inheritance and variation Flashcards
what is the purpose of mitosis?
growth and repair.
what are the three stages of mitosis?
- copies of DNA and organelles are made
- contents of the cell are re-arranged (pulled to the poles)
- cell splits into identical daughter cells
how is mitosis asexual?
doesn’t involve sex cells. only requires one to reproduce.
define ‘variation’:
the differences between individual organisms. to evolve, ‘survival of the fittest’ helps a population to survive, meaning they need to variate and adapt to their environment.
describe asexual reproduction:
- takes place with only one parent
- doesn’t involve gametes
- no mixing of genetic material, so no genetic variation. the offspring are genetic clones
- in eukaryotic organisms (fungi, plants), this is achieved through mitosis
- in prokaryotic organisms, this is achieved through binary fission
which organisms reproduce asexually, and which sexually?
- some plants can do both
- most animals must rely on sexual reproduction
- bacteria all reproduce asexually
what is the purpose of meiosis?
production of gametes (sex cells) for sexual reproduction.
why are gametes haploid cells, and then form diploid cells when forming a normal cell?
- haploid because they only contain half of the genetic material of a normal cell
- diploid because when they combine, they form a normal cell that can grow into a new organism. this normal cell has two sets of genetic info, so it’s diploid
describe the chromosomes in a cell:
23 TYPES of chromosomes in a cell. 46 in total as there is one set from the father (paternal) and one from the mother (maternal). therefore there’s two copies of each chromosome.
what are the two types of base pairs, and what do they make up?
Adenine - Thymine (apple in the tree)
Cytosine - Guanine (car in the garage)
each gene is either the pair Adenine and Thymine, or Cytosine and Guanine.
what is the process of meiosis?
- replicate all of the cell’s DNA by replicating all of their chromosomes - adds an extra arm to each chromosome, so they become an x shape
- these 2-armed chromosomes line up along the centre of the cell in their pairs. which one (paternal/maternal) is on the left/right is totally random. this leads to different DNA in each cell, leading to variation
- the chromosomes (both arms) are pulled to either pole of the cell, and the cell splits in half (first division)
- the chromosomes line up again in the centre, but this time, the two arms are pulled to either side of the cell, and the cell then divides in half again (second division)
- we now have 4 cells that are genetically unique, and these are gametes. 23 chromosomes in each
- in males, these would develop into sperm cells, whereas in females, these would form egg cells
compare the details of asexual and sexual reproduction:
asexual:
- one parent
- fast
- mitosis only
- very limited variation
- identical (clones) to parents
sexual:
- two parents
- slow
- meiosis (divides by mitosis once the zygote is formed)
- significant variation
- different to parents
define sexual reproduction:
involves the fusion of male and female gametes - fertilisation.
- as there are two parents, the offspring contain a mixture of the parent’s genes, so can be genetically different
- this introduces lots of variation over generations as each person is different
what are gametes?
- sex cells that have only half the genetic material of a normal cell - 23 chromosomes. these two gametes fuse together to form a resulting cell with 46 chromosomes
- gametes are made through meiosis
- e.g. sperm and eggs in animals
- e.g. in flowering plants, pollen and eggs
what are the advantages and disadvantages of asexual reproduction?
a:
- only requires 1 organism
- quicker, no need to find a mate. a single organism can quickly colonise a new area in just a few days
- creates exact replica (good for cloning endangered species)
- no energy required
- no need to make gametes
- good in favourable conditions
d:
- no variation (less evolution, less adaptation to new conditions)
- bad in a changed environment. if one organism dies (e.g. due to a disease), as they’re all clones, the rest die. could wipe out entire populations
what are the advantages and disadvantages of sexual reproduction?
a:
- variation (they can adapt to different environments). the population is less likely to be wiped out by a single event
- due to this, evolution can take place, so they can adapt to changing conditions
d:
- must find and impress a mate, which is time consuming, and can spend many resources even though it doesn’t help to aid survival
- requires two parents
- requires gametes to fuse together
- takes longer
- lots of energy used
define evolution:
the process by which a species changes over time, as the most favourable traits are likely to be passed on in each generation
how do fungi reproduce?
- reproduce sexually in poor conditions to generate variation
- also release spores, carried by the wind/water, by asexual reproduction
how do certain plants reproduce?
- plants use sexual reproduction to produce seeds. pollen from one flower must reach the female parts of another flower through pollination, to form seeds - either animal pollinators or the pollen is carried in the wind.
- some plants reproduce asexually. e.g. when strawberry plants send out runners (long shoots from the base of the main plant, which new plants will grow on the end of), or when daffodil bulbs divide
how do malarial parasites reproduce?
