Biology B1 B2 B3 - n/a PDF

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B1 – You and your genes B1.1 What are genes and how do they affect the way that organisms develop?

A GENE is a short section of DNA. Genes carry instructions that control how you develop and function – they are long molecules of a molecule called DNA. Each gene codes for a specific protein by specifying the order in which AMINO ACIDS must be joined together. These proteins can be: JJJJJJJJJ ffffff

STRUCTURAL PROTEIN: Gives the body structure, rigidity and strength E.g. Skin, Hair, Muscles etc FUNCTIONAL PROTEIN: Enables the body to function E.g. Enzymes, Antibodies etc.

The differences between individuals of the same species are described as VARIATIONS. Variations may be due to:  GENOTYPE – The genetic makeup of an organism. The different characteristics that an individual inherits, E.g. whether you have dimples or not.  PHENOTYPE – The observable characteristics the organism has. How the environment changes an individual, E.g. cutting the skin may cause a scar. IDENTICAL TWINS have the same set of genotype however any differences between them is because of environment. CONTINOUS VARIATION shows when some characteristics are controlled by several genes working together e.g. eye colours and height. For instance it was originally believed that eye colour was due to a single gene. It is now known that there are a number of genes coding for different pigments in the iris, mainly on chromosome 15 in humans. This means that there is enormous variation in eye colour. B1.2 Why can people look like their parents, brothers and sisters but not identical to them? A human has 23 PAIRS OF CHROMOSOMES

Parents pass on their genes to their offspring in their sex cells. A pair of chromosomes carries the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called ALLELES. These alleles may be the same (HOMOZYGOUS) on each pair of chromosome, or different (HETROZYGOUS) – For example to give blue eyes or brown eyes. Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father. WHICH CHROMOSOME WE GET FROM EACH PAIR IS COMPLETELY RANDOM. THIS MEANS DIFFERENT CHILDREN IN THE SAME FAMILY WILL EACH GET A DIFFERENT COMBINATION. THIS IS WHY CHILDREN IN THE SAME FAMILY LOOK A LITTLE LIKE EACH OTHER AND A LITTLE LIKE EACH PARENT, BUT ARE NOT IDENTICAL TO THEM. THE CHILD WILL SHARE SIMILARITIES WITH ITS PARENTS DEPENDING ON WHICH CHARACTERISTICS HAVE COME FROM THE FATHER AND WHICH HAVE COME FROM THE MOTHER AND WHICH ONES ARE DOMINANT AND RECESSIVE. An allele can be DOMINANT or RECESSIVE a) An individual with one or both DOMINANT alleles (in a pair of alleles) will show the associated DOMINANT characteristic. b) An individual with one RECESSIVE allele (in a pair of alleles) will not show the associated RECESSIVE characteristic. c) An individual with both RECESSIVE alleles (in a pair of alleles) will show the associated RECESSIVE characteristic. GENETIC DIAGRAMS It is easiest to follow what is happening with the inheritance of gene characteristics by drawing genetic diagrams. FAMILY TREES can be used to trace the inheritance of a characteristic and to work out who must have been carrying a faulty allele. An example of this:

When looking at the possibilities of inheriting and allele, we use a Punnett square diagram. This shows all the possible pairings of alleles from sperm and egg at fertilisation. For example if a male with a dominant A allele and recessive a allele was to mate with the same alleles, the following diagram could be drawn: A

a

A

AA

Aa

a

Aa

aa

This means that three of the four possible offspring would show the dominant characteristics

While only one of the four possible offspring would be recessive for both alleles

A Punnett square diagram can also be used to represent how sex is determined. This is because one of the 23 pairs of chromosomes in a human cell is the sex chromosome. In females the sex chromosomes are the same – they are both X chromosomes. In males they are different – there is an X chromosome and a Y chromosome. X

Y

X

XX

XY

X

XX

XY

Therefore, 50% of the offspring will be female and 50% male. As the process of fertilisation is completely random, some families will only have girls whilst others will only have boys.

