Produced by the Centre for Genetics Education. Internet: http://www.genetics.edu.au

Your (Genetic) Book of Life

Our genetic information, sometimes described as the ‘Book of Life’, can be thought of as being made up of two volumes. Each volume of the book is contributed to a person by one of their parents.

So in your ‘Genetic Book of Life’ (Figures 1.1 & 1.2):

• One volume was inherited from your Mum and one from your Dad
• Both volumes contain 23 chapters each, and together are equivalent to the 23 pairs of chromosomes present in your body cells that contain your genetic information
• The 23 chapters (ie. chromosomes) are made up of a variable number of pages (ie. genes)
• Some of the chapters contain many pages; others only a few. In your cells, some chromosomes contain many thousands of genes; others perhaps only a few thousand (Figure 1.2)
• Careful examination of the words on the pages show that all the words are made up of only three of the four possible letters (triplets): A, T, C & G. In your cells, these letters are the chemical components of DNA

Figure 1.1: Your Genetic Book of Life part 1

Figure 1.2: Your Genetic Book of Life part 2

Just like we read the words on a page to understand what the author is telling us, the body reads the triplets of words in the DNA (our genetic information) to tell us to grow and develop and guide how our cells work in our bodies.

We may also read a book in different circumstances and similarly, our genetic information is ‘read’ by the cells in a background of our personal internal and external environments. This includes our diet, the chemicals that we are exposed to and the other genes in the cells. Books also get older and the pages become brittle or the words are harder to read, our genes are affected by the ageing process. It is important to remember however that our environment also plays a major role in how we develop and how our bodies work by interacting with the genetic information (see Genetics Fact Sheet 11).

Your genetic make-up in more detail

Our bodies are made up of millions of cells. Each cell contains a complete copy of a person’s genetic plan or blueprint. This genetic plan is packaged in the cells in the form of genes.

Chromosomes can be thought of as being made up of strings of genes. The chromosomes, and therefore the genes, are made up of the chemical substance called DNA (DeoxyriboNucleic Acid). The chromosomes are very long thin strands of DNA, that are coiled up like a ball of string as shown in Figure 1.3.

Figure 1.3. Chromosomes are like strings of genes

The chromosomes containing the genes are located in the nucleus (or control centre) of our body cells (Figure 1.4). An exception is our red blood cells, which have no nucleus and so don’t have any chromosomes.

Figure 1.4: Diagram of a human cell

Another place in the cell where DNA is found is in the cell in very small compartments called mitochondria that are found randomly scattered in the cytoplasm outside the nucleus.

• The mitochondria are the energy centres of the cell. Mitochondria contain genes too, although the mitochondrial DNA is one long string of genes and is not arranged as chromosomes
• The genes in bacterial DNA are also arranged in a long string, giving rise to the theory that the mitochondria originated from bacteria that invaded a human cell long ago in evolution. Further information on mitochondria can be found in Genetics Fact Sheet 12

All of the DNA in the human cell (in the nucleus and the mitochondria) makes up the human genome.

Our chromosomes

There are 46 chromosomes in the nucleus of our body cells.

• Of these, 23 came through our mother’s egg and 23 came through our father’s sperm
• When the egg and the sperm join together at the time of conception, they form the first cell of the baby
• The baby’s body cells now have 46 chromosomes, made up of 23 pairs, just like the parents (Figure 1.5)

The genes in the mitochondria are also important for the fertilised egg to divide and grow and for development to occur

• The vast majority of our mitochondria are in the egg from which we arise as the sperm contributes only a very small number of mitochondria to the fertilised egg
• Therefore the genetic information passed on to a baby in the mitochondria largely comes from the mother only, while the genetic information in the nucleus comes from both mother and father

Figure 1.5: At conception the sperm and egg combine

As we age and grow, our cells are continually dividing to form new cells. During this division process, each of the long thin chromosomes coils up tightly, so that each of the 46 individual chromosomes become rod-shaped structures and can be seen when using a microscope (Figure 1.6).

Figure 1.6: Normal chromosome picture (karyotype) from a male 46,XY
(SEALS Genetics Prince of Wales Hospital, Randwick)

In the laboratory, the chromosomes are coloured (stained) with special dyes to produce distinctive banding patterns (Figure 1.6).