- reproduce sexually in the host mosquito
- reproduce asexually in the human host
describe DNA:
DNA = Deoxyribonucleic Acid
- the chemical that all of our genetic material is made of
- base pairs are bases held together by hydrogen bonds
- make up long sections, but can be broken up into 3s, and 3 pairs gives the code for one amino acid
- two polymer chains that come together to form a double helix structure. the monomers in the polymers are called nucleotides/bases (the monomer is a polynucleotide).
what are chromosomes?
the DNA is very long, so can be broken into 46 sections to keep them compact in the nucleus. these sections are very tightly coiled, and these are called chromosomes
- each of our cells has 46 chromosomes
- however, there are only 23 different types, as we have 2 of each type, one from each parent
describe the 23rd chromosome:
the sex chromosomes. women have XX chromosomes, making them female. men have XY chromosomes, making them male
when do chromosomes seem to be in an x-shape?
just before meiosis or mitosis
what are genes?
small sections of DNA (basically a small segment of a chromosome). each gene codes for a specific sequence of amino acids to make a specific protein, as they’re each a particular sequence of bases (determines what type of cell is formed), which include the formation of the enzymes that control cell chemistry.
what is a genome?
the complete genetic makeup/profile of a person. their entire genetic material.
- includes DNA found in the chromosomes (DNA from both the mother and the father), and DNA found in the mitochondria (DNA from just the mother, as it comes from the mitochondria of the egg)
- each human (apart from identical twins) has a unique genome, meaning they have slightly different combinations of proteins inside of them, so they have different phenotypes
describe the Human Genome Project:
- in 2003, scientists from across the globe working together announced they’d worked out the entire human genetic code.
- the first sequence of the human genome took over 10 years to produce, now it only takes 2 weeks.
- they have now sequenced over 1000 people’s genomes, and the aim is to find out as much as possible about human DNA.
- from 1990 to 2003, scientists globally worked together to sequence all 3 billion DNA base pairs that make up the human genome. they have identified around 20,500 human genes and so far found about 2,000 genes linked to disease
describe how we can use the Human Genome Project to understand medicine and inherited disorders:
- the information in the genome could tell you whether you’re at an increased risk of a particular disease, as we know the specific gene that are linked to certain diseases e.g. cancer (there are genes linked to an increased risk of developing disease) and helps us to understand inherited disorders. we can then overcome them by medicine or by repairing faulty genes.
- new personalised drugs and therapies can be tailored to a patient’s specific genetic makeup
- we can predict the risk of an inherited disorder for an individual, so they can make lifestyle choices to reduce this risk. people can get screened to detect health risks early and start treatment sooner
how does the Human Genome Project help us understand human evolution and history?
people across the world can be linked by their DNA patterns, allowing scientists to trace human migration patterns since ancient history, and link us to our ancestors
- we all share most of our genomes, but the small differences between different populations can tell us when they separated
what did the scientists find out about the human genome in the Human Genome Project?
- there are over 3 billion base pairs
- there are 21,000 genes that code for proteins
- you can make many different proteins from the same gene, by switching different parts of it on and off
what are the drawbacks of genetic screening due to the human genome project?
- learning about one’s genetic risks can cause stress and mental health issues
- genetic discrimination may lead to unfair treatment in jobs and insurance for those with certain gene variants. strict regulations are needed to prevent gene-based bias
- there are concerns about the misuse of genetic data by employers and insurers, emphasising the need for privacy protections
which gene increases the risk of breast cancer?
BRCA gene
what are the three parts that nucleotides are made up of?
- phosphate group (little molecule attached to large ribose)
- ribose/sugar (large molecule in the middle)
- nitrogenous base (medium sized molecule attached to the ribose on the other side). this nitrogenous base is either the letter A, T, C, or G, which is what makes each base different.
- every nucleotide has exactly the same phosphate and sugar
how do nucleotides bond together?
the phosphate of one nucleotide bonds to the sugar of the other nucleotide, and this forms a long chain, called a sugar-phosphate backbone
- this effectively forms a protective outer casing around the central bases
- the bases in the middle can then pair up with another polymer strand, holding them together
what do the letters A, C, T, and G stand for?
A = Adenine
C = Cytosine
T = Thymine
G = Guanine
A and T go together, C and G go together (apple in the tree, car in the garage)
- these are called complementary base pairs
what is a genetic code?
the sequence of bases
what are proteins?
polymers of amino acids. contain hundreds of amino acids joined together. there are 20 different amino acids in humans. the specific order and type of amino acids determines the shape of the protein. this determines its function (e.g. enzyme, structural protein, hormone).
- the order of amino acids in a protein is determined by the sequence of bases in the gene for that protein.
how are proteins formed from a sequence of bases?
e.g. ATG GGA CGC ATA TAC TTT
the cell reads the DNA sequence as triplets of bases. each triplet encodes for a specific amino acid in the protein. these amino acids form a long chain (a polypeptide), and this long chain can fold up into a specific protein, with a specific shape and function
describe stage 1 of the process of protein synthesis:
TRANSCRIPTION
- the enzyme RNA polymerase binds to the DNA just before where the gene starts
- just ahead of this enzyme, the two strands of DNA separate, so their bases are exposed
- the RNA polymerase moves along the bases and reads them one by one (this is a template strand), using them make an mRNA strand (the mRNA bases will be complementary to the DNA bases)
- the DNA will then close up after the RNA polymerase has passed over it, so only a small section of DNA is ever exposed
- once the RNA polymerase is finished, it detaches from the DNA, and the DNA can close back up
- we’re left with an mRNA that’s free to leave the nucleus and head to the ribosome
describe stage 2 of the process of protein synthesis:
TRANSLATION
- the mRNA molecule binds to a ribosome, and the ribosome is now ready to start building the protein.
