[I] Biology - B1

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  • Created on: 18-02-14 10:14

What your genes do

Genes make us who we are. They are the instructions that control how organisms develop and function.

You begin as a single cell at conception, when a sperm fertilised the egg. Genes within the cell instructed your body to grow and become the functioning human being that you are today. Genes are found in the nucleus of cells. They instruct the cells to make proteins needed for your body to work.

A gene is a section of a long chemical called DNA. Each gene provides the code for the production of a protein. The genetic material is found within the cell nucleus. The DNA is in long strands that are coiled and found packed into structures called chromosones. 

The strands of DNA are made up of four chemicals that we call "bases", along with phosphate and sugar molecules. It is the order of these four different bases along each piece of the DNA strand that determines the protein code.

Some proteins give an organism rigidity and strength. These are called structural proteins. Collagen is a structural protein that gives the skin its elasticity. It is also a component of connective tissue like ligaments and cartilidge. Keratin is the structural protein that helps make hair and nails in humans, and hooves or feathers in some animals.

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DNA sequencing

Functional proteins enable or speed up chemical reactions in the organism. Examples of functional proteins are the enzymes needed for digestion in animals, such as lipase which digests fats, and amylase which digests starch.

Scientists can now identify the sequence of bases in DNA and determine which genes are located where on the chromosones. The Human Genome Project identified all the 20-25000 genes in human DNA.

The ability to "read" a person's genome (gene set) brings ethical considerations. There have been considerabble benefits to society from the HGP, for example the identification of particular sequences of genes that can lead to or protect against certain diseases. This allows individuals to be screened for genetic orders. 

Some drug companies have identified genes linked to a particular illness, such as breast cancer. They have tried to "patent" (claim ownership of) these genes in order to make a profit by developing drugs that target these genes. This would mean that other companies have to pay to use that gene for research. There is debate about whether this should be allowed. 

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For and against patenting

There are arguments for and against patenting a gene. The benefits of patenting a gene are:

- Drugs are more likely to be developed to prevent disease caused by that gene, as there is likely to be more profit.

- The patent is time-limited, so after a certain amount of time other companies are able to make cheaper versions of the treatment that has been developed

The disadvantages of patenting a gene are:

- Other companies are restricted in their research on that gene, and this could slow the search for a cure or prevention.

- A company should not be allowed to "own" a part of the human genome.

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Why are we all different?

Some differences between individuals are due to genes. An example of this in humans is whether or not you have dimples.

There are other differences that people are not born with, and are entirely due to the ENVIRONMENT. For example, somebody may have a scar from an injury, or may have died their hair. For humans, the "environment" in this context doesn't just mean the physical surroundings, but also the social situation that they live in.

Some differences are due to a combination of genetics and environmental factors working together. If your parents are overweight, you may inherit the genes to make it more likely that you are overweight, but you would only become overweight if you ate too much food and did too little exercise. 

In some cases, several genes work together to determine a feature. A good example of this is eye colour. Human eye colour does not follow a simple pattern of inheritance, because it is determined by a number of different genes. As a result, there is a wide range of eye colours in humans. This is an example of CONTINUOUS VARIATION, where there is a wide range of possible outcomes. Some continuous variation may be affected by the environment (eg: milk yield in cows)

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Genes versus environment

The genetic make up of an individual organism is called the GENOTYPE. The characteristics that an individual displays are called the PHENOTYPE.

The genotype strongly influences the phenotype, but the environment also has an important role to play in determining the physical characteristics of an organism. It is thought that intelligence has a genetic component, but the full potential of an individual can only be reached if they are provided with the stimuli they need by the environment that surrounds them. Similarly, an individual's genotype might enable them to be a successful athlete, but they can only reach that goal if they train well.

Investigations into the relative importance of environmental and genetic factors are often carried out using "twin studies". These compare identical and non-identical twins. Identical twins are produced when one zygote splits into two, and each of the cells produced develops into a baby. Identical twins therefore have identical genotypes. Non-identical or "fraternal" twins grow from two seperate eggs each fertilised by different sperm, so have different genotypes just like other siblings. Twin studies allow scientists to more accurately discover whether "nature" (the genotype), or "nurture" (the environment or upbringing) is more important in the development of a characteristic.

