B6 - Inheritance, Variation and Evolution Flashcards
DNA - definition
> DNA stands for deoxyribonucleic acid.
It’s the chemical that all of the genetic material in a cell is made up from.
DNA is a polymer.
DNA
> It contains coded information - basically all the instructions to put an organism together and make it work.
So it’s what’s in your DNA that determines what inherited characteristics you have.
DNA is found in the nucleus of plant and animal cells, in really long structures called chromosomes.
Chromosomes normally come in pairs.
DNA is a polymer. It’s made up of two strands coiled together in the shape of a double hellix.
Chromosomes - definition
> Chromosomes are really long strands/molecules of DNA.
Gene - definition
> A gene is a small section of DNA found on a chromosome.
Genes
> Each gene codes for a particular sequence of amino acids which are put together to make a specific protein.
Only 20 amino acids are used, but they make up thousands of different proteins.
Genes simply tell cells in what order to put the amino acids together.
DNA also determines what proteins the cell produces, e.g., haemoglobin, keratin.
That in turn determines what type of cell it is, e.g. red blood cell, skin cell.
Genome - definition
> The term for the entire set of genetic material in an organism.
Genome
> Scientists have worked out the complete human genome.
>Understanding the human genome is a really important tool for science and medicine for many reasons.
Why is the human genome important for science and medicine?
- It allows scientists to identify genes in the genome that are linked to different types of disease.
- Knowing which genes are linked to inherited diseases could help us to understand them and could help us to develop effective treatments for them.
- Scientists can look at genomes to trace the migration of certain populations of people around the world. All modern humans descended from a common ancestor who lived in Africa, but humans can be found all over the planet. The human genome is mostly identical in all individuals, but as different populations of people migrated away from Africa, they gradually developed tiny differences in their genomes. By investigating these differences, scientists can work out when new populations split off in a different direction and what route they took.
What is DNA made up of?
Nucleotides
The structure of DNA
> DNA strands are polymers made up of lots of repeating units called nucleotides.
Each nucleotide consists of one sugar molecule, one phosphate molecule and one ‘base’.
The sugar and phosphate molecules in the nucleotides form a ‘backbone’ to the DNA strands. The sugar and phosphate molecules alternate. One of four different bases - A, T, C or G - joins to each sugar.
Each base links to a base with the opposite strands in the helix.
A always pairs up with T and C always pairs up with G. This is called complementary base pairing.
It’s the order of bases in a gene that decides the order of amino acids in a protein.
Each amino acid is coded for by a sequence of 3 bases in the gene.
The amino acids are joined together to make various proteins, depending on the order of the gen’s bases.
There are parts of DNA that don’t code for proteins. Some of these non-coding parts switch genes on and off, so they control whether or not a gene is expressed.(used to make a protein)
What are nucleotides made up of?
> Each nucleotide consists of one sugar molecule, one phosphate molecule and one ‘base’.
The sugar and phosphate molecules in the nucleotides form a ‘backbone’ to the DNA strands. The sugar and phosphate molecules alternate. One of four different bases - A, T, C or G - joins to each sugar.
Complementary base pairing
> Each base links to a base with the opposite strands in the helix.
A always pairs up with T and C always pairs up with G. This is called complementary base pairing.
What does the order of bases in a gene decide?
> It’s the order of bases in a gene that decides the order of amino acids in a protein.
Each amino acid is coded for by a sequence of 3 bases in the gene.
The amino acids are joined together to make various proteins, depending on the order of the gen’s bases.
There are parts of DNA that don’t code for proteins. Some of these non-coding parts switch genes on and off, so they control whether or not a gene is expressed.(used to make a protein).
Proteins - where are they made?
> Proteins are made in the cell cytoplasm on tiny structures called ribosomes.
Proteins - How are they made?
> To make proteins, ribosomes use the code in DNA.
DNA is found in the cell nucleus and can’t move out of it because it’s really big. So the cell needs to get the code from the DNA to the ribosome.
This is done using a molecule called mRNA - which is made by copying the code from DNA.
The mRNA acts as a messenger between the DNA and the ribosome - it carries the code between the two.
The correct amino acids are brouht to the ribosomes in the correct order by carrier molecules.
types of proteins
> When a chain of amino acids has been assembled, it folds into a unique shape which allows the protein to perform the task it’s meant to do. Here are a few examples of types of protein:
- Enzymes.
- Hormones.
- Structural Proteins.
types of proteins - enzymes
> Act as biological catalysts to speed up chemical reactions in the body.
types of proteins - hormones
> Used to carry messages around the body.
>E.g. insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level.
types of proteins - structural proteins
> Are physically strong.
>E.g. collagen is a structural protein that strengthens connective tissues (like ligaments and cartilage).
