Messenger ribonucleic acid (or mRNA) is a single stranded chain of nucleotides (similar to those of DNA) joined together by sugar phosphate bonds.
Differences between mRNA and DNA
- mRNA is single stranded whereas DNA is double stranded (held together by weak hydrogen bonds between the bases belong to the nucleotides of each strand).
- mRNA nucleotides are composed of a ribose sugar, a phosphate and one of four bases whereas DNA nucleotides are composed of a deoxyribose sugar, a phosphate and one of four bases.
- The selection of bases available to each nucleotide differs between mRNA and DNA: mRNA can choose between guanine, cytosine, adenine and uracil; DNA can choose between guanine, cytosine, adenine and thymine.
Role of mRNA in protein synthesis
After DNA has been transcribed into mRNA in the nucleus of a cell, the newly formed mRNA leaves the nucleus through the nuclear membrane and enters the cytoplasm. A ribosome attaches to one end of the mRNA. The mRNA is made up of sets of three nucleotides and the combination of bases present on these is called a codon. In the ribosome, tRNA molecules (with the anticodons complementary to the codons on the mRNA strand) form weak hydrogen bonds to the mRNA. Each tRNA molecule has a specific amino acid joined to it depending on the anticodon. As further tRNA molecules form weak hydrogen bonds with the mRNA strand, the amino acids attached to each tRNA molecule join together with strong peptide bonds to form a growing peptide change (an unfinished protein). Once the amino acid of a tRNA molecule has joined onto the growing peptide chain, the tRNA molecule breaks away. Once the protein chain is complete it is released and is transported by the endoplasmic reticulum to the golgi apparatus where its structure is often changed slightly (for example by adding a carbohydrate section to result in a glycoprotein) before being packaged ready for secretion to the exterior of the cell.
Each tRNA molecule has a specific amino acid joined to it depending on what the anticodon is which, itself, depends on the order of the bases on the mRNA strand, which depends on the order of the bases in the original DNA double helix. This means that the structure of the end protein formed is directly related to the structure of the DNA (or the "genetic code").
This is how all the proteins in nature are made. From globular proteins - enzymes like amylase and compounds such as haemoglobin - to fibrous structural proteins and anywhere in between, all the vital proteins in our bodies are coded for by DNA and synthesised by transcription and translation.
This is a fine example of nature and life never ceasing to amaze!