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biology: inheritance, variation & evolution (evolution (Charles Darwin…
biology: inheritance, variation & evolution
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mutations
insertions are where a new base is inserted into the DNA base sequence where it shouldn't be (3 bases code for particular amino acid)
an insertion changes the way groups of 3 bases are 'read', changing the amino acids they code for.
they can change more than one amino acid as they have a knock-on effect on the bases further in the sequence.
deletions are when a random base is deleted from the DNA base sequence. they change the way that the base sequence is 'read' & have knock-on effects further down sequence.
a mutation is a random change in an organism's DNA - can be inherited.
occur continuously & spontaneously e.g. when chromosome isn't replicated properly.
chance of mutation increased by exposure to certain substances or some types of radiation.
they change the sequence of the DNA bases in a gene, producing a genetic variant.
sequence of DNA bases codes for sequence of amino acids that make up a protein so mutations can lead to changes in the protein.
- most have little or no effect on protein or some will change it to such a small extent that its function or appearance is unaffected.
- some mutations can code for an altered protein changing its shape, affecting its ability to perform its function:
:arrow_right_hook: if shape of enzyme's active site is changed, its substrate can no longer bind to it.
:arrow_right_hook: structural proteins e.g. collagen could lose their strength if their shape is changed, making them useless at providing structure & support.
- mutation in non-coding DNA can alter how genes are expressed.
substitutions mutations are when a random base in the DNA base sequence is changed to a different base
reproduction
sexual reproduction
genetic info. from two organisms (father&mother) is combined to produce offspring which are genetically different to either parent.
mother & father produce gametes by meiosis (egg & sperm cells in animals)
in humans each gamete contains 23 chromosomes
the egg & sperm cell fuse together (fertilisation) to form a cell with the full number of chromosomes
involves the fusion of male & female gametes
2 parents so offspring contain mixture of parents' genes
offspring inherits features from both parents & the mixture of genetic information produces variation in the offspring.
flowering plants have egg cells + pollen
advantages
offspring have a mixture of two sets of chromosomes, so inherits genes from both parents (& therefore features), producing variation in the offspring
variation increases the chance of a species surviving a change in the environment, the change could kill some individuals, the variation is likely to have led to some of the offspring being able to survive in the new environment - survival advantage.
as individuals have characterises that make them better adapted to their environment, they are more likely to breed successfully & pass their genes on to offspring - natural selection.
selective breeding can be used to speed up natural selection, allowing humans to produce animals with desirable characteristics selective breeding is where individuals with a desirable characteristic are bred to produce offspring that have the same desirable characteristic. this can increase food production e.g. by breeding animas that produce a lot of meat.
asexual reproduction
only one parent. no fusion of gametes, no mixing of chromosomes & no genetic variation between parent & offspring. the offspring are genetically identical to the parent - clones.
happens by mitosis, a cell makes a new cell by diving in two. the new cell has exactly the same genetic information as the parent cell - clone.
bacteria, some plants & some animals reproduce asexually
advantages
only needs to be one parent --> uses less energy as organisms don't have to find a mate --> faster
many identical offspring can be produced in favourable conditions
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meiosis
cells divide by meiosis to make gametes only have half the original number of chromosomes.
two cell divisions. in humans only happens in reproductive organs
before the cell starts to divide, it duplicates its genetic information, forming 2 armed chromosomes - one arm is exact copy of other.
after replication, the chromosomes arrange themselves into pairs.
in the first division, the chromosomes 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 father & mother's chromosomes go into each new cell
in the second division, the chromosomes line up in the centre of the cell & the arms of the chromosomes are pulled apart.
4 gametes are produced, each with only a single set of chromosomes. each gamete is genetically different from the others because the chromosomes get shuffled up during meiosis & each gamete only gets half of them at random.
after two gametes have fused during fertilisation, the 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 start to differentiate into different types of specialised cell that make up an organism.
inherited disorders
polydactyly is a genetic disorder where a baby is born with extra fingers or toes - isn't life threatening as doesn't cause any other problems.
- caused by a dominant allele 'D' so can be inherited if just one parent carries the defective allele.
- the parent who has the defective allele will also have the condition as it is dominant.
- 50% chance of child having the disorder if one parent has one D allele.
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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 & in the pancreas.
the allele which causes cystic fibrosis is a recessive allele 'f' carried by about 1 person in 25.
as it is recessive, people with only one copy of the allele wont have the disorder - they are carriers.
for a child to have the disorder, both parents must be either carriers or have the disorder themselves. 1 in 4 chance of a child having the disorder if both parents are carriers.
