B2 Topic 1 - Genes and Enzymes

Animal cells contain:

• Nucleus - contains DNA that control the cell
• Cytoplasm - most chemical reaction happen
• Cell membrane - controls what goes in and out the cell and holds it together 
• Mitochondria - most reactions for respiration occur to releases energy for the cell to work 

Plant cells contain all of the animal cells components and these:

• Rigid Cell Wall - made of cellulose for supporting the cell
• Large Vacuole - contains cell sap (a weak solution of sugar and salts)
• Chloroplasts - where photosynthesis occurs (contains chlorophyll)

Bacteria cells:

• Have no nucleus
• Chromosomal DNA - controls the cell's activities and replication (floats freely in the cytoplasm)
• Plasmid DNA - small loops of extra DNA that aren't part of the chromosome. They contain genes for drug resistance and can be passed between bacteria
• Flagellum (NOT ALL BACTERIA CELLS HAVE THIS) - a long, thin structure that rotates to make the cell move
• Cell wall - support

Microscopes:

• Light microscopes - invented in the 1590s, they let us see nuclei, chloroplasts and mitochondria
• Electron microscopes - invented in 1930s, they let us see in more detail smaller objects such as the structures of mitochondria, chloroplasts and plasmids
• Magnification = length of image / length of specimen

DNA - a double helix of paired bases:

• The two strands are held together by chemicals called bases:

     Adenine (A), Cytosine (C), Guanine (G), Thymine (T)

• Base-pairing -      A and T     or     C and G
• Base pairs are joined by weak hydrogen bonds
• A gene is a section of DNA and the sequence of bases in a gene code for a specific protein

Discovery:

• Rosalind Franklin and Maurice Wilkins worked out that DNA had a helical structure by directing beams of x-rays onto crystallised DNA and looking at patterns the x-rays formed as they bounced off
• James Watson and Francis Crick created a model of the DNA molecule where all the pieces fitted together

Extracting DNA from cells:

1. Chop up an onion and put it in a beaker containing a solution of detergent and salts. The detergent will break down the cell membrane and the salt will make the DNA stick together.

2. Put the beaker in a water bath at 60°C for 15 minutes to denature the enzymes that could digest the DNA and helps to soften the onion cells.

3. Put the beaker in ice to cool the mixture down to stop the DNA breaking down.

4. Put it into a blender for a few seconds to break open the cell walls and release the DNA.

5. Cool the mixture down again and then filter it.

6. Gently add some ethanol to the mixture. The DNA will begin to come out of the solution as it's not soluble in ethanol. It will appear as a stringy-white substance that can be fished out carefully with a glass rod.

DNA is a list of instructions on how to make all the proteins in your body.

A gene codes for a specific protein:

• A gene is a section of DNA that contains the instructions to make a specific protein
• Cells make proteins by stringing amino acids together in a certain order
• Only 20 different amino acids are used to make up thousands of different proteins
• Some proteins help to make other things that aren't made of proteins from substances that come from your diet

The order of bases in a gene tells cells in what order to put the amino acids together: each set of three bases (called a triplet) codes for a particular amino acid

DNA determines which genes are switched on or off so which proteins the cell needs to produce, which determines what type of cell it is

Proteins are made by organelles called ribosomes.

DNA is found in the nucleus and can't move out of it, The cell needs to get the information from the DNA to the ribosome in the cell cytoplasm. This is done by using a molecule called mRNA (which is like DNA but shorter and a single strand). mRNA is like a messenger between the DNA in the nucleus and the ribosome.

1. The two strands of DNA unzip and base pairing ensures it's complementary. This is called TRANSCRIPTION.

2. The mRNA molecule moves out of the nucleus and joins with the ribosome.

3. Amino acids that match the mRNA code are brought to the ribosome by molecules called tRNA.

4. The job of the ribosome is to stick amino acids together in a chain to make a protein. This follows the order of the triplet of bases (called codons) in the mRNA. This is called TRANSLATION.

A mutation is a change to an organisam's DNA base sequence which could affect the sequence of amino acids in the protein. This could affect the shape of the protein and so its function. This could affect the characteristics of the organism.

Mutations can be harmful, beneficial or neutral.

Harmful - A mutation could cause a genetic disorder, such as cystic fibrosis.

Beneficial - A mutation could produce a new characteristic that is beneficial to an organism. such as in the genes of a bacteria plasmid that can make the bacteria cell resistant to antibodies.

Neutral - Some mutations are neither harmful or beneficial, such as they don''t affect a protein's function.

Enzymes are catalysts produced by living things (biological cataylsts in this case):

• Living things have thousands of different chemical reactions going on inside them constantly
• These reactions are carefully controlled to get the right amounts of substances
• Raising temperatures usually increases the rate of a reaction
• Enzymes reduce the need for high temperatures and humans only have enzymes to speed up the useful chemical reactions in the body

A cataylst is a substance which increases the speed of a reaction without being changed or used up in the reaction.

• Enzymes are all proteins and work in the same way to catalyse reactions
• They can work both in and out of cells:

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

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

The substrate is the molecule changed in the reaction.

Every enzyme has an active site where the part joins on to its substrate to catalyse the reaction.

Enzymes only work with one substrate molecule.

For an enzyme to work, the substrate has to fit into the active site. If the substrate's shape doesn't match the active site's shape, then the reaction won't be catalysed. This is called the "lock and key" mechanism.

