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. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar deoxyribose , and a phosphate group.
The two strands of the double helix run in anti-parallel i. The double helix has a right-handed twist, rather than the left-handed twist that is often represented incorrectly in popular media. The DNA bases extend from the backbone towards the center of the helix, with a pair of bases from each strand forming hydrogen bonds that help to hold the two strands together. Under most conditions, the two strands are slightly offset, which creates a major groove on one face of the double helix, and a minor groove on the other.
Each strand is therefore said to be complementary to the other, and so each strand also contains enough information to act as a template for the synthesis of the other. This complementary redundancy is important in DNA replication and repair. Spin the double helix to see the orientation of the sugars and phosphates in the backbone ribbon in the model , the base pairs, major and minor grooves! As for most biological molecules, the structure is important to the function, and the function of DNA is to contain information.
Important properties that are derived from the DNA structure are:. A DNA double helix twists in a right-handed fashion, just as the fingers on the right hand are "pointing" to the right when the right hand forms a "thumbs up. The bases within the double helix interact with each other via hydrogen bonds, but the different bases pairs have different combinations atoms exposed in the major grooves. Proteins that recognize DNA sequences often do so by interacting with particular combinations of base pairs in major grooves based on these exposed atoms.
A mutation occurs, and cytosine is replaced with adenine. What impact do you think this will have on the DNA structure? Adenine is larger than cytosine and will not be able to base pair properly with the guanine on the opposing strand, causing the double helix to bulge at that position.
DNA repair enzymes may recognize the bulge and replace the incorrect nucleotide. Microbial nucleic acids; the desoxypentose nucleic acids of avian tubercle bacilli and yeast. J Biol Chem. The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.
Base pair describes the relationship between the building blocks on the strands of DNA. And each of the nucleotides on one side of the strand pairs with a specific nucleotide on the other side of the strand, and this makes up the double helix. So for example, if there's a G on one side of the strand, there will always be a C on the other. If there's a T on one side of the strand, there will always be an A on the other.
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