Mutations -definition
> Occasionally, a gene can mutate.
A random change in an organism’s DNA.
They can sometimes be inherited.
Mutations
> Mutations occur continuously. They can occur spontaneously, e.g. when a chromosome isn’t quite replicated properly.
However, the chance of mutation is increased by exposure to certain substances or some types of radiation.
what do mutations change?
> Mutations change the sequence of the DNA bases in a gene, which produces a genetic variant.
As the sequence of DNA bases codes for the sequence of amino acids that make up a protein, mutations to a gene sometimes lead to changes in the protein that it codes for.
Effect of mutations
> Most mutations have very little or no effect on the protein. Some will change it to such a small extent that its function or appearance is unaffected.
However, some mutations can seriously affect a protein.
Sometimes, the mutation will code for an altered protein with a change in its shape. This could affects its ability to perform its function. E.g.:
1. If the shape of an enzyme’s active site is changed, its substrate may no longer be able to bind to it.
2. Structural proteins like collagen could lose their strength if their shape is changed, making them pretty useless at providing structure and support.
If here’s a mutation in the non-coding DNA, it can alter how genes are expressed.
Types of mutation
- Insertions.
- Deletions.
- Substitutions.
Mutations - insertions
> Insertions are where a new base is inserted into the DNA base sequence where it shouldn’t be.
You should remember that every 3 bases in a DNA base sequences codes for a particular amino acid.
An insertion changes the way the groups of 3 bases are ‘read’, which can change the amino acids they code for.
Insertions can change more than one amino acid as they have a knock-on-effect on the bases further on in the sequence.
Mutations - deletions
> Deletions are when a random base is deleted from the DNA base sequence.
Like insertions, they change the way that the base sequence is ‘read’ and have knock-on effects further down the sequence.
Mutations - substitutions
> Substitution mutations are when a random base in the DNA base sequence is changed to a different base.
Sexual reproduction - definition
> Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
Sexual reproduction
> In sexual reproduction, the mother and father produce gametes by meiosis.
In humans, each gamete contains 23 chromosomes - half the number of chromosomes in a normal cell.
The egg and sperm then fuse together to form a cell with the full number of chromosomes.
Two parents so offspring inherits features from both parents.
This mixture of genetic information produces variation in the offspring.
Flowering plants can produce this way too. They also have egg cells, but pollen instead of sperm.
Asexual reproduction
> In asexual reproduction, there’s only one parent so the offspring are genetically identical to that parent.
Asexual reproduction happens by mitosis.
The new cell has exactly the same genetic information as the parent cell so it’s a clone.
Bacteria, some plants and some animals reproduce asexually.
No fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring.
Mitosis - definition
> Where an ordinary cell makes a new cell by dividing in two.
Gametes
> Gametes only have one copy of each chromosome, so that when gamete fusion takes place, you get the right amount of chromosomes again.
To make gametes which only have half the original number of chromosomes, cells divide by meiosis.
This process involves two cell divisions.
In humans, it only happens in the reproductive organs (ovaries and testes).
Meiosis
- Before the cell starts to divide, it duplicates its genetic info, forming two armed chromosomes - one arm of each chromosome is an exact copy of the other arm. After replication, the chromosomes arrange themselves into pairs.
- In the first division in meiosis the chromosome pairs line up in the centre of the cell.
- The pairs are then pulled apart so each new cell only has one copy of each chromosome. Some of the father’s chromosomes and some of the mother’s chromosomes.
- In the second division, the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.
- You get 4 gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random.
Gamete fusion
> After 2 gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself.
Mitosis repeats many times to produce lots of new cells in an embryo.
As the embryo develops, these cells then start to differentiate into the different types of specialised cell that make up a whole organism.
Advantages of sexual reproduction as oppose to asexual reproduction
> Offspring from sexual reproduction have a mixture of 2 sets of chromosomes. The organism inherits genes from both parents, which produces variation in the offspring.
Variation increases the chance of a species surviving a change in the environment. While a change in the environment could kill some individuals, it’s likely that variation will have led to some of the offspring being able to survive in the new environment. They have a survival advantage.
Because individuals with characteristics that make them better adapted to the environment have better chance of survival, they are more likely to breed successfully and pass the genes for the characteristics on. This is known as natural selection.
We can use selective breeding to speed up natural selection. This allows us to produce animals with desirable characteristics. This means that we can increase food production.
Selective breeding - defintion
> Where individuals with desired characteristics are bred to produce offspring that have the desirable characteristic too.
Advantages of asexual reproduction as oppose to sexual
> There only needs to be one parent.
This means that asexual reproduction uses less energy than sexual reproduction, because organisms don’t have to find a mate.
This also means it’s faster.
Many identical offspring can be produced in favourable conditions.
