![]() ![]() These nitrogenous bases are covalently bonded via a nitrogen atom to the 1’ carbon of the deoxyribose sugar in a nucleotide (Figure 1a). The purines and pyrimidines differ slightly in structure, but their functional groups are attached to the same basic heterocyclic form. The two pyrimidines found in DNA are thymine (T) and cytosine (C), while the two purines are Adenine (A) and Guanine (G). However, while pyrimidines are six-membered rings, purines consist of a five-membered ring fused to a six-membered ring. Both purines and pyrimidines are heterocyclic aromatic compounds, as they contain nitrogen atoms in their carbon-based ring, which are essential for the hydrogen bonding that holds the two strands of the DNA molecule together. Ī nucleotide can incorporate four main nitrogenous bases, two of which are purines and two that are pyrimidines (Figure 1b). Since the 2’-deoxyribose and the phosphate group are always present, the nitrogenous bases they incorporate distinguish the four DNA nucleotides. A phosphate group covalently binds to the 5’ carbon of 2’-deoxyribose. In the case of DNA, the sugar is 2’-deoxyribose, and thus it has no hydroxyl group attached to its 2’ (pronounced “two prime”) carbon this is in contrast to ribose sugar in RNA, which does not have the 2’ position of its pentose sugar to be reduced (or deoxygenated). A nucleotide comprises a nitrogenous base, a pentose sugar, and at least one phosphate group (Figure 1a). Molecular LevelĪ molecule of DNA is made up of two long polynucleotide chains consisting of subunits known as nucleotides. Specifically, this is done by sequential levels of coiling, starting with DNA wrapping around histone proteins forming a structure known as a nucleosome, then nucleosomes coiling to form chromatin fibers, and then chromatin further condensing into densely packed chromosomes. 4.4 mega-base pairs), they need to utilize a more complex strategy to position their DNA, which, if stretched from end to end, would be two meters long, properly inside a microscopic cellular space. However, because eukaryotes have much more DNA than prokaryotes (3234 mega-base pairs vs. For the specific purpose of decreasing their DNA size to ensure fitting inside a cell, prokaryotes employ DNA supercoiling. In contrast, the ends of eukaryotic DNA molecules do not connect and are thus "free." Prokaryotes typically have one main circular chromosome, while eukaryotes have multiple linear chromosomes of varying sizes. Circular DNA molecules are also found in eukaryotic mitochondrial and chloroplast DNA, evidence that supports the endosymbiotic theory of eukaryotic evolution. One significant difference between prokaryotes' and eukaryotes' DNA structure is that prokaryotic DNA molecules are circular and thus do not have free 5' and 3' ends. In addition, several disorders are due to defects in cellular mechanisms associated with DNA, including replication, DNA repair, and transcription. Mutations in DNA structure can take many forms, such as large or small insertions or deletions of base pairs or inversions and insertions of whole DNA segments between or within chromosomes. The primary issue of concern regarding the DNA structure is variations and mutations in DNA structure as proteins encoded by the mutated DNA generally have altered structure and function, adversely impacting the survival of the cell or organism. It also provided a framework for the subsequent elucidation of the mechanism involved in DNA replication. Their proposed model for DNA structure explained previous observations, such as the equivalent ratios of purines and pyrimidines found in the DNA molecules. In their seminal 1953 paper, Watson and Crick unveiled two aspects of DNA structure: pairing the nucleotide bases in a complementary fashion (e.g., adenine with thymine and cytosine with guanine) and the double-helical nature of DNA. The ability of DNA to function as the material through which genetic information is stored and transmitted is a direct result of its elegant structure. The remarkable structure of deoxyribonucleic acid (DNA), from the nucleotide up to the chromosome, plays a crucial role in its biological function. ![]()
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