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How genes and chromosomes are organised

Genes provide the code that tells a cell what proteins to build. The genes we have determine many of our characteristics, or traits, such as hair colour and eye colour. The genes are connected in long molecules of DNA. These would take up a lot of space if they were stretched out. Instead, they are twisted and folded over into structures called chromosomes for storage. Each chromosome has hundreds of genes.

Chromosomes occur in pairs. Human cells have 23 pairs (46 chromosomes), The chromosomes are found in the nucleus of the cell. The sex cells - sperm and egg cells - are different from the other cells in the body, as they only have 23 chromosomes each, one from each pair. This is so when a sperm and egg cell join together at FERTILISATION, they form a new cell called a ZYGOTE that has 46 chromosomes, 23 from each parent. The zygote can then develop into a human baby with genes from both of its parents.

Pairs of chromosomes usually contain the genes for the same trait in the same position. The alternative forms of the genes are called alleles. Offspring get one of each pair of chromosomes from each parent. This means they have two alleles for each gene, one from each parent. It is possible that both alleles of a gene are the same, in which case the individual is said to be "homozygous" for that trait. If the alleles for a gene are different, the individual is said to be "heterozygous" for that trait. 

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Chromosomal disorders

Mistakes or mutations sometimes occur in the process of creating sex cells or during fertilisation. This can lead to individuals who have different numbers of chromosomes from the usual 46.

If a chromosome abnormality is detected during pregnancy, the parents face the difficult decision of whether or not to continue or terminate the pregnancy. 

This also carries a problem incase a false negative or a false positive is given.

In one of the most common of these disorders, Down's Syndrome, a baby receives an extra copy of chromosome 21, resulting in 47 chromosomes instead of 46. Around one in every thousand babies born is affected, and all will have some degree of learning disability and health problems.

Doctors can advise what the likely outcomes may be for a child with a chromosomal disorder, but cannot make the decision about whether the pregnancy should continue; only the parents can make that decision.

People born with chromisomal disorders, such as Down's syndrome, can live full and rewarding lives.

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Variation in families

In most families, it is easy to see some similarity between parents and children. Parents and children are similar because some of the parents' genes are passed on to their offspring through SEXUAL REPRODUCTION. Half the genes come from the mother and half from the father. The offspring are not identical but show VARIATION. This is because each egg (ovum) and sperm cell contains a combination of genes from the parent, so each child in a family will receive slightly different genes, even though they have the same parents.

A mother and a father have 23 pairs of chromosomes in each of their body cells. Each pair of chromosomes contains hundreds of pairs of alleles, and each allele has a number of possible forms. When an egg or sperm is formed, one of each the 23 pairs of chromosomes is split off into the ovum or sperm cell. An ovum and a sperm cell only contain 23 chromosomes each. At fertilisation, this unique set of 23 chromosomes from the father (in the sperm) joins with the unique set of 23 chromosomes from the mother (in the ovum) to form a set of 23 pairs of chromosomes that is the gene set of the offspring.

Siblings will receive some of the same chromosomes from each of their parents, but they are likely to share only half their chromosomes, due to the random assortment of the pairs of chromsomes in the formation of the ovum and sperm. This mix of chromosomes, and hence mix of alleles, is one of the factors that leads to variation in the offspring.

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Pairs of alleles

Chromosomes occur in pairs in the nucleus of a cell. Each pair of chromosomes has genes with instructions for the same trait on it in the same locations. These corresponding genes are called alleles. The pair of alleles can have different versions of the genes on them.

For example, the gene in a particular location on a chromosome might code for whether or not a person has dimples. The alternative alleles will be for "dimples" or "no dimples". An individual could have any combination of these alleles. Both alleles could be "dimples", "no dimples" or one for "dimples" and one for "no dimples".

Offspring receive one of each pair of chromosomes, and therefore one of each pair of alleles, from each parent. Which alleles they receive determines their physical characteristics.

Traits are passed on from parents to their offspring through genes on chromosomes. Genes for particular trait are located on the same place in each of a pair of chromosomes. These alternative versions of the same gene are called alleles.

Alleles can be dominant or recessive. When the alleles in a pair are different from each other, the trait shown in the offspring is that of the dominant allele. Dominant alleles are written with capital letters in genetics and recessive genes are written with lower-case letters.

