Mindblown: a blog about philosophy.

Recombination is the principal mechanism through which variation is introduced into populations. For example, during meiosis, the process that produces sex cells (sperm or eggs), homologous chromosomes—one derived from the mother and the equivalent from the father—become paired, and recombination, or crossing-over, takes place. The two DNA molecules are fragmented, and similar segments of the chromosome are shuffled to produce…

DNA polymerase adds single nucleotides to the 3′ end of either an RNA or a DNA molecule. In the prokaryote E. coli, there are three DNA polymerases; one is responsible for chromosome replication, and the other two are involved in the resynthesis of DNA during damage repair. DNA polymerases of eukaryotes are even more complicated. In…

Basic mechanisms DNA replication is a semiconservative process in which the two strands are separated and new complementary strands are generated independently, resulting in two exact copies of the original DNA molecule. Each copy thus contains one strand that is derived from the parent and one newly synthesized strand. Replication begins at a specific point on…

Some RNA molecules, particularly those in protozoan mitochondria, undergo extensive editing following their initial synthesis. During this editing process, residues are added or deleted by a posttranscriptional mechanism under the influence of guide RNAs. In some cases as much as 40 percent of the final RNA molecule may be derived by this editing process, rather than being coded…

In prokaryotes the protein coding sequence occupies one continuous linear segment of DNA. However, in eukaryotic genes the coding sequences are frequently “split” in the genome—a discovery reached independently in the 1970s by Richard J. Roberts (the author of this article) and Phillip A. Sharp, whose work won them a Nobel Prize in 1993. The segments of DNA…

DNA metabolism Replication, repair, and recombination—the three main processes of DNA metabolism—are carried out by specialized machinery within the cell. DNA must be replicated accurately in order to ensure the integrity of the genetic code. Errors that creep in during replication or because of damage after replication must be repaired. Finally, recombination between genomes is an important mechanism to provide…

Some RNA molecules, particularly those in protozoan mitochondria, undergo extensive editing following their initial synthesis. During this editing process, residues are added or deleted by a posttranscriptional mechanism under the influence of guide RNAs. In some cases as much as 40 percent of the final RNA molecule may be derived by this editing process, rather than being coded…

In prokaryotes the protein coding sequence occupies one continuous linear segment of DNA. However, in eukaryotic genes the coding sequences are frequently “split” in the genome—a discovery reached independently in the 1970s by Richard J. Roberts (the author of this article) and Phillip A. Sharp, whose work won them a Nobel Prize in 1993. The segments of DNA…

Cleavage Following synthesis by transcription, most RNA molecules are processed before reaching their final form. Many rRNA molecules are cleaved from much larger transcripts and may also be methylated or enzymatically modified. In addition, tRNAs are usually formed as longer precursor molecules that are cleaved by ribonuclease P to generate the mature 5′ end and often have…

Many viruses use RNA for their genetic material. This is most prevalent among eukaryotic viruses, but a few prokaryotic RNA viruses are also known. Some common examples include poliovirus, human immunodeficiency virus (HIV), and influenza virus, all of which affect humans, and tobacco mosaic virus, which infects plants. In some viruses the entire genetic material is encoded in a single…