Sex Determination: The sex of an embryo is determined by a gene on the Y chromosome called the SRY (sex-determining region Y) gene. If the gene is not present i.e. if there are two X chromosomes present, the embryo will develop into a female and ovaries will grow. If the gene is present i.e. both an X and a Y chromosome are present, then testes will begin to develop. Six weeks after fertilisation, the undifferentiated gonads start producing a hormone called ADROGEN. Specialised receptors in the developing embryo detect the androgen. This stimulates the male reproductive organs to grow. B1.3 How can and should genetic information be used? How can we use our knowledge to prevent disease? Different forms of the same gene are called ALLELES. You inherit one allele for each gene from your father and one allele for each gene from your mother. For example, the gene for eye colour has alleles for blue eye colour and alleles for brown eye colour. Your eye colour will depend on the combination of alleles you have inherited from your parents. Some diseases are inherited from our parents though our genes: they are called GENETIC DISORDERS. They occur because of faulty or defective alleles. E.g. CYSTIC FIBROSIS Cystic fibrosis is caused by a recessive allele. You need to inherit two copies of the faulty allele to be born with cystic fibrosis. If you have just one copy, you are a CARRIER

You must have 2 recessive alleles to suffer from the disorder. 25% will suffer from CF 50% are carriers

25% are healthy

Cystic fibrosis affects the cell membranes causing a THICK MUCUS to be produced in the lungs, gut and pancreas. SYMPTOMS – Thick and sticky mucus Breathing problems Chest infections Difficulty digesting food

HUNTINGTON’S DISEASE is another genetic that affects the central nervous system. However is caused by a dominant allele – the presence of just one dominant allele can cause the disease. You only need to inherit one copy of the faulty allele to have Huntington’s disorder, unlike cystic fibrosis, where you need to inherit both copies. You can inherit the disorder if one or both of your parents carry the faulty allele, because it is DOMINANT.

E.g. the mother carries one copy of the Huntington’s allele and has the disorder.

The father does not carry the Huntington’s allele, so he does not have the disorder.

There is a 50% chance of the couple producing a child with the disorder. SYMPTOMS – Uncontrollable shaking Clumsiness Memory Loss Inability to concentrate Mood changes

GENETIC TESTING 





It is now possible to test adults, children and embryos for a faulty allele if there is family history of a genetic disorder. If the test turns out positive, the individual will have to decide whether or not to have children and risk passing on the disorder. This is called PREDICTIVE TESTING FOR GENETIC DISEASES. Genetic testing can also be carried out to determine whether an adult or child can be prescribed a particular drug without suffering from serious side effects. (TESTING AN INDIVIDUAL BEFORE PRESCRIBING DRUGS). E.g. certain people are highly prone to getting liver damage while taking COX-2 inhibitor drugs. A genetic test would ensure that only those patients who do NOT have the prone gene are prescribed the drug. Embryos can be tested for embryo selection. The healthy embryos that do not have the faulty allele are then implanted. This process is called in vitro fertilisation (IVF). The process for embryo selection is called PRE-IMPLANTATION GENETIC DIAGNOSIS (PGD). After fertilisation, the embryos are allowed to divide into eight cells before a single cell is removed from each one for testing. The selected cell is then tested to see if it carries the allele for a specific genetic disease. PGD has risks including inaccuracy in results-healthy embryo not being implanted and it may also decrease the chance of the embryo surviving once it has been implanted.

RISKS OF GENETIC TESTING: Amniocentesis testing

Chorionic Villus Sampling

1% miscarriage risk

2% miscarriage risk

Results at 15-18 weeks

Results at 10 - 12 weeks

Very small risk of infection

Almost no risk of infection

Results not 100% reliable

Results not 100% reliable

However testing adults and foetuses for alleles that cause genetic disorders has implications that need to be considered, including:  

Risk of miscarriage as a result of cell sampling for the genetic test Using results that may not be accurate , including false positives and false negatives

Outcome

Test Result

Reality

True Positive

Subject has the disorder

Subject has the disorder

True Negative

Subject does not have the result

Subject does not have the result

False Positive

Subject has the disorder

Subject does not have the result

False Negative

Subject does not have the result

Subject has the disorder

  

Whether or not to have children Whether or not a pregnancy should be terminated Whether other members of the family should be informed

There are ethical considerations that need to be considered very carefully For example: governments may have the ability to impose genetic tests on individuals by implementing genetic screening programmes, but should they be allowed to do so? There is the potential for genetic testing to be used to produce detailed genetic profiles. These could contain information on everything from ethnicity to whether they are prone to certain conditions (e.g. obesity) or diseases (e.g. cancer).

