Though DNA and RNA have some similarities, RNA is a less stable version of its more famous cousin, DNA. But its relative instability doesn’t make it less important. It is critical for the proper functioning of a cell and thus of the whole living organism. And it turns out that it may have been the key molecule when life started out on Earth 3.8 billion or so years ago.
How Are DNA and RNA Different?
While there are a number of differences between DNA and RNA, it's first worth noting that they are in some ways very similar. For example, they both consist of nucleotides (a five-carbon sugar and a nitrogenous base) connected together in a long row through phosphate groups. What makes DNA and RNA different at the molecular level is their nucleotides. While the nucleotides in RNA have the five-carbon sugar ribose, the nucleotides in DNA have the five-carbon sugar deoxyribose.
Another key difference is the base portion of the nucleotide. While both DNA and RNA have three bases in common, adenine (A), guanine (G), and cytosine (C), they have a different fourth base. In DNA, the fourth base is thymine (T), while in RNA it is uracil (U). These small differences make RNA less stable than DNA.
RNA is unstable under alkaline conditions, and the O-H bond in the ribose makes it more reactive than the relatively stable DNA. Because of its stability, DNA is used to store biological information in a cell. It is important that these instructions not be changed or mutated, and changes are less likely with DNA compared to RNA.
One other big difference between DNA and RNA is that DNA usually comes as two long strands intertwined to form the famous double-stranded DNA helix. The key to this double helix structure being able to form is that the bases pair up in a defined way. In DNA, A always pairs with T and G with C.
RNA is usually a single strand of nucleotides connected together. Even though it is single-stranded, its bases do still pair up—A with U and G with C. This pairing is sometimes important within the RNA strand for a number of reasons. It enables the molecule to form three dimensional shapes and facilitates the pairing of RNA with DNA in the process of transcription—as well as the pairing between RNAs in the process of translation.
What Is the Function of RNA?
RNA plays various critical roles in the cell. One of the most important functions of RNA is turning the information in DNA into proteins which can then carry out the work coded in the cell's genetic instructions. It is also important in regulating when, where, and to what extent a gene should be read in a cell.
To follow the instructions in DNA, a cell must first copy a gene into a form of RNA called messenger RNA (mRNA). This process, which produces RNA from DNA is called transcription. During transcription, a number of different proteins in a cell work together to read the gene, but the key protein is RNA polymerase.
In the first step, initiation, the RNA polymerase lands on the DNA near a gene. In the next step, elongation, the RNA polymerase starts copying the DNA into RNA using base pairing. For example, if there is a T in the DNA strand, the polymerase will add an A to the RNA strand. This continues across the entire length of the gene (often many thousands of bases). Eventually the polymerase stops making RNA and leaves the DNA. This last process is called termination.
In eukaryotes, the RNA is then processed before moving onto the next step. In a process called splicing, a large protein complex called the spliceosome removes pieces of RNA called introns throughout the RNA. Other proteins add a protective cap at the beginning of the RNA and a long string of adenine nucleotides called a polyadenylated tail at the other end. These help to prevent the RNA from being degraded by enzymes in the cell and also facilitate its transportation to the cytoplasm.
This processed RNA, now called messenger RNA, is now ready to be translated into a protein.
In many cases the information contained within DNA needs to be translated into a protein in order for the instructions to be carried out. This process of translation uses three different kinds of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
After the mRNA is synthesized and processed, the newly formed mRNA copy of the gene leaves the nucleus and binds to a structure in the cytoplasm called a ribosome, to begin the process of translation. The ribosome is made up of many different proteins and rRNAs. During translation, the ribosome 'reads' three nucleotides of the mRNA sequence at a time. Each of these three-nucleotide sequences, known as codons, base pairs with an anticodon, a complementary three-nucleotide sequence found on tRNA molecules.
Different anticodons are found on the end of each tRNA molecule. And on the other end, tRNA molecules are attached to a specific amino acid. Thus the complementary base pairing between mRNA and tRNA allows the sequence of nucleotides in mRNA to be converted, or translated, into a sequence of amino acids.
For example, if there is a UUU codon in the mRNA molecule, a tRNA molecule with the anticodon AAA base pairs with it. This tRNA molecule brings a specific amino acid to the ribosome phenylalanine. This process of codon and anticodon pairing continues, and the amino acids are linked together, one after the other, to make the protein. The protein can then go and do its job.
Other Functions of RNA
In addition to the three types of RNA described above, many additional RNAs exist that play other important roles. For example, microRNA (miRNA) is a small type of RNA that can bind to mRNA and affects its ability to be translated. MicroRNAs are important for ensuring that genes are only expressed at the proper time and in the correct cell type.
Another type of RNA called small nuclear RNA (snRNA) assists during the process of splicing. Other important RNAs include long non-coding RNAs (lncRNAs) which like miRNAs look to play an important role in gene expression and ribozymes (RNA molecules that behave just like proteins).
Many scientists think that RNA was a key molecule back when life began 3.8 billion or so years ago. A big reason they think this is that RNA can store information like DNA and can perform some of the functions that proteins can. In this theory, DNA and proteins came later. So without RNA, there might have been no life on Earth.