This negatively charged backbone creates an attractive force between the aqueous, polar environment and the DNA molecule.Ĭonsider what might happen if the negatively charged backbone were on the interior of the molecule and the aromatic bases were on the exterior of the molecule. Can you see why? The backbone is negatively charged because the phosphate groups carry charged oxygen atoms. An important fact to know about DNA is that its backbone is negatively charged. Phosphodiester bonds exist between adjacent nucleotides to create the backbone of DNA. Try to trace the locations of the phosphodiester bond on the diagram below. In the sugar phosphate backbone, the phosphate group of one nucleotide creates a phosphodiester bond with the 3’ carbon of the pentose sugar on the adjacent nucleotide. The phosphate group is attached to the 5’ carbon of the pentose sugar. Recall the structure of a nucleotide and that each nucleotide base contains a phosphate group and pentose sugar. The sugar phosphate backbone consists of a repeating pattern of sugar and phosphate groups bonded together. The DNA backbone is known as a sugar phosphate backbone. Similarly, DNA needs a backbone to hold the double helix structure together. It wouldn’t function as a ladder at all! A ladder needs its side rails to work. This will remind you that adenine (A) bonds with thymine (T), while guanine (G) bonds with cytosine (C). An easy way to remember which nucleotides bond together is by memorizing the letters AT and GC together. These hydrogen bonds provide stability to the DNA, making the guanine and cytosine pair and adenine and thymine pair favorable. When adenine and thymine bond, two hydrogen bonds form. When guanine and cytosine bond, three hydrogen bonds form.
Scientists have discovered exceptions to these rules, but you won’t be tested on them. This bonding is known as Watson-Crick base pairing. In particular, guanine and cytosine almost exclusively bond together while adenine and thymine bond with each other. In DNA, purine bases bond with pyrimidine bases instead of other purines, and vice versa. Memorizing each nucleotide’s structure will be crucial for test day.Įach nucleotide follows strict rules for bonding with other nucleotides. It’s important to remember adenine and guanine still have differences in the organic structure: for instance, in the number of attached carbonyl (C=O) bonds and where they are located. Adenine and guanine are often broadly classified as purine nucleotides, while cytosine and thymine are classified as pyrimidine nucleotides. Adenine and guanine have purine bases, while cytosine and thymine have pyrimidine bases. Take a close look at the structure of nitrogenous bases of the nucleotides. Purines are made of two rings and are larger than pyrimidines, which contain only one ring. While both purines and pyrimidines are aromatic nitrogenous bases, they differ in size and ring structure. There are two classes of nitrogenous bases: purines and pyrimidines. The nucleotides have different nitrogenous bases. In contrast, nucleosides are molecules that only consist of a nitrogenous base and a pentose sugar. Each nucleotide consists of a phosphate group (PO4-), a nitrogenous base, and a pentose sugar that links the other two groups together. What do each of these four nucleotides have in common? All of the bases have a very similar structure. Throughout this guide, you will also see bolded terms that will be important to recall on the exam. It’s helpful to create a list outlining why each component or modification of DNA is necessary for its function. When studying DNA, it’s easy to confuse the names of the bases and building blocks of DNA as well as the different modifications DNA undergoes in the cell. This guide will take a tour through the structure and function of DNA and what can go wrong. As a result, proper DNA regulation is vital to cell survival. Errors in DNA replication can vary significantly, from unnoticeable mutations to life-threatening illnesses, such as cancer. DNA is carefully replicated and inherited from one generation of cells to the next to avoid any errors. DNA stores and selectively expresses the genetic information our cells need to function. DNA, or deoxyribonucleic acid, is one of the two main molecules of nucleic acids (the other being RNA, or ribonucleic acid). Understanding DNA and its function is crucial on the MCAT, especially as DNA is the biological basis of life.
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