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Transcription and translation
Genes provide information for building proteins. They don’t however directly create proteins. The production of proteins is completed through two processes: transcription and translation.
Transcription and translation take the information in DNA and use it to produce proteins. Transcription uses a strand of DNA as a template to build a molecule called RNA.
The RNA molecule is the link between DNA and the production of proteins. During translation, the RNA molecule created in the transcription process delivers information from the DNA to the protein-building machines.
DNA and RNA are similar molecules and are both built from smaller molecules called nucleotides. Proteins are made from a sequence of amino acids rather than nucleotides. Transcription and translation are the two processes that convert a sequence of nucleotides from DNA into a sequence of amino acids to build the desired protein.
These two processes are essential for life. They are found in all organisms – eukaryotic and prokaryotic. Converting genetic information into proteins has kept life in existence for billions of years.
What is Transcription?
Transcription generally refers to the written form of something. In biology, transcription is the process whereby DNA is used as a template to form a complementary RNA strand – RNA is the “written” form of DNA. This is the first stage of protein production or the flow of information within a cell. DNA stores genetic information, which is then transferred to RNA in transcription, before directing the synthesis of proteins in translation. Three types of RNA can be formed: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
Transcription occurs in four stages: pre-initiation, initiation, elongation, and termination. These differ in prokaryotes and eukaryotes in that DNA is stored in the nucleus in eukaryotes, and whereas DNA is stored in the cytoplasm in prokaryotes. In eukaryotes, DNA is stored in tightly packed chromatin, which must be uncoiled before transcription can occur. The production of mRNA from RNA in eukaryotes is particularly more complicated than it is in prokaryotes, involving several additional processing steps.
Pre-initiation, or template binding, is initiated by the RNA polymerase σ subunit binding to a promoter region located in the 5’ end of a DNA strand. Following this, the DNA strand is denatured, uncoupling the two complementary strands and allowing the template strand to be accessed by the enzyme. The opposing strand is known as the partner strand. Promoter sequences on the DNA strand are vital for the successful initiation of transcription. Promoter sequences are specific sequences of the ribonucleotide bases making up the DNA strand (adenine, thymine, guanine,and cytosine), and the identity of several of these motifs have been discovered, including TATAAT and TTGACA in prokaryotes and TATAAAA and GGCCAATCT in eukaryotes. These sequences are known as cis-acting elements. In eukaryotes, an additional transcription factor is necessary to facilitate the binding of RNA polymerase to the promoter region.
RNA polymerase catalyzes initiation, causing the introduction of the first complementary 5’-ribonucleoside triphosphate. Remember that each DNA nucleotide base has a complement: adenine and thymine, and guanine and cytosine. However, the ribonucleotide base complements differ slightly as RNA does not contain thymine, but rather a uracil, and so adenine’s complement is uracil. After the introduction of the first complementary 5’-ribonucleotide, subsequent complementary ribonucleotides are inserted in a 5’ to 3’ direction. These ribonucleotides are joined by phosphodiester bonds, and at this stage, the DNA and RNA molecules are still connected(see Figure 1).
Figure 1: Initiation of transcription. RNAP® refers to RNA polymerase.
Chain elongation occurs when the σ subunit dissociates from the DNA strand, allowing the growing RNA strand to separate from the DNA template strand. This is facilitated by the core enzyme (see Figure 2).
Figure 2: Elongation in transcription
Termination occurs when the core enzyme encounters a termination sequence, which is a specific sequence of nucleotides which acts as a signal to stop transcription. At this point, the RNA transcript forms a hairpin secondary structure by folding back on itself with the aid of hydrogen bonds. Termination in prokaryotes can be assisted by an additional termination factor known as rho(ρ). Termination is complete when the RNA molecule is released from the template DNA strand. In eukaryotes, termination requires an additional step known as polyadenylation in eukaryotes, whereby a tail of multiple adenosine monophosphates is added to the RNA strand.
Figure 3: The main events in each stage of transcription