D.1.1
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Describe four processes needed for the spontaneous origin of life on Earth.
1. Non-living synthesis of organic molecules (H2O, CO2, Ammonia, methane & Hydrogen)
Energy derived from:
ü Volcanic activity
ü Meteorite bombardment
ü UV radiation due to absence of ozone.
ü Frequent thunderstorms & lightning strikes.
ü High temperatures due to greenhouse gases
2. Assembly of these molecules into polymers.
3. Origin of self-replicating molecules that made inheritance possible
4. Packaging of these molecules into membranes.
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D.1.2
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Outline the experiments of Miller and Urey into the origin of organic compounds.
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D.1.3
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State that comets may have delivered organic compounds to Earth.
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D.1.4
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Discuss possible locations where conditions would have allowed the synthesis of organic compounds.
Hydrovents in pacific ocean produce hydrogen sulphate used by bacteria for energy and nutrient rich water that could be used to form organic molecules & form polymers.
Cool environment of space restricts movement of molecules and thus increases likelihood of formation of polymers. Organic molecules could have travelled to earth via comets, meteors and cosmic dust.
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D.1.5
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Outline two properties of RNA that would have allowed it to play a role in the origin of life.
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D.1.6
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State that living cells may have been preceded by protobionts, with an internal chemical environment different from their surroundings.
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D.1.7
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Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere.
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D.1.8
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Discuss the endosymbiotic theory for the origin of eukaryotes.
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D.2.1
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Define allele frequency and gene pool.
Allele Frequency – frequency of specific allele in proportion to all alleles in the gene pool of a population.
Gene pool – All the genes of a population.
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Non-living synthesis of organic molecules
Elements& inorganic molecules combined with energy presumed to have been sufficient for life to begin.
Alternative theories
Organic molecules may have been generated on earth or introduced from space.
Hypothesis that life on earth originated by introduction of complex organic chemicals or even bacteria via comets is called panspermia.
Shower of comets about 4000 million years ago could have introduced complex organic molecules and water to the earth & initiated chemical evolution.
Assembly of polymers from organic molecules
Little to no oxygen in atmosphere at that time, as any oxygen was absorbed by rocks.
Thus, no oxygen to steal electrons away from other atoms (oxidation).
This would have caused a reducing atmosphere which would have made the joining of the simple molecules to form more complex molecules more likely.
Experiments have shown that organic molecules can be formed in a reducing atmosphere.
However, it is very difficult to do when there is an absence of oxygen in atmosphere.
This polymerisation process would allow larger chemicals needed by cells to form.
How were polymers assembled?
In solution, hydrolysis of growing polymer would soon limit the size it could reach.
This has led to a theory that early polymers were assembled on solid, mineral surfaces that protected them from degradation.
Polymers have been synthesized on clay in previous experiments.
Formation of self-replicating molecules allows for inheritance
Currently DNA can replicate but requires enzymes to do this.
Proteins assembled based on information carried on DNA & transcribed into RNA.
Synthesis of DNA & RNA requires proteins.
- Proteins cannot be made without nucleic acids & nucleic acids cannot be made without amino acids
Synthesis of nucleotides & bases could have occurred easily.
Thus, single strand of RNA could have formed.
Complementary base pairing could have resulted in non-enzymatic replication of RNA.
Self-replicating molecules are molecules that can undergo replication
Able to act as template for copies of themselves to be made.
Only biological molecules capable of self-replication are DNA & RNA.
Unlike DNA, RNA sequences are capable of self-replication; it can catalyse its formation from nucleotides in the absence of proteins.
Discovery that certain RNA molecules have enzymatic activity provides possible solution.
These RNA molecules (ribozymes) incorporate both features required of life:
- Storage of information
- Ability to act as catalysts
Active ribosomes can be easily assembled from shorter olignonucleotides (strands of nucleotides).
Ribozymes have been synthesized in laboratory can catalyse exact complements of themselves.
Ribozymes serves as both:
- The template on which short lengths of RNA are assembled, following rules of base pairing
- The catalyst for covalently linking these olignonucleotides.
Evidence provided for these ideas is provided by many of the cofactors that play so many roles in life are based on ribose:ATP GTP
NAD
FAD coenzyme A Cyclic AMP
NAD
FAD coenzyme A Cyclic AMP
Development of lipid bilayer was imitated in laboratory.
Heated amino acids without water & produced long protein chains.
When water was added & mixture cooled, small stable microspheres (coacervates) bubbles of proteins.
Ø Microspheres able to accumulate compounds inside them so that they became more concentrated than outside.
Ø Also attracted lipids & formed lipid protein layer.
Ø If we assume that the coacervates also combined with self-replicating molecules such as RNA, we are looking at a primitive organism
Ø This is thought to have occurred 3.8 billion years ago.
This aggregates microspheres . Protobionts/proto cells contains lipids.
Most successful liposomes (protobiont in presence of lipids) at surviving would have passed on characteristics & developed into early prokaryotes.
