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Red and blue lines placed in a helical arrangement depict the two complementary strands and highlight the three-dimensional structure of DNA.

A circular arrangement of the red and blue lines is used as the simplified form of prokaryotic DNA.

Red and blue lines separated by a thin black line are used as a simple representation of the double-stranded DNA molecule.

Base-pairing -

Two parallel lines are used to emphasize the base-pairing interactions and nucleotide sequence characteristics of the two complementary strands. The "tracks" between the lines are not intended to depict a specific number of base pairs, only the general interaction between complementary strands.

Either a red or a blue line can be depicted as the "top" strand, since DNA is a three-dimensional structure.

Denatured DNA is depicted as separate red and blue lines to emphasize its single-stranded nature.

Figure 7.2 Diagrammatic Representations of the Structure of DNA Although DNA is a double-stranded helical structure, explanatory diagrams may depict it in a number of different ways. In this chapter we will use these color-coded representations.

3' direction and its complement is oriented in the 3' to 5' direction. This also has important implications in the function and synthesis of nucleic acids.

Characteristics of RNA

RNA is in many ways comparable to DNA, but with some important exceptions. One difference is that RNA is made up of ribonucleotides rather than deoxynu-cleotides, although in both cases these are usually referred to simply as nucleotides. Another distinction is that RNA contains the nitrogenous base uracil in place of the thymine found in DNA. Like DNA, RNA consists of a sequence of nucleotides, but RNA usually exists as a single-stranded linear molecule that is much shorter than DNA. ■ ribonucleic acid (RNA), p. 33 ■ nucleotide, p. 31

A fragment of RNA, a transcript, is synthesized using a region of one of the two strands of DNA as a template. In making the RNA transcript, the same base-pairing rules of DNA apply except uracil, rather than thymine, base-pairs with adenine. This base-pairing is only transient, however, and the molecule quickly leaves the DNA template. Numerous different RNA transcripts can be generated from a single chromosome using specific regions as templates. Either strand may serve as the template. In a region the size of a single gene, however, only one of the two strands is generally transcribed. As a result, two complementary strands of RNA are not normally generated.

There are three different functional groups of RNA molecules, each one transcribed from different genes. Most genes encode proteins and are transcribed into messenger RNA (mRNA). These molecules are translated during protein synthesis. Encrypted information in mRNA is deciphered according to the genetic code, which correlates each set of three nucleotides, called a codon, to a particular amino acid. Some genes are never translated into proteins; instead the RNAs themselves are the ultimate products. These

Sugar-phosphate backbone

Sugar-phosphate backbone

Nucleotide

Hydrogen bonds

Sugar-phosphate backbone

Figure 7.3 The Double Helix of DNA The two strands of DNA are antiparallel; one strand is oriented in the 5' to 3' direction, and its complement is oriented in the 3' to 5' direction. Hydrogen bonding occurs between the complementary base pairs; three bonds form between a G-C base pair, and two bonds form between an A-T base pair.

Hydrogen bonds

Sugar-phosphate backbone

Figure 7.3 The Double Helix of DNA The two strands of DNA are antiparallel; one strand is oriented in the 5' to 3' direction, and its complement is oriented in the 3' to 5' direction. Hydrogen bonding occurs between the complementary base pairs; three bonds form between a G-C base pair, and two bonds form between an A-T base pair.

Nucleotide p p

170 Chapter 7 The Blueprint of Life, from DNA to Protein genes encode either ribosomal RNA (rRNA) or transfer RNA (tRNA), each of which plays a different but critical role in protein synthesis.

Regulating the Expression of Genes

While the basic structure of DNA and RNA is relatively simple, the information the molecules encode is extensive and complex. The nucleotide sequence contains genes that encode the amino acid sequence of proteins, and it also encodes mechanisms to regulate expression of those genes. Not all proteins are required by a cell in the same quantity and at all times; therefore, mechanisms that determine the extent and duration of their synthesis are needed.

One of the key mechanisms a cell uses to control protein synthesis is to regulate the synthesis of mRNA molecules. Unless a gene is transcribed into mRNA, the encoded protein cannot be synthesized. The number of mRNA copies of the gene also influences the level of expression. If transcription of a gene ceases, the level of gene expression rapidly declines. This is because mRNA is short-lived, often only a few minutes, due to the activity of enzymes called RNases that rapidly degrade it.

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