Transcription mRNA Synthesis

As described in Chapter 2, ribonucleic acids are single-chain polynucleotides whose nucleotides differ from

DNA in that they contain the sugar ribose (rather than deoxyribose) and the base uracil (rather than thymine). The other three bases—adenine, guanine, and cytosine—occur in both DNA and RNA. The pool of subunits used to synthesize mRNA are free (un-combined) ribonucleotide triphosphates: ATP, GTP, CTP, and UTP.

As mentioned in Chapter 2, the two polynucleotide chains in DNA are linked together by hydrogen bonds between specific pairs of bases—A-T and C-G. To initiate RNA synthesis, the two strands of the DNA double helix must separate so that the bases in the exposed DNA can pair with the bases in free ri-bonucleotide triphosphates (Figure 5-3). Free ribonu-cleotides containing U bases pair with the exposed A bases in DNA, and likewise, free ribonucleotides containing G, C, or A pair with the exposed DNA bases C, G, and T, respectively. Note that uracil, which is present in RNA but not DNA, pairs with the base adenine in DNA. In this way, the nucleotide sequence in one strand of DNA acts as a template that determines the sequence of nucleotides in mRNA.

The aligned ribonucleotides are joined together by the enzyme RNA polymerase, which hydrolyses the nucleotide triphosphates, releasing two of the terminal phosphate groups, and joining the remaining phosphate in covalent linkage to the ribose of the adjacent nucleotide.

Since DNA consists of two strands of polynu-cleotides, both of which are exposed during transcription, it should theoretically be possible to form two different RNA molecules, one from each strand. However, only one of the two potential RNAs is ever formed. Which of the two DNA strands is used as the template strand for RNA synthesis from a particular gene is

Gene Template Strand

FIGURE 5-3

Transcription of a gene from the template strand of DNA to a primary RNA transcript. [Q] EWfl

FIGURE 5-3

Transcription of a gene from the template strand of DNA to a primary RNA transcript. [Q] EWfl

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Genetic Information and Protein Synthesis CHAPTER FIVE

Genetic Information and Protein Synthesis CHAPTER FIVE

determined by a specific sequence of DNA nucleotides called the promoter, which is located near the beginning of the gene on the strand that is to be transcribed (Figure 5-3). It is to this promoter region that RNA polymerase binds. Thus, for any given gene, only one strand is used, and that is the strand with the promoter region at the beginning of the gene. However, different transcribed genes may be located on either of the two strands of the DNA double helix.

To repeat, transcription of a gene begins with the binding of RNA polymerase to the promoter region of that gene. This initiates the separation of the two strands of DNA. RNA polymerase moves along the template strand, joining one ribonucleotide at a time (at a rate of about 30 nucleotides per second) to the growing RNA chain. Upon reaching a "stop" signal specifying the end of the gene, the RNA polymerase releases the newly formed RNA transcript. After the RNA transcript is released, a series of 100 to 200 adenine nucleotides is added to its end, forming a poly A tail that acts as a signal to allow RNA to move out of the nucleus and bind to ribosomes in the cytoplasm.

In a given cell, the information in only 10 to 20 percent of the genes present in DNA is transcribed into RNA. Genes are transcribed only when RNA polymerase can bind to their promoter sites. Various mechanisms, described later in this chapter, are used by cells either to block or to make accessible the promoter region of a particular gene to RNA polymerase. Such regulation of gene transcription provides a means of controlling the synthesis of specific proteins and thereby the activities characteristic of a particular type of differentiated cell.

It must be emphasized that the base sequence in the RNA transcript is not identical to that in the template strand of DNA, since the RNA's formation depends on the pairing between complementary, not identical, bases (Figure 5-3). A three-base sequence in RNA that specifies one amino acid is called a codon. Each codon is complementary to a three-base sequence in DNA. For example, the base sequence T-A-C in the template strand of DNA corresponds to the codon A-U-G in transcribed RNA.

Although the entire sequence of nucleotides in the template strand of a gene is transcribed into a complementary sequence of nucleotides known as the primary RNA transcript, only certain segments of the gene actually code for sequences of amino acids. These regions of the gene, known as exons (expression regions), are separated by noncoding sequences of nucleotides known as introns (intervening sequences). It is estimated that as much as 75-95 percent of human DNA is composed of intron sequences that do not contain protein-coding information. What role, if any, such large amounts of "nonsense" DNA may perform is unclear.

Before passing to the cytoplasm, a newly formed primary RNA transcript must undergo splicing (Figure 5-4) to remove the sequences that correspond to the DNA introns and thereby form the continuous sequence of exons that will be translated into protein (only after this splicing is the RNA termed messenger RNA).

■ One gene-

/- Exons^ Introns^ Nucleus

DNA 1 ^ 1 M '1 PI 1

Primary RNA

Nuclear pore^

^ Transcription of DNA to RNA RNA splicing by spliceosomes .^Nuclear envelope

mRNA

Polypeptide chain | | |

Passage of processed mRNA ' to cytosol through nuclear pore

Translation of mRNA into < polypeptide chain

Cytoplasm

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Myla
    Why only 1 percent of dna is transcribed to rna?
    6 years ago
  • Victoria
    What percentage of human dna is used for coding information for the synthesis of proteins?
    6 years ago

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