Why is mRNA called a messenger RNA?
mRNA carries a written message from DNA . Ribosomes are able to read the message and accordingly proteins are made.
DNA is a huge book where recipes of proteins are written. Each written recipe is called a gene. Each cell of the body stores a complete recipe book in the form of genomic DNA though all recipes are not always required to be cooked.
Our body is made up of different types of cells. Hepatic cells form liver, Osteocytes form bones, Macrophages fight the germs, and so on.
Cells of different tissues require to produce different proteins. For example keratin protein is generated only in skin/nail/hair. Protein components of mucin and chondrin are produced respectively in goblet cells and chondroblasts. All this means that specific cells produce specific proteins.
Thus each cell requires to copy few specific recipes from the DNA stored within it. This is done by Transcription of genes on DNA to generate corresponding RNAs which are called messenger RNA. It is like copying a particular recipe on a page from a huge cook book. Thus a messenger RNA carries a particular information from genomic DNA.
Readable codes for protein synthesis are carried by messenger RNAs from genomic DNA to cellular pool, where ribosomes are able to read the message.
Small subunit of ribosome attaches to messenger RNA, large subunit then joins and protein synthesis begins. The sequence of amino acids in protein is dictated by the sequence of codons on mRNA.