biology topic 1

HideShow resource information

Cells and Microscopy

Plant and animal cells have similarities and differences, they have 4 things in common nucleus (contains DNA, controls it), cytoplasm (contains chemical reaction), cell membrane (controls what goes in and out, holds it together) and the mitochondria (where most reactions for respiration happen). Plant cells have an extra 3 things rigid cell walls (made of cellose gives support), large vacuole (contains cell sap) and chloroplasts (where photostnthesis occurs, contains chlorophyll).

Bacteria cells are a lot smaller they have chromosonal DNA (controls cells activities and replication, floats in cytoplasm), plasmids (small loops of extra DNA that arent apart of chromosone contain genes for things like drug resistance, can be passed between bacteria), flagellum (long hair like structure, rotates to makes bacteria move) and cell walls (supports it).

Cells are studied using microscopes. light microscopes let us see things like nuclei, chloroplasts and mitochondria. Electron microscopes let us see smaller things in more detail like the internal structure of mitochondria and chloroplasts. Magnification is how much bigger the image is than the specimen, Magnification= length of image/ length of specimen. 

1 of 11


A DNA has 2 strands coiled together in a double helix. They're held together by four chemicals called bases (A, C, G T) they are joined together by weak hydrogen bonds. A gene is a section of DNA and the sequence of bases in a gene code for a specific protein.

Watson, Crick, Franklin and Wilkins discovered the structure of DNA, Franklin and Wilkins worked out that DNA ahd a helix structure by directing beams of X-rays onto crystalised DNA and looking at the patterns as they bounced off. Watson and Crick used these ideas to make a model of the DNA molecule where all the pieces fitted together.

Practical to extract DNA from cells:

Chop up an onion and put it into a beaker containing detergent and salt, the detergent wil break down the cell membranes amd the salt will make the DNA stick together. Put the beaker into a water bath at 60 degrees for 15 minutes, this denatures the enzymes that could digest the DNA, then put the the beaker into ice to cool it down to stop it from breaking down, once it is ice cold put it into a blender for a few seconds to break open the cell walls and release the DNA. Cool the mixture down again, then filter it to remove big bits of cell and froth. Gently add some ice cold alcohol to the filtered mixture, the DNA will start to come out of the mixture as its not soluble in cold alcohol, it will appear as a stringy white substance. 

2 of 11

Protein Synthesis

A gene codes for a specific protein, cells make protein by stringing amino acids together in a specific order. Only 20 amino acids are used to make up tjousads of different proteins. The order of the bases in a gene simply tell the cell what order to put the amino acids. each set of three bases (triplets) codes for an amino acid. DNA also determines which genes are turned on or off, this in turn determines the type of cell it is. Some of the proteins help to make all of the other things that arent made of proteins from substances that come from your diet.

Proteins are made by ribosomes. The cell needs to get information from the DNA to the ribosomes in the cell cytoplasm, this is done usong a molecule called mRNA, it uses a uracil (U) instead of T as a base, but it still joins to A. The DNA is used as a template to make mRNA, base pairing ensures its complementary, this step is called transcription. The mRNA strand then moves out of the nucleus and joins with the ribosome. The job of the ribosome is to stick amino acids together in a chain to make poly peptid, this follows the order of the triplet of bases in the mRNA, this step is called translation. The result of all this is that each type of protein gets is own specific number and sequenve of amino acids- the ones described by its DNA base sequence. This is what makes it fold up into the right shape to do its specific job.

Mutations can be harmful (cause a genetic disorder e.g, cystic fibrosis) neutral (they dont effect the proteins function) or beneficial (make bacteria resistant to antibiotics). 

3 of 11


Enzymes are catalysts produced by living things. They reduce the need for high temperatures and we can only have enzymes to speed up the useful chemical reactions in the body. A catalyst us 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 cata;lyse various reactions, they can work inside or outside the cell e.g. DNA repliction, protein synthesis, digestion etc.

