Erwin Center Press Conference
Clemson University President James F. Barker announced a $1.05 million gift from Joe and Gretchen Erwin, co-founders of leading advertising and marketing firm Erwin Penland, to establish the Erwin Center for the Study of Advertising and Communication at the school. The gift creates a unique relationship between the Erwin family, Clemson University and Erwin Penland.
Author(s): No creator set

License information
Related content

The loss of a DNA base causes an abasic site

Hydrolysis of the deoxyribose Cl'–base linkage results in the complete loss of a purine or pyrimidine base, resulting in what is called an abasic site, an event with obvious genetic consequences. This hydrolysis reaction is much more likely to occur at purine bases, resulting in depurination of the DNA (Table 3a<
Author(s): The Open University

4.6 Summary

  1. RNA chains play fundamentally important roles within the cell, including genetic information transfer (mRNA), components of the translation machinery (rRNA in ribosomes and tRNAs) and as regulatory small RNAs.

  2. The tertiary structure of RNA is determined by interactions that maximise base pairing. Despite instability and isolation problems, the tertiary structures of several major cellular RNAs are known.

  3. Transfer RNA struct
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Aptamers

Aptamers are nucleic acid molecules that have been developed to mimic the selective and tight binding of other molecules such as antibodies. In order to identify an aptamer that is capable of binding to a target molecule, a process called Selex (systematic evolution of ligands by exponential enrichment) is utilised. The strategy relies upon a combination of a selective binding assay and amplification by PCR. A ‘library’ of short single-stranded DNA oligonucleotides is synthesised <
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Antisense regulation of gene expression

The term antisense refers to the use of a nucleic acid that is complementary to the coding (i.e. ‘sense’) base sequence of a target gene. When nucleic acids that are antisense in nature are introduced into cells, they can hybridise to the complementary ‘sense’ mRNA through normal Watson-Crick base pairing. Synthetic antisense DNA chains as short as 15–17 nucleotides in length have been used to block specific gene expression by either physically blocking translation of the tar
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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),
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.3 Other structures in DNA

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


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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.
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share