X and Y chromosomes
23 pairs of chromosomes in every human body cell - 22 are matched pairs that control characteristics but the 23rd pair: labelled XY or XX decide the person's sex
males: XY
Y chromosome causes male characteristics
female: XX
combination that allow female characteristics to develop
when making sperm, the X and Y chromosomes are drawn apart in the first division of meiosis - 50% chance that each sperm cell gets an X chromosome and 50% chance for the Y.
when making eggs, the original cell has two X-chromosomes os all the eggs have one X chromosomes.
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genetic diagrams
genes you inherit control the characteristics you develop.
different genes control different characteristics - some characteristics are controlled by a single gene e.g. red-green colour blindness in humans & mouse fur colour.
most characteristics controlled by several genes interacting.
all genes exist in different versions called alleles - humans have two versions of every gene in their body - one on each chromosomes in a pair.
if an organism has two alleles for a particular gene that are the same, it's homozygous for that trait. if the two. alleles for that gene are different, it's heterozygous.
if the two alleles are different, only one can determine what characteristic is present, the allele for the characteristic that is shown is the dominant allele (shown by capital letter) the other one is recessive (lower case letters)
both alleles must be recessive for the organism to display the recessive characteristic but to display the dominant characteristic, the organism can be heterozygous or homozygous dominant as the dominant allele overrules the recessive one if the organism is heterozygous
the genotype is the combination of alleles that the organism has - the alleles work at a molecular level to determine what characteristics the organism has - the phenotype
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family trees: if more people are carriers of a disease but not many are affected by the disease then the allele for it is likely to be recessive
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variation
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different genes
- 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.
- the combing of genes from two parents causes genetic variation - no two of the same species are genetically identical.
- characteristics are determined only by genes, in animals: eye colour, blood group & inherited disorders (e.g. haemophilia, cystic fibrosis)
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most characteristics (e.g. body weight, height, skin colour etc) are determined by a mixture of genetic & environmental factors e.g. the maximum height an animal or plant could grow to is determined by its genes but whether it actually grows that tall depends on its environment (e.g. how much food it gets)
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evolution
theory of evolution: all of today's species have developed from simple life forms that started to develop over three billion years ago.
Charles Darwin used the observations he made on a round-the-world trip, along w/ experiments, discussions & new knowledge of fossils & geology to suggest the theory of evolution by natural selection
- he knew that organisms in a species show wide variation in their characteristics (phenotype variation) & knew that organisms have to compete for limited resources in an ecosystem.
- he concluded that the organisms with the most suitable characteristics for the environment would be more successful competitors & would be more likely to survive - 'survival of the fittest'.
- the successful organisms that survive are more likely to reproduce & 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 & reproduce so are less likely to pass on their genes to the next generation.
- over time, beneficial characteristics become more common in the population & the species. changes - it evolves
Darwin proposed his theory in his book On the Origin of Species in 1859 but his idea was controversial as:
- it went against common religious beliefs about how life on Earth developed - 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 - he didn't know about genes or mutations, weren't discovered until 50 years after his theory was published.
- there wasn't enough evidence to convince many scientists as not many other studies had been done into how organisms change over time.
the relevant scientific knowledge wasn't available at the time so Darwin couldn't give a good explanation for why new characteristics appeared or how individual organisms passed on beneficial adaptations to their offspring.
it is now known 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
speciation occurs when over a long period of time, the phenotype of organisms change so much because of natural selection that a completely new species is formed.
it happens when populations of the same species change enough to become reproductively isolated - they can't interbreed to produce fertile offspring.
species become extinct when:
- 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. volcanic eruption or a collision with an asteroid).
dodos are now extinct. humans hunted them & introduced other animals which ate all their eggs & destroyed the forest where they lived.
Jean-Baptiste Lamarck argued that changes that an organism acquires during its lifetime will be passed on to its offspring - he thought that if a characteristic was used a lot by an organism, it would become more developed during its lifetime & its offspring would inherit the acquired characteristic.
e.g. using his theory/hypothesis, if a rabbit used its legs to run a lot (to escape predators), its legs would get longer & its offspring would be born with longer legs.
scientists come up with different hypotheses to explain similar observations or might develop them because they have different beliefs (e.g. religious) or have been influenced by different people (e.g. other scientists & their way of thinking) or they just think differently.
to find out if it is right they need to find evidence to support or disprove each hypothesis e.g. Lamarck & Darwin had different hypotheses to explain how evolution happens
- Lamarck's hypothesis was eventually rejected because experiments didn't support his hypothesis e.g. if a hamster's fur is dyed bright pink,, its offspring will still be born with the normal fur colour as the new characteristic won't have been passed on.
