10.2 Flashcards
Note 1 —-»
Once biologists had confirmed that DNA was the molecule of heredity, they began to study its structure. DNA and its close chemical cousin RNA are nucleic acids, consisting of long chains (polymers) of chemical units (monomers) called nucleotides.
Nucleotide
A non-membrane-bounded region in a prokaryotic cell where the DNA is concentrated.
Note 2 —-»
At the top left is a view of a DNA double helix. One of the strands is opened up (center) to show two different views of an individual DNA polynucleotide, a nucleotide polymer (chain). The view on the far right zooms in to a single nucleotide from the chain. Each type of DNA nucleotide has a different nitrogen-containing base: adenine (A), cytosine (C), guanine (G), or thymine (T). Because nucleotides can occur in a polynucleotide in any sequence and because polynucleotides can be very long, the number of possible polynucleotides is enormous. The chain shown in this figure has the sequence ACTGG, only one of many possible arrangements of the four types of nucleotides that make up DNA.
Polynucleotide
A polymer made up of many nucleotide monomers covalently bonded together.
Note 3 —-»
Each nucleotide consists of three components: a nitrogenous base (in DNA: A, C, T, or G), a sugar (shown in blue), and a phosphate group (yellow). The nucleotides are joined to one another by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a sugar-phosphate backbone with a repeating pattern of sugar-phosphate-sugar-phosphate. The nitrogenous bases are arranged like ribs that project from the backbone.
Sugar-Phosphate Backbone
The alternating chain of sugar and phosphate to which nitrogenous bases are attached.
Note 4 —-»
The phosphate group has a phosphorus atom (P) at the center with four surrounding oxygen atoms. The sugar has five carbon atoms, called out in red in the figure for emphasis—four in its ring and one extending above the ring. The ring also includes an oxygen atom. The sugar is called deoxyribose because, compared with the sugar ribose, it is missing an oxygen atom. Notice that the C atom in the lower right corner of the ring is bonded to an H atom instead of to an —OH group, as it is in ribose. Hence, DNA is “deoxy”—which means “without an oxygen”—compared to RNA. The full name for DNA is deoxyribonucleic acid: deoxyribo refers to its form of sugar, nucleic because DNA is located in the nuclei of eukaryotic cells, and acid because the phosphate group is in the ionized (negatively charged) form after donating a hydrogen atom.
DNA
A double-stranded helical acid molecule consisting of nucleotide monomers with deoxyribose sugar and the nitrogenous bases adenine, cytosine, guanine, and thymine.
Note 5 —-»
The four nucleotides found in DNA differ only in the structure of their nitrogenous bases. At this point, the structural details are not as important as is the fact that the bases are of two types. Thymine (T) and cytosine (C) are single-ring structures called pyrimidines. Adenine (A) and guanine (G) are larger, double-ring structures called purines. The one-letter abbreviations can be used either for the bases alone or for the nucleotides containing them.
Thymine
A single-ring nitrogenous base found in DNA.
Cytosine
A single-ring nitrogenous base found in DNA and RNA.
Adenine
A double-ring nitrogenous base found in DNA and RNA.
Guanine
A double-ring nitrogenous base found in DNA and RNA.
Note 6 —-»
As its name—ribonucleic acid—implies, its sugar is ribose rather than deoxyribose. Notice the ribose in the RNA nucleotide; unlike deoxyribose, the sugar ring has an —OH group attached to the C atom at its lower-right corner. Another difference between RNA and DNA is that instead of thymine, RNA has a nitrogenous base called uracil (U). Except for the presence of ribose and uracil, an RNA polynucleotide chain is identical to a DNA polynucleotide chain. In this three-dimensional view, each sphere represents an atom; notice that the color scheme is the same as in the other figures in this module. The yellow phosphate groups and blue ribose sugars make it easy to spot the sugar-phosphate backbone. In the next module, we’ll see how two DNA polynucleotides join together in a molecule of DNA.
Compare and contrast DNA and RNA polynucleotides.
Both are polymers of nucleotides consisting of a sugar, a nitrogenous base, and a phosphate. In RNA, the sugar is ribose; in DNA, it is deoxyribose. Both RNA and DNA have the bases A, G, and C, but DNA has a T and RNA has a U.
