Edexcel Biology Topics 3 & 4
Edexcel Biology Topics 3 & 4
Blood group alleles are represented by I^A and I^B and I^O. I^A and I^B are the codominant alleles- this is how the blood group AB exists. However both are dominant over I^O
If the organism has two of the same alleles then it is known as homozygous- this can be dominant or recessive.
If the organism has two different alleles then this is known as heterozygous- here the dominant trait will be expressed.
The inheritance of a single characteristic is called monohybrid inheritance. You can use different diagrams to show this such as punnett squares, genetic diagrams and pedigree diagrams.
Blood group is determined by 3 alleles instead of two and is an example of co dominance- this is where two alleles are not dominant over each other but are dominant over another.
Chromosome- a long coiled up strand of DNA in the shape of an X
Gene- a section of DNA that codes for a particular protein.
Genome- the entirety of an organisms genes.
Allele- the different version of the same gene.
Dominant- occurs more often/ will be displayed over a recessive trait.
Recessive- will only be displayed if there's two of it.
Genotype- the biological expression of a trait/ the DNA of a trait.
Phenotype- the physical expression of a characteristic.
Gamete- a sex cell eg. sperm or egg.
Zygote- fertilised egg cell.
You can predict blood type by using punnett squares however unless it's I^O I^O then it will be most likely, A or B.
Some genetic disorders are sex linked meaning it's due to the X and Y chromosome. Sex linked disorders are mainly prevalent in men since women have 2 X chromosomes so if there is a fault on one of the chromosomes then there will be another version of the gene/alleles to 'override' it. However, the Y chromosome is a lot smaller than the X so if there is a fault on the X chromosome, it is likely that there won't be another copy- resulting in the expression of the genetic disorder even if it's recessive.
Colour Blindness is a sex-linked disorder s the faulty allele it's caused by is on the X chromosome. Usually women aren't colour blind but are a carrier of it meaning it could be expressed in their children as they might carry a faulty allele. The faulty allele doesn't have another copy in men and they only need one version of the recessive allele in order to display it, hence why colour blindness is significantly more common in men.
Most phenotypic features are as a result of multiple genes rather than just single gene inheritance.
The Human Genome Project
This was the project started in conjunction with thousands of scientists across the world to map the entirety of the human genome.
This has allowed scientists to identify 1800 genes related to diseases
It is now about finding what all the genes they sequenced do.
There are loads of benefits to this projects such as:
The prediction and prevention of diseases- advice could be individually tailored to each person in order to prevent diseases.
Inherited disorders such as cystic fibrosis could be earlier diagnosed and it would help to create better treatments or a cure.
Knowing how a disease affects us on a molecular level should make it possible to design more effective treatments with fewer side effects.
They can determine how well an existing drug will work for an individual and the dosage to go with it.
However there are drawbacks-
Increased stress- if someone knew from an early age that they're susceptible to a nasty brain disease, they could panic every time they get headache.
Gene-ism- there could be a pressure on people with certain diseases/ disorders not to have children.
Discrimination by employers and insurers- life insurance would be horrifically high or impossible to get. They may not get hired if they are more genetically inclined to get a disease.
Alleles are displayed with letters, dominant traits always have a capital and recessive traits have a lower case letter. eg. cystic fibrosis is displayed with an 'f' because its a recessive disorder.
Mendel bred pea plants in order to note how characteristics were passed on from one generation to the next.
Mendel crossed a tall pea plant and a small pea plant to see that they produced all tall peal plants.
Then he bred two of the tall offspring together and found that they produced 3 tall offspring for every 1 small pea plant overall. When 2 hybrid plants are crossed, they produce a 3:1 ration of their characteristics.
He also found that it was not only height characteristics that were passed on this way as he showed that certain flower colours were dominant over others as they appeared more often.
He reached 3 conclusions:
Characteristics were passed on by 'hereditary units'.
'Hereditary units' are passed on unchanged from parent to offspring
They can be dominant or recessive.
His work was significant as it allowed the mechanism of inheritance to be explained fully as it never had been before.
Alleles are different versions are different versions of the same gene and can be dominant or recessive.
For a phenotype to express the dominant trait, it needs only 1 dominant allele whereas for the phenotype to display the recessive characteristic, there must be both recessive alleles in the genotype.