Asexual and sexual
> Some organisms can reproduce by both methods depending on circumstances.
Examples of organisms which can reproduce both asexually and sexually.
> Parasite in malaria.
Fungus.
Lots of plant species.
Organisms which can reproduce both asexually and sexually - parasite in malaria.
> Malaria is caused by a parasite that’s spread by mosquitoes.
When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human.
The parasite reproduces sexually when it’s in the mosquito and asexually when it’s in the human host.
Organisms which can reproduce both asexually and sexually - fungus
> Many species of fungus can reproduce both sexually and asexually.
These species release spores, which can become new fungi when they land in a suitable place.
Spores can be produced sexually and asexually.
Asexually-produced spores form fungi that are genetically identical to the parent fungus.
Sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change.
Organisms which can reproduce both asexually and sexually - plants
> Loads of species of plant produce seeds sexually, but can also reproduce asexually.
Asexual reproduction can take place in different ways.
For example, strawberry plants produce ‘runners’. These are stems that grow horizontally on the surface of the soil away from a plant.
At various points of the runner, a new strawberry plant forms that is identical to the original plant.
Another example is in plants that grow from bulbs (e.g. daffodils).
New bulbs can form from the main bulb and divide off. Each new bulb can grow into an identical plant.
Chromosomes
> There are 23 pairs of chromosomes in every human body cell.
Of these 22 are matched pairs of chromosomes that just control characteristics.
The 23rd pair are labelled XY or XX - decide sex. xx is female and XY is male.
Chromosomes - sex
> When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis.
There’s a 50% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome.
A similar thing happens when making eggs.
But the original cell has two X-chromosomes, so all the eggs have one X-chromosome.
Genetic diagrams - definition
> Models used to show all the possible genetic outcomes when you cross together different genes or chromosomes.
Gentic diagram
> Shows possible gamete combinations.
>Punnett square.
Genes and characteristics
> What genes you inherit control what characteristics you develop.
Some characteristics are controlled by a single gene, e.g. mouse fur colour or red-green colour blindness in humans.
However, most characteristics are controlled by several genes interacting.
Alleles - defintion
> Different version of the same gene.
Heterozygous
> If its two alleles for a particular gene are different, then it’s heterozygous.
If the 2 alleles are different, only one can determine what characteristic is present. The allele for the characteristic that’s shown is called the dominant allele (capital letter), the other one is called recessive (small letter).
How can an organism display a recessive characteristic?
> For an organism to display a recessive characteristic, both its alleles must be recessive.
But to dispal
How can an organism display a recessive characteristic?
> For an organism to display a recessive characteristic, both its alleles must be recessive.
cc
How can an organism display a dominant characteristic?
> To display a dominant characteristic the organism can be either CC or Cc, because the dominant allele overrules the recessive one if the organism is heterozygous.
Genotype - definition
> The combination of alleles you have.
Phenotype - definition
> The characteristics you have due to your alleles working at a molecular level to determine this.
Dominant allele - defintion
> The phenotype will be apparent in the offspring even if only one of the alleles is inherited.
Recessive allele - definition
> A phenotype that will only show up in the offspring if both of the alleles coding for that characteristic are present.
Heterozygous - definition
> Individual with different alleles for a characteristic.
Homozygous - defintion
> Individual with two identical alleles for a characteristic.
Cystic Fibrosis
> Cystic fibrosis is a genetic disorder of the cell membranes.
It results in the body producing a lot of thick sticky mucus in the air passages and in the pancreas
The allele which causes cystic fibrosis is a recessive allele. ‘f’, carried by about 1 in 25 people.
Because it’s recessive, people with only 1 copy of the allele won’t have the disorder - they’re known as carriers.
For a child to have the disorder, both parents must be either carriers or have the disorder themselves.
Polydactylyl
> Polydactylyl is a genetic disorder where a baby’s born with extra fingers or toes.
The disorder is caused by a dominant allele, ‘D’, and so can be inherited if just one parent carries the defective allele
The parent that has the defective allele will have the condition too since the allele is dominant.
Embryonic Screening
> During IVF, embryos are fertilised in a lab, and then implanted into the mother’s womb.
Before being implanted, it’s possible to remove a cell from each embryo and analyse its genes.
Many genetic disorders can be detected in this way, such a cystic fibrosis.
It’s also possible to get DNA from an embryo in the womb and test that for disorders.
There are lots of ethical, social and economic concerns surrounding embryo screenin.
For embryos produced by IVF - after screening, embryos with ‘bad’ alleles would be destroyed.
For embryos in the womb - screening could lead to the decision to terminate the pregnancy.
Embryonic Screening - arguments for
- It will help to stop people suffering.
- Treating disorders costs the Government (and taxpayers) a lot of money.
- There are laws to stop it going too far. At the moment parents can’t even select the sex of their baby (unless it’s for health reasons).
