Functions of DNA and RNA
In an earlier module, we briefly reviewed the features of nucleic acids, DNA and RNA, in organisms. DNA and RNA have different purposes in living things:
1. DNA is the master code for life. It is a large molecule that functions to store and transmit genetic information to direct life. Included in this information is the ability to:
- direct it’s own synthesis or replication (used in mitosis/meiosis)
- code for protein synthesis
- in eukaryotes, it is located in the nucleus, in prokaryotes, it floats freely in the cytoplasm
2. RNA is a copy of a single gene in DNA that is responsible for
- delivering genetic information from the DNA in the cell nucleus to the ribosome in the cytoplasm
- assist in the completion of the process of protein synthesis (also known as translation)
Watch the following video from Garland Science Publishing titled The Structure of DNA.
Structure of DNA and RNA
The monomers that make up DNA and RNA are called nucleotides. Each nucleotide is composed of a 5-carbon sugar, a nitrogen base and a phosphate group. A structural formula for a sample nucleotide is shown below.
Note that in DNA, the sugar is deoxyribose, and in RNA the sugar is ribose. Ribose sugar has more oxygen than the sugar in DNA.
There are five nitrogen bases that exist in living things; they are separated into two groups based on the number of rings in their structure. Purines have a double ring of carbon and nitrogen atoms. Adenine (A) and Guanine (G) are purines. Pyrimidines have a single ring of carbon and nitrogen atoms. Cytosine (C), Thymine (T) and Uracil (U) are Pyrimidines.
DNA uses the bases A, T, C and G. RNA uses the bases A, U, C and G. Specific bases are complementary to each other, meaning that they can bond to each other, pairing up along the length of a molecule according to these rules:
Stop and Think: One strand of DNA reads ATGGCA. What would be the sequence on the complementary DNA strand?
Two nucleotides pair to form one of the “rungs” of the ladder in a DNA double helix. The sequence of nucleotides pairs varies from organism to organism, however. The sugar and phosphate groups form the “backbone” or outer support of the DNA ladder-like double helix. The double-stranded structure of DNA makes a double helix, or a twisted ladder.
RNA is only composed of one strand of nucleotides and may take on different 3-dimensional forms: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA or, the ribosome). Find each form of RNA in the image on the right. Each form of RNA plays an important role in protein synthesis, the process where the DNA is used to make proteins to help the organism survive. We will discuss this more in an upcoming lesson.