Measuring the rate of an enzyme-controlled reaction:

1. You can measure the rate of a reaction by using amylase as the enzyme and starch as the substrate.

2. Amylase catalyses the breakdown of starch so you can time how long it takes for the starch to disappear.

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

4. Record the colour change as it will turn blue-black if starch is present.

5. You can use the times to compare reaction rates under different conditions.

Temperature:

• The higher the temperature, the quicker the rate of reaction as there are MORE FREQUENT SUCCESSFUL COLLISIONS
• However, if it is too hot, some bonds will break that hold the enzyme together. This means that the enzyme loses its shape so its active site doesn't fit the shape of the substrate molecule and can't catalyse the reaction so the reaction stops
• The enzyme is denatured
• Each enzyme has an optimum temperature when the reaction is at its fastest. For human enzymes, the optimum temperature is 37°C - the same temperature as the human body

pH levels:

• It the pH is too high or too low, it interferes witht he bonds holding the enzyme together which changes the shape of the active site and denatures the enzyme

Substrate concentration:

• The higher the concentration, the faster the reaction until all the active sites are full

Human DNA is made up of about 25,000 genes which were mapped by scientists and the data made public.

Good: Improving medicine and forensic science

• Predicting and preventing diseases
• Developing new and improved medicines
• Accurate diagnoses
• Improve forensic science

Bad:

• Increased stress
• Gene-ism (pressured to not have children if they have genetic problems)
• Discrimination by emplyers and insurers

Genetic engineering uses enzymes to cut and paste genes:

1. A useful gene is "cut" out from one organism#s chromosome using enzymes

2. Enzymes are them used to cut another organism's chromosome and then insert the useful gene

3. This produces genetically modified (GM) organisms

This can benefit humans by:

• Reducing vitamin A deficiency - golden rice is a variety of GM rice
• Producing human insulin - produced quickly and cheaply to treat diabetes
• Increasing crop yield - resistant to herbicides to make more food without killing any crops

Controversial:

• Reduce farmland biodiversity
• Allergies and safety
• They may get out into the natural environment

• Human body cells are diploid (pairs of chromosomes)
• When a cell divides it makes two cells identical to the original
• This type of cell division is called mitosis and is used when humans (and animals and plants) want to grow or repair or replace damaged cells
• Asexual reproduction uses mitosis so there is no genetic variation

1. The DNA is spread out in long strings

2. The DNA is copied and forms X-shaped chromosomes that are identical

3. The chromosomes line up at the centre of the cell and cell fibres pull them apart and each chromosome goes to opposite ends of the cell

4. Membranes form around each of the sets of chromosome and become nuclei of the two new cells

5. The cytoplasm divides and now there are two new diploid cells that are genetically identical

• Gametes are sex cells and haploid (single - one copy of each chromosome)
• During sexual reproduction (fertilisation) the two gametes combine to form a new cell which will grow to become a new organism
• The resulting cell (zygote) has the correct number of chromosomes (diploid - two copies of each chromosome)
• Meisos of sexual reproduction over asexual reproduction is a huge advantage as it creates variation in the offspring

1. The cell begins to divide and duplicates its DNA

2. In the first division, the chromosome pairs line up in the centre of the cell and are pulled apart so that each new cell has only one copy of each chromosome

3. Each new cell has a mixture of both the mother's and father's chromosomes

4. In the second division, the chromosomes line up in the centre of the cell and are pulled apart

5. There are now 4 haploid gametes that each have a single set of chromosomes in it

Cloning is a type of asexual reproduction as it produces cells that are gentically identical to the original cell

1. Adult cell cloning involves taking an unfertilised egg cell and removing the nucleus

2. A nucleus is taken from an adult body cell (such as a skin cell). This is a diploid nucleus containing the full number of chromosomes

3. The diploid nucleus is inserted into the "empty" egg cell

4. The egg cell is then stimulated by an electric shock to divide by mitosis (like a normal embryo)

5. When the embryo is a ball of cells, it's implanted into the surrogate mother to grow into a genetically identical copy (clone) of the original adult body cell

Benefits:

• Shortage of organ transplants
• Studies for a greater understanding of the development of the embryo, ageing, and age-related disorders
• Preserve endangered species

Problems:

• "Reduced gene pool" - fewer different alleles in a population. If a population are too closely related and a new disease appears it could wipe out the species and there is no allele giving resistance to the disease
• Shortened lifespan
• Risks - failure, genetic defects and poor immune systems so are more likely to suffer from diseases and illnesses

Embryonic stem cells can turn into any type of cell:

1. A fertilised egg can divide by mitosis to produce the embryo of the new cell

2. The cells in the embryo undifferentiatied to begin with and are called embryonic stem cells

3. Stem cells are able to divide to produce either more stem cells or different types of a specialised cell (such as blood cells)

4. The stem cells become more specialised, called differentiated. This is when the embryo starts to develop a recognisably human body with organs and systems (in most animal cells, the ability to differentiate is lost at an early stage buy lots of plant cells don't lose this ability ever)

Adult humans only have stem cells in certain places, such as bone marrow. These aren't as versatile as embryonic stem cells as they can only differentiate into certain types of cells.

Benefits:

• Cures for some diseases, such as sickle cell anaemia
• Replace damaged specialised cells

Controversial:

• Potential human life lost
• Unwanted embryos
• Banned in many countries