Embryonic screening - arguments against
- It implies people with genetic problems are ‘undesirable’ - this could increase prejudice.
- There may come a point where everyone wants to screen their embryos so they can pick the most ‘desirable’ one, e.g. they want a blue-eyed, blond-haired, intelligent boy.
- Screening is expensive.
Mendel - background
> Gregor Mendel was an Austrian monk who trained in mathematics and natural history at the University of Vienna.
On his garden plot at the monastery in the mid 19th century, Mendel noted how characteristics in plants were passed on from one generation to the next.
The results of his research were published in 1866 and eventually became the foundation of modern genetics.
Mendel - his experiment
> He did an experiment with pea plants.
First cross:
-a tall pea plant and a dwarf pea plant are crossed and all offspring are tall pea plants.
Second cross:
-two pea plants from the 1st set of offspring are crosses (so both tall) and in the offspring there are 3 tall pea plants and one dwarf pea plant.
Mendel had shown that the height characteristic in pea plants was determined by separately inherited ‘hereditary units’ passed on from each parent.
The ratios of tall and dwarf plants in the offspring showed that the unit for tall plants, T, was dominant over the unit for dwarf plants, t.
Important conclusions from Mendel’s experiment.
> Mendel reached 3 important conclusions about heredity in plants:
- Characteristics in plants are determined by ‘hereditary units’.
- Hereditary units are passed on to offspring unchangeed from both parents, one unit from each parent.
- Hereditary units can be dominant or recessive - if an individual has both the dominant and the recessive unit for a characteristic,the dominant characteristic will be expressed.
Acceptance of Mendel’s work
> Mendel’s work was cutting edge and new to the scientists of the day. They didn’t have the background knowledge to properly understand his findings - they had no idea about genes, DNA and chromosomes.
It wasn’t until after his death that people realised how significant his work was.
Using Mendel’s work as a starting point, the observations of many different scientists have contributed to the understanding of genes we have today.
Development of our understanding of genetics
- In the late 1800s, scientists became familiar with chromosomes. They were able to observe how they behaved during cell division.
- Then in the early 20th century, scientists realised that there were striking similarities in the way that chromosomes and Mendel’s ‘units’ acted. Based on this, it was proposed that the ‘units’ were found on the chromosomes. We now know these ‘units’ as genes.
- IN 1953, the structure of DNA was determined. This allowed scientists to go on and find out exactly how genes work.
Variation
> Even organisms of the same species will usually look at least slightly different.
These differences are called the variation within a species, and there are two types of variation - genetic variation and environment variation.
Causes of genetic variation
> All plants and animals have characteristics that are in some ways similar to their parents’.
This is because an organism’s characteristics are determined by the genes inherited from their parents.
These genes are passed on in sex cells (gametes), from which the offspring develop.
Most animals get come genes from mother and some from the father.
This combining of genes from two parents causes genetic variation - except for identical twins, no two of the species are genetically identical.
Gene - defintion
> Genes are codes inside your cells that control how you are made.
What characteristics are determined only by genes in animals?
> Eye colour, blood group and inherited disorders.
Environment variation.
> Characterstics are also influenced by environment.
The environment, including the conditions that organisms live and grow in, also causes differences between members of the same species - this is called environmental variation.
For example, a plant grown in plenty of sunlight would be luscious and green, but the same plant grown in darkness would grow tall and spindly and have yellow leaves. These are environmental variations.
Environmental variation covers a wide range of differences - from losing your toes in an accident , to getting a suntan, to having yellow leaves.
Genetic and environmental variation
> Most characteristics (e.g. body weight, height, skin colour, condition of teeth, academic or athletic prowess) are determined by a mixture of genetic and environment factors).
For example, the maximum height that an animal or plant can grow is determined by its genes. But whether it actually grows that tall depends on its environment (how much food it gets).
Mutations and variation
> Mutations introduce variation.
Mutations are changes to the sequence of bases in DNA.
Mutations can lead to changes in the protein that a gene codes for.
Most mutations have no effect on the organism’s phenotype. Dome have a small influence on the phenotype so only alter characteristics slightly.
However, although it’s rare, mutations can result in a new phenotype being seen in a species.
If the environment changes, and the new phenotype makes an individual more suited to the new environment, it can become common throughout the species relatively quickly by natural selection.
Theory of evolution
> All of today’s species have evolved from simple life forms that first started to develop 3 billion years ago.
Charles Darwin
> Darwin came up with a really important theory about evolution.
He used the observations he made on a huge round-the-world trip, along with experiments, discussions and new knowledge of fossils and geology, to suggest the theory of evolution by natural selection.
Darwin’s observations
> Darwin knew that organisms in a species show a wide variation in their characteristics (phenotypic variation). He also knew that organisms have to compete for limited resources in an ecosystem.
