Introduction

Module 1.3 Introduced you to the structure and function of nucleic acids (double stranded DNA; deoxyribonucleic acid and single stranded RNA; ribonucleic acid). Nucleic acids are specialised for the storage and transmission of genetic information. They are important in cell division and reproduction, where information is transferred to the new DNA.

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DNA and RNA

DNA is stored in the nucleus of eukaryotic cells. DNA has two main biological functions. It contains the genetic material responsible for inheritance and it directs and regulates protein synthesis. The biological function of DNA includes vertical gene transfer (transmission of stored information from mother to daughter cells) as well as DNA replication (duplicate copy of DNA during cell division). In eukaryotes, DNA is a linear stranded double helix wrapped around histones (proteins that act as spools around which DNA is wound) to form a nucleosome. DNA contains four bases arranged in pairs: cytosine, guanine, adenine and thymine.

RNA is structurally similar to DNA however is single stranded, includes ribose (not deoxyribose) and replaces thymine with uracil.

RNA is important in protein synthesis. There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA).

mRNA - Messenger RNA is a linear molecule which carries a copy of the gene to be expressed from the nucleus. Every group of three bases (triplet = codon) codes for an amino acid. mRNA is short, unstable, single stranded RNA corresponding to a gene encoded within DNA. It is an intermediary between DNA and protein, it directs synthesis of the protein it encodes.

tRNA - Transfer RNA is a single strand of RNA which folds due to base pairing to form this an unusual shape (like a cloverleaf). It has a triplet anti-codon site and an attachment site for a specific amino acid. tRNA is short (70-90 nucleotides), stable RNA with extensive intramolecular base pairing; contains an amino acid binding site and an mRNA binding site. tRNA carries the correct amino acid to the site of protein synthesis in the ribosome.

rRNA - Ribosomal RNA and proteins form the ribosome which is the structure where protein synthesis takes place. Longer, stable RNA molecules make up 60% of ribosome’s mass. rRNA ensures alignment of mRNA, tRNA, and ribosome during protein synthesis, and catalyses peptide bond formation between amino acids. More about this in module 2.4.

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Watch this video to undertand the structure and function of DNA and RNA.