Genetics, revised edition. Karen Vipond
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– National Institutes of Health, Bethesda, Maryland, USA, for their permission to reproduce illustrations in this book.
Most of the illustrations in this book have either been sourced from the National Library of Medicine or are the Author’s own. However, if any copyright material has been used inadvertently without permission or acknowledgement I, as the Author, apologise and will make any necessary correction(s) at the first opportunity.
Dedication
To Joshua and Libby who have inherited half my genes, and to Phil who also contributed half his genes. Also to the rest of my family with whom I share the same genetic material.
| LEARNING OUTCOMES |
The following topics are covered in this chapter:
• Cellular structure and function;
• Chromosomes;
• The cell cycle;
• Deoxyribonucleic acid (DNA);
• Protein synthesis;
• Mitochondrial DNA.
INTRODUCTION
The activities that occur within cells give us an understanding of how human traits are inherited. Knowledge of cellular function gives rise to the understanding of how the body works. The human body is made up of trillions of cells, many of which have specialised functions. Despite this, all cells share certain features:
• cells arise from the division of pre-existing cells;
• cells interact, they send and receive information;
• cells produce proteins for growth repair and normal body functioning;
• cells contain all the genetic instructions for the body.
All cells in the body behave in this way apart from red blood cells. Red blood cells are not considered to be true cells by the time they reach the blood stream as they do not contain a nucleus. Cells are the basic building blocks of all living matter.
CELL STRUCTURE
Cells have many parts, each with a specialised function. Any structure within the cell that has a characteristic shape and function is termed an organelle. Most organelles are too small to be seen through a light microscope but can be seen with an electron microscope (see Figure 1.1).
Figure 1.1 Cell structure
Plasma membrane
This is the outer lining of the cell. It is composed of a bilipid layer through which certain molecules can enter the cell (endocytosis) and wastes can exit (exocytosis).
Figure 1.2 The outer lining of the cell
Nucleus
The nucleus functions as the control centre of the cell (Figure 1.3). It contains DNA (Deoxyribonucleic Acid) which is the cell’s genetic material. A double membrane separates the contents of the nucleus from the rest of the cell.This nuclear membrane (also called the nuclear envelope) is perforated by nuclear pores.
Figure 1.3 Nucleus
Nucleolus
The nucleolus (Figure 1.4) is a morphologically distinct area within the nucleus which is involved in the production of Ribonucleic Acid (RNA).
Figure 1.4 Nucleolus
Cytoplasm
Cytoplasm is a gel-like fluid that contains all the organelles and the enzymatic systems which provide energy for the cell.
Cytoskeleton
The cytoskeleton is a network of fibres made from the protein tubulin (Figure 1.5). This provides the structural framework of the cell and functions in cellular shape, cell division and cell motility, as well as directing movement of the organelles within the cell.
Figure 1.5 Cytoskeleton
Endoplasmic reticulum
The endoplasmic reticulum is an organelle that processes the molecules made by the cell (Figure 1.6). The endoplasmic reticulum transports these molecules to their specific destinations.
Figure 1.6 Endoplasmic reticulum
Ribosomes
Ribosomes are organelles that provide the sites for protein synthesis (Figure 1.7). Ribosomes are attached to the endoplasmic reticulum as well as freely floating in the cytoplasm.
Figure 1.7 Ribosomes
Golgi body
The Golgi body is a structure that packages the molecules produced by the endoplasmic reticulum ready for transport out of the cell (Figure 1.8).
Figure 1.8 Golgi body
Mitochondria
Mitochondria are organelles that convert energy gained from food into a form that the cell can use (Figure 1.9). Adenosine triphosphate (ATP) is the main source of energy used by the cell. These organelles have their own genetic material and can make copies of themselves.
Figure 1.9 Mitochondria
Lysosomes
Lysosomes are organelles that break down bacteria and other foreign bodies, as well as recycling worn out cell components (Figure 1.10).
Figure 1.10 Lysosomes
Peroxisomes
Peroxisomes are responsible for the detoxification of foreign compounds and the oxidation of fatty acids (Figure