- the amino acids are brought to the ribosome by tRNA (transfer RNA). these molecules have the amino acid at the top, and an anti-codon at the bottom - a sequence of three bases complementary to the three bases on the mRNA. it’s these three bases on the mRNA that code for the amino acid on the tRNA
- the ribosome moves along and the process repeats, and the first tRNA molecules can detach, leaving the amino acid
- the amino acid chain will now detach from the ribosome
once the protein chain/polypeptide (of amino acids in the correct order) is complete, it’s bound together by the ribosome and it folds into its unique shape. the shape enables the protein to do its specific job.
what is another word for a triplet?
codon
how do we know the correct amino acid will always be brought?
each type of tRNA molecule is specific to a particular triplet on the mRNA, so we know it always brings down the correct amino acid
what mRNA complementary base pairs are there?
DNA mRNA
Cytosine - Guanine
Guanine - Cytosine
Thymine - Adenine
Adenine - Uracil (the mRNA doesn’t have a thymine base, it’s instead replaced with a uracil base, represented by a U)
why must we use mRNA for protein synthesis instead of just the DNA strand?
take a copy of the gene (on mRNA), as the entire DNA strand can’t leave the nucleus to access the ribosome (outside of the nucleus).
what are the differences between mRNA and DNA?
- mRNA is much shorter than DNA (only a single gene long)
- only a single strand, rather than a double strand
- doesn’t contain the base thymine, and instead uracil
what are the main uses of proteins?
- enzymes
- hormones
- structural proteins (adds strength to cells and tissues)
what are alleles?
we have two copies of each chromosome (1 from each parent). therefore, we have 2 copies of each gene. therefore we have two alleles. they can be the same allele, or different
- one chromosome in the pair comes from the father, and the other comes from the mother.
ALLELES ARE BASICALLY THE DIFFERENT VERSIONS OF A GENE.
what is the difference between genes, chromosomes, and DNA?
- chromosomes are the biggest. they are molecules made up of many coiled up sections of DNA.
- DNA is responsible for building and maintaining your human structure.
- genes are segments/sequences of your DNA, that code for certain characteristics.
what are the two alleles for the eye colour gene?
- one causes blue eyes. the recessive gene (b), meaning there’s less of a chance as you need two, not just one.
- one causes brown eyes. the dominant gene (B), meaning there’s a greater chance as you only need one.
what does homozygous mean?
where the two alleles are the same. (e.g. a bb person is homozygous for the blue eye allele).
what does heterozygous mean?
where the two alleles are different. (e.g. Bb is heterozygous, as there’s one blue eye and one brown eye allele).
what does genotype mean?
it tells us which alleles a person has for a particular gene (tells us the entire collection of alleles that are present). (e.g. a person with 2 copies of the blue eye allele has the ‘bb’ genotype).
what does phenotype mean?
a characteristic caused by a particular genotype. (e.g. a ‘bb’ genotype causes blue eyes. blue eyes is the phenotype).
what is the difference between a dominant and a recessive allele?
- a dominant allele always produces its characteristic, even if there’s only one in the genotype. represented with an upper-case letter.
- recessive alleles (represented with lower-case letter) only produces its characteristic if both of the alleles in the genotype are recessive.
which gender matches with which chromosome?
- XY
- XX
XY = male
XX = female
who are James Watson and Francis Crick?
discovered structure of DNA. realised it’s made up of two chains of nucleotide pairs that encode genetic information.
- used Rosalind Franklin’s image of DNA to make their discovery.
who is Rosalind Franklin?
- the discovery of the structure of the DNA was made possible by her X-ray diffraction work with Wilkins, although he took the credit for it and she didn’t.
- she created Photo 51, where in her x-ray diffraction work, the lighter diamond shapes above and below the darkened x suggested a double helix pattern. provided a lot of structural information about DNA.
- she was unaware that Wilkins, Watson, and Crick had used her x-ray photograph, so they didn’t have permission to use her data. they excluded her from winning the Nobel prize.
who is Erwin Chargraff?
- found that in DNA, the ratios of A to T and G to C are equal. provided clues into the chemical pairings that made up DNA. found that the amounts of A, T, C, and G varied from species to species.
- observed the amounts of the 4 nitrogenous bases found in different samples of DNA. paper chromatography was used to separate the substances in the DNA and UV spectrophotometry was used to count the amount of each base in each sample.
- discovered part of the structure of DNA, allowed Watson and Crick to make their final discovery.
what are some examples of inherited diseases?
- cystic fibrosis
- sickle cell anaemia (however, anaemia is not inherited)
- colour blindness
how does an inherited disorder come about?
the result of a gene mutation. not always negative, but can affect your vision/quality of life.
what is a mutation?
a change to a DNA base sequence (e.g. a C changes to a G).
- these mutations happen spontaneously all the time, especially when DNA is being duplicated for cell division
what two things INCREASE THE RISK of mutations?