Darwin concluded that organisms with the most suitable characteristics for the environment would be more successful competitors and would be more likely to survive. This idea is called the ‘survival of the fittest’.
The successful organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring.
The organisms that are less well adapted would be less likely to survive and reproduce, so they are less likely to pass on their genes to the next generation.
Over time, beneficial characteristics become more common in the population and species change - it evolves.
Development of Darwin’s theory
> Darwin’s theory wasn’t perfect.
Because the relevant scientific knowledge wasn’t available at the time, he couldn’t give a good explanation for why new characteristics appeared or exactly how individual organisms passed on beneficial adaptations to their offspring.
We know know that phenotype is controlled by genes.
New phenotypic variations arise because of genetic variants produced by mutations.
Beneficial variations are passed on to future generations in the genes that parents contribute to their offspring.
Phenotypic variation - definition
> Phenotypes are traits or characteristics of an organism that we can observe, such as size, color, shape, capabilities, behaviors, etc. …
Phenotypes can be caused by genes, environmental factors, or a combination of both.
Phenotypic variation, then, is the variability in phenotypes that exists in a population.
Speciation - definition
> The development of a new species.
Speciation
> Over a long period of time, the phenotype of organisms can change so much much because of natural selection that a completely new species is formed. This is called speciation.
Speciation happens when populations of the same species change enough to become reproductively isolated - this means that they can’t interbreed to produce fertile offspring.
Extinction - definition
> Extinction is when no individuals of a species remain.
Why do species become extinct?
- The environment changes too quickly (e.g. destruction of habitat).
- A new predator kills them all (e.g. humans hunting them).
- A new disease kills them all.
- They can’t compete with another (new) species for food.
- A catastrophic event happens that kills them all (e.g. a volcanic eruption or a collision with an asteroid).
The extinction of dodos
> Dodos are now extinct.
Humans not only hunted them, but introduced other animals which ate all their eggs, and we destroyed the forests where they live.
They really didn’t stand a chance…
Opposition to Darwin’s evolution
> When Darwin proposed his theory in his book ‘On the Origin of Species’ in 1859, his idea was very controversial for various reasons:
- It went against common religious beliefs about how life on Earth developed - it was the first plausible explanation for the existence of life on earth without the need for a ‘Creator’ (God).
- Darwin couldn’t explain why these new, useful characteristics appeared or how they were passed on from individual organisms to their offspring. But then he didn’t know anything about genes or mutations - they weren’t discovered until 50 years after his theory was published.
- There wasn’t enough evidence to convince many scientists, because not many other studies had been done into how organisms change over time.
- Lamarck had different ideas.
Lamarck
> There were different scientific hypotheses about evolution around at the same time as Darwins’, such as:
- Jean-Baptiste Lamarck (1744-1829) argued that changes that an organism acquires during its lifetime will be passed on to its offspring - e.g. he thought that if a characteristic was used a lot by an organism, then it would become more developed during its lifetime, and then the organism’s offspring would inherit the acquired characteristic.
- For example, using this theory, if a rabbit used its legs to run a lot, its legs would get longer. The offspring would then be born with longer legs.
Hypotheses
> Often scientists come up with different hypotheses to explain similar observations.
Scientists might develop different hypotheses because they have different beliefs or have been influenced by different people.
The only way to find out whose hypothesis is right is to find evidence to support or disprove each one.
Lamarck’s hypotheseis - rejection
> Eventually, Larmarck’s hypothesis was rejected because experiments didn’t support his hypothesis.
Darwin’s hypothesis - approval
> The discovery of genetics supported Darwin’s idea because it provided an explanation of how organisms born with beneficial characteristics can pass them on.
Other evidence was also found by looking at fossils of different ages (fossil record) - this allows you to see how organisms developed slowly over time.
The relatively recent discovery of how bacteria are able to evolve to become resistant to antibiotics also further supports evolution by natural selection.
Selective breeding- definition
> Selective breeding is when humans artificially select the plants or animals that are going to breed do that the genes for particular characteristics remain in the populations.
Why are organisms selectively breed?
> Organisms are selectively bred to develop features that are useful or attractive, for example:
- animals that produce more meat or milk.
- crops with disease resistance.
- dogs with a good, gentle temperament.
- decorative plants with big or unusual flowers.
Basic process of selective breeding
- From your existing stock, select the ones which have the characteristics you desire.
- Breed them with each other.
- Select the best of the offspring, and breed them together.
- Continue this process over several generations, and the desirable trait gets stronger and stronger.
>In agriculture, selective breeding can be used to improveyields.
>Selective breeding is nothing new - people have been doing it for 1000s of years. It’s how we’ve ended up with edible crops and wild plants and how we got domesticated animals like cows and dogs.
