AS Biology Unit 2 topic 3

When a plant starts to flower, cells in the meristem become specialised to form the organs that make up the flower

The expression of genes across the meristem determines which structures will form. 

3 Genes determine which type of specialised organ will be produced in each area of the meristem.

GENE A-expressed sepal form

GENE C--carpels form

GENE A AND B--petals

GENE B AND C--stamens

Master genes produce mRNA which codes for signal proteins which switch on the appropriate genes

Synthesis of the proteins coded for by these genes results in the development of specialised cells in each floral garden

During the development of an animal, the animal must also lose some cells by programmed cell death which is known as APOPTOSIS.

There is a small group of cell 'suicide' genes and when they are expressed, this causes the nucleus and the cytoplasm to fragment and break down.

Genes that prevent death are expressed in most cells, and during development cells have to switch on their 'suicide' genes in order for them to die.

This explains why cell death does occur throughout the embryo

Programmed cell death occurs in particular places like between where the fingers will be. In the embryo brain, many millions of extra cells are produced--of which at least half are die by controlled cell death

The characteristics of an organism such as its' height, shape, blood group, sex are known as its' PHENOTYPE. Differences in phenotype between members of the population are caused by differences in:

• genetic make-up/ genotype
• the environment in which an individual develops

Some characteristics are controlled almost completely by the organisms' genotype, with the environment having little or no effect. e.g. a persons' blood group is controlled by the genes inherited. The genes code for protein antigens on the surface of their red blood cells. A persons' blood group is not affected by the environment in which they develop. These characteristics are controlled by genes at a single locus and show DISCONTINUOUS VARIATION. 

They have phenotypes that fall into discrete groups with no overlap

Characteristics that are affected by both genotype and environment often show CONTINUOUS VARIATION. Human height is one of them<--this means that a person can be any height within the human range. The most common height can be found somewhere in the middle of the extremes of the range. 

Characteristics that show continuous variation are controlled by genes at many loci, known as POLYGENIC INHERITANCE and also by the environment, either directly or by influencing gene expression.

• Polygenic--when a number of genes are involved in the inheritance of a characteristic, rather than just one
• Examples of polygenic inheritance are: height and skin colour
• Diabetes, CHD, Alzheimer's, schizophrenia and some cancers have genes AND the environment contributing to the development of the disease.<--known as MULTIFACTORIAL

Height, weight and skin pigmentation all involve POLYGENIC INHERITANCE. Each allele has a small effect on the characteristic, and the effects of several alleles combine to produce the phenotype of an individual.

HEIGHT

• Some evidence that taller men have more children, which would result in a gradual change in the genetic make-up of the population
• Greater movements of people have resulted in less inbreeding leading to taller people
• Better nutrition, especially increased protein, has resulted in greater growth of children
• Improved health, with a reduction in infectious diseases, has occurred through improved sanitation, clean water supplies, vaccination and antibiotics
• The end of child labour has allowed more energy to go into growth
• Better heating of houses and better quality clothing reduces the amount of energy needed to heat the body, so again more energy can go into growth

It is widely accepted that a person's height is determined by an interaction between the effects of their genes for height and environmental influences such as diet. A person may have genes for being tall but not achieve their full height potential because of malnutrition

We do not know for certain which of the possible reasons for change in height is the most important. However, most people think that better diet is the most significant factor.

Differences in hair colour are largely genetically determined due to variation in the amount and type of pigment the hair contains. But the environment can influence hair colour in some cases.

making melanin:

Melanin is made in special cells MELANOCYTES found in the skin and at the root of the hair in the follicle. These are activated by melanocyte-stimulating hormone (MSH). 

There are receptors for MSH on the surface of the melanocyte cells. The melanocytes place melanin into organelles called melanosomes. The melanosomes are transferred to nearby skin and hair cells where they collect around the nucleus, protecting the DNA from harmful UV light. 