Enzymes have special shapes so they can 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, where it joins onto its substrate, enzymes usually only work with one substrate, they have a high specificity for their substrate. This is because for the enzyme to work the substrate has to fit into the active site, if the shapes dont match, the reaction wont be catalysed, this is the 'lock and key' mechanism.

you can measure the rate of a reaction by using amylase as the enzyme and starch as the substrate. Take a drop of the amylase and starch mixture and put it into a drop of iodine solution on a spotting tile, record the colour change, it'll turn blue- black if starch is present. Note the time when the solution no longer turns blue- black the starch has been broken down by the amylase. You can use the times to compare reaction rates under different conditions.

4 of 11

More on Enzymes

Enzymes like it warm but not too hot. Changing the temperature changes the rate of an enzyme catalysed reaction. A higher temperature increases the rate at first because more heat means more energy so the enzymes and substrates move around more so they're more likely to collide and react. if it gets too hot, some bonds holding the bond together break, this causes it to lose its shape so the active sight no longer fits its substrate so it cant catalyse the reaction. The enzyme is denatured- it wont go back to its normal shape if it cools down. Each enzyme has its own optimum temperature when the reaction goes fastest, this is the temperature just before it gets too hot. The optimum temperature for the most important human enzyme is about 37 degrees.

Enzymes also need the right pH, if its too high or low it inteferes with the bonds holding the enzyme together, causing it to denature. All enzymes have an optimum pH, it is often neutral pH 7 but not always.

Substrate concentration also affects the rate of reaction up to a point, the higher the concentration the faster the rwcation, it's more likely to collide with an enzyme and react. This is only true up until a point, after that the enzymes active sites are all full so adding more substrate will do nothing.

5 of 11

The Human Genome Project

Thousands fo scientists from all over the world collaborated on the human genome project, the idea was to find every single human gene. Human DNA is made up of 25000 genes. The collaborstion of all the scientists meant that the genes were found quicker and the data could be made public, we have now found all of the genes. 

Whats good about it- We can predict and prevent disease, we could all get individually tailore advice on the best diet and lifestyle to avoid out likely problems and doctors could check us regularly to ensure early detection and treatment. Cures could be found for genetic diseases. Develop new and better medicines, we could have medicine specifically designed for us based on how our individual body will react to disease and the possible treatments. Accurate diagnosis, some diseases are hard to test for, but if we know the genetic cause, accurate testing will be a lot easier. Improve frensic science, it could be possible to figure out what a suspect looks like from DNA found at the scene of the crime.

Whats bad about it- Increased stress, of someone knew they were susceptible to a nasty brain disease they could panic every time they got a headache. Gene-ism, people with a genetic problem could be pressured to not have children. Discrimination by employers and insurers, life insurance could become impossible to get if you have any genetic liklehood of a serious disease.

6 of 11

Genetic engineering

Genetic engineering uses enzymes to cut and paste genes. The basic idea is to moe useful genes from one organisms chromosomes into the cell of another. This technique produces geetically modified (GM) organisms.

Genetic engineering could benefit humans by reducing vitiman A deficiancy, golden rice is a variety of GM rice, it contains genes from organisms that enable it to produce beta-carotene, so we can grow this in places where people suffer from vitiman A defficiancy. Producing human insulin, the human insulin gene can be inserted into bacteria to produce human insulin, it can be produced quickly and cheaply to treat diabetes. Increasing crop yield, GM crops have their genes modified to make them resistant to herbicides, fields can be sprayed with these herbicide and all plants except GM crops are killed.

Genetic engineering is a controversial topic, there are worries about long term effects of it, it might acidentally create problems. Some concerns are that growing GM crops will affect the number of weeds and flowers around the crops, reducing farmland diversity. Not everyone is convinced GM crops are safe, people are concerned they may develop allergies. A big concern is that transplanted genes may get out into the natural enviroment.