Problem with selective breeding.
> The main problem with slective breeding is that it reduces the gene pool - the number of different alleles in a population. This is because the farmer keeps breeding from the ‘best’ animals or plants - which are all closely related. This is known as inbreeding.
Inbreeding can cause health problems because there’s more chance of the organisms inheriting harmful genetic defects because of inbreeding - e.g. pugs often have breathing problems.
There can also be serious problems if a new disease appears, because there’s not much variation in the population. All the stock are closely related to each other, so if one of them is going to be killed by a new disease, the others are also likely to succumb to it.
1. Selective breeding. 2. Reduction in the number of different alleles. 3. Less chance of any resistant alleles being present in the population.
Genetic engineering - definition
> Where a gene responsible for a desirable characteristic is transferred from one organism’s genome into another organism, so that it also has the desired characteristic.
Genetic Engineering - process
- A useful gene is isolated from one organism’s genome using enzymes and is inserted into a vector.
- The vector is usually a virus or a bacterial plasmid depending on the type of organism that the gene is being transferred to.
- When the vector is introduced to the target organism, the useful gene is inserted into its cell(s).
> In some cases, the transfer of the gene is carried out when the organism receiving the gene is at an early stage of development. This means that the organism develops with the characteristic coded for by the gene.
What have scientists used genetic engineering for?
> Bacteria have been genetically modified to produce human insulin that can be used to treat diabetes.
Genetically modified crops have had their genes modified e.g. to improve the size and quality of their fruit, or make them resistant to disease, insects and herbicides.
Sheep have bee genetically engineered to produce substances, like drugs, in their milk that can be used to treat human diseases.
Scientists are researching genetic modification treatment for inherited diseases caused by faulty genes, e.g. by inserting working genes into people with the disease. This is called gene therapy.
Genetic engineering - controversy
> Genetic engineering is an exciting area of science, which has the potential for solving many of our problems (e.g. treating disease, more efficient food production etc.), but not everyone thinks it’s a great idea.
There are worries about the long-term effects of genetic engineering - that changing an organism’s genes might accidentally create unplanned problems, which could get passed on to future generations.
Pros of GM crops
> The characteristics chosen for GM crops can increase the yield, making more food.
People living in developing nations often lack nutrients in their diets. GM crops could be engineered to contain the nutrient that’s missing. For example, ‘golden rice’ is a GM rice crop that contains beta-carotene - lack of this substance causes blindness.
GM crops already being grown in some laces, often without any problems.
Cons of GM crops
> Some people say that growing GM crops will affect the number of wild flowers (and so the insect population) that live in and around the crops - reducing farmland biodiversity.
Not everyone is convinced that GM crops are safe ad some people are concerned that we might not fully understand the effects of eating them on human health. E.g. people are worried they may develop allergies to the food - although there’s probably no more risk for this than for eating usual foods.
A big concern is that transplanted genes may get out into the natural environment. For example, the herbicide resistance gene may be picked up by weeds, creating a new ‘superweed’ variety.
Tissue Culture
> This is where a few plant cells are put in a growth medium with hormones, and they grow into new plants - clones of the parent plant.
These plants can be made very quickly, in very little space, and will be grown all year.
Tissue culture is used by scientists to preserve rare plants that are hard to reproduce naturally and by plant nurseries to produce lots of stock quickly.
Cuttings
> Gardeners can take cuttings from good parent plants, and then plant them to produce genetically identical copies -clones- of the parent.
These plants can be produced quickly and cheaply. This is an older, simpler method than tissue culture.
The cuttings are kept in moist conditions until they are ready to plant.
How to clone plants?
- Tissue culture
2. cuttings
How can you make animal clones?
> By using embryo transplants
>Adult cell cloning
Embryo transplants
> Farmers can produce cloned offspring from their best bull and cow using embryo transplants.
- Sperm cells are taken from a prize bull and egg cells are taken from a prize cow. The sperm are then used to artificially fertilise an egg cell. The embryo that develops is then split many times (to form clones) before any cells become specialised.
- These cloned embryos can then be implanted into lots of other cows where they grow into baby calves (which will all be genetically identical to each other).
- Hundreds of ‘ideal’ offspring can be produced every year from the best bull and cow.
Adult cell cloning
1.Adult cell cloning involves taking an unfertilised egg cell and removing its nucleus. The nucleus is then removed from an adult body cell and is inserted into the ‘empty’ egg.
2.The egg cell is then stimulated by an electric shock - this makes it divide just like a normal embryo.
3.When the embryo is a ball of cells, it’s implanted into the womb of an adult female. It grows into a genetically identical copy (clone) of the original adult body cell as it has the same genetic information.
>This technique was used to make Dolly - the famous cloned sheep.
