Rna

7.3 Gene Expression

Gene expression involves two separate but interrelated processes, transcription and translation. Transcription is the process of synthesizing RNA from a DNA template. During translation, information encoded on an mRNA transcript is deciphered to synthesize a protein.

Transcription

The enzyme RNA polymerase catalyzes the process of transcription, producing a single-stranded RNA molecule that is complementary and antiparallel to the DNA template (figure 7.7). To describe the two strands of DNA in a region that is transcribed into RNA, the terms minus (-) strand and plus (+) strand are sometimes used (table 7.2). The strand that serves as the template for RNA synthesis is called the minus (-) strand, whereas its complement is called the plus (+) strand. Recall that the base-pairing rules of DNA and RNA are the same, except that RNA contains uracil in place of thymine. Therefore, because the RNA is complementary to the (-) strand, its nucleotide sequence is the same as the (+) strand, except it has uracil in place of thymine. Likewise, the RNA transcript has the same 5' to 3' direction, or polarity, as the (+) strand.

In prokaryotes, an mRNA molecule can carry the information for one or multiple genes. A transcript that carries one gene is called monocistronic (a cistron is synonymous with a gene). Those that carry multiple genes are called polycistronic. Generally, the proteins encoded on a polycistronic message are all involved in a single biochemical pathway. This enables the cell to express related genes in a coordinated manner.

Table 7.2 Components of Transcription in Prokaryotes

Component

Comments

-strand

Strand of DNA that serves as the template for RNA synthesis; the resulting RNA molecule is complementary to this strand.

+strand

Strand of DNA complementary to the one that serves as the template for RNA synthesis; the sequence of the resulting RNA molecule is analogous to this strand.

Promoter

Nucleotide sequence to which RNA polymerase binds to initiate transcription.

RNA polymerase

Enzyme that synthesizes RNA using single-stranded DNA as a template; synthesis always occurs in the 5' to 3' direction.

Sigma (a) factor

Component of RNA polymerase that recognizes the promoter regions. A cell may have different types of c factors that recognize different promoters.These may be expressed at different stages of cell growth, enabling the cell to transcribe specialized sets of genes as needed.

Terminator

Sequence at which RNA synthesis stops; the RNA polymerase falls off the DNA template and releases the newly synthesized RNA.

174 Chapter 7 The Blueprint of Life, from DNA to Protein

Duplicate of original molecule

174 Chapter 7 The Blueprint of Life, from DNA to Protein

Duplicate of original molecule

Template strand

A sequence of nucleotides, called a promoter, identifies the region of DNA that will be transcribed into RNA.

A sequence of nucleotides, called a promoter, identifies the region of DNA that will be transcribed into RNA.

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