7 of 11


Mitosis makes new cells for growth and repair. Human body cells are diploid, this means they have two versions of each chromosomes- one from the mother and the other from the father. When a cell divides it makes two cells identical to the original, this type of cell divison is mitosis, it's used when humans want to grow or to replace cells that have been damaged.

Mitosis results in two identical cells. In a cell thats not dividing the DNA is spread out in long strings. If the cell gets the signal to divide the DNA is copied and forms X- shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other. The chromosomes then liene up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell. Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells. Lastly the cytoplasm divides. 

Asexual reproduction also uses mitosis Some organisms reproduce by mitosis, this is an example of asexual reproduction, the offspring have exactly the same genes as the parent so theres no genetic variation.

8 of 11


Gametes are sex cells, they have half the usual number of chromosomes, in sexual reproduction two gametes combine to form a new cell. Gametes are haploid- this means they only have one copy of each chromosome, this is so that when two combine, the resulting cell (zygote) has the right number of chromosomes

Meiosos ivolves two divisions, it only happens in the reproductive organs. Meiosis is when a cell divides to produce four haploid nuclei whose chromosomes are not identical.

Division 1- before the cell starts to divide, it duplicates its DNA, one arm of each copy is an exact copy of the other. In the first division in meiosis the chromosome pairs line up in the centre of the cell, they're then pulled apart, so each new cell has one copy of each chromsome. Each new cell will have a mix of the mothers and fathers chromosomes, mixing up the allels in this way creates variation, this is a huge advantage of sexual reproduction over asexual.

Division 2- The chromosomes line up again in the centre of the cell, the arms of the chromosomes are pulled apart. You get four haploid gametes, each with only a single set of chromosomes in it. 

9 of 11

Cloning Mammals

Cloning is a type of asexual reproduction. Adult cell cloning involves taking an unfertalised egg cell and removing its nucleus, a nucleus from an adult body cell is inserted into the 'empty' egg cell. The egg cell is then stimulated by an electric shock- this makes it dovide by mitosis like a normal embryo. When it is a bag of cells it is implanted into an adult femaleto grow into an identical copy of of the original adult cell 

Cloning has many uses, it could help with the shortage of organs for transplants. The study of animal clones could lead to greater understanding the developement of the embryo and of age related illnesses. It could also be used to preserve endangered species.

There are many issues surrounding cloning. Cloning mammals leads to reduced gene pool, so if a population is closely related and a new disease appears, the whole population could be wiped out. Cloned mammals might not live as long, dolly the sheep only lived for six years, she was put down because she had lung disease and arthiritis, these are usually more common in older sheep, Dolly was cloned from an older sheep her true age could have been older, but she could have just been unlucky. Other risks associated to cloning are that it often fails, clones are often born with genetic defects and cloned mammals immune systems are sometimes unhealthy- so they suffer from more diseases.

10 of 11

Stem Cells

Embryonic stem cells can turn into any type of cell. To start with the cells in the embryo are all undifferentiated, stem cells are able to divide to produce more stem cells or different types of specialised cell. In most animals the ability to differentiate is lost at an early stage, but lots of plant cells dont ever lose this ability. Adult humans only have stem cells in certain places lime bone marrow, they arent as versatile at the ones in embryos. 

Stem cells may be ably to cure diseases. Scientists have experimented with extracting stem cells from very early embryos and growing them. Under certain conditions they will differentiate into specialised cells. It might be possible to use stem cells to create specialised cells to replace those damaged by injury or disease, this potential for new cures is the reason for the huge scientific interest in stem cells. Before this can happen, a lot of research needs to be done, there are also ethical concerns about this- some poeple are strongly against it as embryos are a potential for human life, so shoukdnt be experimented on, others beieve that curing patients who are suffering should be more important than the potential life of embryos. In some countries stem cell research is banned, it is allowed in the UK under strict guidelines. 

11 of 11


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all genes and enzymes resources »