However how will the information be used?  

Employers could potentially refuse to employ someone who possessed certain alleles Insurers may not cover a person who had genes that made them more likely to suffer a heart attack.

B1.4 How is a clone made? Clones are genetically identical individuals. Bacteria, plants and some animals can reproduce ASEXUALLY to form clones that are genetically identical to their parent. ASEXUAL REPRODUCTION only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes and no mixing of genetic information. As a result are genetically identical to the parent and to each other. Identical human twins are also clones – ANY DIFFERENCES BETWEEN THEM ARE DUE TO ENVIRONMENTAL FACTORS. Plants- asexual reproduction in plants can take a number of forms: Some plants such as strawberries produce shoots called RUNNERS. These eventually break off and become new strawberry plants, clones of the original.

Other plants grow BULBS. When bulbs are planted they grow into genetically identical plants. Again the environment will alter them. No two organisms can occupy the same space in the universe so the environment will always be different for individuals, even if they are clones.

Animals – clones in animals can occur naturally and artificially: Clones of animals occur naturally when, during the earliest stages after fertilisation, the developing embryo splits into two, they have the same genes. As the genes came from both parents they are not clones of either parent, but they are natural clones of each other. It is now possible to make clones artificially by taking the nucleus from an adult body cell and transferring it into an empty, unfertilised egg cell. Cloning depends on cells that have the potential to become any cell type in the body. These are called STEM CELLS. ADULT STEM CELLS are unspecialised cells that can develop into many, but not all types of cell. EMBRYONIC STEM CELLS are unspecialised cells that can develop into ANY type of cell, including more embryonic stem cells.

As a result of being unspecialised, stem cells from embryos and adults offer the potential to treat some illnesses. For example – skin can grow as a treatment for serious burns and sight can now be restored to people who are blind due to damage of their corneas. The majority of cells of multicellular organisms become specialised during the early development of the organism. This is because after the zygote has divided four times to reach the 16 cell stage, the majority of cells in the embryo start to become specialised.

B2 – Keeping Healthy B2.1 How do our bodies resist infection? MICROORGANISMS are organisms that are too small to see with the naked eye. They include BACTERIA, VIRUSES and FUNGI. They can be beneficial to us (e.g. the bacteria that live in our intestines can produce certain vitamins) or they can cause us harm (e.g. bacteria that cause food poisoning). PATHOGENS are microorganisms that cause infectious diseases. BACTERIA and VIRUSES are the main PATHOGENS. Bacterium 1000-5000 nm

Size

Virus 20-300 nm

Appearance

Description

Examples of diseases caused

Bacteria come in many shapes and sizes. Bacteria are living CELLS and can multiply rapidly in favourable conditions. Once inside the body, they release poisons or TOXINS that can make us feel ill.

   

Tonsillitis Tuberculosis Plague Cystitis

Viruses are many times smaller than bacteria. They are among the smallest ORGANISMS known. Viruses can only reproduce inside host cells, damaging them when they do so. Once inside they take over the cell and make hundreds of thousands of copies of themselves. Eventually, the virus copies fill the whole host cell and it bursts open. The viruses then pass out through the bloodstream, the airways or by other routes  Flu  Measles  AIDS  Common cold

Symptoms of an infectious disease are caused by damage done to cells by microorganisms or the poisons (TOXINS) they produce. In the correct conditions (with warmth, moisture, nutrients) bacteria can multiply rapidly. The human body provides ideal conditions for microorganisms to grow. In the body, there is water, oxygen, food and heat, as well as different pH levels.

