4.3 Hairpin formation and micro-RNAs

A class of small RNA molecules called micro-RNAs (miRNAs) has been identified in recent years. The roles of these small RNAs are only just beginning to be understood, but many are expressed only at specific developmental stages. Indeed, the first observations of miRNAs were made in C. elegans because of their mutant developmental phenotypes. The genes that encode these miRNAs are called mir genes (pronounced ‘meer’) and have now been identified within the genomes of v
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4.2 The structure of tRNA

Transfer RNAs are small and compact molecules. Comparisons of the base sequences of many tRNAs led to the predicted four-leaf clover structure shown in Figure 18a, which follows the rule of maximising base-pairing interactions. This structure was largely confirmed by analysis with single-strand nucleases.

Two of the four main arms of the tRNA molecule are named according to their function, i.e. binding to the mRNA trinucleotide that encodes a specific amino acid (anticodon arm),
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4.1 The varied structures of RNA

RNA is a versatile cellular molecule with the ability to adopt a number of complex structural conformations. Although RNA is often thought of as a single-stranded molecule it is actually highly structured.

SAQ 19

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Summary of Section 3

  1. Watson–Crick base pairing arises due to hydrogen bonding between A and T and G and C and spatial limitations within the hydrophobic core of the helix.

  2. DNA commonly folds into the B-form helix; other forms such as Z-DNA form in vitro. A-form helices are formed primarily by duplex RNA.

  3. The twisting of DNA around its helical axis results in torsional stresses that promote the formation of high-energy alternative confo
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Triplex structures

An unusual form of three-stranded structure, called triplex DNA, can arise in vitro when a single-stranded region of DNA pairs with a paired duplex DNA helix through additional hydrogen bonding between the bases of all three strands.

SAQ 18

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3.3 Other structures in DNA

We will finish our discussion of DNA structure by examining two cases of unusual structures that can arise.


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The fluidity of torsional stress along the DNA chain

The fluid changes in conformation and free energy of the DNA helix are influenced by many processes including the binding of proteins, some of which may have a regulatory function. Thus binding of a protein in one position along a DNA chain could result in alterations in the topology of the DNA, and hence changes in free energy availability, both locally and at some distance from the binding site. Changes in torsional energy may serve as an indicator of the state of the surrounding helix. For
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Torsional energy can be taken up by alternative DNA conformations

The energy introduced into DNA by twisting has great potential as a regulatory mechanism, since the free energy can be stored in a variety of different high-energy conformations along the chain.

SAQ 14

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3.1 The helical structure of DNA

Having outlined the general principles of nucleic acid structures, we will now focus on how these principles influence the formation of specific structures found in DNA.

The helical structure of DNA arises because of the specific interactions between bases and the non-specific hydrophobic effects described earlier. Its structure is also determined through its active synthesis; that is, duplex DNA is synthesised by specialist polymerases upon a template strand. Within the helix, the two
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Summary of Section 2

  1. Nucleic acids are intrinsically highly flexible molecules.

  2. The chemical properties of nucleic acid components are primary determinants in structure formation.

  3. The formation of nucleic acid structures is driven by base pairing and stacking interactions between the hydrophobic bases. In DNA, these interactions drive the formation of the double helix, whose structure is maintained under torsional stress by twisting. RNA second
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2.4 Analysis of nucleic acids by electrophoresis and hybridisation

Nucleic acids can be separated according to size by gel electrophoresis, most commonly performed using a horizontal gel (Figure 7a). This is in contrast to the vertical gel electrophoresis set-up, which is generally used for analysis of proteins.

The size of DNA molecules is usually expressed in terms of the number of
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2.3 Analysing nucleic acid structures

In studying nucleic acid structures, many different experimental approaches can be adopted. In many cases, nucleic acid structures are examined in vitro, under non-physiological conditions, such as after denaturation or chemical synthesis. Nucleic acids within a cell are formed under very specific conditions and the structures that they adopt are influenced not only by the nature of their synthesis (by DNA or RNA polymerases), but by ancillary proteins that influence their folding. Nev
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Base pairing

Nucleic acid folding patterns are dominated by base pairing, which results from the formation of hydrogen bonds between pairs of nucleotides. In nucleic acids, as in proteins, the highly directional nature of this hydrogen bonding is the key to secondary structure.

SAQ 5


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2.2 General features of higher-order nucleic acid structure

Polynucleotide chains are intrinsically flexible molecules and have the potential to form many different higher-order structures. Their flexibility derives from rotation around bonds in the sugar-phosphate backbone (Figure 3b). In vivo, the structures that form are obviously determined by both the proteins that synthesise the nucleic acid chains (polymerases) and the ancillary proteins that bind to and modify them. We will discuss these aspects of structure later in this unit. What dri
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2.1 The primary structure of nucleic acids

We now know the detail of the order of individual bases, i.e. the genome sequence, of many of the organisms listed in Table 1. In Section 2 we will focus on the structures of nucleic acids within the cell, and we will start this discussion by outlining some of the general principles that apply to all nucleic acid structures.
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1.3 Nucleic acids and the flow of genetic information

The ‘flow’ of information from an organism's genome to the synthesis of its encoded proteins is referred to as the central dogma and emphasises the crucial roles that nucleic acids play within the cell (Figure 2). The synthesis of proteins (translation) is directed by the base order in mRNA, copied directly from that in the DNA of the genes by transcription. Translation involves RNAs in the form of the ribosome and tRNAs. In this unit we will be focusing on the relationship between
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1.2 Nucleic acids: genetic, functional and structural roles in the cell

The first role that one immediately thinks about for nucleic acids is that of an inherited genetic material, principally in the form of DNA. In some cases, the inherited genetic material is RNA instead of DNA. For example, almost 60% of all characterised viruses have RNA genomes and these are more common in plant viruses than in animal viruses. There is considerable variation in the amount of genetic material present within organisms (Author(s): The Open University

1.1 Early observations

Some of the earliest observations of macromolecules within living cells were of nucleic acids in the form of chromosomes. These long dark-staining objects, which became visible in the nucleus of cells at specific stages of cell division, were large enough to be detected using primitive light microscopes. Giant polytene chromosomes, found in certain cells such as the salivary gland cells of Drosophila (see Figure 1a), contain many thousands of copies of each chromosomal DNA align
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3.4 Second messengers

In the previous section, we have discussed the principles of second messengers (Section 1.5) and, in particular, those produced by PLC (IP3 and DAG) and PI3 kinase (PI(3,4)P2 and PI(3,4,5)P3). We shall now consider the roles and mechanisms of action of the other chief mediators, which are Ca2+ ions, cAMP and cGMP
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3.3 Lipid-modifying enzymes

The internal surface of the plasma membrane provides a useful environment for spreading signals received by surface receptors around the cell. Several specialist enzymes are activated by membrane-bound receptors, creating large numbers of small lipid-soluble second messenger molecules, which can diffuse easily through the membrane. These enzymes all use phosphatidylinositol (PI) and its derivatives as their substrates. PI itself is a derivative of glycerol: the OH group on carbon atom
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