![]() The sugar and phosphate of the nucleotides form the backbone of the structure, while the nitrogenous bases are stacked inside. The two strands are anti-parallel in nature that is, the 3′ end of one strand points in one direction, while the 5′ end of the other strand points in that direction (Figure 3). Photo credit Madeline Price Ball Wikimedia. There is a 5′ and 3′ end to both chains of nucleotides, which are antiparallel to each other. Notice that adenine (a purine) and thymine (a pyrimidine) are connected together with 2 hydrogen bonds, while guanine (a purine) and cytosine (a pyrimidine) are connected by three hydrogen bonds. When nucleotides are joined together into a chain, the 5′ phosphate of one nucleotide is attached to the 3′ hydroxyl group of the next nucleotide, thereby forming a 5′-3′ phosphodiester bond. What this means is that when nucleotides are joined together in a chain, there will always be a free 3′ OH group (from the sugar) at one end of the chain and a free 5′ phosphate at the other end (Figure 3). The 5′ carbon is attached to a phosphate group. At the 3′ position, there is always a hydroxyl (OH) group that is a part of the sugar. We don’t particularly care about the 1′, 2′, or 4′ positions – you’ll never hear them mentioned again. The carbon atoms of the five-carbon sugar are numbered in order starting from the carbon connected to the nitrogenous base: 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”). The sugar is deoxyribose in DNA and ribose in RNA. Since a purine is “2 rings” across and a pyrimidine is “1 ring” across (Figure 2), the diameter of the DNA double helix remains constant at “3 rings” (Figure 3).įigure 2: Each nucleotide is made up of a sugar, a phosphate group, and a nitrogenous base. Adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen bonds. The base pairs are stabilized by hydrogen bonds (a weak type of bond that forms between partially positive and partially negative atoms). Adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. Watson and Crick’s model proposed that the two strands of nucleotides interact through base pairing between the nucleotides: A pairs with T and G pairs with C. One good way to remember this is that cytosine, thymine, and pyrimidine all contain the letter “y”. ![]() Pyrimidines are smaller in size they have a single six-membered ring structure. Adenine and guanine are both purines, while cytosine and thymine are pyrimidines. The purines have a double ring structure with a six-membered ring fused to a five-membered ring. The nucleotide is named depending on the nitrogenous base: adenine (A), thymine (T), cytosine (C), and guanine (G). The nucleotides are joined together in a chain by covalent bonds known as phosphodiester bonds. Scientists already knew that nucleotides contain the same three important components: a nitrogenous base, a deoxyribose (5-carbon sugar), and a phosphate group (Figure 2). This is actually a really interesting story of “sexism in the sciences” – there’s a movie called “The Secret of Photo 51” that you can find on YouTube if you’re interested.īased on Rosalind Franklin’s X-ray diffraction photograph, and work by other scientists, Watson and Crick proposed that DNA is made up of two strands of nucleotides that are twisted around each other to form a right-handed helix. Unfortunately, by then Franklin had died (of ovarian cancer, likely caused by exposure to X-rays), and Nobel prizes are not awarded posthumously (after death). In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Unfortunately, Watson and Crick gained access to Franklin’s data without her knowledge or approval. (credit a: modification of work by Marjorie McCarty, Public Library of Science) Scientist Rosalind Franklin discovered (b) the X-ray diffraction pattern of DNA, which helped to elucidate its double helix structure. Figure 1 The work of pioneering scientists (a) James Watson, Francis Crick, and Maclyn McCarty led to our present day understanding of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin’s data because Crick had also studied X-ray diffraction (Figure 1). ![]() In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand the structure of DNA. Pauling had discovered the secondary structure of proteins using X-ray crystallography. Other scientists like Linus Pauling and Maurice Wilkins were also actively exploring this field. In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. ![]()
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