The form of growth is known as EXPONENTIAL GROWTH. It follows the formula: X(t) = a x b t/Ƭ X = the quantity of bacteria at a given time T = time

a = amount of bacteria at start b = growth factor Ƭ = time taken to double

When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the disease. The body’s first line of defence is its NATURAL BARRIERS which include:    

Skin Chemicals in tears Chemicals in sweat Stomach acid

The body’s first line of defence is called PASSIVE IMMUNITY, which means preventing the PATHOGEN from entering in the first place. If a pathogen manages to get into the body, the second line of defence takes over which is called ACTIVE IMMUNITY. The WHITE BLOOD CELLS have key functions in this. White blood cells are part of the body’s immune system and can:   

Destroy pathogens by engulfing and digesting them Produce antibodies to destroy pathogens Produce antitoxins to neutralise the toxins released by the microbe

Each microorganism has its own markers made out of protein on its surface – these markers are called ANTIGENS. ANTIBODIES recognise microorganisms by the ANTIGENS that they carry on their surface therefore a different ANTIBODY is needed to recognise each different type of microorganism. MEMORY CELLS are a type of white blood cell that can respond quickly when it meets a microorganism for the second time. They produce the right antibody very fast for the particular microorganism and destroy it before you feel unwell. This is described as being IMMUNE to a disease. B2.2 What are vaccines and antibiotics and how do they work?

VACCINATION involves exposing the body’s immune system to a weakened or harmless version of the pathogen in order to stimulate white blood cells to produce antibodies. If the body is re-infected by the same microorganism, memory cells produce antibodies quickly so that the microorganism is destroyed before damage is done. This is how vaccination works: ❶ Injection of vaccine A safe form of the diseases – cause microorganism is injected into the body ❷ Immune response triggered Although the microorganism is safe, the antigens on its surface still cause the white blood cells to produce specific antibodies ❸ Memory cells remain in body Long after the vaccination, memory cells patrol the body. If the disease-causing microorganism infects the body again, the white blood cells can attack it very quickly. In order to prevent an EPIDEMIC of a disease in a population it is important that as many as individuals as possible are vaccinated. If more than 95% of the population is vaccinated then the unvaccinated will be protected too because the risk of coming in contact with an infected person will be very small. There is no guarantee that all vaccines and drugs (medicines) are risk free. People have GENETIC DIFFERENCES, so they may react to a vaccine or a drug in different ways – these are called SIDE EFFECTS. ANTIMICROBIALS are chemicals that kill, or inhibit bacteria, fungi and viruses. ANTIBIOTICS are a type of antimicrobial that are only effective against bacteria but NOT viruses Over a period of time, bacteria can become RESISTANT to antimicrobials. MUTATIONS (random changes) can take place in the genes of microorganisms. This leads to new strains of bacteria and fungi that are no longer affected by the antimicrobial. These reproduce and pass on the resistance – as a result, the antimicrobial is no longer effective. To prevent resistance to antimicrobials increase:  

Doctors should only prescribe them when completely necessary Patients should always complete a course of antibiotics, even if they are feeling better.

❶ Harmful bacteria enter the body ❸ ❷ Bacteria multiply – Patient begins to feel unwell ❷ ❹ ❸ Prescribed with antibiotics by doctor ❶

❹ Number of bacteria now lower than originally entered the body (but not all dead) ❺ All harmful bacteria now destroyed.

B2.3 What factors increase the risk of heart disease? The HEART is a muscular organ in the circulatory system. It beats automatically, pumping blood around the body to provide cells with oxygen and dissolved food for RESPIRATION. The blood removes carbon dioxide and water as waste products. The muscle cells in the heart need a constant supply of oxygen and nutrients, and for their waste products to be removed. So the heart requires its own blood supply in order to keep beating Blood from the rest of the body enters the RIGHT ATRIUM of the heart. It then moves into the RIGHT VENTRICLE before being pumped to the lungs. When the oxygenated blood returns to the heart, it enters the LEFT ATRIUM. It then moves into the LEFT VENTRICLE before being pumped to the rest of the body. The heart is a DOUBLE PUMP in the circulatory system because blood returns twice.

Arteries, Veins and Capillaries

ARTERIES carry blood AWAY from the heart TOWARDS the organs. Substances cannot pass through the artery walls. Thick, elastic, muscular wall to cope with the high pressure in the vessels

VEINS carry blood from the organs back to the heart. Substances cannot pass through the walls o...