- the discovery of genetics supported Darwin's idea as it provided an explanation of how organisms born with beneficial characteristics can pass them on (via genes). other evidence also found by looking at fossils of different ages (the fossil record) - allowing you to see how changes in organisms developed slowly over time.
- relatively recent discoveries of how bacteria are able to evolve to become resistant to antibiotics further supports evolution by natural selection.
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selective breeding
when humans artificially select the plants or animals that are going to breed so that the genes for particular characteristics remain in the population. organisms are selectively bred to develop features that are useful or attractive.
e.g. animals that produce more meat or milk.
crops with diseases resistance
dogs with a good, gentle temperament.
decorative plants with big or unusual flowers
- from the existing stock, individuals with the desired characteristics are selected.
- they are bred with each other.
- this process is continued over several generations, & the desirable trait gets stronger & stronger. eventually all the offspring will have the characteristic.
in agriculture, selective breeding can be used to improve yields e.g. to improve meat yields, a farmer could breed together the cows & bulls with the best characteristics for producing meat e.g. large size. After doing this for several generation, the farmer would get cows with a very high meat yield.
has been done for thousands of years, it is how we got edible crops from wild plants & domesticated animals like cows & dogs.
the main problem is that it reduces the gene pool - the number of different alleles in a population because the farmer keeps breeding from the "best" animals or plants which are closely related - inbreeding.
inbreeding can cause health problems as there is more chance of the organisms inheriting harmful genetic defects when the gene pool is limited. some dog breeds are particularly susceptible to certain defects because of inbreeding e.g. pugs often have breathing problems.
there can be serious problems if a new disease appears as there is not much variation in the population. all the stock are closely related so if one of them is going to be killed by a new disease, the others are also likely to succumb to it.
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genetic engineering
to transfer a gene responsible for a desirable characteristic from one organism's genome into another organisms so that it also has the desired characteristic.
- a useful gene is isolated (cut) from one organism's genome using enzymes & is inserted into a vector.
- the vector is usually a virus or a bacterial plasmid depending on the type of organism the gene is being transferred to.
- when the vector is introduced to the target organism, the useful gene is inserted into its cell(s).
scientists use this method for a lot:
- bacteria have been genetically modified to produce human insulin that can be used to treat diabetes.
- genetically modified (GM) crops have had their genes modified e.g. to improve the size & quality of their fruit, or. make them resistant to disease, insects & herbicides.
- sheep have been genetically engineered to produce substances, like drugs, in their milk that can be used to treat human diseases.
- scientists are researching genetic modification treatments for inherited diseases caused by faulty genes e.g. by inserting working genes into people with the disease - gene therapy.
in some cases the transfer of the gene is carried out when the organism receiving the gene is at an early stage of development (e.g. egg or embryo) so the organism develops with the characteristic coded for by the gene.
genetic engineering has the potential to solve many of our problems e.g. treating diseases, more efficient food production etc. but not everyone agrees with it because there are worries about the long-term effects of it - that changing an organism's genes might accidentally create unplanned problems which could get passed on to future generations.
:heavy_minus_sign: growing GM crops will affect the number of wild flowers (& so population of insects) that live in & around the crops - reducing farmland biodiversity.
:heavy_minus_sign: not everyone is convinced that GM crops are safe & some people are concerned that we might not fully understand the effects of eating them on human health e.g. people may develop allergies to the food - although there is probably no more risk for this than for eating usual foods.
:heavy_minus_sign: concern that transplanted genes may get out into the natural environment e.g. the herbicide resistance gene may be picked up by weeds, creating a new 'superweed' variety.
:heavy_plus_sign: characteristics chosen for GM crops increase the yield, making more food
:heavy_plus_sign: people living in developing nations often lack nutrients in their diets but GM crops can be engineered to contain the nutrient that's missing e.g. 'golden rice' is a GM rice crop that contains beta-carotene - lack of this substance causes blindness
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cloning
cloning plants
tissue culture:
- a few plant cells are put in a growth medium with hormones & they grow into new plants - clones of the parent plant. these plants can be made quickly in very little space & be grown all year.
- tissue culture is used by scientists to preserve rare plants that are hard to reproduce naturally & by plant nurseries to produce lots of stock quickly.
cuttings:
- gardeners can take cuttings from good parent plants & then plant them to produce genetically identical copies (clones) of the parent plant.