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Genetic diagrams

In genetics, a diagram called a Punnett square is used to show clearly all the possible outcomes for a particular trait, and how likely the outcomes are for a particular pair of alleles. It is represented by a two by two grid, with one side featuring the possible alleles that could be passed on by the female and the other featuring the possible alleles that could be passed on by the male. 

For example, if the mother didn't have any dimples (dd) and her alleles were dd, and the father had one gene for dimples and one gene for no dimples (dD), then the Punnett square would feature dd, dd, Dd, Dd. So there is a 50% chance that the child will have dimples.

Another way to show genetic inheritance is using a family tree diagram. These diagrams have a key to help you to interpret them, and show how a trait is passed through a family.

Huntington's disease is a serious genetic disorder caused by a faulty dominant allele. A family tree can show the inheritance of the dominant gene for Huntington's disease.

The GENOTYPE of an organism is the combination of alleles that the organism has. The PHENOTYPE of an organism is observable in its physical characteristics.

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Sex determination

One of the 23 pairs of chromosomes found in human cells is the pair called the "sex chromsomes". In a karyotype, or picture of human chromosomes, the sex chromosomes are always shown as the last pair. Sex chromosomes come in two types, X and Y. An individual can have the sex chromosomes XX, in which case she is female, or XY in which case he is male.

Whenever fertilisation occurs, there is the same chance that the baby will be male or female.

XX = Female

XY = Male

A female has the sex chromosomes XX, so each egg cell will have the sex chromosomes X. A male has both X and Y chromosomes, so half of the sperm cells will have the X sex chromosome and half will have the Y chromosome. 

The Punnett square shows how the sex chromosomes can be combined in the ofspring. Two possible combinations give a male baby and two possible combinations give a female baby. Remember, the X chromosome is always listed first when writing combinations of sex chromosomes.

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The sex-determining gene

It is not the presence of the Y chromosome in itself, but the presence of a gene on the Y chromosome that determines whether or not an embryo will develop male characteristics or not. The gene is known as the SEX-DETERMINING GENE. It is thought that the sex-determining gene on the Y chromosome triggers the development of testes in the growing embryo. If this gene is not present, then ovaries develop and the embryo is a female.

If you look at the pair of sex chromosomes, the X and Y, you can see that there are parts of the X chromosome in a male that have no "matching" alleles on the Y chromosome. Normally, alleles exist as a pair so each can have an alternative allele that "dominates". Genes that appear in the region of the X chromosome where there are no alleles on the Y chromosome are shown as traits in the individual even if they are recessive. These genes are said to be sex-linked.

Examples of genes found in the section of the X chromosome are the gene for haemophilia (a blood-clotting disorder) and red-green colour blindness. Because there is no "matching" allele on the Y chromosome to "dominate" these genes, males in particular suffer from these conditions.

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Disorders caused by a single gene

Some disorders are caused by a single faulty allele. When a single faulty allele causes a disease in this way, we say it is a dominant disorder. This means that anyone with one allele for the disease (out of the pair) will get the disease. An example of a single-gene disorder that is dominant is Huntington's disease.

Symptons of Huntington's disease usually begins when people are aged between 30 and 50 years old. These include a tremor, clumsiness, memory loss, inability to concentrate and mood changes. 

Because the symptons do not appear for so long, many people who are not screened do not know that they are sufferers (carriers) until they have already had children.

Whether or not you have a gene disorder, you need a good understanding of how they are passed on through families. A person can carry a gene for a disorder without knowing it.

This becomes important if they meet and want to start a family with another "carrier" of the disorder.

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Disorders caused by recessive genes

Disorders can also be caused by recessive alleles. The only way to get a recessive trait is to have both recessive alleles. Disorders caused by a recessive gene include cystic fibrosis. Sufferers have alleles "cc". This serious genetic disorder begins in early childhood and is caused by a faulty protein in the cell membranes. Sufferers of cystic fibrosis have the following symptons: thick, gluey mucus, particularly affecting their lungs, difficulty breathing, tendency to chest infestions and a difficulty digesting food. 

The inheritance of cystic fibrosis can be shown in a family tree. When a person is a CARRIER for cystic fibrosis they have the alleles "Cc". When they have cystic fibrosis they have the alleles "cc" and when they have no trace of cystic fibrosis they have "CC" alleles. 

When two carriers have children together (Cc and Cc), they have a one in four chance of having a child with cystic fibrosis. 