People with more receptors have darker skin and hair; they have more protection against sunburn. UV light increases the amount of MSH and also of MSH receptors, making the melanocytes more active and causing the skin to darken.

Hair does not appear darker because although more melanin is produced, UV light causes chemical and physical changes to melanin and other proteins in hair cells. Hair lightens due to destruction of melanin by UV light. 

To make melanin, animals use an enzyme called TYROSINASE which catalyses the first step along a chemical pathway changing the amino acid into melanin.

MAOA is an enzyme that catalyses the breakdown of a neurotransmitter in the brain involved in the regulation of behaviour, including the response to stress. Some individuals have random mutations in the MAOA gene and produce no enzyme. They exhibit aggressive and sometimes violent behaviour. GM mice which lack the MAOA gene are also aggressive. These observations led scientists to suggest there might be a connection between the gene and violent behaviour but studies did not show a clear link.

Childhood maltreatment was associated with more antisocial behaviour as adults. No direct link was found between MAOA levels and subsequent antisocial behaviour.

Cancers occur when the rate of cell multiplication is faster than the rate of cell death-->causes the growth of a tumour, often in tissues with a high rate of mitosis such as the lung, bowel, gut or bone marrow. Cancers are caused by damage to DNA. DNA is easily damaged by physical factors such as UV light or asbestos. It can also be damaged by chemicals, known as carcinogens, which may be in the environment or can be produced by cell metabolism. 

Mutations can also occur when cells divide. If DNA is copied incorrectly in gamete formation, an inherited form of cancer can result. 

Oncogens code for the proteins that stimulate the transition from one stage in the cell cycle to the next. Mutations in these genes can lead to the cell cycle being continually active which could cause excessive cell division, resulting in a tumour.

Tumour Repressor Genes produce supressor proteins that stop the cycle. Mutations inactivating these genes mean there is no brake on the cell cycle. One example of a tumour suppressor protein p53. This protein stops the cell cycle by inhibiting the enzymes at the G1/S transition, preventing the cells from copying its' DNA. In cancer, cells lack p53 which means that there is nothing to stop the cell cycle from going into the 'S' phase. such cells have lost control of the cell cycle. Loss of tumour suppressor proteins has been linked to skin, colon, bladder and breast cancer. 

There needs to be damage to more than one part of the cell control system for cancer to occur. This make cancer very unlikely in any particular cell, but because the body contains so many cells dividing and changing over a lifetime, cancers will occasionally occur. Cancers are more likely in older people as they have accumulated more mutations

Inherited Cancer

Most common cancers occur more frequently in close relatives of cancer patients suggesting an inherited component. Many gene defects have been identified that predispose people to cancers including bowel cancer, ovarian cancer, prostate cancer, retinal cancer and some types of leukaemia.

Environment and Cancer

DAMAGE FROM THE ENVIRONMENT CAN BE EITHER CHEMICAL OR PHYSICAL

The greatest chemical risk of all is from smoking, smoking increases the likelihood of many forms of cancer, especially lung cancer, through the action of carcinogens in tar. Tar lodges in the bronchi and causes damage to DNA in the surrounding epithelial cells

UV light physically damages DNA in skin cells. Sometimes, a mole which has been affected by UV light may start to grow bigger and can develop into a tumour. If a tumour is not removed, cancer cells can sometimes spread to other parts of the body, carried in the blood and lymphatic systems. New cancers may then form in other parts of the body

Diet is also linked to cause and prevention of cancer although the links are not always clear. A diet with plenty of fresh fruit and vegetables provides antioxidants which destroy radicals. Radicals are chemicals from the diet, from environmental factors like smoke and UV or produced by the cell's own metabolism, which contribute to ageing and cancer through DNA damage

Several cancers are triggered by virus infection e.g. liver cancer can follow some types of hepatitis and cervical cancer can follow infection by the papilloma (genital wart) virus. A virus' RNA may even contain an oncogene, which it has picked up from one its hosts and then transfers to the cells it infects.