Only one of the two strands of dna is transcribed because
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If cells are fed radioactive RNA precursors, then the labeled RNA shows up first of all in the nucleus, indicating that the RNA is synthesized there. In a pulse-chase experiment, a brief pulse of labeled RNA precursors is given. These precursors are incorporated into RNA molecules. The cells are then transferred to medium with unlabeled RNA precursors. This “chases” the label out of the RNA because, as the RNA breaks down, only the unlabeled precursors are used to synthesize new RNA molecules. The pulse-chase protocol enables one to track a population of RNA molecules, synthesized almost simultaneously, over time. In samples taken after the chase, the labeled RNA is found in the cytoplasm (Figure 10-1). Apparently, the RNA is synthesized in the nucleus and then moves into the cytoplasm, where proteins are synthesized. Thus, RNA is a good candidate as an information-transfer intermediary between DNA and protein.
Figure 10-1
RNA synthesized during one short time period is labeled by feeding the cell a brief “pulse” of radioactive RNA precursors, followed by a “chase” of nonradioactive precursors. In an autoradiograph, the labeled RNA appears (more...)
In 1957, Elliot Volkin and Lawrence Astrachan made a significant observation. They found that one of the most striking molecular changes when E. coli is infected with the phage T2 is a rapid burst of RNA synthesis. Furthermore, this phage-induced RNA “turns over” rapidly, as shown in the following experiment. The infected bacteria are first pulsed with radioactive uracil (a specific precursor of RNA); the bacteria are then chased with cold uracil. The RNA recovered shortly after the pulse is labeled, but that recovered somewhat longer after the chase is unlabeled, indicating that the RNA has a very short lifetime. Finally, when the nucleotide contents of E. coli and T2 DNA are compared with the nucleotide content of the phage-induced RNA, the RNA is found to be very similar to the phage DNA.
The tentative conclusion from the two aforedescribed experiments is that RNA is synthesized from DNA and that it is somehow used to synthesize protein. We can now outline three stages of information transfer (Figure 10-2): replication (the synthesis of DNA), transcription (the synthesis of an RNA copy of a part of the DNA), and translation (the synthesis of a polypeptide directed by the RNA sequence).
Figure 10-2
The three processes of information transfer: replication, transcription, and translation.
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Complementarity and asymmetry in RNA synthesis
The similarity of RNA to DNA suggested that transcription may be based on the complementarity of bases, which is also the key to DNA replication. A transcription enzyme, RNA polymerase, could carry out transcription from a DNA template strand in a fashion quite similar to replication.
In fact, this model of transcription is confirmed cytologically (Figure 10-3). The fact that RNA can be synthesized with DNA acting as a template is also demonstrated by synthesis in vitro of RNA from nucleotides in the presence of DNA by using an extractable RNA polymerase. Whatever source of DNA is used, the RNA synthesized has an (A + U)/(G + C) ratio similar to the (A + T)/(G + C) ratio of the DNA (Table 10-1). This experiment does not indicate whether the RNA is synthesized from both DNA strands or from just one, but it does indicate that the linear frequency of the A–T pairs (in comparison with the G–C pairs) in the DNA precisely corresponds to the relative abundance of (A + U) in the RNA. (These points are difficult to grasp without drawing some diagrams; Problem 2 at the end of this chapter provides some opportunitites to clarify these notions.)
Figure 10-3
Tandem repeats of ribosomal RNA (rRNA) genes being transcribed in the nucleolus of Triturus viridiscens (an amphibian). (rRNA is a component of the ribosome, a cellular organelle.) Along each gene, many RNA polymerase molecules are attached and transcribing (more...)
Table 10-1
Nucleotide
Figure 10-1
RNA synthesized during one short time period is labeled by feeding the cell a brief “pulse” of radioactive RNA precursors, followed by a “chase” of nonradioactive precursors. In an autoradiograph, the labeled RNA appears (more...)
In 1957, Elliot Volkin and Lawrence Astrachan made a significant observation. They found that one of the most striking molecular changes when E. coli is infected with the phage T2 is a rapid burst of RNA synthesis. Furthermore, this phage-induced RNA “turns over” rapidly, as shown in the following experiment. The infected bacteria are first pulsed with radioactive uracil (a specific precursor of RNA); the bacteria are then chased with cold uracil. The RNA recovered shortly after the pulse is labeled, but that recovered somewhat longer after the chase is unlabeled, indicating that the RNA has a very short lifetime. Finally, when the nucleotide contents of E. coli and T2 DNA are compared with the nucleotide content of the phage-induced RNA, the RNA is found to be very similar to the phage DNA.
The tentative conclusion from the two aforedescribed experiments is that RNA is synthesized from DNA and that it is somehow used to synthesize protein. We can now outline three stages of information transfer (Figure 10-2): replication (the synthesis of DNA), transcription (the synthesis of an RNA copy of a part of the DNA), and translation (the synthesis of a polypeptide directed by the RNA sequence).
Figure 10-2
The three processes of information transfer: replication, transcription, and translation.
Go to:
Complementarity and asymmetry in RNA synthesis
The similarity of RNA to DNA suggested that transcription may be based on the complementarity of bases, which is also the key to DNA replication. A transcription enzyme, RNA polymerase, could carry out transcription from a DNA template strand in a fashion quite similar to replication.
In fact, this model of transcription is confirmed cytologically (Figure 10-3). The fact that RNA can be synthesized with DNA acting as a template is also demonstrated by synthesis in vitro of RNA from nucleotides in the presence of DNA by using an extractable RNA polymerase. Whatever source of DNA is used, the RNA synthesized has an (A + U)/(G + C) ratio similar to the (A + T)/(G + C) ratio of the DNA (Table 10-1). This experiment does not indicate whether the RNA is synthesized from both DNA strands or from just one, but it does indicate that the linear frequency of the A–T pairs (in comparison with the G–C pairs) in the DNA precisely corresponds to the relative abundance of (A + U) in the RNA. (These points are difficult to grasp without drawing some diagrams; Problem 2 at the end of this chapter provides some opportunitites to clarify these notions.)
Figure 10-3
Tandem repeats of ribosomal RNA (rRNA) genes being transcribed in the nucleolus of Triturus viridiscens (an amphibian). (rRNA is a component of the ribosome, a cellular organelle.) Along each gene, many RNA polymerase molecules are attached and transcribing (more...)
Table 10-1
Nucleotide
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1. They are formed by the aggregate of a large number of atoms or molecules which generally have diameters less than 1 nm, e.g., sols of gold, sulphur etc.
2. Lyophobic sols: These sols are usually formed by the inorganic materials like metals, their sulphides etc.They are irreversible in nature i.e. once precipitated cannot return the colloidal sol by simply addition of the dispersion medium.
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