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5.1 Introduction

In some ways, proteins can be thought of as molecular machines, that through evolution have become highly specialised and efficient. Despite the somewhat static representations of proteins that you have met so far, proteins are in fact dynamic molecules. Not only are there internal movements, with conformational changes that are integral to protein function and regulation of function, but proteins, by virtue of their specific interactions with other cellular components, are essential to all t
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4.2 Amino acid sequence homologies and why they occur

Consider two genes encoding proteins that have 50% of their amino acid sequence in common.

  • How can this sequence homology be explained in terms of evolution?

  • The most parsimonious explanation is that the similarities result from the fact that the two organisms share a common evolutionary past and that the genes encoding
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3.5 Summary of Section 3

  1. Protein domains allow segregation of different functions in the same protein. They can have a binding function, a structural function or a catalytic function.

  2. Binding domains mediate interactions between proteins of related function (such as those in a signalling cascade) and often are important in regulation of activity. Interactions via these binding domains are often dependent on the phosphorylation state of one of the binding partners. Exa
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2.4.2 Lipid-linked proteins and lipoproteins

Lipid-linked proteins are proteins that have been covalently modified by addition of one or more lipid groups. Note that the term lipoprotein, though sometimes used to describe lipid-linked proteins, is strictly applicable only to those proteins that associate with lipids non-covalently. These proteins have quite distinct functions. Lipoproteins serve to transport triacylglycerols and cholesterol in the blood plasma. We will not be discussing them any further at this point.

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2.4.1 Glycosylation

Glycosylation of a protein entails the covalent attachment of carbohydrate groups (typically oligosaccharides) and the resulting modified protein is called a glycoprotein. Covalent attachment of sugar residues to proteins occurs in the endoplasmic reticulum (ER) and Golgi apparatus. The oligosaccharide chains usually contain less than 15 sugar residues but are very diverse and are often branched. They are linked to the protein component via either the –OH groups of serine and threoni
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2.4 The covalent modification of proteins

Many proteins are modified by the covalent linking of groups that can affect their function and/or localisation in the cell. Such covalent modifications occur after synthesis and folding of the polypeptide component. The main types of covalent modification and their functions are listed below.

  1. Methylation/acetylation of amino acids at the N-terminal tails of histone proteins in eukaryotes can affect the structure of chromatin and ultimately gene
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1.4.4 Covalent cross-linkages stabilise protein structure

Proteins that are secreted by the cell, or are attached to the extracellular surface of the plasma membrane, can be subject to more extreme conditions than those experienced by intracellular proteins. Often, covalent cross-linkages stabilise these proteins by connecting specific amino acids within a polypeptide or between polypeptide chains in multisubunit proteins (see below). Typically such a linkage will be a covalent sulfur–sulfur bond which forms between the –SH groups of two cystein
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1.4.1 Motifs and supersecondary structures

Supersecondary structures or motifs are particular arrangements and combinations of two or three secondary structures, often with defined topology (or connectivity). Table 3: view document describes some of the most common of these.

The term ‘motif’ is also used to describe a consensus sequence of amino acids, i.e. a partial sequence c
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1.2. Getting started

Registering is free and only requires a user name, email address and password. Sign up to iSpot.(Right-click on the link to open it in a new window or tab). Watch the video on how to register.


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8.6 Line spectra: Activity 8 Quasar redshifts

Activity 8: Quasar redshifts

Read Peterson section 1.3.5 (pages 16 and 17) by clicking the link below.

8.4 Line spectra: line flux and equivalent width

In general, astronomical objects emit both continuous emission and lines superimposed on this continuous emission. The equivalent width is a useful way of describing the relative strength of a line compared to the continuous emission at nearby wavelengths. The flux level of the continuous spectrum is called the continuum level, and at an emission line the spectrum rises above this level, while at an absorption line the spectrum dips below this level, as Author(s): The Open University

8.3 Line spectra: ions and spectral lines

For obvious reasons optical astronomy developed earlier than radio and X-ray astronomy, and astronomers are able to learn many things from analysis of optical emission. Just as in the radio band, optical pictures, i.e. the spatial distribution of emission, can be informative. Even when a source is not spatially resolved, astronomers can still deduce some information about what it might look like close-up. These ‘visualizations’ of what is happening in a particular source result from analy
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7.9 Compton scattering

Electromagnetic radiation interacts strongly with electrons. If a photon encounters an electron, there is a high probability that a scattering interaction will occur. In the low-energy non-relativistic regime, i.e. where h Author(s): The Open University

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7.2 Free-free radiation

The blackbody spectrum is emitted when thermally emitting matter is optically thick. Optically thin matter can also emit thermal radiation. Whenever a charged particle is accelerated it emits electromagnetic radiation. When the acceleration is due to the electric field of another charged particle the emitted radiation is called free-free emission or bremsstrahlung. (Bremsstrahlung is a German word meaning ‘braking radiation’.) The radiation emitted by an optically thin, ther
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5 Distances in extragalactic astronomy

Recall Hubble's law:

This relationship between redshift, z, of a distant galaxy which is given by

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3.3 Luminosities

The issue of whether QSO redshifts are of cosmological origin was unambiguously settled by the work illustrated in Figure 5 (particularly the two leftmost panels). When sensitive enough observations are made, the galaxies in which the quasars reside can be detected. The relatively faint emission from the surroundi
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3.2 QSO spectra

It only became apparent that these quasi-stellar objects were not stars when their spectra were examined. At first astronomers could not interpret their spectra because the spectral lines did not appear at appropriate wavelengths for atoms of any known chemical element. The spectrum shown in Figure 4 provided the
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3.1 AGNs

It is clear that the objects studied by Fath and Seyfert, such as those shown in Figure 1 and Figure 2, are bright nuclei at the centres
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6 Inheritance of more than one pair of contrasting characters

We have considered the inheritance of one pair of contrasting characters that involves the segregation of the two copies of one gene in maize. A single chromosome carries many genes, of the order of 2 000 to 4 000 in each human chromosome, each carrying its own information, and each with its specific location, its own locus, on the chromosome. In a pair of homologous chromosomes, there will be many pairs of genes strung along their length.

These observations raise some intriguing questi
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1.3.4 Predicting the outcome of crosses

By knowing the pattern of inheritance of genes as described above, it is possible to make some predictions about the phenotypes and genotypes of each generation in breeding experiments. This section considers some examples of such predictions.

First consider whether it is possible to determine the genotype for certain characters, such as grain colour, from observing an organism's phenotype.

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