Category: Nucleic acids

The process of translation uses the information present in the nucleotide sequence of mRNA to direct the synthesis of a specific protein for use by the cell. Translation takes place on the ribosomes—complex particles in the cell that contain RNA and protein. In prokaryotes the ribosomes are loaded onto the mRNA while transcription is still ongoing. Near the 5′…

Small segments of DNA are transcribed into RNA by the enzyme RNA polymerase, which achieves this copying in a strictly controlled process. The first step is to recognize a specific sequence on DNA called a promoter that signifies the start of the gene. The two strands of DNA become separated at this point, and RNA polymerase begins copying from…

RNA provides the link between the genetic information encoded in DNA and the actual workings of the cell. Some RNA molecules such as the rRNAs and the snRNAs (described in the section Types of RNA) become part of complicated ribonucleoprotein structures with specialized roles in the cell. Others such as tRNAs play key roles in protein synthesis, while mRNAs…

It is extremely important that the integrity of DNA be maintained in order to ensure the accurate workings of a cell over its lifetime and to make certain that genetic information is accurately passed from one generation to the next. This maintenance is achieved by repair processes that constantly monitor the DNA for lesions and activate appropriate…

Site-specific recombination involves very short specific sequences that are recognized by proteins. Long DNA sequences such as viral genomes, drug-resistance elements, and regulatory sequences such as the mating type locus in yeast can be inserted, removed, or inverted, having profound regulatory effects. More than any other mechanism, site-specific recombination is responsible for reshaping genomes. For…

General recombination, also called homologous recombination, involves two DNA molecules that have long stretches of similar base sequences. The DNA molecules are nicked to produce single strands; these subsequently invade the other duplex, where base pairing leads to a four-stranded DNA structure. The cruciform junction within this structure is called a Holliday junction, named after Robin…

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…