BASIC CELL BIOLOGY

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).

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).

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.

Nucleolus

The nucleolus (Figure 1.4) is a morphologically distinct area within the nucleus which is involved in the production of Ribonucleic Acid (RNA).

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.

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.

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.

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).

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.

Lysosomes

Lysosomes are organelles that break down bacteria and other foreign bodies, as well as recycling worn out cell components (Figure 1.10).

Peroxisomes

Peroxisomes are responsible for the detoxification of foreign compounds and the oxidation of fatty acids (Figure 1.11).

CHROMOSOMES

Each of the trillions of cells in the body, with the exception of red blood cells, has a nucleus. Within each nucleus are structures called chromosomes. Chromosomes are not usually visible under a light microscope, but when a cell is about to divide, the chromosomes become denser and can be viewed at this stage.

Chromosome structure

A chromosome is composed of DNA and proteins and includes structures that enable it to replicate and remain intact (see Figure 1.12). During cell division, chromosomes have a constriction point termed a centromere. The centromere divides each chromosome into two sections or 'arms'. The long arm is referred to as the q arm and the short arm as the p arm (p for petite).

The location of the centromere gives the chromosome its characteristic shape and can be used to describe the location of specific genes.

Telomeres

Telomeres are distinctive structures found on the end of each arm of the chromosome. They are made up of the same short sequence of DNA, which is replicated about three thousand times. The function of the telomeres appears to be twofold.

1. They protect the chromosome by 'capping' off the ends to prevent them from sticking or joining onto other chromosomes.

2. Due to the way that chromosomes are replicated, the ends of the chromosomes are not copied. Telomeres shorten during every cell replication, but the loss of DNA within the telomeres protects against loss of essential DNA within the chromosome itself.

Chromosome numbers

Chromosomes exist in pairs. Although not actually joined together, each pair has a characteristic length. The human cell nucleus has 23 pairs of chromosomes; in other words, 46 individual chromosomes. One chromosome from each pair is inherited from the father and one from the mother. Twenty-three individual chromosomes are inherited from each parent. The total number of chromosomes in each cell is called the diploid number (diploid 46) while the number of pairs is called the haploid number (haploid 23). Of the 23 pairs of chromosomes, 22 pairs are termed autosomes and do not differ between the sexes. For ease of identification, these autosomes are numbered from 1 to 22. The chromosomes are numbered according to length, with chromosome number 1 being the longest and chromosome 22 being the shortest. The remaining two chromosomes are known as the sex chromosomes. These two chromosomes are not numbered but are known as the X chromosome and the Y chromosome. The Y chromosome determines maleness. A female will have two X chromosomes while a male will have one X and one Y chromosome.

Karyotype

The chromosome complement within the nucleus is called a karyotype. Charts called karyographs (see Figure 1.13) display chromosomes in pairs in size order. The 22 paired autosome...