Most people have heard of DNA, but ribonucleic acid (RNA) is its lesser known cousin. They have many similar features, but quite different functions.
There are different types of RNA, the three main types being messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). If you have been following the cell organelle series, you will know that rRNA is part of the structure of ribosomes and is made in the nucleolus (part of the nucleus). In this article, we will take a closer look at the structure and function of mRNA and tRNA.
We already know that DNA consists of two polynucleotide strands joined together with hydrogen bonds between complementary bases (perhaps go and revise that article if you are unsure what that means). However, RNA is just a single polynucleotide strand. Also, the nucleotides are slightly different.
Firstly, an RNA nucleotide contains the pentose sugar called ribose instead of deoxyribose. Secondly, the base thymine is not found in an RNA nucleotide; it is replaced with a new base called uracil (U). So the four bases in RNA are adenine, uracil, cytosine, and guanine. Uracil can bond in a complementary base pair with adenine. The nucleotides are joined together with phosphodiester bonds to form a sugar-phosphate backbone along the molecule. Now, we need to look at mRNA and tRNA separately as they have quite different functions.
Messenger RNA (mRNA)
Messenger RNA is quite literally a messenger. It carries the genetic code from the nucleus to the ribosomes, acting as the middle man between DNA and proteins (don’t forget that genes code for proteins). DNA itself is too big to leave the nuclear pores, but mRNA is much smaller and can squeeze out. The process of converting DNA code to mRNA code is called transcription – we will look at this in more detail in this article. Briefly, transcription is when a strand of mRNA that is complementary to a gene (a section of a DNA strand) is made. This mRNA strand leaves the nucleus and attaches to a ribosome, where translation into an amino acid sequence can begin.
An mRNA strand is a single strand of nucleotides joined with phosphodiester bonds. Each three bases codes for one amino acid; a three base sequence is called a codon. Codons are complementary to the anticodons on tRNA.
Transfer RNA (tRNA)
The function of tRNA is to bring amino acids to the ribosomes, and make sure that amino acids are joined in the correct order according to the mRNA code. This process is called translation (detailed here).
tRNA has a clover leaf shape; it is still a single polynucleotide strand, but it folds round on itself and complementary bases pair together with hydrogen bonds in some places. At one end there is an amino acid binding site. On the bottom loop there is a sequence of three bases called an anticodon.
The anticodon determines which amino acid the tRNA is carrying, and is complementary to a codon found on mRNA. For example the anticodon AUG is complementary to the mRNA codon UAC, and means that the tRNA will be carrying the amino acid called methionine. We will look at this in more detail when we look at translation.
Summary
- RNA is single polynucleotide strand – a strand of nucleotides joined together with phosphodiester bonds.
- RNA nucleotides contain a pentose sugar called ribose, a phosphate group, and a base. Uracil (U) replaces thymine (T) in RNA.
- Messenger RNA (mRNA) carries genetic code from the nucleus to the ribosomes. One codon (three bases) codes for one amino acid.
- Transfer RNA (tRNA) carries amino acids to the ribosomes. They have an anticodon (three bases) which is complementary to an mRNA codon.
