Why transcription is necessary

Figure 1. The Transcription Process. Transcription Initiation. Figure 3. Figure Detail. Figure 2. Figure 4: Eukaryotic core promoter region. Genetics: A Conceptual Approach , 2nd ed. All rights reserved. Strand Elongation. Transcription Termination.

Figure 5: Rho-independent termination in bacteria. Inverted repeat sequences at the end of a gene allow folding of the newly transcribed RNA sequence into a hairpin loop.

This terminates transcription and stimulates release of the mRNA strand from the transcription machinery. References and Recommended Reading Connelly, S. Genes and Development 4 , — Dennis, P. Journal of Molecular Biology 84 , — Dragon. Journal of Biological Chemistry , — Kritikou, E. Methods in Molecular Biology , 23—37 Logan, J. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.

Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Applications in Biotechnology. DNA Replication. Jumping Genes. Discovery of Genetic Material. Gene Copies. No topic rooms are there. Or Browse Visually.

Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. The Success Code. Why Science Matters. The Beyond. Plant ChemCast. A simplified version of bacterial DNA replication is described in Figure 2. DNA biosynthesis proceeds in the 5'- to 3'-direction. This makes it impossible for DNA polymerases to synthesize both strands simultaneously. A portion of the double helix must first unwind, and this is mediated by helicase enzymes.

The leading strand is synthesized continuously but the opposite strand is copied in short bursts of about bases, as the lagging strand template becomes available. The resulting short strands are called Okazaki fragments after their discoverers, Reiji and Tsuneko Okazaki. Strangely, DNA polymerases cannot initiate DNA synthesis de novo , but require a short primer with a free 3'-hydroxyl group.

Pol III can then take over, but it eventually encounters one of the previously synthesized short RNA fragments in its path. The gap is filled by DNA ligase, an enzyme that makes a covalent bond between a 5'-phosphate and a 3'-hydroxyl group Figure 3. The initiation of DNA replication at the leading strand is more complex and is discussed in detail in more specialized texts.

DNA replication is not perfect. This leads to mismatched base pairs, or mispairs. DNA polymerases have proofreading activity, and a DNA repair enzymes have evolved to correct these mistakes.

Occasionally, mispairs survive and are incorporated into the genome in the next round of replication. These mutations may have no consequence, they may result in the death of the organism, they may result in a genetic disease or cancer; or they may give the organism a competitive advantage over its neighbours, which leads to evolution by natural selection.

Transcription is the process by which DNA is copied transcribed to mRNA, which carries the information needed for protein synthesis. Transcription takes place in two broad steps. The mechanism of transcription has parallels in that of DNA replication. As with DNA replication, partial unwinding of the double helix must occur before transcription can take place, and it is the RNA polymerase enzymes that catalyze this process. Unlike DNA replication, in which both strands are copied, only one strand is transcribed.

The strand that contains the gene is called the sense strand, while the complementary strand is the antisense strand. The mRNA produced in transcription is a copy of the sense strand, but it is the antisense strand that is transcribed. The DNA molecule re-winds to re-form the double helix. The pre-messenger RNA thus formed contains introns which are not required for protein synthesis.

In alternative splicing, individual exons are either spliced or included, giving rise to several different possible mRNA products. Each mRNA product codes for a different protein isoform; these protein isoforms differ in their peptide sequence and therefore their biological activity. The mRNA molecule is elongated and, once the strand is completely synthesized, transcription is terminated. The newly formed mRNA copies of the gene then serve as blueprints for protein synthesis during the process of translation.

Further Exploration Concept Links for further exploration translation transcription unit gene expression frameshift mutation nonsense mutation RNA DNA enhancer promoter differentiation gene expression transcription factor intron exon chromatin histones mutation helicase transcriptome phosphate backbone poly-A tail nuclear pore primase TATA box hairpin loop mRNA DNA polymerase mRNA chromatin remodeling cis-regulatory element RNA polymerase catabolite repression methylation.

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