I emphasize the importance of ‘the central dogma’ pretty regularly throughout the semester in General Biology. This idea represents one of the core theories of biology and helps to explain an enormous amount about life. This ‘dogma’ explains how the information contained in DNA is replicated prior to cell division – and used to make drafts of that information (RNA) that can guide the construction of proteins that get the work of the cell done.
In the current unit, we are expanding this theory and examining the processes and the molecules they create more closely. Fortunately, each of these processes is elegantly illustrated in a set of animations available on the HHMI website.
The first video presents a model of replication. The model (as shown) is correct in its idea, but is not intended to be a model of HOW replication occurs, only how, in the words of Watson and Crick, “… [T]he specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
The actual mechanism of DNA replication is complicated by the anti-parallel arrangement of the paired DNA strands and the fact that DNA Polymerase, the enzyme responsible for copying the DNA, can work only in one direction.
The action of the DNA polymerase, along with some additional enzymes (Helicase and Ligase) is illustrated mechanistically in the following animation:
Once DNA is replicated, during S Phase of the cell cycle, the cell is ready to divide and provide one complete copy of the DNA to each of the two daughter cells. In this way, DNA replication allows for the continuity of genetic information from one generation (of cells or whole organisms) to the next.
Throughout the cell’s life, it is necessary to produce proteins to accomplish the work of that particular cell. Again, the information contained in the DNA is copied, this time to a messenger RNA (mRNA) strand, and the instructions to make the protein are carried into the cytoplasm. This process, called Transcription, is carried out primarily by the enzyme, RNA polymerase, as illustrated below:
Once an mRNA is constructed, it is transported from the nucleus (where the DNA resides) into the cytoplasm. There, a Ribosome will coordinate the recruitment of transfer RNAs (tRNA) bearing specific Amino Acid building blocks called for to synthesize the protein. This process, called translation, is illustrated by HHMI below: