B2 - Edexcel Revision

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  • Created by: Payall
  • Created on: 09-06-13 17:13

Animal, Plant and Bacterial Cells

All living things are made of cells.

- Nucleus =
contains DNA that controls what the cell does
- Cytoplasm = 
gel-like substance where most of the chemical reaction takes place
- Cell Membrane =
holds the cell together and controls what goes in and out
- Mitochondria =
where most of the reactions for respiration takes place

- Rigid Cell wall =
made of cellulose, gives support for the cell
- Large Vacuole =
contains cell sap ( a weak solution of salts and sugars)
- Chloroplasts =  
where photosynthesis occurs

- Chromosomal DNA =
one longular cirular chromosome, controls the cells activities
- Plasmids =
loops of extra DNA. Contains genes for things like drug resistance.
- Flagellum =  
Long hair-like structure that allows the bacterium to move
- Cell Wall =
supports the cell

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DNA molecule has two strands coiled togther in a double helix shape. Strands are held together by chemicals called bases. The four bases are adenine. thymine, guanine and thymine. Adenine pairs with thymine and cytosine pairs with guanine.Bases are held together by weak hyrdogen bonds.

Rosalind Franklin and Maurice Wilikns found out that DNA had a helical structure by directing beams of x-rays of cyrstalised DNA and looking at the patterns the x-rays formed.

James Watson and Francis Crick used these ideas along with the base pairing knowledge and made a model where all the pieces fitted together.

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Protein Synthesis

Protein are made by organelles called ribosome.Here are the steps to protein synthesis:

1) The two DNA strands unzip. The DNA is used as a template for mRNA. This step is called TRANSCRIPTION.

2) The mRNA molecules moves out of the nucleus and joins with a ribosome.

3) Molecules called tRNA bring amino acids that matches the mRNA molecule to the ribosome.

4) The job of the ribosome is to stick the amino acids in a chain called polypeptides. This step is called TRANSLATION.

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A mutation is a change to an organisms DNA base sequence. There are three main types:

HARMFUL = A mutation could cause a genetic disorder for example cystic fibrosis

BENEFICIAL = A mutation could produce a new characteristic beneficial to the organism 

NEUTRAL = They dont affect a proteins function.

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Enzymes: Catalysts

A cataylst is a substance which increases the speed of the reaction wihtout being changed or used up in the reaction. There are three enzyme controlled chemical reactions that happen in our bodies

DNA REPLICATION - enzymes help copy a cell's DNA before it divides by mitosis or meiosis.

PROTEIN SYNTHESIS - enzymes hold amino acids in place and form bonds between them

DIGESTION - various enzymes are secreted into the gut to digest different food molecules.

Enzymes have an active site - the part where it joins to its substrate to catalyse the reaction. Enzymes have a high specificty for their substrate. If the substrate doesn't match the active sites shape then the reaction won't be catalysed. This is like the 'lock and key mechanism'

Enzymes like it warm but not too hot otherwise they will be denatured. They also like the right pH, usually at pH 7 but sometimes at pH 2. Finally the subtrate concerntration affects the rate of the reaction.

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The Human Genome Project

The main plan was to map out the 25,00 or so Human genes. Thousands of scientists around the world worked together .








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Genetic Engineering

The basic idea is to cut out the useful gene from one organisms chromosome into the cells of another:

1) A useful gene is cut out from one organisms chromosome using enzymes.
2) The same enzyme is then used to cut another organisms chromosome and then to insert the useful gene. 

There are 3 example of how G.E can benefit humans:
- Reducing vitamin A deficiency - helps prevent thousands from children from going blind.
- Producing human insulin - used to treat diabetes very quickly and cheaply.
- Increasing crop yield-  making crops that are resistant to herbicides for example.

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Mitosis and Meiosis

Mitosis makes new cells for growth and repair. When a cell divides it makes two cells identical to the original cell- each with a nucleus containing the same number of chromosomes as the original one. Asexual reproduction also uses Mitosis e.g strawberry plants form runners in this way.

Gametes are sex cells ( ova in females and sperm in males). Gametes are haploid meaning they only have one copy of each chromosome, so when the sperm and ova combine you get 46 chromsomes. 

Meiosis involves two divisions and only happen in the reproductive organs. It produces four haploid nuclei whose chromosomes are not identical.

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Cloning part A

Cloning is a type of asexual reproduction. Here's how it's done:

1) Take an unfertilised egg cell and remove its nucleus.

2) A nucleus is taken from an adult body cell. This is a diploid nucleus containing the ful number of chromosome. 

3) The diploid nucleus is inserted into the empty egg cell

4) The egg cell is then stimulated by an electric shock - this makes it divide by mitosis.

5) When the embryo is a ball of cells, its implanted into an adult femail.

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Cloning part B

Cloning has many uses:

- help with shortage of organs for transplants

- help have a greater understanding of the development of the embryo 

- could help preserve endangered species.

Although there are may issues surrounding cloning:

- It could lead a reduced gene pool which basically means that there are fewer different allels in the population.

- Cloned animals might not live as long

- It often fails and takes several attempts to get perfect

- Some cloned animals could be born with genetic defects.

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Stem cells

Stem cells are able to divide to produce either more stem cells or different typees odfspecialised stem cells. The process of stem cells becoming specialised is called differentiation. Adult humans only have stems cells in certain places for example bone marrow.

Stem cells may be able to cure many diseases.

- Sickle cell anaemia can sometimes be cured with a bone marrow transplant.

- scientists are able to expermient with them

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On page two the word thymine is written twice. One of them should be cytosine

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