The central dogma of molecular biology describes how, typically, DNA is transcribed to messenger RNA, which, in turn, is translated into proteins that carry out vital cellular functions. Mutations introduce change into this process—ultimately leading to new or lost functions. Much of cellular metabolism is concerned with translating the genetic message of genes into specific proteins. A gene usually
codes for a messenger RNA (mRNA) molecule, which ultimately results in the formation of a protein. When the ultimate molecule for which a gene codes (a protein, for example) has been produced, we say that the gene has been expressed. The flow of genetic information
can be shown as flowing from DNA to RNA to proteins,
Transcription is the synthesis of a complementary strand of
RNA from a DNA template.
PROKARYOTE TRANSCRIPTION: Transcription begins when RNA polymerase binds to the DNA at a site called the promoter. Only one of the two DNA strands serves as the template for RNA synthesis for a given gene. Like DNA, RNA is synthesized in the 5′ S 3′ direction. RNA synthesis
continues until RNA polymerase reaches a site on the
DNA called the terminator. Transcription allows the cell to produce short-term copies of genes that can be used as the direct source of information for protein synthesis. Messenger RNA acts as an intermediate between the permanent storage form, DNA, and the process that uses the information, translation-this happens in the cytoplasm.
EUKARYOTES TRANSCRIPTION: takes place in the nucleus. The mRNA must be completely synthesized and moved through the nuclear membrane to the cytoplasm before translation can begin. In addition, the RNA undergoes processing before it leaves the nucleus. In eukaryotic cells, the regions of genes that code for proteins are often interrupted by noncoding DNA. Thus, eukaryotic genes are composed of exons, the regions of DNA expressed, and introns, the intervening regions of DNA that do not encode protein. In the nucleus, RNA polymerase synthesizes a molecule called an RNA transcript that contains
copies of the introns. Particles called small nuclear ribonucleoproteins, abbreviated snRNPs and pronounced "snurps," remove the introns and splice the exons together. In some organisms, the introns act as ribozymes to catalyze their own removal.
Genetic recombination refers to the exchange of genes between two DNA molecules to form new combinations of genes on a chromosome. If a cell picks up foreign DNA (called donor DNA in the figure), some of it could insert into the cell's chromosome—a process called crossing over—and some of the genes carried by the chromosomes are shuffled. The DNA has recombined, so that the chromosome now carries a portion of the donor's DNA.
During the process of transformation, genes are transferred
from one bacterium to another as "naked" DNA in solution.
Conjugation is mediated by one kind of plasmid, a circular piece of DNA that replicates independently from the cell's chromosome. However, plasmids differ from bacterial
chromosomes in that the genes they carry are usually not
essential for the growth of the cell under normal conditions.
Conjugation differs from transformation in two major ways.
First, conjugation requires direct cell-to-cell contact. Second, the conjugating cells must generally be of opposite mating type; donor cells must carry the plasmid, and recipient cells usually do not.
transduction- In this process, bacterial DNA is transferred from a donor cell to a recipient cell inside a virus that infects bacteria, called a bacteriophage, or phage.
plasmids- plasmids are self-replicating, gene-containing, circular pieces of DNA about 1-5% the size of the bacterial chromosome. They are found mainly in bacteria but also in some eukaryotic microorganisms, such as Saccharomyces cerevisiae.
transposons-small segments of DNA that can move (be
"transposed") from one region of a DNA molecule to another. These pieces of DNA are 700 to 40,000 base pairs long.