Cloning -cons
> You get a ‘reduced gene pool’ - this means there are fewer different alleles in a population. If a population are all closely related and a new disease appears, they could all be wiped out - there may be no allele in the population giving resistance to it.
It’s possible that cloned animals might not be as healthy as normal ones, e.g. Dolly the sheep has arthritis, which tends to occur in older sheep.
Some people worry that humans might become cloned in the future. If it was allowed, any success may follow many unsuccessful attempts, e.g. baby born with disability.
Cloning - pros
> Cloning quickly gets you lots of ‘ideal’ offspring.
But the study of animal clones could lead to greater understanding of the development of the embryo, and of ageing and age-related disorders.
Cloning could also be used to help preserve endangered species.
Fossils - definition
> Fossils are the remains of organisms from many thousands of years ago, which are found in rocks.
Fossils - info + how they form
> They provide the evidence that organisms lived ages ago.
Fossils can tell us a lot about how much or how little organisms have change and evolved over time.
Fossils form in rocks in one of three ways:
1. From gradual replacement by minerals.
2. From casts and impressions.
3. From preservation in places where no decay happens.
Formation of fossils - gradual replacement by minerals
- Things like teeth, shells, bones etc., which don’t decay easily,can last a long time when buried.
- They’re eventually replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part.
- The surrounding sediments also turn into rock, but the fossil stays distinct inside the rock and eventually someone digs it up.
Formation of fossils - from casts and impression
- Sometimes, fossils are formed when an organism is buried in a soft material like clay. The clay later hardens around it and the organism decays, leaving a cast of itself. An animal’s burrow or a plant’s roots (rootlet traces) can be preserved as casts.
- Things like footprints can also be pressed into these materials when soft, leaving an impression when it hardens.
Formation of fossils - preservation
> In amber and tar pits there’s no oxygen or moisture so decay microbes can’t survive.
In glaciers it’s too cold for the decay microbes to work.
Peat bogs are too acidic for decay microbes.
Beginnings of life
> Fossils show how much or how little different organisms have evolved as life has developed on Earth over millions of years. But no one knows how life began:
-lots of hypotheses.
-maybe first life forms came into existence in a primordial swamp or under the sea here on Earth. Maybe simple organic molecules were brought to Earth on comets - these could have then become more complex organic molecules, and eventually very simple life forms.
There hypotheses can’t be supported or disproved as there’s a lack of good, valid evidence:
-many early forms of life were soft-bodied, and soft tissue tends to decay away completely - so the fossil record is incomplete.
-fossils that did form millions of years ago may have been destroyed by geological activity.
Species - definition
> A species is a group of similar organisms that can reproduce to give fertile offspring.
Speciation - definition
> Speciation is the development of new species.
Speciation occurs when populations of the same species become so different that they can no longer successfully interbreed to produce fertile offspring.
what can lead to speciation?
> Isolation and natural selection
Isolation - speciation
> Isolation is where populations of a species are separated.
This can happen due to a physical barrier e.g. floods and earthquakes can cause barriers that geographically isolate some individuals from the main population.
Conditions on either side of the barrier will be slightly different, e.g. they may have different climates.
Because the environment is different on each side, different characteristics will become more common in each population due to natural selection operating slightly differently on the population.
Natural selection - speciation
> Each population shows genetic variation because they have a wide range of alleles.
In each population, individuals with characteristics that make them better adapted to their environment have a better chance of survival and so are more likely to breed successfully.
So the alleles that control the beneficial characteristics are more likely to be passed on to the next generation.
Eventually, individuals from different populations will have changed so much that they won’t be able to breed with one another to produce fertile offspring.
The two groups will have become separate species.
Alfred Russel Wallace
> Alfred Russel Wallace was a scientist working at the same time as Charles Darwin.
He was one of the early scientists working on the idea of speciation.
His observations greatly contributed to how we understand speciation today.
Our current understanding developed as more evidence became available over time.
Theory of Speciation
> During his career, Alfred Russel Wallace independently came up with the idea of natural selection and published work on the subject together with Darwin in 1858.
This then prompted Darwin to publish ‘On the Origin of Species’ in 1859.
Observations made by Wallace as he travelled the world provided lots of evidence to support the theory of evolution by natural selection.
For example, he realised that warning colours are used by some species (e.g. butterflies) to deter predators from eating them and that this was an example of a beneficial characteristic that had evolved by natural selection.
It’s this work on warning colours and his work on speciation that he’s most famous for.
What can bacteria evolve to become?
> Antibiotic-resistant.
Bacteria evolution
> Like all organisms, bacteria sometimes develop random mutations in their DNA. These can lead to changes in the bacteria’s characteristics, e.g. being less affected by a particular antibiotic. This can lead to antibiotic-resistant strains forming as the gene for antibiotic resistance becomes more common in the population.
