DNA, Genes and chromosomes

Introduction

Assignment two will firstly explain about DNA, genes and chromosomes. It will then evaluate some pre- disposed genetic factors that affect normal human functioning and look a range of different diseases. It will then evaluate pre-disposed environmental factors that may also affect normal human functioning and discuss another range of diseases.

Genetic information is contained in nucleic acids, which are the molecules that hold the information. All living cells and viruses contain information and there are two types of nucleic acid, deoxyribonucleic acid (DNA), the self replicating genetic material in living cells and ribonucleic acid (RNA). The structure of DNA was worked out by Watson and Crick in the 1950s. Nucleic acids are made of units called nucleotides and an individual nucleotide is in three parts which combine by condensation reactions. These are phosphoric acid, pentose sugar, which in DNA is deoxyribose and in RNA ribose and there is an organic base comprising of five and divided into two groups. The DNA is a double stranded polymer of nucleotides (polynucleotide) comprising of many million nucleotide units. Its structure is in the form of double helix which is maintained by hydrogen bonding and it contains four organ bases, adenine, guanine, cytosine and thymine.

A gene is the unit of heredity, comprising of a length of DNA that influences an organisms form and function. The protein produced when a gene is expressed produces a characteristic and each gene occupies its own position on the chromosome called the locus. Different forms of the same genes are called allele and there may be different alleles of the same gene with slightly different DNA structure.

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The chromosome is one long coiled DNA molecule which has genes dotted along its length. The genetic material of each cell is packaged together in the nucleus as chromosomes and each one of these contains very long DNA molecules. The human body has 46 chromosomes and in each body cell the chromosomes are in pairs, called homologous and a full set of chromosomes is called the karotype. Chromosomes make copies of each other so that when they divide, each daughter cell receives an exact copy of the genetic information. This is called replication and results in two DNA molecules.

The Human Genome Project started in 1990 and it was a huge task to determine the order of bases in the human genome as well as identifying all the genes formed by the bases. Its other aims were to find the location of the genes on the 23 chromosomes and store the information on a data base. The purpose of collating the information was for scientists to know which sections of DNA, on which chromosomes are responsible for many inherited diseases. The main uses of genetic testing are in carrier screening, pre-implantation genetic diagnosis, new born baby screening, and for prediction testing of onset disorders such as Huntington disease, onset cancers and Alzheimer’s disease. Using a sample of DNA it is possible to find out whether a person is carrying a faulty gene which causes a disease such as cystic fibrosis, or to identify genes that play a contributory role in diseases such as breast cancer. From results it is possible to eliminate all risk of the disease by correcting the faulty allele.

Sexual reproduction produces genetic variation amongst individuals in a population.

Mitosis is when cell division takes place and it results in growth or repair of body tissues which is not to be confused with Meiosis which is the cell division that produces gametes (sex cells). In mitosis, one cell divides into two identical cells and in meiosis one cell divides into four daughter cells that a genetically unique. A species must change to its environment if it is to survive and the genotype of an organism gives it the potential to show a particular characteristic. Characteristics that are determined by a number of genes are called continuous variation such a height and characteristics that are clear cut are by a single gene are called discontinuous variation. The origins of variation are either non-inheritable or heritable. The environment has a huge role in determining phenotypic variation and factors in humans can include diet and exercise. Heritable variations are when an organism, for example, inherits genes which will determine its eventual size, although this can depend on nutritional influences.

Monohybrid inheritance is when a characteristic controlled by a single gene is passed on from one generation to another. Examples of genetic diseases that are passed on in this way are Huntington’s disease and cystic fibrosis. The gene can be either dominant or recessive. Huntingdon’s disease is due to a mutation in a single gene that occurs on chromosome 4. Every cell nucleus has two copies on the gene and the codes for the protein are Huntingdin. People who develop the disease carry a mutation in one of copy of the Huntingdin gene. Huntingdin is concentrated in areas of the brain and that degeneration of the gene is called Huntingdon’s disease.

Huntingdon’s disease is rare but another more common disease is cystic fibrosis which is caused by a recessive allele. To inherit the disease both parents have to be carriers of the defective alleles. In the UK one person in 2000 suffers from this condition and people that develop the disease produce a thick sticky mucus from the epithelial cells lining some passages in the body. The pancreatic duct can become blocked so food digestion can not complete and the bronchioles and alveoli of the lungs can become blocked. The normal allele of the cystic fibrosis gene makes an important protein called CFTR. Normally CFTR will transport chloride ions through the plasma membrane, however, the mutated allele causes production of a channel protein that does not transport the ions so the person who is homozygous suffers from cystic fibrosis.

The full amount of alleles and their combination a person has is called their genotype and some of these are recessive and some dominant. The effect that these alleles have is called the phenotype. Different alleles of a gene do not have to be recessive or dominant and if two alleles both produce a protein that can function then the alleles can be codominant. An example of a disease that is codominance is Sickle cell anaemia, where a mutant allele of a normal haemoglobin gene causes one amino acid in the two beta polypeptide chains to be different. The shape of the molecule is altered and the red blood cells can be crescent or ‘sickle’ shaped. These can be easily damaged and the number of working cells decreases the amount of oxygen going to the tissues. The heart works harder and the defective cells join together making the blood sticky. This can result in many side effects including kidney failure, heart attack and strokes. The spleen is over burdened and can stop its ability to remove bacteria from the blood so infections can be common.

Another type of faulty cell division is called non disjunction where the daughter cell receives two copies of a chromosome and the other gets none. This can result in the condition called Down’s syndrome where chromosome 21 is affected. The genetic condition is known as trisomy, where a person inherits an extra copy of one chromosome. People with the syndrome have three copies of chromosome 21 rather than two and this additional genetic material affects the balance of the body and results in characteristic physical and intellectual features.

Many people have alleles of genes which can make them much more susceptible to certain diseases. The disease may only develop if the person become in contact with something in the environment such as a chemical. An example of this would be lung cancer as some smokers die from cancer in middle age, while others carry on well into old age without being affected. The expression of genes can also be affected by environmental factors such as diet, disease and temperature during development. Mutagenic agents can cause gene mutations in tissues which then grow abnormally. There much scientific disagreement about a person’s intelligence as is it determined by genes or by the environment that they grow up in.

Asthma is a condition that tends to run in families that are prone to allergies. Although there are many factors that cause and influence asthma there is no single gene that is involved, although scientists are searching for the gene involved which may lead to a cure. The condition affects the bronchioles that carry air in and out the lungs which become swollen or narrowed and excess mucus is produced. It is a chronic condition and symptoms are wheezing, shortness of breath and a tight feeling in the chest. There are environmental factors which increase the risk of the disease such as being brought up in a house that has a pet, exposure to cigarette smoke in the uterus or in early life, air pollution and being born at the time of year when pollen is at is highest.

Coronary heart disease is a condition in which genetics and environmental factors determine which humans get the disease. The disease is caused by a blockage of the coronary arteries which supply blood to the heart. In a healthy heart the walls are smooth and the blood flows easily, but the disease develops when material blocks the walls of the arteries causing narrowing of the vessels and possibly a complete blockage. This can lead to a wide range of cardiac problems including angina. The disease often occurs within the same family which can indicate that there maybe genetic link between people with the condition. It is difficult to establish if there is a direct correlation between family members due to genes or whether it is the environmental factors which they all are exposed to. Some of the main environmental factors that increase the risk of developing the disease are smoking, lack of exercise, obesity, unhealthy diet, mental stress, alcohol and coffee. It has been proved that genetic factors have an influence on cholesterol levels, but overall, it seems that a combination of genetics and the environment would best explain the family link to heart disease.