- carcinogens (harmful chemicals, e.g. in cigarette smoke)
- certain types of radiation (e.g. x-rays, gamma rays)
describe a harmless mutation in a section of bases of a gene:
e.g. ATG GGA … , and this creates a certain protein shape.
this could change to: ATG GGG, and it could still encode the same protein shape, by not changing the amino acid sequence. this is because different base triplets can sometimes encode for the same amino acid, or it may only affect a protein very slightly (look slightly different, but work in the same way)
this mutation has no effect on the protein’s shape/function (phenotype). this is the case for most mutations.
what happens when a mutation actually has an effect on the amino acid produced from the base triplet?
- the protein has a different amino acid sequence, and this has altered its shape.
- this can have a dramatic effect on its function.
- e.g. the active site on an enzyme may have changed shape, so it can no longer attach to the substrate. it can’t form an enzyme-substrate complex, so can no longer catalyse a reaction
- e.g. if it changes the shape of a structural protein (e.g. collagen), it may lose its strength
what occurs rarely in terms of mutations?
- very rarely leads to a changed, new phenotype
- sometimes, a new phenotype can be beneficial if the environment changes, and this can lead to a rapid change in the species, through reproduction
- e.g. resistance to a disease
where do most mutations happen?
non-coding DNA
describe the non-coding DNA in some chromosomes:
- these regions switch genes on and off.
- tell genes when to produce proteins.
- e.g. the gene coding for haemoglobin in a nerve cell may be switched off, as there’s no need for it, so it’s not expressed
- mutations in these non-coding regions can affect how genes are switched on or off. e.g. a gene may be turned on that is supposed to be turned off.
- as a result, the cell would produce a protein that it is not meant to have at that time. this could have a significant effect on a cell (e.g. uncontrolled mitosis, leading to cancer).
what are inherited disorders?
a group of conditions that can be passed on in certain alleles, and so can be inherited from a person’s parents
what is cystic fibrosis?
- disorder of the cell membranes, resulting in a thick sticky mucus being released in different parts of the body, e.g. airways of the lungs, pancreas
- controlled by a single gene.
- the allele for normal cell membranes is dominant (C), and the allele for defective cell membranes is recessive (c). therefore, in order to have cystic fibrosis, the person must have 2 copies of the defective allele.
- a person with the Cc genotype does not have cystic fibrosis, but instead a carrier of the disorder.
what is polydactyly?
- people who have extra fingers or toes, but doesn’t usually cause any other problems
- caused by a dominant allele (only need one copy of the polydactyly allele to have the disorder)
- not possible to be a carrier of this disorder, or of any dominant allele
what is the use and advantages of embryo screening?
- embryos tested to see if they have the alleles for inherited disorders
- ## alleles without the defective alleles are implanted into the woman, and develop into healthy offspring
- reduces the overall amount of suffering (fewer people will have health problems)
- saves money (treating genetic disorders is expensive)
what are the issues around embryo screening?
- expensive. people believe the money should be spent elsewhere in the health service
- often a large number of embryos are created, but only a small number are implanted. that means some healthy embryos are destroyed, and some think that’s unethical
- in the future we may be able to screen embryos to produce offspring with desirable features (e.g. taller/more intelligent offspring). some find this unethical, and is illegal in the UK
- implies that people with genetic disorders are less desirable, increasing prejudice
what is gene therapy?
the treatment of an inherited disorder by giving the patient a healthy copy of the faulty gene.
- at the moment, however, this is still experimental, and we don’t know the potential effects of modifying gene on other genes
how does gene therapy work?
giving a person a healthy version of a gene, and hoping it will fix the problem
- however, the faulty gene would be in all of the cells of the body, so we’d need to transfer this new gene to every cell, which is difficult. we would then transfer the gene at an early stage of development, as then as the person develops, the healthy gene will be passed onto all cells
go into more depth on cystic fibrosis: what are its symptoms, why?
- most common life-threatening disease in the UK, affects 1 in 2500 people, affects around 100,000 people globally
- usually develops in early childhood
- build-up of sticky mucus, affects the lungs and digestion
- can cause recurring chest infections (due to bacteria being stuck in the chest mucus); wheezing, coughing, shortness of breath; difficulty putting on weight. can develop other conditions such as diabetes, weakened bones (osteoporosis), male infertility (affects sperm ducts/testes), diarrhoea.
- this is because it clogs the pancreatic duct and limits digestive enzyme secretion, limiting the absorption of fats, proteins, and vitamins.
how would you test for cystic fibrosis during the person’s life?
the sweat test. checks for high levels of chloride in the sweat - people with CF have higher amounts of sodium and chloride in their sweat.
what are the treatments for cystic fibrosis?
- antibiotics to prevent/treat chest infections
- medicines to thin the mucus and make it easier to cough it up
- medicines to widen the airways and reduce inflammation
- medicines to help digestion
- physiotherapy loosens the mucus
- enzyme and vitamin tablets help keep the levels up
- oxygen gas improves breathing
what are the two types of embryonic screening?
CHORIONIC VILLUS SAMPLING (CVS): sampling of embryonic cells between 10-12 weeks old. takes a sample of tissue from the developing placenta, from the chorionic villi, and test these cells. it’s a diagnostic test (gives a yes/no answer), but only tests for chromosome disorders such as down syndrome.