He found that height characteristics were passed by on 'hereditary units' and that they could be 'dominant' or not which we now classify as alleles and dominant and recessive.
Sex is determined at fertilisation because the sperm will carry either an X chromosome or a Y chromosome. The egg is always X. There is a 50% chance of it being a girl or a boy due to the sperm, what the sperm carries will determine the sex of the zygote. If it's XX then it's a girl and if it's XY then it's a boy. The chances of each can be shown here:
In a population there will be genetic and environmental variation. Usually genetic variation in a population will be quite high due to natural mutations and the inheritance of those in offspring, also the mixing of gametes by sexual reproduction. Environmental variation is something that the place an organism is in will affect eg. amount of light or water for a plant.
There are also acquired characteristics which happen through the lifetime of an organism but aren't to do with the environment or genes and so can't be passed down. An example is the loss of a limb.
Most variation in a phenotype is as a result of both the environment and the genetic variation. For example: the maximum height that a plant can grow to is determined by its genetics but whether it will actually reach that height is determined by the environment.
Mutations occur in the DNA and change the base sequence. When they occur within a coding region for a protein, they result in an allele, a different version of a gene. They don't always have a big effect on the phenotype of an organism- most are
.Occasionally, mutations can be
and may correct a fault that had occurred previously and so better the function or efficiency of a protein. Sometimes these can have a very small effect on the phenotype. Mutations can also be
, they cause a change which may mean the protein can no longer carry out its function and this means it will have a significant effect on the phenotype.
Darwin was the guy that came up with the theory of evolution by natural selection. He spent 5 years on a voyage around the world studying plants and animals on a ship called HMS Beagle. He noticed that there were variation in members of the same species- finches in the Galapagos- and that those characteristics most suited to the environment were more likely to survive. He noticed these characteristics could be passed on to offspring. He wrote his theory of evolution by natural selection.
Alfred Russell Wallace was a scientist working at the same time as Darwin. He came up with the idea of natural selection independently too. Him and Darwin published their papers on evolution together and acknowledged each other's work although they didn't always agree on the mechanisms involved in natural selection. Wallace's observations provided lots of evidence to help support the theory of evolution by natural selection.
is a fossil dated to 4.4 million years ago which was found in Ethiopia. The structure of her feet showed that they climbed trees but was suited to bipedalism- as also shown in the structure of her hands. She had the same brain size of a chimpanzee and had long arms and shot legs which is very reminiscent of apes.
is a fossil dated to 3.2 million years ago found in Ethiopia and has a mixture of ape and human features. Lucy had arched feet which was more adapted to walking than climbing. She walked upright and more efficiently than Ardi's. The size of her legs and arms were between what you'd expect between an ape and a human. Her brain was slightly larger than Ardi's but still similar to the size of a chimps.
Turkana boy or
who was dated to 1.6 million years and found in Kenya. He walked better than Ardi and Lucy due to the structure of his legs and feet. He had short arms and long legs which was much more similar to humans than apes. His brain size was much larger than Lucy's- similar to human brain size.
There were tools that aided the dating and the evaluation of evolution for each of the homo species.
- made simple stone tools called pebble tools by hitting rocks together to make sharp flakes- could be used to scrape meat from bones or crack bones from meat.
- sculpting rocks into shapes to produce more complex tools like simple hand axes. These could be used to hunt, dig, chop and scrape meat from bones.
- More complex tools, evidence of flint tools, pointed tools and wooden spades.
- flint tools widely used. Pointed toils including arrowheads, fish hooks, buttons and needles appeared around 50,000 years ago.
Fossils can be dated using multiple methods such as statigraphy (older rock layers are normally found below younger layers so tools or fossils in deeper layers are usually older) and carbon dating.
Bacteria provides evidence for evolution by showing that bacteria develop random mutations in their DNA which allows them to be more adapted to their environment in which antibiotics (a selection pressure) are present. They can create new alleles, which can change the bacteria's characteristics like that the bacterium could become less affected by a particular antibiotic. For the bacterium, the ability to resist the bacteria is a big advantage. In a host who's being treated to get rid of the infection, a resistant bacterium is better able to survive than a non-resistant bacterium and so it lives for longer and reproduces for longer.
This leads to the allele for antibiotic resistance being passed on to lots of offspring- natural selection. This is how it spreads and becomes more common in a population of bacteria over time.
