Which nucleotides are used to build dna

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 DNA is twisted beyond the double helix in what is known as supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes.

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. Another area of science that relies heavily on DNA sequencing is comparative genomics, in which researchers compare the genetic material of different organisms in order to learn about their evolutionary history and degree of relatedness. DNA sequencing has also aided complex disease research by allowing scientists to catalogue certain genetic variations between individuals that may influence their susceptibility to different conditions.

How can all people benefit from DNA sequencing? More about sequencing. Franklin, Watson, and Crick all published articles describing their related findings in the same issue of Nature in Most cells are incredibly small. For instance, one human alone consists of approximately trillion cells. Yet, if all of the DNA within just one of these cells were arranged into a single straight piece, that DNA would be nearly two meters long!

So, how can this much DNA be made to fit within a cell? The answer to this question lies in the process known as DNA packaging , which is the phenomenon of fitting DNA into dense compact forms Figure 7. During DNA packaging, long pieces of double-stranded DNA are tightly looped, coiled, and folded so that they fit easily within the cell. Eukaryotes accomplish this feat by wrapping their DNA around special proteins called histones , thereby compacting it enough to fit inside the nucleus Figure 8.

Together, eukaryotic DNA and the histone proteins that hold it together in a coiled form is called chromatin. It is impossible for researchers to see double-stranded DNA with the naked eye — unless, that is, they have a large amount of it. Modern laboratory techniques allow scientists to extract DNA from tissue samples, thereby pooling together miniscule amounts of DNA from thousands of individual cells.

When this DNA is collected and purified, the result is a whitish, sticky substance that is somewhat translucent. To actually visualize the double-helical structure of DNA, researchers require special imaging technology, such as the X-ray diffraction used by Rosalind Franklin. However, it is possible to see chromosomes with a standard light microscope, as long as the chromosomes are in their most condensed form.

To see chromosomes in this way, scientists must first use a chemical process that attaches the chromosomes to a glass slide and stains or "paints" them. Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual chromosomes as a result of the staining process are unique to each pair of chromosomes, so they allow researchers to distinguish different chromosomes from one another.

Then, after a scientist has visualized all of the chromosomes within a cell and captured images of them, he or she can arrange these images to make a composite picture called a karyotype Figure This page appears in the following eBook. Aa Aa Aa. What components make up DNA? Figure 1: A single nucleotide contains a nitrogenous base red , a deoxyribose sugar molecule gray , and a phosphate group attached to the 5' side of the sugar indicated by light gray.

Opposite to the 5' side of the sugar molecule is the 3' side dark gray , which has a free hydroxyl group attached not shown. Figure 2: The four nitrogenous bases that compose DNA nucleotides are shown in bright colors: adenine A, green , thymine T, red , cytosine C, orange , and guanine G, blue. Although nucleotides derive their names from the nitrogenous bases they contain, they owe much of their structure and bonding capabilities to their deoxyribose molecule.

The central portion of this molecule contains five carbon atoms arranged in the shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol '. Of these carbons, the 5' carbon atom is particularly notable, because it is the site at which the phosphate group is attached to the nucleotide. Appropriately, the area surrounding this carbon atom is known as the 5' end of the nucleotide.

Opposite the 5' carbon, on the other side of the deoxyribose ring, is the 3' carbon, which is not attached to a phosphate group. This portion of the nucleotide is typically referred to as the 3' end Figure 1. When nucleotides join together in a series, they form a structure known as a polynucleotide.

At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the 3' end of the adjacent nucleotide through a connection called a phosphodiester bond Figure 3. It is this alternating sugar-phosphate arrangement that forms the "backbone" of a DNA molecule. Figure 3: All polynucleotides contain an alternating sugar-phosphate backbone.