Enzymes RNA polymerase

Due to the large amount of DNA present within the nucleus it must be carefully packaged. In the resting cell DNA is tightly compacted around basic histone proteins, excluding the binding of the enzyme RNA polymerase II, which activates the formation of mRNA. This conformation of the chromatin structure... 

Transcription The process of copying a DNA sequence into an RNA molecule catalysed by the enzyme RNA polymerase. 

RNA polymerase Core enzyme RNA polymerase I rRNA RNA polymerase II mRNA snRNA RNA polymerase III tRNA, 5S RNA... 

The most detailed molecular information about the transcription cycle is available in bacterial systems. The synthesis of RNA is initiated at the promoter sequence by the enzyme RNA polymerase. A single RNA polymerase type is responsible for the synthesis of messenger, transfer, and ribosomal RNAs. 

Elongation is the function of the RNA polymerase core enzyme. RNA polymerase moves along the template, locally unzipping the DNA double helix. This allows a transient base pairing between the incoming nucleotide and newly-synthesized RNA and the DNA template strand. As it is made, the RNA transcript forms secondary structure... 

Most of the DNA sequences which are transcribed give rise to mRNA, which is subsequently translated into protein. However, the most abundant species of RNA are ribosomal RNA (rRNA) and transfer RNA (tRNA), which do not code for protein but function in the process of translation. They are formed by a high level of transcription of a relatively small number of genes (called rRNA and tRNA genes). In bacteria, transcription of all genes is brought about by the enzyme RNA polymerase. 

For reduplication, the chains are separated and on each a new, complementary strand is synthesized by enzymes called DNA polymerases [652J. For protein biosynthesis, the DNA is copied (transcribed) into the messenger ribonucleic acid (mRNA) by the enzyme RNA polymerase (Fig. 20.2) where, in contrast to DNA, the deoxyribose is replaced by ribose and thymine by the equivalent uracil. Here again, the Watson-Crick base pair plays the crucial role so that the mRNA sequence is complementary to the DNA sequence. 

Actinomycin D is a selective inhibitor of RNA synthesis but its use in cancer chemotherapy is severely limited owing to its toxicity. It is thought that the two cyclic peptide moieties of the antibiotic whose three-membered aromatic ring system is intercalated into doublestranded DNA occlude the minor groove of duplex DNA and impede the progression of the enzyme, RNA polymerase. 

The synthesis of RNA from a DNA template is called transcription and is catalyzed by the enzyme RNA polymerase (Figure 5.24). RNA polymerase requires the following components ... 

The selectivity of this antibiotic is interesting since both bacterial cells and mammalian cells contain the enzyme RNA polymerase. However, as we have seen, the enzyme in bacterial cells contains a peptide chain not present in mammalian RNA polymerase. 

We begin our consideration of transcription by examining the process in bacteria such as E. coli. The E. coli RNA polymerase is a very large (MOO kd) and complex enzyme consisting of four kinds of subunits (Table 29.1). The subunit composition of the entire enzyme, called the holoenzyme, is a >(3p a. The (T subunit helps find a promoter site where transcription begins, participates in the initiation of RNA synthesis, and then dissociates from the rest of the enzyme. RNA polymerase without this subunit (a2PP ) is called the core enzyme, which contains the catalytic site. 

The synthesis of proteins is a more complex process (Fig. 124—11). Proteins consist of chains of amino acids in very specific seqnences. As in DNA synthesis, the double helix must unwind. However, in protein synthesis, only the portion of the DNA molecnle that codes for the desired protein is exposed. The enzyme RNA polymerase matches free complementary RNA nncleotides to the exposed DNA nucleotides, and the resultant chain of nncleotides is called mRNA. This process is called transcription. The mRNA travels to ribosomes in the cytoplasm, where protein synthesis occurs. Each three nucleotides of the mRNA chain compose a codon, whose seqnence is specific for a particular amino acid. The codon is recognized by tRNA, which then carries the amino acid to the ribosome, where it is added to the growing peptide chain. This process is known as translation. The... 

A process called transcription generates messenger RNAs (mRNA). A large enzyme, RNA polymerase II, functions as a catalyst to synthesize long chains of single-stranded ribonucleic acids in the presence of Mg2+ ions and the four rNTPs. It codes for proteins and occurs in almost all sizes. 

Enzyme - RNA polymerase I is a complex enzyme, containing 13 subunits totaling over 600,000 daltons. It is responsible for synthesizing the large 45S pre-rRNA transcript that is later processed into mature 28S, 18S, and 5.8S ribosomal RNAs (rRNAs). At least two transcription factors are known to be required, but there is no need for an elaborate transcriptional apparatus characteristic of pol II transcription (see here), because only a single kind of gene is transcribed. 

Nobel Prize winner Sydney Brenner showed that a sequence of three nucleotide bases in a row encode each specific amino acid. RNA strands complementary to a DNA strand are responsible for the process called transcription, which is brought about by the action of the enzyme, RNA polymerase. 

Genes are DNA sequences that are copied by the action of the enzyme RNA polymerase, and are passed on by transfer RNA to ribosomes, where proteins are synthesized in a process called translation. Ribosomes assemble amino acids into polypeptide chains messenger RNA provides a template to join together the correct sequence of amino acids. 

The present detailed knowledge of the mechanism of DNA transcription to produce RNA rests largely upon studies with bacteria, particularly E. coli. It is convenient to discuss transcription in bacteria first. The enzyme RNA polymerase is responsible for transcription of all genes in bacteria. 

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