AMNIOCENTIS: (sampling of embryonic cells between 15-16 weeks). take fluid from around the foetus, this fluid contains the cells used for screening. this causes a small chance of miscarriage, but lab errors are rare.
what is a limitation with genetic family trees?
only shows phenotypes and not genotypes
describe the chromosome pairs in a cell:
- humans contain 23 pairs of chromosomes in normal body cells
- 22 of the chromosome pairs contain the genes which determine inherited characteristics only
- one of the chromosomes contain the genes that determine sex
what sex chromosomes do males and females have, and what are the chances of either sex in a baby?
males = XY
females = XX
- 50% chance of either sex
what is variation?
the differences in the characteristics (phenotypes) of individuals in a population
- most of our characteristics are formed by the interaction of our genes with our environment
what is a common cause of genetic variation?
mutations. if the protein is different, due to a change in an organism’s DNA code, the phenotype may also change
- most of the time, this results in a negative change, but it can sometimes be beneficial (e.g. resistance to a disease, improved abilities)
- these organisms with beneficial mutations are more likely to survive and reproduce, passing on their genes to the next generation
SURVIVAL OF THE FITTEST/NATURAL SELECTION (the fittest individuals are being selected to survive)
name some forms of genetic variation:
the alleles that individuals have inherited
- includes hair colour, eye colour
name some environmental variation:
- the colour of some flowers depends on the pH of the soil
- language in humans
name some forms of variation that are a combination of genes and the environment:
- height in humans
- some people have alleles that make them likely to grow taller
- however, their diet must include enough calcium to allow their bones to fully develop
what do scientists believe about where species come from?
- scientists believe that life first developed on Earth over 3 billion years ago
- these first life forms were simple (e.g. single cells)
- all living things have evolved from this simple life form
describe the wide range of genetic variation in a population of rabbits:
- every rabbit will have a slightly different combination of alleles that it inherited from its parents
- some rabbits will have alleles for thicker fur, better eyesight, or better hearing
how may an allele for thicker fur in rabbits lead to evolution through natural selection?
- rabbits which have inherited the alleles for thicker fur are more likely to survive the colder temperatures than those with thinner fur
- the rabbits with thicker fur can survive the cold and then reproduce
- their offspring could inherit the alleles for thicker fur, and these are more likely to survive the cold and reproduce
- over many generations, this allele will become more common among the population of rabbits
how many an allele for better hearing/eyesight lead to evolution through natural selection in rabbits?
- a predator moves into the area, e.g. a fox
- the rabbits with alleles for better eyesight/hearing have an advantage
- these rabbits are more likely to detect the fox as it approaches, so they’re more likely to survive and reproduce
- these beneficial alleles will be passed onto the offspring, and over several generations, these alleles will be widespread
define evolution:
the change in the inherited characteristics of a population over time through a process of natural selection
- this could lead to a change in the whole species, or even a development of an entirely new species
what does the theory of evolution suggest we evolved from?
simple life forms that first developed over 3 billion years ago
what determines when two populations become different species?
two populations of one species can become so different in phenotype that they can no longer interbreed to produce fertile offspring
name four examples of selective breeding:
- domestic dogs have been bred to have a more gentle nature
- food crops (e.g. wheat) have been bred to be resistant to disease
- animals (e.g. cows) have been bred to produce more meat/milk
- plants have been bred to produce large/unusual flowers
what is selective breeding?
the best plants or animals in a population are bred together in the hope of getting even better offspring
how would we selectively breed large cows for meat?
- take a mixed population of cows. select the largest male and female (take the organisms with the desired characteristics in the population)
- breed these together. sexual reproduction produces variation in the offspring, so usually they’d be a mixture of larger and smaller animals
- constantly selecting the largest male and female offspring to breed together means eventually all offspring will be large
what is a problem with selective breeding?
- reduces the gene pool of a population (the collection of different alleles in a population). the best individuals are usually closely related as they both have the good genes, so breeding them together can lead to inbreeding
- inbreeding can cause some breeds to be prone to disease or inherited defects
- e.g. inbred dog breeds can develop inherited disorders such as joint problems, heart disease, or epilepsy
- a small gene pool also leads to less variation within a population, meaning an incoming pathogen, for example, will affect all of them, and possibly wipe out the entire population
what occurs in genetic engineering?
genes from one organism (e.g. humans) are cut out an transferred to the cells of a different organism (e.g. bacteria)
- the genome of the bacteria is modified and now contains a human gene
- the genes from an organism with a desirable characteristic are taken and transferred to another organism, so this organism has the same trait. this organism is genetically modified (GM)
- can do this intraspecies or interspecies
describe the genetic engineering of insulin:
- insulin = hormone involved in blood glucose regulation in humans
- people with type 1 diabetes cannot make their own insulin, so must inject themselves with it regularly
- bacteria have been genetically modified to contain the human insulin gene, and these now produce human insulin. this can then be purified and used for type 1 diabetes
describe the genetic engineering of crops:
- can transfer genes into plants to produce genetically modified (GM) crops
- these generally produce a greater yield, or can be disease/insect resistant, or can produce bigger/better fruits (more food for less money, which is important in developing countries, where famine is common)
- some GM crops are resistant to herbicides, meaning farmers can spray their fields to kill weeds without killing the crops too
- crops can be modified to contain nutrients, e.g. golden rice protects people from blindness, due to beta carotene
what are the disadvantages of GM crops?