Miller & Urey
Stanley Miller & Harold Urey worked on trying to confirm some of these ideas regarding pre-biotic earth.
In 1953, Miller set up an apparatus to simulate conditions on the early earth.
Apparatus contained warmed flask of water simulating the primeval sea & atmosphere of water vapour, hydrogen, methane, nitrogen, ammonia & CO2.
View Miller& urey experiment animation.
Water was boiled, while a condenser cooled the atmosphere, raining water and any dissolved compounds back to the minature sea.
Simulated environment produced many types of amino acids and other organic molecules leading them to conclude that the pre-biotic synthesis of organic molecules was possible.
This spontaneous generation of organic molecules was supported by investigation of meterorites.
In 1970, a meteorite was found to contain 7 different amino acids, 2 of which are not found in living things on earth.
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Mitochondria and chloroplasts were once free living bacteria cells:
- Mitochondria aerobic bacteria
- Chloroplasts photosynthetic bacteria
These cells were “swallowed up” by other cells by endocytosis cells engulfed but not eaten.
Mitochondria provide additional energy (aerobic respiration) and receives protection
Chloroplast provide food by photosynthesis & receives protection.
Prokaryotes are similar to mitochondria and chloroplasts:
- Similar size
- Similar ribosomes (70S)
- Contain DNA that is different from the nucleus
- Surrounded by double membrane
- Formation of new organelles resembles binary fission
Four eukaryotic kingdoms:
· Protoctista
· Fungi
· Plantae
· Animalia
Eukaryotic cells have some advantages over prokaryotic cells so the early eukaryotes survived & proliferated.
Species & speciation
D2.1 Define allele frequency and gene pool.
Allele frequency – Frequency of specific allele relative to other alleles in a population. Frequency of allele for green feathers is 80% and frequency for yellow feathers is 20%.
Gene pool – Sum of all the individual genes of a population.
D2.2 State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations.
Evolution involves the change in allele frequency in a population’s gene pool over a number of generations. This change in allele frequency can be caused by mutations and/or environmental factors. Organisms that have inherited traits that are well suited to the environment are able to survive and pass on their genes. The frequency of these alleles increases and become more common the in the population. These changes constitute evolution.
D2.3 Discuss the definition of the term species.
Common definition of species is – A group of potentially or actually interbreeding populations in nature to produce viable offspring. The problems with this definition are:
Ø Some species only reproduce asexually, e.g. most species of bacteria.
Ø Organisms of some species cannot physically mate with other organisms of the same species, e.g. Canis familiaris.
Ø Some species occasionally mate to produce hybrids (offspring of dif. Species)
D2.4 Describe 3 examples of barriers between gene pools.
Speciation is the formation of new species from existing species due to barriers between gene pools.
Sympatric speciation happens within the same geographical area and can be caused by different behavioural patterns and organisms living in different niches within the habitat. For example sexual selection occurs when closely related species cannot mate each other because of different courtship behaviour.
Allopatric speciation occurs in different geographical areas because of barriers between gene pools. Differing environments on either side of the barrier cause have different selection pressures. For example, cichlids (fish) occur in several East African Lakes
For example,
D2.5 Explain how polyploidy can contribute to speciation.
Polyploid organisms have more than two sets of homologous chromosomes, e.g. plants and tetraploid red viscacha rat. During sexual reproduction, chromosomes pair up during meiosis. Mutation in embryonic mitosis can cause all chromosomes to fail to separate at anaphase causing a tetraploid cell, leading to a tetraploid organism. Triploid hybrid is sterile and acts as a barrier between diploid and tetraploid forms and over time other differences could accumulate leading to speciation.
D 2.1 Define allele frequency & gene pool.
Allele frequency – Frequency of an allele relative to all alleles in a population.
Gene pool -
D 2.2 State that evolution involves change in allele frequency in a population’s gene pool over a number of generations.
D 2.4 Describe three examples of barriers between gene pools. Examples include geographical isolation, hybrid infertility, temporal isolation and behavioural isolation.
If a species is separated into two groups by geographical isolation, one species could potentially diverge into two. If environments on either side of barrier are different, each environment will select for different traits. Two isolated groups cannot interbreed, so there is no gene flow between them.
After a long period of isolation & selection, groups on either side of barrier may become different to one another so they cannot interbreed when put together.
Therefore, one species is evolved into two.
Genetic isolation between species can occur in a no. of ways, all the result of reproductive isolation.
Pre-zygotic isolation – meaning that the zygotes are not formed because the gametes never meet.
Post-zygotic isolation – meaning that the zygotes don’t develop.
Two species may have evolved in such a way that they are active at different times of day or night. May evolve different reproductive seasons.
Thus isolated in time, the two groups are not likely to interbreed.
The two species occupy different habitats in a similar region. May have been separated by natural disasters (earthquakes) or rivers. Could be difference between ground & tree dwellers.
