In essence both tRNA (transfer RiboNucleic Acid) and mRNA (messenger RiboNucleic Acid) are composed of the same elements. Both are single-stranded nucleotide chains. A nucleotide is a unit composed of a nitrogenous base, a ribose and a phosphate. The ribose and phosphate elements are common to all RNA, but what makes RNA such an important structure in cell biology is the nitrogenous base, which can be one of four different types (Adenine (A), Cytosine (C), Guanine (G) or Uracil (U)). What this means is that when an RNA molecule is formed (i.e. a strand of nucleotides), it may have any combination of these four nitrogenous bases in a row; for example, a partial RNA molecule could be annotated as CAGUGCC; this would mean that the first nucleotide has Cytosine (C) as the nitrogenous base, Adenine (A) as the second base, Guanine (G) as the third, and so on. On a very basic level all RNA then, has a similar appearance. The difference between tRNA and mRNA becomes more obvious when their role in the cell, more importantly in protein synthesis, is understood.
Messenger RNA (mRNA) is so called, because it is used to carry the information from genes (comprised of a similar, yet chemically distinct nucleic acid, DNA) when making protein. DNA could be described as a hard copy of the instructions for constructing proteins. As with any important documentation it is prudent to use a copy of the source data when performing a process to avoid accidental damage or wear and tear. This is where mRNA comes into play, during a process called transcription. As the name suggests, transcription involves creating a replica of the data stored in the DNA, essentially creating an mRNA photocopy of the protein coding instructions.
In order to make this copy it is first necessary to access the stored information. DNA is a double stranded molecule. Each strand is similar in structure to the RNA molecule discussed earlier, however the nitrogenous bases are joined together protecting the information from the exterior. If you imagine a closed zip, where each side is a single strand of DNA with the teeth representing the bases, then you have an idea of the structure. During transcription a protein (Transcription Factor) temporarily ‘unzips’ the DNA molecule exposing the teeth (i.e. the nitrogenous bases). Single mRNA nucleotides then bind to any DNA with a matching base, creating a single strand mRNA with a copy of the DNA coding information.
When completed the mRNA detaches from the gene. At this stage the mRNA exists as a line of nucleotides with different bases (AGGGUCG, etc.) which reflects the sequence of bases on the DNA. The unique series of bases carries the instructions for building proteins. The nitrogenous bases are grouped into sets of three, each triplet (Codon) coding for a single amino acid. The process for constructing protein is known as translation, as the mRNA base code is translated into the relevant amino acid sequence for the relevant protein. During translation the single strand mRNA is bound to a large molecule (Ribosome). The ribosome attaches to the first codon of the mRNA strand. tRNA molecules are then used to attach protein elements (Amino Acids) in the order dictated by the base sequence.
Although comprised of identical nucleotides to mRNA, tRNA is folded into a three-dimensional shape. To one side of this structure amino acids bind; on the other is a set of three nitrogenous bases (anti-codon). As the ribosome moves along the mRNA molecule, a tRNA molecule towing the appropriate amino acid binds to the mRNA. This process of binding is similar to that seen in transcription, with the tRNA codon attaching to its complimentary mRNA codon. The ribosome then shifts to the next mRNA codon which binds the correct tRNA sequence, building up a string of amino acids. In this way, the code of bases in the original DNA molecule are used to dictate the correct sequence of amino acids in a given protein.
Although the actual cellular processes involved in RNA and protein synthesis are much more detailed than that described, the explanation of the roles of tRNA and mRNA illustrates the different functions of each. Also, some of the specific codon/anti-codon information has been been simplified ensure clarity. Some excellent graphic representations of transcription and translation are available on YouTube which may help to visualise the process.