- could be unsafe. e.g. harmful to insects or wild flowers
- some people feel we need to do more research on the health effects of eating GM crops
- the plants could make their way into the wild, introduce competition and change the entire ecosystem. however, this is unlikely as the plants have been modified to survive in the farmer’s field, not in the wild
what are the main steps in genetic engineering?
- identify the gene we want to transfer (e.g. human gene, animal/plant gene)
- use enzymes to isolate this gene
- transfer the gene into a small circle of DNA called a plasmid (from a bacteria). we could also use a virus
- the plasmid/virus will transfer the DNA from one organism to the cells of the target organism, therefore they’re vectors. the organism’s cells will take up the vector and the contained useful genes and start producing a protein that the gene codes for
when would we transfer genes?
- always transfer the gene at an early stage in an organism’s development
- e.g. if we were transferring the gene into an animal, we would do it during the early embryo stage
- this would ensure that all of the cells receive the transferred gene, so the organism develops with the characteristic we want
name some examples of genetic engineering that has been done:
- modified sheep to produce drugs in their milk that we can extract and use to treat diseases
- modified bacteria to produce the hormone insulin that we can harvest and use to treat diabetes
- modifying crops to increase the size and quality of their fruit, to become resistant to diseases, insects, and herbicides, etc.
what is the advantage of cloning plants?
as the clone is genetically identical to the original plant (mitosis), we know exactly what its characteristics will be (e.g. the colour of the flowers)
how would we clone a plant through taking cuttings?
- good if we want just a few clones
- gardeners have been using this method for a long time
- a small piece of the plant is removed (e.g. growing shoot/branch) and the end is dipped in rooting powder and then planted
- rooting powder contains plant hormones and encourages it to develop roots
- produces a genetically identical clone of the original plant
what is tissue culture (micropropagation)?
- find a plant with desirable characteristics to clone
- take very small pieces of plant tissue (explants) from the tips of stems, using scalpel/razor/scissors/knife
- sterilise the explants to remove any microorganisms
- place the explants in a nutrient medium (e.g. agar, containing growth hormones) and let them grow into small masses of cells called calluses
- transfer (with tweezers/forceps) the calluses to the soil where they can grow into plantlets (baby plants)
- ## the plantlets can be transferred to their own pots to develop into genetically identical adult plants
- conditions must be sterile as we don’t want any microorganisms such as bacteria or fungi to enter
what nutrients should be added into the growth medium during micropropagation?
- nitrates for amino acids/proteins
- glucose/sucrose for energy/respiration
- magnesium for chloroplasts/chlorophyll
name some precautions that must be taken to ensure the healthy growth of the plantlets during micropropagation:
- sterile/aseptic conditions
- temperature
- light
- humidity
- growth hormones
what are the advantages of cloning via cuttings vs micropropagation?
- cuttings is quicker and cheaper
- cuttings requires less technical expertise/equipment
- don’t have to worry about sterilisation so much for cuttings
what are the benefits of tissue culture?
- useful in commercial plant nurseries
- allows growers to produce thousands of genetically identical plants quickly and cheaply
- gardeners can be certain the plants will have the characteristics they want
- tissue culture can also be used to preserve a rare species of plants
how do we clone mammals through embryo transplants?
- use a horse as the example
- start with a sperm and egg cell from horses with the desired characteristics
- fertilise the egg, and allow it to develop into an early stage embryo. however, the cells in the embryo must not have started to specialise
- use a glass rod to split this embryo into two
- transplant the embryo into two host mothers. the embryos will grow and develop, and when the animals are born, we will have two genetically identical offspring (clones)
what are the problems with embryo transplants?
as we start with a sperm and an egg (sexual reproduction, therefore variation), we cannot be certain that the offspring will have the characteristics we want
- therefore, use adult cell cloning, as we can be certain of the characteristics the clone will have
describe the steps of adult cell cloning:
- use sheep as an example
- remove a cell from the animal we want to clone (e.g. skin cell)
- remove the diploid nucleus from the cell, containing the animal’s genetic information
- take an unfertilised egg cell from the same species, e.g. any female sheep
- remove the nucleus from the unfertilised egg, so it now contains no genetic material at all
- insert the nucleus from the original adult body cell into the empty egg cell
- stimulate the egg cell by giving it an electric shock, causing it to act like a zygote and divide to form an embryo
- when the embryo has developed, it’s inserted into an adult female (surrogate mother) to continue its development
- the clone is then birthed
what are the advantages of cloning over selective breeding?
