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.
There are four nitrogenous bases in DNA, two purines adenine and guanine and two pyrimidines cytosine and thymine. A DNA molecule is composed of two strands. Chemical structure of the four nitrogenous bases in DNA. Notice how the purines A and G are composed of two ring structures, whereas the pyrimidines T and C are composed of one ring structure. The DNA of all species has the same four nitrogen bases…. Think of a strand of DNA. Each base pairs with a specific partner, allowing us to determine their percentages: adenine and thymine are always equal, and cytosine and guanine are always equal.
In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine. DNA is a polymer of nucleotides. Nitrogenous bases hold the strands together via hydrogen bonding. These bases are adenine A , thymine T , guanine G , and cytosine C. Adenine A can only pair with thymine T , and guanine G can only pair with cytosine C. Numerous nucleotides are bonded together by phosphodiester bonds to form a single molecule of DNA.
The pattern of nucleotides is used to store and transmit hereditary information. A nucleotide is composed of a phosphate group, a nitrogenous base, and a pentose sugar deoxyribose. Cytosine and guanine form three hydrogen bonds with each other, while adenine and tyrosine only form two hydrogen bonds.
This means that strands of DNA with a higher percentage of cytosine and guanine will have higher melting points. Since we are looking for the sequence with the lowest melting point, we want the lowest percentage of cytosine and guanine, and the highest percentage of adenine and thymine. What is the percentage of cytosine bases in the section of DNA?
This is because adenine will always pair with thymine, so there will be just as many thymine bases as adenine bases. What percentage of the bases in the sample are thymine? Guanine will pair with cytosine. From this knowledge, we can assume that there will be an equal number of guanine and cytosine residues in the sample. Each guanine must have a cytosine counterpart. Like cytosine and guanine, adenine and thymine must be present in equal amounts in order to form proper base pairs.
We can reasonably assume that half of the remaining DNA will consist of each residue. What percent of the sample would be cytosine?
There are four nitrogenous bases found in DNA: adenine, thymine, cytosine, and guanine. Adenine always binds with thymine, and cytosine always binds with guanine. Since certain bases always appear in pairs, they will have equal percentages of the DNA composition.
The percentage of adenine will equal the percentage of thymine, and the percentage of cytosine will equal the percentage of guanine. We know that cytosine and guanine pair together and will be present in equal amounts, so we can divide this final total by 2 to find our answer. What is the DNA's composition of the other bases? We can use Chargaff's rule to find the remaining compositional percentages.
Adenine always pairs with thymine, so their percentages will be equal. Cytosine always pairs with guanine, so their percentages will also be equal.
Think of a strand of DNA. Each base pairs with a specific partner, allowing us to determine their percentages: adenine and thymine are always equal, and cytosine and guanine are always equal. In RNA, with this pairing absent, there is no correlation between the base percentages. A polymer is a macromolecule that is made up of subunits that are repeated or very similar. These subunits are called monomers.
DNA is a polymer made up of monomer units called nucleotides. Nucleotides are made up of a phosphate group, a five-carbon sugar deoxyribose, in the case of DNA , and a variable nitrogenous base. There are four different nucleotides that make up the polymer of DNA: thymine, cytosine, adenine, and guanine.
These four nucleotides belong to two different classes based on structure. Adenine and guanine are purines that have two carbon-nitrogen rings.
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