The nature of biological information, how it is copied and passed on, how it is read and interpreted, and how it gives rise to the cellular activities that we can observe, is the subject of this chapter. Another kind of information is also considered, towards the end of the chapter- the molecular information that cells receive from, and send to, each other. Overlaid on the instructions in the genes, this information provides cells with ongoing clues about both their own inner state and the environment around them. The interplay of these two kinds of information is responsible for the form and behavior of all living organisms.
- 7.0: Prelude to Information Processing
- As creatures used to regarding ourselves as exceptional, humans must surely be humbled to realize that the instructions, for making one of our own, reside in a molecule so simple that scientists, for a very long time, did not believe could possibly contain enough information to build even a simple cell. But a large body of evidence, built up over the past century, supports Larison Cudmore’s assertion that the information for making you and me (and all the other kinds of living things in the worl
- 7.1: Genes and Genomes
- For many years, scientists wondered about the nature of the information that directed the activities of cells. What kind of molecules carried the information, and how was the information passed on from one generation to the next? Key experiments, done between the 1920s and the 1950s, established convincingly that this genetic information was carried by DNA. In 1953, with the elucidation of the structure of DNA, it was possible to begin investigating how this information is passed on, and is used
- 7.2: DNA Replication
- The only way to make new cells is by the division of pre-existing cells. Single-celled organisms undergo division to produce more cells like themselves, while multicellular organisms arise through division of a single cell, generally the fertilized egg. Each time a cell divides, all of its DNA must be copied faithfully so that a copy of this information can be passed on to the daughter cell. This process is called DNA replication.
- 7.3: DNA Repair
- It is evident that if DNA is the master copy of instructions for an organism, then it is important not to make mistakes when copying the DNA to pass on to new cells. Although proofreading by DNA polymerases greatly increases the accuracy of replication, there are additional mechanisms in cells to further ensure that newly replicated DNA is a faithful copy of the original, and also to repair damage to DNA during the normal life of a cell.
- 7.4: Transcription
- In the preceding sections, we have discussed the replication of the cell's DNA and the mechanisms by which the integrity of the genetic information is carefully maintained. What do cells do with this information? How does the sequence in DNA control what happens in a cell? If DNA is a giant instruction book containing all of the cell's "knowledge" that is copied and passed down from generation to generation, what are the instructions for? And how do cells use these instructions for?
- 7.5: RNA Processing
- So far, we have looked at the mechanism by which the information in genes (DNA) is transcribed into RNA. The newly made RNA, also known as the primary transcript is further processed before it is functional. Both prokaryotes and eukaryotes process their ribosomal and transfer RNAs.
- 7.6: Translation
- Translation is the process by which information in mRNAs is used to direct the synthesis of proteins. As you have learned in introductory biology, in eukaryotic cells, this process is carried out in the cytoplasm of the cell, by large RNA-protein machines called ribosomes. Ribosomes contain ribosomal RNA (rRNA) and proteins. The proteins and rRNA are organized into two subunits, a large and a small.
- 7.7: Gene Expression
- The processes of transcription and translation described so far tell us what steps are involved in the copying of information from a gene (DNA) into RNA and the synthesis of a protein directed by the sequence of the transcript. These steps are required for gene expression, the process by which information in DNA directs the production of the proteins needed by the cell.
- 7.8: Signaling
- It is intuitively obvious that even unicellular organisms must be able to sense features of their environment, such as the presence of nutrients, if they are to survive. In addition to being able to receive and respond to information from the environment, multicellular organisms must also find ways by which their cells can communicate among themselves.
Thumbnail: DNA double helix. Image used with permission (public domain; NIH - Genome Research Institute).