The nucleotide is named according to the nitrogenous base it contains. The phosphate group of one nucleotide bonds covalently with the sugar molecule of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. In its natural state, each DNA molecule is actually composed of two single strands held together along their length with hydrogen bonds between the bases.
Watson and Crick proposed that the DNA is made up of two strands that are twisted around each other to form a right-handed helix, called a double helix. Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and G pairs with C.
In other words, adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. Adenine and thymine are connected by two hydrogen bonds, and cytosine and guanine are connected by three hydrogen bonds.
The diameter of the DNA double helix is uniform throughout because a purine two rings always pairs with a pyrimidine one ring and their combined lengths are always equal. Figure 9. There is a second nucleic acid in all cells called ribonucleic acid, or RNA. Each of the nucleotides in RNA is made up of a nitrogenous base, a five-carbon sugar, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine.
Molecular biologists have named several kinds of RNA on the basis of their function. For this reason, the DNA is protected and packaged in very specific ways. In addition, DNA molecules can be very long. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure the cell that is not visible to the naked eye. The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features Figure 9.
Most prokaryotes contain a single, circular chromosome that is found in an area in the cytoplasm called the nucleoid. The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4. So how does this fit inside a small bacterial cell? The earliest DNA sequencing methods were time consuming, but a major breakthrough came in with the development of the process called Sanger sequencing.
Sanger sequencing is named after English biochemist Frederick Sanger, and it is sometimes also referred to as chain-termination sequencing or dideoxy sequencing. Some 25 years after its creation, the Sanger method was used to sequence the human genome, and, with the addition of many technological improvements and modifications, it remains an important method in laboratories across the world today. How does Sanger sequencing work? Understanding DNA replication.
Setting up the sequencing experiment. Adding ddNTPs. Figure 2: The four ddNTPs. Figure 3: By adding together information about all of the truncated strands, researchers can determine the nucleotide sequence of the DNA target. The sugar-phosphate backbone is depicted as gray, horizontal cylinders stacked end-to-end.
Each cylinder is attached to a thin rectangle, representing the nucleotide. Gray nucleotides have an unknown chemical composition. Green nucleotides represent adenine, and orange nucleotides represent cytosine. The sequence of nucleotides is: two gray, green, orange, gray, orange, two gray, green, 5 gray, green, gray. In the bottom DNA strand, eight nucleotides are base paired with the upper strand on the right side. The second sugar-phosphate group is colored black instead of gray, indicating that it contains a dideoxy-ribose sugar, and the first nucleotide is off-set to indicate that it is not bound to the DNA chain.
The sequence of the paired nucleotides is: red thymine , blue guanine , orange, blue, green, orange, red, blue. In a smaller diagram to the left of the larger chain, examples of resulting truncated nucleotide chains help decipher the DNA sequence.
Under the heading ddTTP, three nucleotide chains are shown. The first chain contains 14 nucleotides, with a red ddTTP inserted in the left-most position, truncating synthesis. The second chain contains 8 nucleotides, also truncated with a ddTTP. The third chain contains only 2 nucleotides, truncated after ddTTP addition. Under the heading ddGTP, two nucleotide chains are shown.
The first chain contains 13 nucleotides, truncated after ddGTP addition. The second chain contains 11 nucleotides, also truncated after ddGTP addition. After complete analysis with all four ddNTPs, the final nucleotide sequence is shown in the right panel. Nucleotides are represented by different colored rectangles: red for thymine, blue for guanine, green for adenine, and orange for cytosine.
Below the sequenced strand, examples of truncated strands from the four reactions are shown. Reading the sequence: Now and then. How is DNA sequencing used by scientists?
In recent years, DNA sequencing technology has advanced many areas of science. For example, the field of functional genomics is concerned with figuring out what certain DNA sequences do, as well as which pieces of DNA code for proteins and which have important regulatory functions. An invaluable first step in making these determinations is learning the nucleotide sequences of the DNA segments under study.
Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated December 10, Each nucleotide is a polymer made up of three parts: A five-carbon sugar 2'-deoxyribose in DNA or ribose in RNA A phosphate molecule A nitrogenous nitrogen-containing base.
Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. The 5 Kinds of Nucleotides. What Are the 3 Parts of a Nucleotide? How Are They Connected? Nitrogenous Bases - Definition and Structures. Thymine Definition, Facts, and Functions. The Difference Between Purines and Pyrimidines.
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