1 Natural groups

Darwin made extensive observations on a great many creatures, including mammals, and noticed that species fell into natural groups, e.g. lions, tigers and leopards have many similarities, and resemble cats. On the basis of his observations, he was able to place mammals in distinct groups.

His work has continued, and we now recognise that mammals have evolved from a common ancestor, and have branched into many different groups, or ‘Orders’. The animation below shows the different O
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

Learning outcomes

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

  • develop an appreciation of the huge variety of different mammals that exist on Earth today;

  • see how fossil evidence can help us to understand evolutionary history;

  • understand how the structure of DNA can help us to detect differences between different species;

  • apply the techniques of DNA analysis to work out which mammals are most closely related to each other;

  • appreciate t
    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

Acknowledgements

Grateful acknowledgement is made to the following:

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

Figures

Figures 4 and 8 Alber
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

7.1 Introduction

In all eukaryotic cells, proteins that are destined for the plasma membrane or secretion are synthesised in the rough endoplasmic reticulum and enter the Golgi apparatus where they undergo a variety of post-translational modifications, before transfer to the cell surface in secretory vesicles.

  • Which post-translational modifications of proteins occur in which compar
    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

6.2 Endocytosis

Fluid-phase uptake by pinocytosis can be broadly categorised according to the size of the endocytic vesicle and this also relates to how the vesicle is coated (Figure 35). The rate of internalisation is directly proportional to (i) the concentration of extracellular molecules, (ii) the volume enclosed by the vesicle and (iii) the ra
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

4.5 Summary

  1. Targeting sequences at the N-terminus of proteins direct translation across the ER, and act as signals for import to the nucleus, mitochondrion and chloroplasts. Sequences at the C-terminus control traffic through the ER and the Golgi and to peroxisomes.

  2. Glycosylation is directed by signal sequences that act as targets for N-linked glycosylation in the ER and O-linked glycosylation in the Golgi apparatus. Glycosylation and remodelling of polys
    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 Peptide signal sequences

The distinct chemistry of proteins at the N- and C-termini provides protein molecules with two positionally and chemically unique sites for post-translational modifications and with the means to control their spatial and temporal interactions and position. This feature of proteins is crucial for a variety of biological processes from protein degradation to protein sorting for specific cellular compartments. The N- and C-termini of proteins have distinct roles, and we have already emphasised t
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

3.6 Membrane fusion mediated by viral proteins

Until now, we have focused on the transport of material between different intracellular membrane-bound compartments and fusion of cytoplasmic membranes. This type of fusion is endoplasmic fusion. Another type of membrane fusion, called ectoplasmic fusion, is used by enveloped viruses to infect cells (enveloped viruses have an outer phospholipid bilayer). The biophysical and structural studies of viral proteins involved in the processes of membrane fusion provide a foundation 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

3.4 The function of Rab proteins in directing traffic

The SNARE proteins are just one component of the vesicle targeting system. Other participants in this process are the Rab family of GTPases, which regulate traffic between different cellular compartments and which are implicated in directing vesicles to their appropriate target compartments. The Rab family is the largest family of GTPases, with more than 30 members. They are distributed in specific organelles where they mediate the assembly of distinctive groups of proteins. Moreover,
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 Fusion of vesicles with the target membrane

In this section, we shall look at how vesicles fuse with the appropriate target membrane. The targeting of different classes of transport vesicles to their distinct membrane destinations is essential in maintaining the distinct characteristics of the various eukaryotic organelles. Because coat proteins, such as clathrin, are found in different trafficking pathways, it follows that other proteins in the coat must specify the direction of transport of a particular vesicle and its ultimate desti
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 Introduction

In the following sections, we shall describe the sequential steps involved in the movement of vesicles from one membrane to another (see Figure 9). Some of these steps are quite well defined, but for others there are gaps in our knowledge. Although we have emphasised the importance of proteins as cargo, vesicles also transfer membra
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

2.5 The endocytic pathways and lysosomes

Endocytosis is the process by which cells internalise molecules from the outside, and it includes pinocytosis, the uptake of small soluble molecules in vesicles, and phagocytosis, the internalisation of large insoluble particles. These are two ends of a spectrum as seen microscopically, but the receptors, the subsequent intracellular trafficking pathways and the fate of the internalised molecules, vary depending on the cell type and its functions. The endocytic pathway co
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 Exocytosis and the secretory pathways

Exocytosis is the process by which molecules are released to the outside of the cell. This includes the release of proteins to the plasma membrane and the release of secreted molecules into the extracellular fluid. All eukaryotic cells need a system to transport molecules to their plasma membrane, and many cells secrete proteins into the extracellular environment. In addition, cells in multicellular organisms communicate with each other via a variety of signalling molecules, which are
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 Sorting for the basolateral and apical zones of the plasma membrane

Many cells are permanently polarised, and this means that surface proteins are selectively localised to different areas of the plasma membrane, depending on their function. For example, endothelial cells have adhesion molecules on the surface that contacts the basal lamina, but receptors that take up molecules from the blood (e.g. the transferrin receptor – see below) are located on the surface of the cell that is in contact with the blood. Cell surface molecules can normally diffuse latera
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 Glucose metabolism

We are now in a position to draw together the major concepts and components of signalling, and show how they operate in one well-understood system, namely the regulation of the storage or release of glucose in the human body. From this, you will be able to recognize archetypal pathways represented in specific examples, you will be able to appreciate how the same basic pathways can be stimulated by different hormones in different tissues, and you will see how opposing hormones activate separat
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

5.3.3 Phosphorylation of proteins as a means of regulating activity

Phosphorylation is an important mechanism for regulating the activity of many proteins, either switching on or switching off some activity of the protein.

  • What protein that we have already discussed is both positively and negatively regulated by phosphorylation?

  • Src kinase activity is switched on by dephosphorylation 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

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
    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 Chaperones help polypeptides to fold

We have seen how steric restrictions and energetic considerations specify preferred polypeptide conformations and ultimately determine a protein's three-dimensional structure. It is possible, of course, that there may be more than one energetically favourable conformation for a polypeptide. This is particularly true for large polypeptides. For a protein with a specific function in the cell, misfolding will affect its activity. Indeed, the misfolded protein may actually have some aberrant unde
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

1.7 Summary of Section 1

  1. Protein structure is described in terms of four levels of organisation: primary, secondary, tertiary and quaternary.

  2. The primary structure of a protein is the sequence of amino acids of which it is composed and ultimately determines the shape that the protein adopts.

  3. The peptide group formed between two amino acid residues has a rigid planar structure and these planar groups can rotate around the Cα–N and Cα<
    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

1.6 Fibrous proteins

Most of the proteins described so far have been globular proteins. There are, however, some distinctive features that characterise fibrous proteins and we present here a general overview of these. Elongated fibrous proteins frequently play a structural role in the cell. They do not readily crystallise but tend to aggregate along their long axis to form fibres. X-ray diffraction studies of these fibres, in contrast to analysis of protein crystals, provides only very limited information on the
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