DNA is essentially a storage molecule. It contains all of the instructions acell needs to sustain itself. These instructions are found within genes, which are sections of DNA madeup of specific sequences of nucleotides. In order to be implemented, theinstructions contained within genes must be expressed, or copied into a formthat can be used by cells to produce the proteins needed to support life.
The instructions stored within DNA are read and processed by a cell in twosteps: transcription and translation. Each of these steps is a separatebiochemical process involving multiple molecules. During transcription, a portion of the cell”s DNA serves as a template forcreation of an RNA molecule. (RNA,or ribonucleic acid, is chemicallysimilar to DNA, except for three main differences described later on in thisconcept page.) In some cases, the newly created RNA molecule is itself afinished product, and it serves an important function within the cell. In othercases, the RNA molecule carries messages from the DNA to other parts of thecell for processing. Most often, this information is used to manufactureproteins. The specific type of RNA that carries the information stored in DNAto other areas of the cell is called messengerRNA, or mRNA.
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Transcription begins when an enzyme called RNA polymerase attaches to the DNA template strand and beginsassembling a new chain of nucleotides to produce a complementary RNA strand. There are multiple types of types of RNA. In eukaryotes, there are multiple types of RNA polymerase which make the various types of RNA. In prokaryotes, a single RNA polymerase makes all types of RNA. Generally speaking,polymerases are large enzymes that work together with a number of otherspecialized cell proteins. These cell proteins, called transcription factors, help determine which DNA sequences should betranscribed and precisely when the transcription process should occur.
The first step in transcription is initiation. Duringthis step, RNA polymerase and its associated transcription factors bind to theDNA strand at a specific area that facilitates transcription (Figure 1). Thisarea, known as a promoter region,often includes a specialized nucleotide sequence, TATAAA, which is also calledthe TATA box (not shown in Figure1)
Figure 2:RNA polymerase (green) synthesizes a strand of RNA that is complementary to the DNA template strand below it.
Once RNA polymerase and its relatedtranscription factors are in place, the single-stranded DNA is exposed andready for transcription. At this point, RNA polymerase begins moving down theDNA template strand in the 3″ to 5″ direction, and as it does so, it stringstogether complementary nucleotides. By virtue of complementary base- pairing,this action creates a new strand of mRNA that is organized in the 5″ to 3″direction. As the RNA polymerase continues down the strand of DNA, more nucleotidesare added to the mRNA, thereby forming a progressively longer chain ofnucleotides (Figure 2). This process is called elongation.
Figure 3: DNA (top) includes thymine (red); in RNA (bottom), thymine is replaced with uracil (yellow).
Three of the four nitrogenous bases that make up RNA — adenine (A), cytosine (C), and guanine (G) — are also found in DNA. In RNA, however, a base called uracil (U) replaces thymine (T) as the complementary nucleotide to adenine (Figure 3). This means that during elongation, the presence of adenine in the DNA template strand tells RNA polymerase to attach a uracil in the corresponding area of the growing RNA strand (Figure 4).
Figure 4: A sample section of RNA bases (upper row) paired with DNA bases (lower row). When this base-pairing happens, RNA uses uracil (yellow) instead of thymine to pair with adenine (green) in the DNA template below.
Interestingly, this base substitution is not the only difference between DNA and RNA. A second major difference between the two substances is that RNA is made in a single-stranded, nonhelical form. (Remember, DNA is almost always in a double-stranded helical form.) Furthermore, RNA contains ribose sugar molecules, which are slightly different than the deoxyribosemolecules found in DNA. As its name suggests, ribose has more oxygen atoms than deoxyribose.
Thus, the elongation period of transcription creates a new mRNA molecule from a single template strand of DNA. As the mRNA elongates, it peels away from the template as it grows (Figure 5). This mRNA molecule carries DNA”s message from the nucleus to ribosomes in the cytoplasm, where proteins are assembled. However, before it can do this, the mRNA strand must separate itself from the DNA template and, in some cases, it must also undergo an editing process of sort.
Figure 5:During elongation, the new RNA strand becomes longer and longer as the DNA template is transcribed. In this view, the 5″ end of the RNA strand is in the foreground. Note the inclusion of uracil (yellow) in RNA.
Figure 6:In eukaryotes, noncoding regions called introns are often removed from newly synthesized mRNA.
“, “182”, “http://www.thedailysplash.tv/thedailysplash.tv_education”, “A schematic shows two strands of RNA against a white background. One extends from the upper left corner to the mid-right side. The other strand forms a loop, with the two ends pinched together and nearly touching the first strand. The sugar-phosphate backbone is depicted as a segmented white cylinder. Nitrogenous bases are represented as blue, green, yellow, or red vertical rectangles extending downward from each segment on the sugar-phosphate backbone. The loop represents a section of mRNA, called an intron, that has been removed from the coding sequence.”)” class=”inlineLinks”>Figure Detail
As previously mentioned, mRNA cannot perform its assignedfunction within a cell until elongation ends and the new mRNA separates from theDNA template. This process is referred to as termination. In eukaryotes, the process of termination can occur inseveral different ways, depending on the exact type of polymerase used duringtranscription. In some cases, termination occurs as soon as thepolymerase reaches a specific series of nucleotides along the DNA template,known as the termination sequence.In other cases, the presence of a special protein known as a termination factor is also required fortermination to occur.
Figure 7:In eukaryotes, a poly-A tail is often added to the completed, edited mRNA molecule to signal that this molecule is ready to leave the nucleus through a nuclear pore.
Once termination is complete, the mRNA molecule falls offthe DNA template. At this point, at least in eukaryotes, the newly synthesizedmRNA undergoes a process in which noncoding nucleotide sequences, called introns, are clipped out of the mRNAstrand. This process “tidies up” the molecule and removes nucleotides that are not involved in protein production (Figure 6). Then, a sequence ofadenine nucleotides called a poly-A tailis added to the 3″ end of the mRNA molecule (Figure 7). This sequence signalsto the cell that the mRNA molecule is ready to leave the nucleus and enter thecytoplasm.
Once an mRNA molecule is complete, that molecule can go on to play a keyrole in the process known as translation. During translation, the information that is contained within the mRNA isused to direct the creation of a protein molecule. In order for this to occur,however, the mRNA itself must be read by a special, protein-synthesizingstructure within the cell known as a ribosome.