A cell (Fig. 2.1) is the basic building unit of life. All animal cells are similar in their components, although they may have different functions. Similar cells are grouped together to create tissues which carry out specific functions. For example, there are four main types of tissue that make up the human body: muscle, epithelial, nervous and connective tissues.

Fig 2.1 The human cell

Activity

Recall and refresh

Read your biology lecture notes or Waugh and Grant (2010) (see References) or a similar textbook to refresh your memory of the components of a human cell.

NMC Domain 3: 3.2

All cells have the ability to carry out complex tasks, such as the uptake of nutrients and converting this into energy. They are also able to replicate in order to replace damaged or old cells. All the instructions needed to build and maintain the body’s functions are contained in the DNA. Each instruction is carried on a unique piece of DNA called a gene. Each gene codes for particular proteins which control the function and structure of the individual cell. All the genetic codes make up what is known as the human genome. It is a bit like an instruction manual and the genes are chapters in the book with specific information. Genes can be turned on or off depending on the job a cell needs to do.

Each cell contains a complete copy of our genome in the form of 23 separate pairs of chromosomes (one set from each of our parents) (Fig. 2.2). For each chromosome and each gene, we have two slightly different copies.

Fig 2.2 Human chromosomes

DNA is made up of individual molecules called nucleotides, which are in turn made up of a sugar (deoxyribose), a phosphate and a nitrogenous base. The DNA molecule is comprised of two chains of nucleotide bases, arranged in a double helix (Fig. 2.3). There are four bases which are grouped into two types: purines (adenine (A) and guanine (G)) and pyrimidines (thymine (T) and cytosine (C)). Each base is paired up with another base: A pairs with T and C pairs with G. Each base is a slightly different length which gives the double helix its twisted shape. The bases can occur in any sequence. It is the sequence of the bases that makes up the instruction, a bit like the words of an instruction manual. Depending on the sequence of the bases, a particular protein will be produced. The proteins in turn will enable the cell to function in a particular way, including cell replication.

Fig 2.3 DNA: the double helix

Cell cycle

From the time of conception, all of our cells continue to multiply in order for us to grow into an adult. Once we reach adulthood our cells only divide when there is need to repair and replace old damaged cells and to reproduce. To do this, cells go through a process called the cell cycle (Fig. 2.4). The phases of the cell cycle are:

• G0: resting phase, not in cell division.

• G1 and G2: the cell builds up energy and prepares for the next stage.

• S: DNA doubles by splitting the double helix.

• M: mitosis where the cell splits into two cells.

Fig 2.4 The cell cycle.

(Reproduced with permission from Kearney N, Richardson A (2005) Nursing patients with cancer: principles and practice. Churchill Livingstone (Fig. 5.4, page 78))

By replacing old or damaged cells in a controlled manner, the number of cells in the body remains fairly constant. When cells divide, one cell becomes two. One of these will mature and take on a highly specialised function. The other will rest until it is needed to replicate once again, this is dependent on the cell type. Some cells rarely divide once they are mature; for instance, liver cells divide once every 1 or 2 years. Some cells never divide once they are specialised and are irreplaceable (staying in the G0 phase), such as nerve cells. Some cells can replicate on demand in response to physiological need, for example the endometrium, mammary glands, etc. Others continually replicate, such as blood cells, skin and hair cells, germ cells, bone marrow and the cells lining the stomach (which multiply at least twice a day).

However, all cells have a limited life span, although this varies depending on the cell function and type. Most normal cells divide about 40–60 times before they die of old age. The life span of a cell is controlled by the ends of chromosomes known as telomeres. Each time the cell divides, the telomeres get shorter and eventually the cell cannot divide and dies.

Normal cells only divide when they are needed, such as during growth (from conception to adulthood) and to replace old and damaged cells. This replication is carefully controlled by a number of checkpoints (see Fig. 2.4), which ensure that cell division only occurs when really necessary and occurs accurately. Certain genes control cell division, known as proto-oncogenes. These genes code for proteins and growth factors which signal to the nucleus of the cell to turn the cell cycle on, a bit like an accelerator of a car. There are four main groups of proteins produced by the proto-oncogenes: growth factors, growth factor receptors, signal transducers and nuclear proto-oncogenes and transcription factors.

There are other genes that turn cell division off by coding for proteins that slow everything down, a bit like the brake of a car. These are known as tumour suppressor genes. One of the key tumour suppressor genes is known as the ‘guardian of the genome’ or ‘p53’. This controls whether the cell goes into G0 to rest or starts the active cell division process by entering G1. The tumour suppressor genes and the proto-oncogenes work together in order to keep the cell cycle regulated and ensure that cells only divide when they are needed.

During the cell cycle, natural errors occur and the DNA becomes damaged. Normal cells have the ability to repair small amounts of damage by activating repair genes which make the necessary corrections. However, if the damage is too great then the cell will destroy itself by committing cell suicide or apoptosis. This prevents the mutation being passed on to future cells and causing cancer. This may explain why some people might disregard health promotion messages and base their health belief in personal experience: ‘My granny smoked 50 a day until she was 95 years old and she didn’t develop cancer!’ Smoking may well have caused damage to granny’s DNA but she may have had very good repair genes or genes that initiated apoptosis, removing the damaged DNA – she did not, therefore, develop cancer.

Unfortunately there is no way of knowing which individuals have mutated or missing repair genes and/or whose cells lack the ability to recognise and to commit cell suicide. Therefore, health promotion is extremely important. We need to prevent or minimise damage to DNA/genes in the first place by healthy lifestyle choices. Second, we need to raise awareness so that people know the signs and symptoms of cancer, ensuring early diagnosis.