The two groups may become so different that they no longer identify with each other’s courtship behaviour. Could release different chemical signals (pheromones) as a result of mutations and thus cannot interbreed.
Audio & visual mating signals may also change.
The two groups may become so different that they can no longer physically interbreed.
If copulation is prevented, there will be no gene flow between these two groups.
Hybrids (heterozygotes) are produced but fail to develop to maturity & fail to produce functional gametes.
F1 hybrids are fertile but the F2 generation fail to develop or are infertile.
Speciation is process by which one or more species arise from previously existing species
Single species may give rise to new species (intraspecific speciation)
Two different species may give rise to a new species (intraspecific hybridisation).
If intraspecific speciation occurs whilst the populations are physically separated (allopatric speciation).
If process of speciation occurs while populations are occupying same geographical area
Occurs when geographical barrier produces a barrier to gene flow due to spatial separation.
Organisms are unable to meet & reproduce, leading to reproductive isolation.
Adaptations to a new environment will change the allele and genotype frequencies.
Prolonged separation of populations will lead to two genetically isolated populations, even if the barrier is removed. Barriers could be mountain ranges, rivers (has to be permanent). This means that speciation can also occur through random forces, rather than through natural selection. E.g. Charles Darwin’s Galapagos Finches.
D 2.6 Compare allopatric and sympatric speciation. Speciation: the formation of a new species by splitting of an existing species. Sympatric: in the same geographical area. Allopatric: in different geographical areas.
D 2.7 Outline the process of adaptive radiation.
Define allele frequency and gene pool.
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1
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D.2.2
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State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations.
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1
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D.2.3
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Discuss the definition of the term species.
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3
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D.2.4
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Describe three examples of barriers between gene pools.
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2
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Examples include geographical isolation, hybrid infertility, temporal isolation and behavioural isolation.
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D.2.5
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Explain how polyploidy can contribute to speciation.
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3
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Avoid examples involving hybridization as well as polyploidy, such as the evolution of wheat.
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D.2.6
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Compare allopatric and sympatric speciation.
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3
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Speciation: the formation of a new species by splitting of an existing species.
Sympatric: in the same geographical area.
Allopatric: in different geographical areas.
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D.2.7
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Outline the process of adaptive radiation.
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2
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D.2.8
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Compare convergent and divergent evolution.
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3
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D.2.9
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Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.
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3
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Gradualism is the slow change from one form to another. Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution. Volcanic eruptions and meteor impacts affecting evolution on Earth could also be mentioned.
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D.2.10
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Describe one example of transient polymorphism.
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2
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An example of transient polymorphism is industrial melanism.
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D.2.11
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Describe sickle-cell anemia as an example of balanced polymorphism.
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2
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Sickle-cell anemia is an example of balanced polymorphism where heterozygotes (sickle-cell trait) have an advantage in malarial regions because they are fitter than either homozygote.
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Option D.3 Evolution
Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K. 5 marks
Each radioactive isotope has a half-life which is the time required for the amount of radioactive material to halve, caused by radioactive decay.
14C has a half-life of 5730 years that expresses the rate of decay of 14C. 14C is used for dating samples that are 1,000-100,000 years old.
40K is used for dating samples over 100,000 years old due to a half-life of 1.25 billion years.
Isotopes are extracted from sample and measured in proportion to 14C and 40K relative to breakdown products of 14N or 40Ar.
The 14C/14N ratio decreases over time at a predictable rate.
The 14C/14N ratio is compared with decay curve to determine age of sample.
· accurate dating of fossils allows accurate sequencing of fossils
§ select 14C for young samples / samples from 1,000 to 100,000 years old
· select 40K for older samples / samples over 100,000 years old
· extract isotopes from sample
· measure isotopes in sample as proportion of 14C / 40K relative to breakdown products 14N / 40Ar
· 14C/14N ratio decreases over time at a predictable rate / half-life = 5730 years
· 40K/40Ar ratio decreases over time at a predictable rate / half-life = 1,250,000 years
· compare 14C/14N / 40K/40Ar ratio with decay curve to determine age of sample
Define half-life
Deduce the approximate age of materials based on simple decay curve for radioisotope.
Describe the major anatomical features that define humans as primates.
Compare allopatric and sympatric speciation.
Both involve separation of gene pools. Allopatric speciation occurs when species diverge due to a geographical barrier whereas sympatric speciation occurs in the same or overlapping geographical region. A break-away population can be formed during migration that is likely to have different frequencies of alleles than the main population. This could cause it to be more susceptible to different selection pressures.
Both involve reproductive isolation/separation of gene pools; sympatric is speciation due to isolation of populations living in the same geographical area whereas allopatric is speciation due to geographic isolation;
Explain how polyploidy can contribute to speciation.
Polyploid organisms have more than two sets of homologous chromosomes. Polyploidy prevents
Triploid hybrid is sterile and acts as a barrier between diploid and tetraploid forms and over time other differences could accumulate leading to speciation.
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