Research has also shown that the instructions stored within DNA are "read" in two steps: transcription and translation. In some cases, the RNA molecule itself is a "finished product" that serves some important function within the cell. Often, however, transcription of an RNA molecule is followed by a translation step, which ultimately results in the production of a protein molecule. Visualizing Transcription. 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.
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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. Several different mechanisms of alternative splicing are known, two of which are illustrated in Figure 6. Alternative splicing contributes to protein diversity - a single gene transcript RNA can have thousands of different splicing patterns, and will therefore code for thousands of different proteins: a diverse proteome is generated from a relatively limited genome.
Splicing is important in genetic regulation alteration of the splicing pattern in response to cellular conditions changes protein expression. Perhaps not surprisingly, abnormal splicing patterns can lead to disease states including cancer. This process, catalyzed by reverse transcriptase enzymes, allows retroviruses, including the human immunodeficiency virus HIV , to use RNA as their genetic material. The mRNA formed in transcription is transported out of the nucleus, into the cytoplasm, to the ribosome the cell's protein synthesis factory.
Here, it directs protein synthesis. The ribosome is a very large complex of RNA and protein molecules. Each three-base stretch of mRNA triplet is known as a codon , and one codon contains the information for a specific amino acid. This tRNA molecule carries an amino acid at its 3'-terminus, which is incorporated into the growing protein chain. The tRNA is then expelled from the ribosome. Figure 7 shows the steps involved in protein synthesis.
DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies.
The DNA code contains instructions needed to make the proteins and molecules essential for our growth, development and health. If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website?
Open survey. In: Facts In the Cell. When the information stored in our DNA is converted into instructions for making proteins or other molecules, it is called gene expression. Gene expression is a tightly regulated process that allows a cell to respond to its changing environment.
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