These bases contain nitrogen in their ring compounds. This is one of the reasons why a transversional base pairing change can have such disastrous effects on the structure of a protein as hydrogen bonds will not occur between two purines or two pyrimidines [4]. Before Watson and Crick presented the structure of DNA, Erwin Chargaff in the s discovered a chemical technique in which he could determine the molar concentration of any one of the bases in a source of DNA.
From what Chargaff discovered he noticed some patterns in the molar concentrations of the bases, from his results he devised some rules [5]. In the DNA double helix, as well as the bases being complementary base-paired they are also stacked on top of one another. These bases also have interactions Van der Waals happening between one another which also contribute towards the DNAs structure.
Base stacking in this way creates a hydrophobic core on the DNA [6]. When this base-pairing happens, RNA uses uracil yellow instead of thymine to pair with adenine green in the DNA templat. When this base-pairing happens, RNA uses uracil yellow instead of thymine to pair with adenine green in the DNA template below. This image is linked to the following Scitable pages:. During the process of transcription, DNA is converted to RNA, a complex molecule that is able to carry genetic information outside of the cell's nucleus.
This page guides you through the biochemical steps of this process. Comments Close. The Comment you have entered exceeds the maximum length. Submit Cancel. Comments Please Post Your Comment. Strand elongation. Once RNA polymerase and its related transcription factors are in place, the single-stranded DNA is exposed and ready for transcription. At this point, RNA polymerase begins moving down the DNA template strand in the 3' to 5' direction, and as it does so, it strings together 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. This process is called elongation. 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.
In this view, the 5' end of the RNA strand is in the foreground. Note the inclusion of uracil yellow in RNA. Termination and editing. Figure 6: In eukaryotes, noncoding regions called introns are often removed from newly synthesized mRNA. 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. This process is referred to as termination. In eukaryotes, the process of termination can occur in several different ways, depending on the exact type of polymerase used during transcription. In some cases, termination occurs as soon as the polymerase 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 for termination 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. At this point, at least in eukaryotes, the newly synthesized mRNA undergoes a process in which noncoding nucleotide sequences, called introns , are clipped out of the mRNA strand.
This process "tidies up" the molecule and removes nucleotides that are not involved in protein production Figure 6. Then, a sequence of adenine nucleotides called a poly-A tail is added to the 3' end of the mRNA molecule Figure 7. This sequence signals to the cell that the mRNA molecule is ready to leave the nucleus and enter the cytoplasm.
What's next for the RNA molecule? More on transcription. How are polymerases different in prokaryotes and eukaryotes? How is bacterial transcription unique?
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