A fossil is any trace of an animal or plant that lived a long time ago and are commonly found in rocks. Generally, the deeper the rock, the older the fossil. By arranging the fossils in chronological order, gradual changes in organisms can be observed.
Theories of evolution have influenced modern biology by classification and antibiotic resistance and conservation. It shows how things have evolved from the common ancestors and continue to evolve.
Those individuals with characteristic that make them better adapted to the selection pressures in their environment and therefore are more likely to breed successfully
This means the useful alleles are going to be passed onto the offspring,
Some will be lesser adapted and will be less likely to survive and reproduce. The beneficial characteristics become more common in the population overtime.
This is known as survival of the fittest
A pentadactyl limb is a limb with 5 digits which can be seen in many species like mammals and reptiles.
The similarity between bone structure of pentadactyl limbs of different species provides evidence that these species have evolved from a common ancestor.
Individuals in a population show genetic variation because of differences in their alleles- new alleles are created through mutations.
Things like predation, competitions for resources and disease act as selection pressures.
Living things are classified into 5 kingdoms which are:
The further subdivisions in the taxonomic system are: phylum, class, order, family, genus, species.
Over time as technology increased and so did our scientific understanding. We can now determine the sequences of DNA bases in different organisms' genes and compare them- the same thing can be done with RNA sequences. This can determine how closely related they are. This led to a reform of the taxonomic system and a suggestion of sorting the kingdoms into Domains called Archea, Eukarya and Bacteria since Woese found that some members of the Prokaryote kingdom were not as closely related as originally thought. This kingdom would be divided into Archea and Bacteria.
Tissue cultures can be useful in medical research because it means that you can carry out all kinds of experiments on tissues in isolation. It means that you can look at the effects of a particular substance or environmental change on the cells of a single tissue without complication from other processes in the whole organism.
Selective breeding is artificially selecting the plants or animals that are going to breed so that a certain characteristic will remain in a population. They
It can be very useful in
for example. genetic variation means that some cattle will have better characteristics for producing meat than others. To improve meat yields, a farmer could select cows and bulls with these characteristics and breed them together.
It can be very useful in
too. For example, In several studies investigating the reasons behind alcoholism, rats have been bred with either a strong preference for alcohol or a weak preference for alcohol. This has allowed researchers to observed the differences in their behaviour
However, it can have disadvantages. One is that it reduces the gene pool because all the best animals are always used for breeding and are all closely related- this is known as inbreeding. Inbreeding can cause health problems because there's more chance of organisms inheriting harmful genetic defects when the gene pool is limited. This leads to ethical considerations. A limited gene pool can also cause problems if a new disease appears. There's mot much variation so there's less chance of resistance alleles being present.
A tissue culture is growing cells on an artificial growth medium. Whole plants can be grown via tissue culture and it's very easy and useful since plants can be made all year round and very quickly. These plants are clones - genetically identical organisms- they can be grown by removing several small pieces of the parent plant (best from shoot tip or root tip) and grow the tissue on a growth medium containing nutrients and growth hormones and is done under aseptic technique. As the tissues produce shoots and roots they can be moved to potting compost to carry on growing.
Organisms are selectively bred for features that are useful or attractive. Eg. animals that make a lot of milk or meat or crops that are disease resistance.
The DNA you want to insert is cut with restriction enzyme and the vector DNA is cut open using the same restriction enzyme.
The vector DNA and concerned DNA are left with the complimentary sticky ends and are 'glued' back together with DNA ligase. The plasmid is now a recombinant plasmid. The recombinant plasmid is reinserted back into the bacterium. When the bacterium reproduces it will reproduce with the ability of whatever the gene which was translated gives.
Crops in agriculture can be genetically modified to be resistant to herbicides which can increase crop yields by allowing farmers to kill weeds without affecting crops.
In medicine, it can be used to engineer bacteria to produce human insulin or transplanting human genes into sheep and cows for useful proteins. However there are concerns about genetically modifying things since we don't know the long term effects of it and there are ethical concerns. Also, GM crops could get a transplanted gene to the environment and weeds creating 'superweeds'.
