8.1 Introduction

The average human cell has around two metres of DNA within its nucleus. In the interphase nucleus, in which transcription and replication are going on, this DNA is packaged into nucleosomes that are variably compacted, through association with H1, into larger 30nm fibres. In fact, the average nucleus most likely contains DNA with a continuum of chromatin configurations, ranging from highly open 10 nm fibres, through to 30 nm fibres and fibres that are even more tightly packed together, call
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Core histone tail modification regulates DNA compaction

SAQ 34

What effect would neutralising the positive charges on the octamer N-terminal tails have upon the compaction of DNA by H1?

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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 histone proteins

The genes for the histone proteins are very highly conserved across eukaryotes, reflecting their importance in DNA packaging. The histone family consists of five groups of proteins, histones H1, H2A, H2B, H3 and H4. An examination of their amino acid content gives us clues as to how the histones fulfil their role in DNA packaging. Rather like the polyamines in bacteria, these proteins are highly positively charged, with up to 20% of their amino acids being lysine or arginine, the charged side
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The DPS protein compacts the eubacterial chromosome during stress

When an E. coli cell enters into stationary phase, transcription and cell division cease completely. In such cells, the normal chromatin components, such as those described above, are replaced by a negatively charged protein called DPS. The interaction between DPS and DNA appears to be a specialised bacterial adaptation to survive starvation. In normal conditions of growth, the DNA within the bacterial cell is distributed evenly throughout the entire cytoplasm. In stationary cells, how
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7.2 The eubacterial chromosome

Some of the diverse roles of chromatin components can be illustrated by examining the E. coli chromosome. Like most prokaryotes, E. coli has a single chromosome consisting of a single double-stranded circular DNA molecule. There is no nucleus present, but the E. coli DNA is within a discrete entity in the cytoplasm called the nucleoid. The nucleoid contains a multitude of proteins and is in close proximity to the ribosomes, where translation occurs. In addition to
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7.1 Introduction

Until now, we have discussed DNA primarily as a double helix, but in its natural state within the cell it is found packaged as a complex mixture with many different proteins and other components. You have already seen examples of proteins with specific roles to play, such as topoisomerases and the proteins with various DNA binding domains, but in this section we will turn our attention to the proteins that serve to pack and organise the DNA into what we call chromatin.

The packaging of
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2.1.1 Ron revisited

In Case Report 1 you met Ron. Ron is 59 years old, has a BMI of 31 and a central obesity ratio of 0.96, indicating that he is moderately obese, with the fat concentrated in his abdomen, rather than his hips. Clearly a number of factors, some environmental, others relating to Ron's age and all interacting with Ron's genetic constitution, may help to provide an explanation. Let's look at some of these factors in a little more detail. It is clear that Ron enjoys a comfortable lifestyle with litt
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1.5 Obesity

At the time of writing (2004) 20% of the adult population of the UK is classified as obese. The number of obese children has doubled since 1982, 10% of six year olds and 17% of fifteen year olds are now classified as obese. As shown in Table 4, obesity is recognized when the BMI exceeds 30 and occurs quite simply wh
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7.5 Outstanding issues

  • Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes.

  • Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years.

  • It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat
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7.2 Types of active galaxy

  • All active galaxies have a compact, energetic nucleus – an AGN.

  • Seyfert galaxies are spiral galaxies with bright, point-like nuclei which vary in brightness. They show excesses at far infrared and other wavelengths, and have strong, broad emission lines.

  • Quasars resemble very distant Seyfert galaxies with very luminous nuclei. They are variable. About 10% are strong radio sources thought to be powered by jets of material
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7.1 The spectra of galaxies

  • The spectrum of a galaxy is the composite spectrum of the objects of which it is composed.

  • The optical spectrum of a normal galaxy contains contributions from stars and HII regions. An elliptical galaxy has no HII regions and has an optical spectrum that looks somewhat like a stellar spectrum but with rather fainter absorption lines. A spiral galaxy has both stars and star-forming regions, and its optical spectrum is the composite of its st
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Radio-loud AGNs

The second model (Figure 36b) is similar to the first, but now the engine is producing a pair of jets that will eventually end in a pair of lobes, as seen in radio galaxies and some quasars.

Looking at the model from the side, one expects to see narrow lines in the spectrum (but not broad lines) and two
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Introduction

There are a wide range of different interactions between ‘science and the public’. Examples range from visiting a museum, or indulging in a science-related hobby, to reading a newspaper article about a breakthrough in the technique of therapeutic cloning, to attending a protest meeting about plans to build a waste disposal unit near to a residential area. Some such interactions are largely one-way; being a member of the audience for a ‘go-hear’ lecture, visiting a museum or‘‘liste
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10 Risk assessment exercise

After reading this unit you might like to carry out a risk assessment of your office environment or a nearby office and one of the following:

  1. a display-screen user risk assessment;

  2. a laboratory-based risk assessment;

  3. a field-work risk assessment for a proposed field expedition.


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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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7.6 Personal protective equipment

Figure 12
Figure 12 Personal protective equipment (PPE) for working with highly corrosive liquids

In most laboratories, laboratory coats and
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7.5 Control measures

7.5.1 Control measures to avoid exposure

There are four main methods of exposure to chemicals:

  1. Inhalation – This is the main method of exposure to volatile solvents and gases.

  2. Skin absorption – Certain chemicals possess the ability to penetrate through pores of skin (for example, mercury compounds and hydr
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7.3 Disposal requirements

Figure 9
Figure 9 Disposal canister

When carrying out a risk assessment, you must consider disposal requirements. For example, any chemical d
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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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