Genetic testing can be carried out for a particular condition if a person is likely to be at a greater risk of the disorder because it occurs in their family. It is possible to test people to find out if they have the gene for Huntington's disease or cystic fibrosis. This allows people to get the treatment that they need and provides them with information that might help them plan for their future, for example, whether they plan to have a family. 

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Carrying genes and passing them on

Cystic fibrosis is a recessive condition. This means that you only have the disorder if you have two faulty alleles. If one parent has cystic fibrosis then they know that any child they have is at a higher risk of suffering from the disorder, but is is also quite possible for two parents who have no signs of cystic fibrosis to give birth to a child who has the disorder. This is because people can have the gene for the disorder without being aware of it. This is called being a CARRIER of the condition. A carrier for cystic fibrosis has one faulty allele.

In vitro fertilisation (IVF) is a technique that can help infertile couples to conceive a baby. The method involves taking control of the mother's hormones, and causing her to overproduce ripe eggs in her ovaries. These eggs are then collected and mixed with a sample of her partner's sperm. Some of the eggs are fertilised and begin to develop into human EMBRYOS. A number (usually no more than three) of well-developed embryos are then implanted into the mother's uterus via a hollow tube through her cervix. Some of these embryos may implant in the uterus wall and develop into fetuses.

Embryos are usually implanted back into the mother two or three days after the eggs have been collected. There are usually between four and eight cells at that stage.

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Regulating work with embryos

IVF is very useful as it allows doctors to investigate the genetic make-up of the embryos prior to implantation in the mother in a process known as "embryo screening". In families where there is a known history of a genetic disorder such as cystic fibrosis, the embryos can be screened to find out if they have inherited the faulty genes. This screening allows doctors to remove any embryos that would suffer from the disorder and only implant genetically normal embryos back into the mother. Embryos that are not required can sometimes be frozen for future use, donated for research or used by infertile couples through embryo donation. Unwanted embryos are destroyed.

Screening embryos prior to implantation and only using healthy embryos is called pre-implantation genetic diagnosis (PGD). Procedures like PGD and fertility treatments, along with embryo research, are carefully monitored in the UK. Guidelines are provided to clinics and research centres about how embryos are used, making sure ethical and moral considerations are taken into account.

When decisions are controversial about whether treatment such as PGD should take place, often "ethics committee" will discuss and take the decision. This committee usually consists of a number of health care professionals, but may also include other people not involved in medicine, to help provide the most balanced view possible.

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Genetic screening and testing

GENETIC SCREENING can be carried out for a particular condition even when there is no family history of a disorder. Genetic screening of large numbers of individuals is sometimes used to identify sufferers of genetic disorders as early as possible, in order to minimise the damage such disorders can cause.

A number of screening tests are carried out on newborn babies to make sure they are not suffering from rare genetic disorders.Almost all babies in the UK are checked using a "blood spot test" where a drop of blood is taken from the baby's heel and tested for disorders. These conditions are very rare, but consequences of not spotting early signs can be devastating.

There are many other reasons why people are screened. For example, a patient may be tested before a doctor prescribes a certain drug, to see how effective that drug would be for that individual, or to check for an allergy to the drug. Some genetic tests are carried out during pregnancy. These involve CELL SAMPLING - collecting cells from the fetus while it is developing inside the mother's uterus. A long needle can be used to take a sample of some of the baby's cells, which can be tested for alleles linked to certain diseases. Another test could be chorionic villus samping, whereby a sample of cells from the placenta is taken through the mother's cervix or her abdomen. They both carry risks of miscarriage. For many, the risk is worth taking to find out whether their baby will suffer from a serious disorder.

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Implications of genetic testing

The possibility of genetic testing raises a number of important ethical considerations. For example, if an individual is diagnosed with a disorder such as Huntington's disease, should they have to tell other people, such as their place of work or their health insurance provider? Should they be compelled to share the information with their family?

Occasionally, a genetic test can give a FALSE POSITIVE result, where the individual does not have a disorder but the test had a positive result, or a FALSE NEGATIVE result, where the test was negative but the individual does have the disorder. Different tests have different levels of accuracy. Aminocentesis is the most accurate test, giving the correct result over 99% of the time.