• Each chromosome has been arranged in pairs and in order of size
• At one point along their length, each chromosome has a constriction, called the centromere
• The centromere divides the chromosomes into two ‘arms’: a long arm and a short arm

The chromosomes are numbered from the largest (chromosome number 1) to the smallest (chromosome number 22): these numbered paired chromosomes are called autosomes. Figure 1.7 shows a drawing of one of these autosomes (chromosome number 7), illustrating its characteristic banding pattern and the centromere.

There are also two chromosomes that have been given the letters X and Y: these are the sex chromosomes. The X chromosome is much larger than the Y chromosome.

Women have

• 46 chromosomes (44 autosomes plus two copies of the X chromosome) in their body cells: 46,XX
• 23 chromosomes (22 autosomes plus an X chromosome) in their egg cells

Men have

• 46 chromosomes (44 autosomes plus an X and a Y chromosome) in their body cells: 46,XY
• 23 chromosomes (22 autosomes plus an X or Y chromosome) in their sperm cells

Figure 1.7: The chromosome 7 pair showing the banding pattern

Our genes

The DNA making up each chromosome is usually coiled up tightly. If we imagine it stretched out, it would look like beads on a string (Figure 1.3).

• Each of these beads is called a gene
• Each gene is a piece of genetic information
• Thousands of genes make up each chromosome

Since the chromosomes come in pairs, there are two copies of the genes. The exception to this rule applies to the genes carried on the sex chromosomes: the X and Y.

• Since men have only one copy of the X chromosome, they have only one copy of all the genes carried on the X chromosome
• Women have two copies of the X chromosome in their cells and so they have two copies of the genes carried on the X chromosome
• So that men and women have the same number of X chromosome genes ‘active’ in their cells, in women one of the X chromosomes is ‘switched off’ or inactivated. Genetics Fact Sheet 14 describes this process in more detail
• The genes on the Y chromosome are responsible mainly for the development of ‘maleness’ only

The number of human genes

There are about 20,000 genes located on one of the 23 chromosome pairs found in the nucleus or on long strands of DNA located in the mitochondria. Each of these genes make up the human genome.

Information about the location and the sequence of ‘letters’ in each gene is stored in a database that is publicly accessible. This information was largely collected by the work done as part of the Human Genome Project (see Genetics Fact Sheet 24).

Although the project’s completion was celebrated in April 2003, and understanding how the letters are arranged in the genes (sequencing) is essentially finished, the exact number of genes in our genome is still unknown.

Moreover, finding out what the information in our genes tells our bodies to do will still take many years.

Our genes have an important role in our cells

Each gene has its own specific location on the chromosome and is a piece of the genetic material that does one particular job.

All of the 20,000 or so genes contain a different `packet’ of information necessary for our bodies to grow and work. Our genes also contain the information for how we look: the colour of our eyes, how tall we are, the shape of our nose, etc. The information is in the form of a chemical (DNA) code (the genetic code) (see Genetics Fact Sheet 4).

• The DNA code is made up of very long chains of four basic building blocks (nucleotide bases): Adenine (A) and Guanine (G), and Thymine (T) and Cytosine (C)
• A chromosome consists of two of these DNA chains running in opposite directions; the bases pair up to form the rungs of a ladder twisted into the now famous double helix (Figures 1.8 & 1.9)
• Pairing of the bases follows strict rules: base A can only pair with base T, and vice versa; and base G can only pair with base C, and vice versa. Roughly three billion of these base pairs of DNA make up the human genome
• The DNA message is in fact made up of three-letter ‘words’ composed of combinations of these chemical ‘letters’ A, G, C and T

In summary, genes can be defined as segments of DNA that issue instructions to the cells by these chemically coded ‘messages’ to make a product (protein) that the cells can use. There may be hundreds, or even thousands, of three-letter words in each gene message (Figure 1.8).

• That is why the DNA that makes up the genes is often called ‘coding DNA’

Figure 1.8: The information in the genes

Figure 1.9: The DNA helix

The DNA ‘string’ between the genes is often called ‘non-coding DNA’. It was originally referred to as ‘junk DNA’ as it appeared that this DNA did not contain the information for gene products that the cells use and produce.