Crops can be genetically modified to be pest resistance which can improve crop yields and reduce the need for chemical pesticides. There's a bacterium called
which produces a toxin that kills many of the insect larvae that are harmful to crops.The gene for Bt toxin is inserted into crops, such as corn and cotton, which then produce the toxin in their stems and leaves- making them resistant to the insect pests. The toxin is specific to insect pests- harmless to humans, animals and other insects. However, the long term effects of exposure to Bt crops aren't yet known. The insects that feed on the crops are constantly exposed to the toxin so there's a chance that they'll develop resistance and no longer be killed by it.
GMOs can be used to increase food production for the rapidly increasing global population and increase food security. Some crops can be engineered to combat certain deficiencies such as Golden Rice which helps with Vitamin A deficiency.
However, many people argue that the food is not the issue or a shortage of food it's just that they have no money to buy it. Therefore poverty must be tackled first. There's fears that countries may become dependent on companies who sell GM products and also sometimes, the soil is the reason that the crops fail and so even GM crops won't grow.
Using GMOs is a relatively new way of improving crop yields but may not always be helpful because, for example, if soils are poor then applying fertilisers is likely to be the best way of improving yields but excess fertilisers can cause eutrophication. Pests can be controlled without the use of GM methods such as biological control. This can have longer lasting effects than chemical pesticides and be less harmful to wildlife but introducing non indigenous species can cause issues like them becoming a pest.
Advantages of asexual reproduction:
the reproduction cycle is quicker
doesn't require 2 mates
this allows an organism to colonise a new area very rapidly.
Disadvantages of asexual reproduction:
there's no genetic variation in the population so they're more susceptible to disease.
Advantages of sexual reproduction:
there's genetic variation in the population so they're more resistant to disease.
natural selection and evolution can take place because there is two parents.
Disadvantages of sexual reproduction:
there's a requirement to make a mate which can be an issue is the organism is isolated.
Sexual reproduction cycle takes more time.
Meiosis is the type of cell division that includes only gametes. It produces 4 genetically different daughter cells with a haploid nucleus. It forms gametes because it has two divisions rather than one like mitosis. It is vital to reproduction.
DNA strands are polymers made of repeating units called nucleotides. Each nucleotide has a sugar molecule, a phosphate molecule and a base (A, C, T, G). The sugar and the phosphate molecules alternate. A DA molecule has two strands coiled together in the shape of a double helix. Each base links to the complimentary base in the opposite helix. A links to T and C links to G. The complimentary pairs are joined together by weak hydrogen bonds.
mash up strawberries and put them into a solution of detergent and salt. Mix well.
The detergent will break down the cell membranes to release the DNA and the salt will cause the DNA to stick together.
Filter the mixture to get the froth and big, insoluble bits of cell out,
Gently add ice cold alcohol to the filtered mixture.
The dna will appear on the top as a stringy white precipitate which can be removed with a glass rod.
Proteins are made from folded up amino acid chains. Certain codons of bases code for an amino acid eg. ATG = met.
Each amino acid chain folds up to give a specific, different shape which allows different functions. This is why enzymes have different active site shapes. Many regions of DNA are non-coding which means they don't code for an amino acid. However, these regions are still involved in protein synthesis.
A mutation is a rare, random change to an organism's DNA base sequence that can be inherited. If a mutation happens in a gene, it produces a genetic variant- a different version of the gene. The genetic variant may code for a different sequence of amino acids which may change the shape of the final protein and so its activity. For example, the activity of an enzyme might increase, decrease or stop altogether. This could end up changing the characteristics of an organism.
RNA Polymerase binds to a non coding region in front of a gene.
It unzips the two strands and moves along the template strand adding complimentary mRNA bases to form a strand of mRNA. mRNA has uracil (u) instead of thymine (t). Once made, the mRNA molecule moves out of the nucleus via the nuclear pore.
the mRNA strand binds to the ribosome so the protein can be assembled
Amino Acids are brought to the ribosome by another RNA molecule called transfer RNA (tRNA)
The order in which the amino acids are brought to the ribosome matches the order of the codons in the mRNA.
Part of the tRNA's structure is called an anticodon and they're complimentary to the codon for the amino acid.
The amino acids are joined together by the ribosome and each anticodon has a peptide at the top which join together and form a polypeptide chain.
This folds up to make a protein.
If a mutation happens in the region of DNA, then it could affect the ability of the RNA polymerase to bind to it. It might make it easier or more difficult. How well RNA Polymerase can bind to this region of DNA will affect how much mRNA is transcribed and therefore how much of the protein is made. The phenotype of the organism may be affected.