription, and occurs in the nucleus of eukaryotic cells. The mRNA will be "read" by a ribosome and the protein the gene codes for will synthesized by the ribosome - this is translation and occurs in the cytoplasm of both eukaryotic and prokaryotic cells.

Some terminology:

The mRNA is the molecule that carries the genetic code and is decoded by ribosomes to synthesize the gene product. That makes mRNA a coding strand of nucleic acid.

 

The nucleotide sequence of the template strand of a gene is complementary to the mRNA. The non-template strand of the gene is (of course) complementary to the template strand of the gene; therefore the nucleotide sequence of the non-template strand of the gene is identical to the nucleotide sequence of the mRNA (except where the mRNA has U intead of T).

 

This is why the non-template strand of the gene is sometimes referred to as the coding strand and the template strand as the non-coding strand.

 

In viruses that use RNA as their genetic material RNA that can be directly translated by ribosomes is referred to as + strand or sense strand. A nucleic acid strand that is complementary to a + strand is - strand, or antisense.

 

If a DNA strand that is complementary to the + strand of a virus or to a mRNA molecule is allowed to bind, forming an RNA-DNA hybrid, the RNA can't be read by a ribosome. This is called antisense technology and can be used to prevent translation of undesirable messenger or + strand RNA.

 

So of course you can refer to the non-template strand of a gene (the nucleic acid sequence is identical to the mRNA sequence) as sense strand and the template strand (complementary to the mRNA) as the antisense strand.

The starting point for transcription, where RNA polymerase unwinds the DNA and begins to make a complementary mRNA copy, is the promoter site; the region of DNA that is the endpoint of transcription is the terminator site; mRNA is synthesized in the 5’ to 3’ direction.

So how does the RNA polymerase "know" which strand of DNA to copy? 

First of all, since RNA polymerase synthesizes mRNA from 5' to 3' (just like DNA polymerase) the DNA template strand has to be read from 3' to 5' (just like when DNA polymerase uses a DNA template strand to synthesize a new complementary DNA strand).

 

The key is the promoter site - there are a couple of base sequences that are in the promoter site, one 10 bases before the start of the coding region and another 35 bases before the coding region.  These sequences are recognized by the RNA polymerase and line it up to read the correct strand of DNA.

 

Click here for McGraw-Hill transcription animation

A word of caution here: Transcription looks a lot like DNA replication, but it isn't the same thing. There is no replication fork, there is no leading strand/lagging strand, and the mRNA doesn't remain bound to the template strand of the gene.

Translation is the process in which the information in the nucleotide base sequence of mRNA is used to dictate the amino acid sequence of a protein.

The mRNA associates with ribosomes, which consist of rRNA and protein.

Three-base segments of mRNA that specify amino acids are called codons. (The three-base segments of DNA that the codons are transcribed from are called triplets and the three base segments on tRNA that help align the correct amino acid on the ribosome are called anticodons).   The genetic code refers to the relationship between the nucleotide base sequence of DNA (the triplets), the corresponding codons of mRNA, and the amino acids for which the codons code.   Specific amino acids are attached to molecules of tRNA. Another portion of the tRNA has a three base sequence called an anticodon.   Notice that the anticodons are complementary and antiparallel to the codons, the codons are complementary and antiparallel to the triplets.   Notice also if two things are both complementary to a third thing (i.e. the tRNA anticodon and the DNA triplet that are complementary to the mRNA codon AUG) they are identical to each other (except for the U in RNA instead of T as in DNA - the anticodon that is complementary to AUG is UAC and the triplet that is complementary to AUG is TAC).

The genetic code is degenerate; that is, most amino acids are coded for by more than one codon.   Of the 64 codons, 61 are sense codons (which code for amino acids, and 3 are nonsense codons (which do not code for amino acids and are stop signals for translation).   The start codon, AUG, codes for methionine.

The base pairing of codon and anticodon at the ribosome results in specific amino acids being brought to the site of protein synthesis.

The ribosome moves along the mRNA strand as amino acids are joined to forma growing polypeptide; mRNA is read in the 5’ to 3’ direction.

Translation ends when the ribosome reaches a stop codon on the mRNA.

Click here for McGraw-Hill translation animation.

In prokaryotes, translation can begin before transcription is complete.

In eukaryotes, coding regions of a gene (the expressed regions, or exons) are often interrupted by noncoding regions (intervening sequences, or introns).

In the nucleus, RNA polymerase synthesizes an RNA transcript containing exons and introns.

The introns must be removed from the RNA transcript before the resulting mRNA can be translated – ribozymes remove the introns and splice the exons together. The mRNA is then moved through the nuclear membrane and into the cytoplasm, where translation takes place.