• It is increasingly clear that the non-coding DNA has a very important role to play
• That role is still largely unknown but is likely to include regulating which genes are ‘switched on’ and which are ‘switched off’ in each cell
• Studies of this non-coding DNA are useful for forensic investigations and determining biological relationships (see Genetics Fact Sheet 22)

Genes contain recipes for the body to make proteins - the Book of Life is like a recipe book for our bodies

The DNA message in the genes is like a recipe for an essential component of the body, such as a protein. Chains of the protein building blocks (amino acids) called polypeptides, fold into more complex structures. These structures (proteins) have a specific function or role in cells.

• Proteins may be made up of a number of different polypeptides
• That can mean that a number of different genes are concerned with coding for that protein

The Genetic Book of Life is made up of recipes for proteins - it is like a recipe book for our bodies. In this Book, each three-letter word (triplet) tells the cell to produce a particular amino acid, or to start or stop reading the words.

• The sequence of three-letter words in the gene enables the cells to assemble the amino acids in the correct order to make up the protein or polypeptide
• The genetic code for each amino acid is virtually identical across all living organisms
• When the instructions in a gene are to be ‘read’ because the cell needs to make a particular protein, the DNA making up the gene unwinds and the message is ‘translated’ into a chain of amino acids
• When the whole message has been translated, the long chain of amino acids folds itself up into a distinctive shape that depends upon its sequence, and is now known as a ‘protein’

Some of the proteins form building blocks for structures within the cells such as the protein called keratin, from which hair is made; others are called enzymes which help carry out chemical reactions, such as digesting food. Others form communication networks within and between cells.

• Each gene message can be ‘read’ by the cell in a number of different ways
• Each gene can provide a message to the cell to make two or three different proteins
• That is why the number of proteins known to exist in the cells is more than the number of genes

Not all our genes are ‘switched on’ all the time

Our bodies have many different types of cells such as those in the skin, muscle, liver and brain.

• While all of these different types of cells contain the same genes, each cell requires particular proteins to function correctly
• Therefore, different genes are active in different cell types, tissues and organs, producing the necessary specific proteins
• Not all the genes in the cell are ‘switched on’ (active) in every cell

For example, the genes that are active in a liver cell are different from the genes that are active in a brain cell. This is because these cells have different functions and therefore require different genes to be active.

Some genes are only switched on during the development of the baby. After birth they are no longer needed to be active as their ‘job’ has been completed.

Changes to the genetic code

When the code in a gene is changed in some way, there is a different message given to the cells of the body. These changes include a variation in the sequence of letters in the message or a deletion or insertion of either individual letters or one or more whole words. Even a deletion of the whole gene can occur.

Changes to genes can occur for a variety of reasons including exposure to radiation or certain chemicals. Ageing however, is one of the most common causes of genetic changes. As our bodies age, our cells need to be continually replaced: the cells (and their genetic make-up) are copied over and over again as time goes by. Sometimes mistakes occur in this copying process, and changes in the genes build up in our cells.

Other changes to genes do not seem to make any difference to the way the message is read or to its meaning to the cell. These types of changes in genes are quite common. Nevertheless these ‘neutral’ gene changes can sometimes be associated with an increased susceptibility to a genetic condition, for example, schizophrenia (Genetics Fact Sheet 58).

Some gene changes make the gene faulty so that the message is not read correctly or is not read at all. A change in a gene that makes it faulty is called a mutation. A faulty (mutated) gene may cause a problem with the development and functioning of different body systems or organs and result in a genetic condition (see Genetics Fact Sheets 2, 4 & 5).

We are all born with several faulty gene copies that usually cause no problem

Importantly, some faulty (mutated) genes may not cause any problem. We are all born with several faulty genes. Indeed having a faulty gene can be beneficial as discussed in Genetics Fact Sheet 5.

When faulty genes are contained in the egg or sperm cells, they can be passed on to children (inherited). The faulty gene may be in these cells because that person inherited it from one or both parents. Sometimes, however, a mutation can occur for unknown reasons in an egg or sperm cell and may cause a genetic condition. An individual conceived from that egg or sperm cell will be the first in the family to have the condition but which may then be passed down to his or her children and future generations. Genetics Fact Sheets 8, 9, 10 & 11 discuss the patterns of inheritance of these faulty genes in more detail. Fact sheets 4 & 5 discuss changes to the genetic code in more detail.

Other Genetics Fact Sheets referred to in this Fact Sheet: 2, 4, 5, 8, 9, 10, 11, 12, 14, 22, 24, 58