To make matters worse, because bacteria are so rapid at reproducing, they can evolve quite quickly.
Antibiotic resistant bacteria
> For the bacterium, the ability to resist antibiotics is a big advantage. It’s better able to survive, even in a host who’s being treated to get rid of the infection, and so it lives for longer and reproduces many more times. This increases the population size of the antibiotic resistant strain.
Antibiotic-resistant strains are a problem for people who become infected with these bacteria because they aren’t immune to the new strain and there is no effective treatment. This means that the infection easily spreads between people. Sometimes drug companies can come up with a new antibiotic that’s effective, but ‘superbugs’ that are resistant to most known antibiotics are becoming more common.
MRSA is a relatively common ‘superbug’ that’s really hard to get rid of. It often affects people in hospitals and can be fatal if it enters their bloodstream.
Problem with antibiotic resistance
> For the last few decades, we’ve been able to to deal with bacterial infections pretty easily using antibiotics. The death rate from infectious bacterial diseases (e.g. pneumonia) has fallen dramatically.
But the problem of antibiotic resistance is getting worse - partly because of the overuse and inappropriate use of antibiotics, e.g. doctors prescribing them for non-serious conditions or infections caused by viruses.
The more often antibiotics are used, the bigger the problem of antibiotic resistance becomes, so it’s important that doctors only prescribe antibiotics when they really need to :
-It’s not that antibiotics actually cause resistance - they create a situation where naturally resistant bacteria have an advantage and so increase in numbers.
It’s also important that you take all the antibiotics a doctor prescribes for you:
-Taking the full course makes sure that all the bacteria are destroyed, which means that there are none left to mutate and develop into antibiotic-resistant strains.
Farming and antibiotics.
> In farming, antibiotics can be given to animals to prevent them becoming ill and to make them grow faster.
This can lead to the development of antibiotic-resistant bacteria in the animals which can then spread to humans, e.g. during meat preparation and consumption.
Increasing concern about the overuse of antibiotics in agriculture has led to some countries restricting their use.
What’s being done to stop spread of antibiotic resistant bacteria?
> The increase in antibiotic resistance has encourage drug companies to work on developing new antibiotics that are effective against the resistant strains.
Unfortunately, the rate of development is slow, which means we’re unlikely to be able to keep up with the demand for new drugs as more antibiotic-resistant strains develop and spread.
It’s also a very costly process.
Why does the gene for antibiotic resistance become more common in bacteria?
> The gene for antibiotic resistance becomes more common in the population because of natural selection.
Classification - definition
> Classification is organising living organisms into groups.
Classification
> Traditionally, organisms have been classified according to a system first proposed in the 1700s by Carl Linnaeus, which group living things according to their characteristics and the structures that make them up.
In this system (known as the Linnaean system), living things are first divided into kingdoms.
The kingdoms are then subdivided into smaller and smaller groups.
Linnaean System order
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species
How had classification system changed over time
> As knowledge of the biochemical processes taking place inside organisms developed and microscopes improved (which allowed us to find out more about the internal structures of organisms), scientists put forward new models of classification.
In 1990, Carl Woese proposed the three-domain system. Using evidence from new chemical analysis techniques such as RNA sequence analysis, he found that in some cases, species thought to be closely related in traditional classification systems are in fact not as closely related as first thought.
The three-domain system
> In the three-domain system, organisms are first of all split into three large groups called domains;
1. Archaea
2. Bacteria
3. Eukaryota.
These are then subdivided into smaller groups - kingdom, phylum, class, order, family, genus, species.
The three-domain system - archaea
> Organisms in this domain are primitive bacteria. They’re often found in extreme places such as hot springs and salt lakes.
The three-domain system - bacteria
> This domain contains true bacteria like E. coli and Staphylococcus.
The three-domain system - eukaryota
> This domain includes a broad range of organisms including fungi, plants, animals and protists.
Binomial System
> In the binomial system, every organism is given its own two-part Latin name.
The first part refers to the genus that the organism belongs to. This gives you information on the organism’s ancestry. The second part refers to the species. E.g. humans are known as Homo sapiens. ‘Homo’ is the genus and ‘sapiens’ is the species.
The binomial system is used worldwide and means that scientists in different countries or who speak different languages all refer to a particular species by the same name - avoiding potential confusion.
Evolutionary trees
> Evolutionary trees show how scientists think different species are related to each other.
They show common ancestors and relationships between species.
The more recent the common ancestor, the more closely related the two species - and the more characteristics they’re likely to share.
Scientists analyse lots of different types of data to work out evolutionary relationships.
For living organisms, they use the current classification data (e.g. DNA analysis and structural similarities). For extinct species, they use info from the fossil record.
