What makes nucleotides unique

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. The DNA is wrapped tightly around the histone core. This nucleosome is linked to the next one by a short strand of DNA that is free of histones.

This fiber is further coiled into a thicker and more compact structure. At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted. They are approximately nm in width, and are found in association with scaffold proteins. In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining.

There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted. Concept in Action. Watch this animation of DNA packaging.

The DNA molecule is a polymer of nucleotides. The four reaction products were then separated by gel electrophoresis, a process that organizes DNA fragments in order of size. This enabled researchers to assess the lengths of the truncated strands in each sample. This was important, because the end of each truncated strand was used to determine the position at which a ddNTP was added to the strand, thereby halting DNA elongation.

This page appears in the following eBook. Aa Aa Aa. How do researchers "read" gene sequences? Determining the order of the nucleotides within a gene is known as DNA sequencing. 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. Skip to main content. This illustration introduces nucleotide and the terminology used to describe them.

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