- all offspring are identical (no variation)
- quicker
- more offspring are produced
- no need for natural mating/two parents
what do you remove when you ‘enucleate’ a cell?
the nucleus
describe Charles Darwin:
- took part in a global expedition in the 1800s
- collected a vast number of different plants and animals, he was fascinated by the variety
- spent many years studying geology and fossils - many species of animals and plants alive today are similar to extinct species
- said that natural selection is the driving force behind the gradual development of species over time
describe transgenic animals:
- using genetic engineering, scientists can place human genes into the DNA of other organisms, e.g. cows, sheep, goats
- once an organism has DNA from another species, it’s ‘transgenic’
- we can genetically engineer cows, sheep, and goats to produce helpful human proteins in their milk. if a cow had the human insulin gene, it would produce the human insulin protein, which can be extracted and used
- instead of genetically engineering organisms each time, we can often clone the transgenic organisms
what was Darwin’s observation surrounding fossils?
older layers of rock contained fossils of less complex organisms, while more recent layers show more complex organisms
describe the theory of evolution through natural selection:
- within a species, there is a wide range of genetic variation for any characteristic/trait
- individuals with characteristics most suited to the environment are more likely to survive to breed successfully
- the characteristics that have enabled these individuals to survive are then passed onto the next generation
what was the public reaction to Darwin’s theory?
- published his findings in ‘On the Origin of Species’
- extremely controversial theory, only gradually accepted
- at the time, a lot of people strongly believed that God made all the animals and plants on Earth, and Darwin’s theory challenged this idea
- many scientists believed he didn’t have enough evidence to back his theory up
- ## genetics and DNA wasn’t understood until 50 years after Darwin’s theories were published
- Darwin’s theory has now been proven multiple times. we can see antibiotic resistance in bacteria, and looked at fossil records
what is Lamarck’s theory?
suggested that when a characteristic is regularly used in an organism’s lifetime, it becomes more developed. this strengthened characteristic is then passed onto the offspring
- e.g. giraffes started with shorter necks adapted to lower vegetation. they stretched their necks to reach higher branches for food, resulting in longer necks during their lifetime. this acquired longer neck was passed onto offspring, and onto successive generations
what is the big problem with Lamarck’s theory?
- we now know that in the vast majority of cases, changes that occur in an organism’s lifetime cannot be passed onto offspring, as they do not affect the DNA sequence of an organism
- Lamarck’s theory is therefore incorrect, and scientists accept Darwin’s theory as how species change
what was Darwin’s and Wallace’s theory on why giraffes have long necks? (countering Lamarck’s theory)
- some giraffes had longer necks than others, due to variation within the species
- giraffes with longer necks were better adapted to their environment, so could eat the leaves from taller trees
- giraffes with longer necks had a higher chance of surviving and reproducing, so passed on the traits of their long necks to their offspring
- over many generations, this produced modern giraffes with long necks
describe Alfred Russel Wallace:
- travelled globally looking at different animals and plants
- interested in the warning colouration in animals, and how they’d evolved (e.g. bright red frogs could warn predators that it’s poisonous)
- independently proposed his own theory of evolution through natural selection, and also came up with the idea of speciation
- him and Darwin realised they had the same theory, so they jointly published their findings
what is speciation?
the evolutionary process by which populations evolve to form distinct, new species that can no longer interbreed and create fertile offspring
what factors may cause speciation?
a combination of both:
- isolation (a physical barrier, e.g. river/mountain range, separates 2 different populations of the same species)
- natural selection
what is a species?
a group of genetically similar organisms that are able to interbreed and produce fertile offspring
how do new species form?
- two populations become isolated (by a geographical barrier, e.g. river/mountain)
- these populations have different gene pools/have developed different mutations
- each population was exposed to different environmental conditions (e.g. weather, water levels)
- natural selection occurs in the 2 populations, favouring the alleles that best suit each environment
- the two populations become genetically different and can no longer interbreed to produce fertile offspring
describe Gregor Mendel:
- carried out many experiments on pea plants, and looked at their characteristics (shape/colour of pod, colour of flowers, height)
- from this, he realised that characteristics aren’t ‘blended’ during inheritance. e.g. the shape of a pea pod has no effect on the colour of the flower.
- Mendel said that characteristics are determined by inherited units (genes), which don’t change when passed onto descendants
- also showed that some characteristics can be masked and then reappear in later generations - recessive alleles
describe an example of one of Mendel’s experiments:
- took a green pea pod and a yellow pea pod. crossed the two plants together. the offspring were all yellow pea plants
- took 2 of these yellow offspring pea plants. this time, 1 in every 4 pea plants were green (the yellow plants still had the hereditary units for green pods, they just weren’t being expressed, as they were recessive)
- concluded that there must be inherited units passed on from generation to generation, and that they can either be recessive or dominant
- Mendel also did these experiments with other traits, e.g. height of plant, colour of flowers, but each time he found the same pattern, suggesting all of these characteristics are passed down in this dominant/recessive way
what was the changing scientific reaction to Mendel’s discovery?
- published his research in a scientific paper
- ## many scientists held onto the idea that characteristics are blended, and forgot about his work, despite it being a major discovery
- in the late 1800s, scientists looked at how chromosomes behave during cell division, and rediscovered Mendel’s work on cell genetics
- by the early 1900s, they realised that Mendel’s units (genes) behaved in a similar way to chromosomes. they realised that genes must be located on chromosomes
- in the mid 1900s, scientists determined the structure of DNA and how genes function
- later, in 2003, we were able to figure out the entire human genome
what are fossils?
the remains of organisms from millions of years ago which are found in rocks
- they’re important as most of the organisms that ever lived are now extinct, so the only way to learn about them is by looking at what they left behind
- by studying organisms, evidence is provided for evolution, as we can see the small, incremental changes that took place over millions of years
what are the three ways fossils could form?
