FIGURE 6.01 Transcription in Its Simplest Form
The two strands of the DNA to be transcribed are separated locally. The top strand serves as a template for building a new RNA molecule.
DNA "merely" stores genetic information. Putting the information to use requires RNA and (usually) protein.
Messenger RNA carries the information for making proteins from the genes to the cytoplasm.
The DNA double helix must be opened up for RNA polymerase to read the template strand and make RNA.
For a cell to operate, its genes must be expressed. The word "expressed" means that the gene products, whether proteins or RNA molecules, must be made. The DNA molecule that carries the original copy of the genetic information is used to store genetic information but is not used as a direct source of instructions to run the cell. Instead, working copies of the genes, made of RNA, are used. The transfer of information from DNA to RNA is known as transcription and RNA molecules are therefore sometimes referred to as transcripts. Genes may be subdivided into two major groups: those whose final product is an RNA molecule (e.g., tRNA, rRNA, assorted regulatory RNAs—see below) and those whose final product is protein. In the latter case, the RNA transcript acts as an intermediary and a further step is needed to convert the information carried by the RNA to protein. This process is discussed in Ch. 8. The type of RNA molecule that carries genetic information encoding a protein from the genes into the rest of the cell is known as messenger RNA, or mRNA. Since the great majority of genes encode proteins, we will deal with these genes first.
For a gene to be transcribed, the DNA, which is double stranded, must first be pulled apart temporarily, as shown in Figure 6.01. Then, RNA is made by RNA polymerase. This enzyme binds to the DNA at the start of a gene and opens the double helix. Finally, it manufactures an RNA molecule.
The sequence of the RNA message is complementary to the template strand of the DNA from which it is synthesized. Apart from the replacement of thymine in DNA with uracil in RNA, this means that the sequence of the new RNA molecule is identical to the sequence of the coding strand of DNA; that is, the strand not actually used as a template during transcription. Note that RNA, like DNA, is synthesized in the 5'-to 3'- direction (Fig. 6.02). Other names for the template strand are the non-coding or anti-sense strand; other names for the coding strand are non-template or sense strand. Only one of the strands of DNA is copied in any given transcribed region. [But note coding strand The strand of DNA equivalent in sequence to the messenger RNA (same as plus strand)
messenger RNA (mRNA) The molecule that carries genetic information from the genes to the rest of the cell
RNA polymerase Enzyme that synthesizes RNA using a DNA template transcription Process by which information from DNA is converted into its RNA equivalent template strand Strand of DNA used as a guide for synthesizing a new strand by complementary base pairing
The DNA is shown in its double helical form. After local separation of the strands, the new RNA is synthesized so that it base pairs with one of the DNA strands—the template strand. The other DNA strand is inactive and is called the coding strand. The enzyme RNA polymerase synthesizes single-stranded RNA in the 5'- to 3'-direction. The sequence of bases in the RNA is the same as in the coding strand of DNA, except that uracil substitutes in RNA for thymine in DNA. The bases of the mRNA are complementary to those of the template DNA strand; note that uracil base pairs with adenine.
that the two different strands of the DNA may each be used as templates in different regions of the chromosome.]
Each mRNA carries information from only a short stretch of the DNA.
Short Segments of the Chromosome Are Turned into Messages
Although a chromosome carries hundreds or thousands of genes, only a fraction of these are in use at any given time. In a typical bacterial cell, about 1,000 genes, or about 25% of the total, are expressed under any particular set of growth conditions. Some genes are required for the fundamental operations of the cell and are therefore expressed under most conditions. These are known as housekeeping genes. Other genes vary in expression in response to changes in the environment. During cell growth and metabolism, each gene or small group of related genes is used to generate a separate RNA copy when, and if, it is needed. Consequently, each cell contains many different RNA molecules, each carrying the information from a short stretch of DNA.
In the cells of higher organisms, which have many more genes than do bacteria, the proportion of genes in use in a particular cell at a particular time is much smaller. Different cells of multi-cellular organisms express different selections of genes depending on their specialized roles. For example, in the human female, genes related to the menstrual cycle are largely unique and expressed in a timed sequence to provide functionality to the organism. In addition, gene expression varies with the stage of development. Embryonic genes are often expressed only at certain times. Thus, the control of gene expression is much more complex in higher organisms, although the basic principles are the same.
Terminology: Cistrons, Coding Sequences and Open Reading Frames
A cistron is a structural gene, which is a coding sequence or segment of DNA encoding a polypeptide. It was defined originally as a genetic unit by complementation using the cis/trans test. Nowadays, the terms cistron and structural gene also include DNA sequences that code for RNA molecules that function as RNA without being translated into proteins (e.g., rRNA, tRNA, snRNA, etc.). An open reading frame (ORF)
cistron Segment of DNA (or RNA) that encodes a single polypeptide chain housekeeping genes Genes that are switched on all the time because they are needed for essential life functions open reading frame (ORF) Sequence of bases (either in DNA or RNA) that can be translated (at least in theory) to give a protein structural gene Sequence of DNA (or RNA) that codes for a protein or for an untranslated RNA molecule
How Is the Beginning of a Gene Recognized? 135
DNA Promoter StIgcneral DNA Promoter Structural genes
The typical situation in eukaryotes is to have one structural gene produce monocistronic RNA and th is, in turn, be translated into a single protein. In bacteria, it is common to see several structural genes transcribed under the control of a single promoter. The RNA produced is polycistronic and yields several separate proteins.
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