It also influences how they bond to things like the nitrogenous bases, as well as the presence of uracil in place of thymine in the base pairs for RNA. This also affects the stability of each acid. For one, DNA is far less reactive than RNA even in an alkaline solution, as lacking the additional hydroxyl bond on its second carbon like RNA the extra oxygen molecule leads to less chance of bonding with outside substances.
RNA, however, will react with more readily with other substances because of that OH- in an attempt to balance itself. Even physically, these changes can be observed based on the depth of the grooves on each strands, with RNA featuring much deeper depressions on its surface that allow substances to pull it apart easier. Other fixtures within the body are also set up to naturally reject enzymes or other substances that might wish to pull DNA apart save for during replication or when read by the RNA.
Conversely, RNA is continually built, used, and broken down as part of its natural life cycle and is not as closely guarded as DNA, able to navigate out of the nucleus and to different parts of the cell as needed. This resiliency does not extend to radiation, though, as ultra-violet rays are more likely to do serious damage to DNA as opposed to RNA. This stems largely from the fact that DNA is less likely to be broken apart, meaning mutation or damage typically stays within a gene affected by radiation while an affected RNA molecule is broken down and repurposed as it normally would be.
Additionally, to accomplish all these functions, DNA is almost exclusively found in a double helix while RNA must be in a single strand. Sugar is one of the fundamental parts of DNA. 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. This backbone is formed when the 3' end dark gray of one nucleotide attaches to the 5' phosphate end light gray of an adjacent nucleotide by way of a phosphodiester bond. How is the DNA strand organized?
Figure 4: Double-stranded DNA consists of two polynucleotide chains whose nitrogenous bases are connected by hydrogen bonds. Within this arrangement, each strand mirrors the other as a result of the anti-parallel orientation of the sugar-phosphate backbones, as well as the complementary nature of the A-T and C-G base pairing.
Figure Detail. Figure 6: The double helix looks like a twisted ladder. How is DNA packaged inside cells? Figure 7: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes. What does real chromatin look like? Compare the relative sizes of the double helix, histones, and chromosomes.
Figure 8: In eukaryotic chromatin, double-stranded DNA gray is wrapped around histone proteins red. Figure 9: Supercoiled eukaryotic DNA. How do scientists visualize DNA? Figure This karyotype depicts all 23 pairs of chromosomes in a human cell, including the sex-determining X and Y chromosomes that together make up the twenty-third set lower right.
Watch this video for a closer look at the relationship between chromosomes and the DNA double helix. What are karyotypes used for? Who is James Watson? What do we know about Francis Crick?
Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. The Success Code. 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 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.
0コメント