This code contains only ones and zeros, and think of all the things your computer can do. The DNA alphabet can encode very complex instructions using just four letters, though the messages end up being really long. For example, the E.
The human genome all the DNA of an organism consists of around three billion nucleotides divided up between 23 paired DNA molecules, or chromosomes. The information stored in the order of bases is organized into genes : each gene contains information for making a functional product. The genetic information is first copied to another nucleic acid polymer , RNA ribonucleic acid , preserving the order of the nucleotide bases.
In order for DNA to function effectively at storing information, two key processes are required. First, information stored in the DNA molecule must be copied, with minimal errors, every time a cell divides.
This ensures that both daughter cells inherit the complete set of genetic information from the parent cell. Second, the information stored in the DNA molecule must be translated , or expressed.
In order for the stored information to be useful, cells must be able to access the instructions for making specific proteins, so the correct proteins are made in the right place at the right time.
Figure 4. Both copying and reading the information stored in DNA relies on base pairing between two nucleic acid polymer strands. Recall that DNA structure is a double helix see Figure 4.
The sugar deoxyribose with the phosphate group forms the scaffold or backbone of the molecule highlighted in yellow in Figure 4. Bases point inward. Complementary bases form hydrogen bonds with each other within the double helix. See how the bigger bases purines pair with the smaller ones pyrimidines. This keeps the width of the double helix constant. More specifically, A pairs with T and C pairs with G.
As we discuss the function of DNA in subsequent sections, keep in mind that there is a chemical reason for specific pairing of bases.
Insulin is responsible for regulating blood sugar levels. The insulin gene contains instructions for assembling the protein insulin from individual amino acids. Changing the sequence of nucleotides in the DNA molecule can change the amino acids in the final protein, leading to protein malfunction. If insulin does not function correctly, it might be unable to bind to another protein insulin receptor.
On the organismal level of organization, this molecular event change of DNA sequence can lead to a disease state—in this case, diabetes. The order of nucleotides in a gene in DNA is the key to how information is stored. For example, consider these two words: stable and tables. Both words are built from the same letters subunits , but the different order of these subunits results in very different meanings.
In DNA, the information is stored in units of 3 letters. Use the following key to decode the encrypted message. This should help you to see how information can be stored in the linear order of nucleotides in DNA.
When comparing prokaryotic cells to eukaryotic cells, prokaryotes are much simpler than eukaryotes in many of their features Figure 5. Most prokaryotes contain a single, circular chromosome that is found in an area of the cytoplasm called the nucleoid.
Figure 5. A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid. In prokaryotic cells, both processes occur together. 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? Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases.
This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.
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