- these plants can be produced quickly & cheaply & is an older, simpler method than tissue culture.
cloning animals
embryo transplants:
farmers can produce cloned offspring
- sperm cells are taken from a prize bull (e.g.) & egg cells are taken from a prize cow. the sperm are 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 be implanted into lots of other cows where they grow into baby calves which will be genetically identical to each other.
- hundreds of "ideal" offspring can be produced every year from the best bull & cow for example.
adult cell cloning involves taking an unfertilised egg cell & removing its nucleus.
- the nucleus is then taken from an adult body cell (e.g. skin cell) & is inserted into the 'empty' egg cell.
- the egg cell is stimulated by an electric shock which makes it divide like a normal embryo.
- when the embryo is a ball of cells, it's implanted into the womb of an adult female where it grows into a genetically identical copy (clone) of the original adult body cell as it has the same genetic information.
this technique used to create Dolly the sheep.
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fossils
fossils are the remains of organisms from many thousands of years ago which are found in rocks.
they provide the evidence that organisms were around then. they tell us a lot about how much or how little organisms have evolved over time.
gradual replacement by minerals:
- teeth, shells, bones etc don't decay easily so 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 to rock but the fossil stays distinct inside the rock.
from casts & impressions:
- sometimes fossils are formed when an organism is buried in a soft material like clay which later hardens around it & the organism decays, leaving a cast of itself.
- e.g. an animal's burrow or plant's roots (rootlet traces) can be preserved as casts.
- footprints can be pressed into these materials when soft, leaving an impression when it hardens.
from preservation in places where no decay happens:
- in amber (clear yellow stone made from fossilised resin) & tar pits, there is no oxygen or moisture so decay microbes can't survive.
- in glaciers it is too cold for the decay microbes to work.
- peat bogs are too acidic for decay microbes.
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speciation
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isolation is where populations of a species are separated & can happen due to a physical barrier e.g. floods & earthquakes can cause barriers that geographically isolate some individuals from the main population.
:arrow_right_hook: conditions on either side of the barrier will be slightly different e.g. different climates so different characteristics become more common in each population due to natural selection operating differently on the populations.
- each population shows genetic variation as 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 so are more likely to breed successfully.
- the alleles that control the beneficial characteristics are more likely to be passed on to the next generation.
eventually, individuals from the 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 have become separate species.
Alfred Russel Wallace - scientist working at same time as Darwin.
his observations greatly contributed to the understanding of speciation - current understanding developed as more evidence became available over time.
independently came up with the idea of natural selection & published work on the subject together with Darwin in 1858 - prompted Darwin to publish On the Origin of Species 1859.
observations he made as he travelled the world provided evidence to support the theory of evolution by natural selection e.g. he realised that warning colours are used by some species (e.g. butterflies) to deter predators from eating them - example of a beneficial characteristic that had evolved by natural selection.
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classification
organisms classified according to a system proposed in 1700s by Carl Linnaeus which groups living things according to their characteristics & the structures that make them up.
:arrow_right: Linnaean system - living things divided into kingdoms which are subdivided into smaller groups: phylum, class, order, family, genus, species.
as knowledge of the biochemical processes taking place inside organisms developed & microscopes improved --> find out more about internal structures of organisms, scientists put forward new models of classification.
1990 Carl Woese proposed the three-domain system, using evidence gathered from new chemical analysis techniques such as RNA sequence analysis, found that some species thought to be closely related in traditional classification systems are not as closely related as first thought.
domains:
- archaea - organisms are primitive bacteria - found in extreme places e.g. hot springs & salt lakes.
- bacteria - true bacteria like E. Coli & Staphylococcus - often look similar to Archaea but there are biochemical differences between them.
- eukaryota broad range of organisms including fungi, plants, animals & protists.
subdivided into kingdom, phylum, class, order, family, genus, species.
binomial system every organism has its own two-part Latin name --> first part refers to the genus the organism belongs to & the 2nd part refers to the species e.g. humans - Homo sapiens.
used worldwide & means scientists in different countries or who speak different languages all refer to a particular species by the same name - avoiding potential confusion.
evolutionary trees show how scientists think different species are related to each other.
:arrow_right_hook: they show common ancestors & relationships between species --> the more recent common ancestor, the more closely related the two species & the more characteristics they're likely to share.
scientists analyse different types of data to work out evolutionary relationships:
- for living organisms they use current classification data e.g. DNA analysis & structural similarities.
- for extinct species they use info. from the fossil record.