For example, eukaryotic mRNAs last for several hours, whereas the typical prokaryotic mRNA lasts no more than five seconds. The additional steps involved in eukaryotic mRNA maturation create a molecule that is much more stable than a prokaryotic mRNA. The newly transcribed eukaryotic mRNAs must undergo several processing steps before they can be transferred from the nucleus to the cytoplasm and translated into a protein. In this way, a specific protein can rapidly reach a high concentration in the bacterial cell. In contrast, the presence of a nucleus in eukaryotic cells precludes simultaneous transcription and translation.įigure 9.17 Multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcripts into polypeptides. In a prokaryotic cell, by the time termination occurs, the transcript would already have been used to partially synthesize numerous copies of the encoded protein because these processes can occur concurrently using multiple ribosomes (polyribosomes) ( Figure 9.17). On termination, the process of transcription is complete. Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that result in RNA polymerase stalling, leaving the DNA template, and freeing the mRNA transcript. Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all ( Figure 9.15).įigure 9.16 During elongation, RNA polymerase tracks along the DNA template, synthesizes mRNA in the 5' to 3' direction, and unwinds then rewinds the DNA as it is read. In most cases, promoters exist upstream of the genes they regulate. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter. The region of unwinding is called a transcription bubble. Transcription requires the DNA double helix to partially unwind in the region of mRNA synthesis. In both prokaryotes and eukaryotes, transcription occurs in three main stages: initiation, elongation, and termination. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. With the genes bound in the nucleus, transcription occurs in the nucleus of the cell and the mRNA transcript must be transported to the cytoplasm. Transcription: from DNA to mRNAīoth prokaryotes and eukaryotes perform fundamentally the same process of transcription, with the important difference of the membrane-bound nucleus in eukaryotes. However, as we shall see in the next module, the translation to protein is still systematic, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on. The translation to protein is more complex because groups of three mRNA nucleotides correspond to one amino acid of the protein sequence. The copying of DNA to mRNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every complementary nucleotide read in the DNA strand. The flow of genetic information in cells from DNA to mRNA to protein is described by the central dogma ( Figure 9.14), which states that genes specify the sequences of mRNAs, which in turn specify the sequences of proteins.įigure 9.14 The central dogma states that DNA encodes RNA, which in turn encodes protein. The Central Dogma: DNA Encodes RNA RNA Encodes Protein This module discusses the details of transcription. Through the processes of transcription and translation, a protein is built with a specific sequence of amino acids that was originally encoded in the DNA.
How to transcribe dna to rna code#
The mRNA then provides the code to form a protein by a process called translation. To do this, the DNA is “read” or transcribed into an mRNA molecule. In both prokaryotes and eukaryotes, the second function of DNA (the first was replication) is to provide the information needed to construct the proteins necessary so that the cell can perform all of its functions.
Explain the main steps of transcription.Learning Objectives By the end of this section, you will be able to:
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