GCSE Biology (B2)(Edexcel)

Based on the dark blue CGP revision guide


How was the structure of DNA discovered?

Rosalind Franklin and Maurice Wilkins discovered that DNA has a helical structure by directing beams of x-rays on to crystallised DNA and looking at the patterns the x-rays formed as they bounced off.

James Watson and Francis Crick used these ideas, along with the knowledge that the amount of A+G matched the amount of T+C, to make a model of the DNA molecule where all the pieces fitted together.

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Describe the practical to extract DNA from cells?

1. Place some onion into some detergent+salt. The detergent breaks down the cell walls and the salt helps the DNA stick together.

2. Put the mixture into a water bath at 60C for 15 minutes. This de-natures the enzymes that could digest the DNA and softens the onion cells.

3. Cool the mixture down in ice to prevent the DNA breaking down.

4. Blend the mixture for a few seconds to break open the cell walls and release (but not break up) the DNA.

5. Cool the mixture down again then filter it to remove the big bits.

6. Gently add ice cold alcohol (eg ethanol) to the mixture. The DNA will come out of the solution as it's not soluble in cold alcohol. It has the appearence of a stringy white substance.

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Describe the events of protein synthesis?

1. DNA unzips to form single strands, this becomes MRNA. This is called TRANSCRIPTION.

2. The MRNA strand leaves the nucleus and joins to a ribosome.

3. TRNA brings the amino acids that match the MRNA code to the ribosome.

4. The ribosome sticks amino acids together in a chain to make a polypeptide (protein). The protein made is dependant on the order of the base triplets (codons) in the MRNA. This is called TRANSLATION.

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How do enzymes catalyse reactions?

Chemical reactions usually involve things being split apart or joined together.

The substrate is the molecule changed in the reaction.

Every enzyme has an active site (the part where it joins to the substrate to catalyse the reaction).

Enzymes usually only work with one substrate (high specificity). 

For the enzyme to work the substrate has to fit in to the active site. 

If the shapes don't match then the reaction won't be catalysed 

This is called the "lock and key mechanism"

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Measuring the rate of an enzyme controlled reactio

Use amylase as the enzyme to catalyse the breakdown of starch (the substrate).

You can time how long it takes for the starch to disappear.

Regularly take a drop of the amylase+starch mixture and put it onto a drop of iodine on a spotting tile.

The mixture will turn blue/black if starch is present.

Repeat this experiment under different conditions to explore different reactions rates.

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Effect of substrate concentration on reaction rate

Substrate concentration affects the rate of reaction up to a point.

The higher the substrate concentration the faster the reaction because the enzyme is more likely to meet up with a substrate particle.

This is only true up to a point though. After that there are so many substrate particles that all active sites are full so a further increase in substrate particle concentration has no affect anymore.

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Advantages of the human genome project?

1. Predict and prevent diseases. If doctors knew what genes lead to a disease then we could get individual advice on diet and lifestyle to avoid the disease. Also more regular checks could be made to ensure erly diagnosis. Furthermore, cures could be found for genetic disease such as cystic fibrosis and sickle cell anaemia.

2. Develop new and better medicines. In the future we could have medicines designed especially for us based on our bodies reaction to the disease and treatments. More effective treatments can be deisgnedby studying their effect on us.

3. Accurate diagnosis. Some diseases are hard to test for (eg Alzheimer's) but if we know the genetic cause then accurate testing becomes easier.

4. Improve forensic science. Scientists can produce a 'DNA fingerprint' from a crime scene which could be matched against suspects. In the future suspects may be able to identify a criminals appearence from their DNA.

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Disadvantages of the human genome project?

1. Increased stress. If someone knew from an early age that they're susceptible to a disease then they could panic every time they get mild symptoms (eg a headache for a brain disease)

2. Gene-ism. People with genetic problems could come under pressure to not have children.

3. Employer+insurance discrimination. Life insurance could become impossible or very expensive for people with a likelihood of getting a genetic disease. Employers may not hire people who are likely to get a disease.

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Why is genetic engineering controversial?

Some people say growing GM crops will reduce the amount of weeds, flowers and wildlife or a reduction in farm biodiversity.

Some people think GM crops are unsafe because of a risk of developing allergies. However GM crops are no more dangerous than eating any non GM food.

Some people think that transplanted genes may escape into the natural environment and wreak havoc by creating herbicide resistance in weeds for example.

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