The results of genetic tests on fetuses often raise very difficult decisions for parents. If they have received news that their child is suffering from a genetic abnormality, they must decide whether to continue with the pregnancy or terminate it. Parents need a lot of support and information at this time. Some couples who are aware that they are likely to pass on a genetic abnormality to a child may decide not to have a family at all. Others decide that they will not have testing and will cope with a child with a disorder if they have one. Genetic disorders have a very wide range of effects, and some people live productive, happy lives despite having a genetic disorder.

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Cloning

Clones are individuals with identical genes. Bacteria, plants and even some animals can reproduce asexually to form clones. In ASEXUAL reproduction, only one parent is involved so the offspring has identical DNA to the parent.

Plants can reproduce asexually in a variety of ways. Some plants, such as brambles, send out runners, which are long shoots that grow along the surface or just below the surface of the ground. The runners send out roots and grow into an identical plant a short distance away from the parent plant. Plants can also produce bulbs that grow into offspring with identical DNA to the parent plant.

Some animals can reproduce asexually, although this is quite unusual. An animal that can reproduce in this way is the hydra. When a hydra reproduces asexually, the offspring "buds" out from the body of the parent. Echinoderms, such as starfish, can grow back from a small piece of tissue.

As clones have identical DNA, any differences between clones and their parents are likely to be due to the environment they are in, rather than their genes. There are advantages to cloning, if the parent is successful in their habitat - the offspring is likely to be successful as well. Also, all their energy can be put into the offspring. A disadvantage is no genetic variation/diversity.

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Animal and human clones

Apart from the few animals that can reproduce asexually, there are other ways that animal clones can occur. Identical siblings have identical DNA, so they are clones of each other, rather than of their parents. Identical siblings, most commonly twins, are produced when the fertilised egg splits, producing two or more genetically identical individuals.

Clones of animals can also be created artificially. This process is complex, and is the method by which Dolly the sheep and Snuppy the dog were created.

The nucleus containing the genetic material is extracted from an adult body cell and inserted into an empty egg cell. This gives the egg cell a full set of genes, without being fertilised. If the egg begins to develop into an embryo, it can be implanted into a suitable female - a surrogate mother. The cell develops normally to become an individual with identical DNA to the "donor".

It is illegal to create clones of humans in most countries. Although scientists can create embryos, these cannot be implanted into surrogate mothers. In 2005, British scientists cloned human embryos as part of a research into what is known as "theraputic cloning", but these embryos were only used for research. 

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Stem cells

A human embryo develops from a single cell into a baby with all the different organs and cells that make up its body. This happens because the unspecialised EMBRYONIC STEM CELLS that form in the first few days of life can be developed into any type of cell. Embryonic stem cells can be removed from the embryo at around a week old when it has only about 100 cells and is a ball smaller than a full stop.

ADULT STEM CELLS are only found in certain parts of the body. These cells can repair or replace cells by becoming different type of cell. For example, the bone marrow contains adult stem cells that are able to form several different types of blood cell. But adult stem cells can only develop into some other types of cell. 

Once cells have become a particular type, or specialised, they cannot change. This process is called DIFFERENTATION, and occurs before 12 weeks in a human pregnancy.

Stem cell technology could transform medicine. For example, stem cells have already been used to grow skin tissue. Technology like this could be used to grow features to help people with severe facial disfigurement, such as those with serious burns.

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The potential of stem cells

Stem cells are extrememly useful to scientists as they can be used to produce any other type of cell. It might allow treatments or even cures to be developed for many diseases. However, using embryonic stem cells is controversial. This is because the stem cells are generally taken from living human embryos, usually those that are surplus to requirements after fertility treatments. 

Scientists are working on ways of creating strm cells that can become any human cell by "reprogramming" adult body cells, so that embryos do not need to be destroyed. 

in-vitro fertilised egg > after 5-7 days the egg is a ball of cells > after 7-10 days stem cells can be removed > the stem cells are cultured in a lab, they can then be differentated into all different kinds of cell (red blood cell/nerve cell/muscle cell)

Human stem cells have many potential uses. Stem cells that have been differetiated into particular specific body cells could be used to test drugs for their effectiveness when treating disease. Embryonic stem cells are the most useful as they can become any cell, it operates on a "switching on and off" scheme. A very important application of stem cell research is the possibility of creating cells and tissues. Stem cell therapies could provide a renewable source of cels to treat disorders like spinal injury, Alzheimer's and heart disease. Intensive research is being carried out on how stem cells could be used in this way and promising results are shown.

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