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What are genes?
Genes are the coded instructions for the appearance and make up of a living organism. The genetic inheritance we receive from our parents at the moment of our conception fixes much of our later development, determining hereditary characteristics as varied as whether we will have brown eyes or suffer from a life threatening disease such as cystic fibrosis.
Our bodies are made up of over 100 million, million cells. Each cell is a highly organized place. The cell has often been compared to a factory with workers, assembly lines and powerhouses. Raw materials are taken in and new products built. Some are used in the cell itself, while others are packaged and exported. (The cell breaks down and recycles material.) At its heart lies a nucleus; the central command centre, which oversees the cell’s activity every second of the day. Each nucleus houses two sets of 23 chromosomes; 46 in all. One of each pair is inherited at conception from each of your parents. 22 of the 23 pairs of chromosomes appear identical: these are the autosomes. The remaining pair, the sex chromosomes, differ between males and females. Females carry two X chromosomes and males carry one X and one Y chromosome. (See Inherited genetic disorders).
The core of each chromosome is an elongated string of DNA, or deoxyribonucleic acid. DNA is the messenger of inheritance and the stuff that genes are made of. If all the DNA in the 46 chromosomes of one human cell DNA carries a blueprint, a set of instructions needed to build a living being and to control the day-to-day activities of each cell in your body. We call these coded instructions genes; they are plans to build proteins, a group of molecules that are the workhorses of the cell. Many tens of thousands of different proteins go to make up the structure of your body and to ensure that it functions properly. Together, their combined action defines what we call life, and their normal function ensures our continuing health. Imagine that you have just eaten a meal before you sit down to read the Teacher’s Pack and that you are suffering from a sore throat. As you read this pack, your immune system is making antibodies to combat your sore throat, and your fingernails are growing, imperceptibly perhaps, but the cells are making new proteins. Without being conscious of the fact, you are digesting your lunch and this requires enzymes to help breakdown the food. The red blood cells in your bloodstream only last around 120 days, so your body is busy replenishing your supply. All this and more is the effect of your body’s cells reading instructions written in your genes and translating those instructions into the proteins that you require to live. Proteins are built from smaller sub-units called amino acids. There are about 20 different amino acids, and these can be assembled into an almost infinite number of sequences to build tens of thousands of different proteins. To build a protein, the cell must know the order in which the amino acids should be joined up. The function of a gene is to specify the sequence of the amino acids, and these detailed instructions are carried, in coded form, on the DNA molecule. To understand how this is possible, it is necessary to understand DNA’s own structure. In 1953, the English physicist Francis Crick and the American biologist James Watson showed that DNA was in the form of a double helix. Two intertwined sequences form a ladder of bases along the molecule. The inherited genetic instructions lie in these sequences of bases running along the DNA double helix. The genetic code is written in triplets of bases, like words of three letters - AAA, for example, or TAC. Each triplet specifies a particular amino acid - because the four letters can be arranged into 64 different triplets, there is plenty of capacity to code all 20 amino acids. The code for a particular protein is a specific length of the DNA called the ‘gene’ for that protein. A copy of the genetic message is made from the DNA and then passed to the cytoplasm of the cell where it is decoded to produce a protein. The core of each chromosome is an elongated string of DNA, or deoxyribonucleic acid. DNA is the messenger of inheritance and the stuff that genes are made of. HOW IS DNA REPLICATED?Cells reproduce simply by dividing in two. After an egg has been fertilized
by a sperm, in the course of sexual reproduction, the result is a single
cell. Each cell has its own unique set of genetic instructions inherited
in equal parts from the mother and the father. The fertilized egg, guided
by its own unique set of genetic instructions, starts to divide. It
first forms two cells, which divide to form four, which divide to form
eight, and so on. The end result, is an adult human being with over
a hundred million million cells. With a few exceptions, such as the
germ-line cells, which contain just one set of chromosomes, each of
our cells contains the same genetic information as this fertilized egg.
Watson and Crick’s double helix discovery also neatly explains
how messages are copied from one generation to the next. The two complementary
strands are untwisted and the necessary building blocks to construct
a new partner are SummaryOur bodies are made up of over 100 million million cells. Most of these cells contain 23 pairs of chromosomes with one of each pair coming from either parent. The chromosomes in turn are made up of DNA. We can imagine a ‘gene’ as a link in the DNA chain - each gene carries instructions for a particular characteristic. In this way individual characteristics are passed down through generations, as we receive copies of our parents’ genes. We all have about 100 000 pairs of genes, which is part of the reason why we are all unique, unless of course we are one of identical twins. PATTERNS OF INHERITANCEIn The Gift, RYAN refers to the ‘Mendelian pattern of inheritance’ as he rehearses his arguments with ANNIE for genetic testing for Friedreich’s ataxia. Our modern understanding of inheritance starts in the 19th century with the Augustine monk Gregor Mendel. In his spare time he studied inheritance in many plant species. He published his discoveries on his work with sweet pea plants in an obscure scientific journal, The Transactions of the Brunn Natural History Society. For centuries prior to Mendel’s birth, farmers knew that if they bred from the cows that gave the most milk, or from the wheat with the largest grains, they were likely to get these useful characteristics again. The results of breeding, however, did not always turn out just as expected. Sometimes the offspring had useful characteristics; at other times they did not. Useful characteristics that did not appear in the ‘children’, sometimes reappeared in ‘grandchildren’. In 1865, Mendel provided the first scientific explanation for the puzzle of inheritance through a series of experiments carried out by breeding pea plants with parents of different types or varieties. Other people had carried out similar work before, but had been unable to identify any clear patterns from their observations. This was mainly because they had looked at the overall appearance of the plants, which seemed to show that the offspring were a ‘blend’ of features from both parents. In the same way clear patterns of inheritance are hard to see in humans; most people seem a mix of their parents. However, some clear patterns can be traced, especially with inherited diseases. Mendel’s breakthrough was to focus his attention on a few carefully chosen characteristics such as the shape of the seeds and the height of the plant. By crossing peas with different characteristics, Mendel demonstrated the effects of the gene for tallness were concealing the gene for shortness. He therefore described tallness as ‘dominant’ over shortness. In following experiments, he was able to show that other characteristics were ‘recessive’ and required the same information from both parents to display that particular characteristic. First generationWhen Mendel crossed tall and short plants, all the first generation were tall. Each of these parents contained a pair of genes that controlled the plant size (tall or short). Each plant passes on one short gene. The dominant gene for tallness was concealing the copy for shortness. Second generation
Mendel then created a second generation from the first and this produced four combinations of genes:
As the gene for tallness is dominant, in three out of four cases, a tall plant was produced. The same pattern of recessive inheritance is evident with many inherited disorders including Friedreich’s ataxia. The gene is recessive so that people who carry one normal and one Friedreich’s ataxia gene are physically normal. If both parents carry one copy of the Friedreich’s ataxia gene then there is a one in four chance of a child inheriting two copies of the Friedreich’s ataxia gene. Such children will develop the disease in young adult life.
SummaryFrom the early work done by Mendel and others, five basic principles that govern human inheritance have been identified:
This depends on which copy of the gene is passed on by the parent of a twin. In the first generation, tallness is dominant over shortness. In the second generation the gene for ‘shortness’ is revealed. |
