Nucleosomal DNA packaging into a 30 nm fibre: the role of histone H1

When chromatin is isolated from the nucleus and examined under the electron microscope, it can be seen as a 30 nm fibre. This fibre is formed through the action of the histone H1 on the nucleosomal DNA in the 10 nm fibre. In contrast to the other histone proteins, H1 does not contain the histone fold motif.

Compaction of the 10 nm fibre to give the 30 nm fibre is achieved by interaction of the H1 protein with both the linker DNA and the histone octamers, as shown in Figure 31
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The histone fold and formation of the nucleosome

We have seen how in the eubacterial chromosome, bending DNA serves to facilitate its compaction. A similar process occurs in eukaryotic cells in that DNA is bent and wrapped around a protein unit. In this case, the core unit is a protein–DNA complex termed a nucleosome. The nucleosome comprises the core histone proteins H2A, H2B, H3 and H4 arranged in a structure known as the core histone octamer, with an associated length of DNA. In order to understand how the nucleosome is a
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The deamination of DNA

If you look back to Figure 4a, you will notice that cytosine carries an –NH2 group. This group is liable to undergo what is called a deamination reaction, resulting in the formation of a carbonyl group. What you might notice is that deamination of cytidine generates the nucleoside uridine (Author(s): The Open University

Quadruplex structures

The ends of linear chromosomes are protected from potential damage by special elements called telomeres. In many organisms, telomeres consist of long stretches of DNA that contain many thousands of copies of G-rich repeat sequences. They are easily detected by FISH (Subsection 2.4), as shown in the case of human telomeres in Fig
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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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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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3.2 Higher-order DNA structures: DNA twisting and torsional effects

As discussed earlier, the helical nature of DNA results for the most part from the properties of the bases, their interactions and the geometry of the helix itself. There is, however, another important contributor to the structure of DNA that is found within the cell. The DNA helix is actually under a torsional stress due to what is called DNA twisting, which arises when the two strands of the helix are twisted around the axis, as shown in Figure 11a.


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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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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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Glossary

Click on the link below to open the unit glossary.

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9.3 Health

The leader must be informed of any problems of mental or physical health that may affect safety during field-work. This may include, for instance, information on diabetes, asthma or epilepsy; students should also inform the leader if they require extra assistance. All work handling living organisms, soil or water may give some risk of infection, and protection in the form of gloves, masks, etc., may need to be carried. Supervisors should give advice concerning particular health hazards that m
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7.2 Information sources

7.2.1 How do I find the information required to carry out COSHH risk assessments?

The best source of information is the material safety data sheet (MSDS). By law (CHIP3) this should accompany any chemical that is purchased. However, if this is not available, or the chemical is old, then copies can be obtained from the manufacturer's website or information can sometimes be found in t
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5.1 Basic do's and don'ts and lone working

Some basic ‘do's and don'ts’ are:

  • Laboratory coats must be worn at all times.

  • When handling chemicals or sharps (any sharp object that can cause injury, particularly to the hands), observe good laboratory practice by wearing gloves. Latex or nitrile gloves are best, depending on the application.

  • There should be no eating, chewing gum, drinking, smoking or applying cosmetics in any laboratory.

  • No p
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4.2 Why do I need to know about first aid?

It takes only 3 to 4 minutes for a blocked airway to kill someone, but it can take more than 8 minutes for an ambulance to arrive on the scene. A simple procedure like opening an airway can save someone's life while you are waiting for professional help to arrive.

If you are working with harmful substances (chemicals, biological agents and dusts) you must know the first aid treatment if you are exposed. Do not expect a nurse or a doctor to know everything about every harmful substance.
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1 History of health and safety

The discipline of health and safety is relatively modern, only developing in the last century. However, throughout the ages people have voiced their concerns about people being exposed to harmful substances. Hippocrates mentions in the 4th century BC that lead miners and workers tended to suffer from diseases. The phrase ‘mad as a hatter’ was coined because mercury used in the hat industry caused mental illness. In 1775 Pott reported that chimney s
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Learning outcomes

By the end of this unit you should be able to:

  • understand the legal framework of the Health and Safety at Work etc. Act 1974 and Regulations associated with it;

  • understand the employers’, employees’ and visitors’ duties;

  • evaluate hazards and risks in order to carry out a risk assessment;

  • understand the legal requirement to report any accident or dangerous occurrence;

  • develop risk assessments for scientific laborat
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Introduction

Ths unit is an adapted extract from the course Postgraduate research skills in science (STM895)

This unit is designed to introduce you to the concepts of health and safety within a science laboratory or in the field. There are a number of legal requirements that must be adhered to before carrying out work in a laboratory. One of these is the necessity to carry out risk assessments on the chemical and biological agents that are to be used as part of your practical work activities. As par
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References

Bauer, M. W. and Gaskell, G. (2002) Biotechnology: The Making of a Global Controversy, Cambridge University Press.
Bowring, F. (2003) Science, Seeds and Cyborgs, Verso, London.
Campbell, S. (2004) A genetically modified survey, Spiked 
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Acknowledgements

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions.

Figures

Figure 4 BP (2
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