- parts of organisms that haven’t decayed, e.g. when the conditions needed for decay are absent (low temperature, lack of oxygen/water in amber,tar pits, glaciers - too cold - , peat bogs - too acidic - for example)
- if parts of the organism (e.g. shells, teeth, bones) are slowly replaced by minerals during the decay process, forming rock-like substance in the same shape and size as the original structures
- could be the preserved traces/casts/impressions of organisms in soft material such as clay. as the clay hardens, we’re left with a gap the same size/shape as the organism left (e.g. footprints, burrows, plant root spaces)
what is the problem with fossils?
- many of the earliest forms of life were soft-bodied organisms (no shell/skeleton), which don’t form fossils as they decayed quickly
- any fossils that did form have been destroyed by changes to the rock in the Earth’s crust (eartquakes, volcanoes, movement of tectonic plates)
- there are therefore very few fossils of the early forms of life, and large gaps in time with no fossils, so scientists cannot be certain on how life began
what do fossils show?
that a huge number of species have become extinct
what could cause a species to become extinct?
- catastrophic event (e.g. asteroid)
- environmental changes too quickly (e.g. changing weather patterns)
- new disease/predator
- new, more successful species evolves and competes with the pre-existing species, e.g for water
how long does evolution usually take?
millions of years
how quickly can bacteria reproduce?
under ideal conditions, can reproduce every twenty minutes - due to this, they can evolve rapidly
what are antibiotics used for?
- in the 1940s, doctors began to treat bacterial diseases using antibiotics such as penicillin
- antibiotics kill bacteria
- they’re now widely used in medicine
- also used in farming, to prevent animals from developing bacterial diseases
what is antibiotic resistance?
in the last few years, certain strains of bacteria are no longer killed by antibiotics. these bacteria have evolved and are now antibiotic resistant
- a common antibiotic resistant strain is MRSA
how does antibiotic resistance happen?
- it’s possible that a mutation (as they happen so often) could make a bacterium resistant to antibiotics
- if we use an antibiotic on a bacteria population, all of the bacteria are killed apart from the antibiotic resistant bacterium
- this survives and reproduces very quickly, without competition, and the population of this resistant strain rises, forming a colony
- as the antibiotic isn’t effective anymore, the person is still affected, so can pass on this resistant bacteria to others. if they then go and get the same antibiotic, it won’t work
what is a superbug?
bacteria that is resistant to lots of different types of antibiotics
what is MRSA?
- a superbug
- relatively common, as its resistance makes it so hard to kill
how can we reduce the development of antibiotic resistant strains of bacteria?
- doctors shouldn’t prescribe antibiotics inappropriately, e.g. no point in using an antibiotic to treat a virus, and don’t prescribe them for mild bacterial infections that will clear quickly anyway
- patients should make certain to complete their course of antibiotics, ensuring all of the bacteria are killed and none can survive to mutate and form resistant strains
- restrict the use of antibiotics in farming, which is usually just so the livestock don’t get ill in the first place, or so they are healthier
what is the problem with the development of new antibiotics?
- takes a long time
- extremely expensive
- as new antibiotic resistant bacteria emerge all the time, it’s unlikely that we’ll be able to keep up
what did Carl Linnaeus do?
- in the 1700s, he began to classify species into different categories based on their (bone) structure and characteristics
- divided all living organisms into 2 kingdoms: animal and plant
- then divided each kingdom into a number of smaller categories
what is wrong with simple names for animals?
- different countries have different names for animals
- these names don’t tell us much about how the animals are related
what categories did Linnaeus group organisms into?
- kingdom (e.g. plant/animal)
- phylum
- class
- order
- family
- genus
- species
how is every organism named?
- binomial naming system, mainly latin
- named from their genus and species - naming organisms just by the species name isn’t enough, as multiple species could have the same name
what are the benefits of the binomial naming system?
- each species has a unique name
- it lets scientists discuss individual species
what is the drawback of the classification system?
- based on visible characteristics
- however, there have been major advances since then (e.g. we can now use microscopes to analyse internal structures, and can analyse an organisms biochemistry, e.g. its DNA and look for similarities with other species)
what is the 3 domain system?
- ## developed by Carl Woese, compared the biochemistry (RNA sequences) of different organisms, and found that some species are less related than we thought
- these domains are placed above kingdoms
ARCHAE/PRIMITIVE BACTERIA: different type of prokaryotic cell that lives in extreme conditions e.g. hot springs
TRUE BACTERIA: tiny, single-celled prokaryotic organisms
EUKARYOTA: animals, plants, fungi, protists (amoeba)
what are evolutionary trees?
- show the evolutionary relationships between different species/groups by linking common ancestors. the more recent the common ancestor, the more closely related the species are
- scientists, to make one, can use classification data on living organisms (e.g. their structure DNA)
- for extinct organisms, scientists must use fossils. this is a problem, as the fossil records for many species are incomplete
