Fundamental #1: DNA to mRNA to Protein
I talk a lot about “gene expression” in this blog. What is a gene? What is actually being “expressed”?
A gene is a small section of a chromosome that carries the instructions necessary to make a protein (there are exceptions to everything I’m saying, but we won’t get into those now). A gene is made up of the molecule DNA. DNA is made of two strands of nucleic acids linearly hooked together. There are 4 types of nucleic acids, which are abbreviated by their first letter (A, C, G, or T).
The sequence of a gene tells the cell what type of protein to make. Proteins are made of amino acids (like tryptophan or cysteine, etc) attached in a linear strand. Proteins can fold into intricate shapes which help them perform various functions around the cell. If DNA is the instruction manual, then proteins are the workers. They can act as enzymes which speed up chemical reactions (like even in the process of making more proteins or DNA). Other proteins act as scaffolding or as intercellular signals or for transport within the cell.
Because DNA is inside the nucleus and proteins are made in the cytoplasm, there has to be an intermediate messenger in this process and that is mRNA (m = messenger). RNA is also made of nucleic acids, but has only one strand. When a gene is ready to be expressed, enzymes will make a copy of the DNA sequence in a single strand of mRNA, in a process called transcription. We say that the mRNA is “complimentary” to the DNA sequence. The mRNA goes out of the nucleus and encounters a ribosome, which is the factory that translates the nucleic acid code into the amino acids of proteins. A sequence of three nucleotides will be translated into one particular amino acid.
|The "Central Dogma" of Molecular Biology|
All the cells in our body have the same sequence of DNA, which includes all of our genes. However, only a subset of these genes is expressed depending on the cell type, the developmental age and environmental factors. There’s no point in expressing the gene for eye color in any other cells besides the iris. It’s up to every cell to figure out which genes to transcribe and when. Gene expression can be regulated by signals from other cells (like hormones). Environmental factors can also change which genes are expressed through a process known as epigenetics. The regulation of transcription and translation is a fundamental mechanism our bodies use to adapt and change. We are able to learn and remember things only because our brain cells are adjusting expression of particular genes.
Side note: Like I said, this is a simplified description of molecular biology. Whole textbooks are devoted to this subject and there are many exceptions to the rule. For instance, RNA can do a lot more than just act as a messenger. RNA can actually act as an enzyme too and it plays an important role in the ribosome during translation. There are also tons of small inhibitory RNAs that will “silence” gene expression. The “central dogma” of molecular biology is always getting more and more complicated.