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Prokaryotes and eukaryotes
Prokaryotes:
-Bacteria, cyanobacteria
-smaller
-Cellular organisation unicellular or colonial
-Cell walls made of sugars and peptides
-Some have flagella made of flagellin
-No membrane bound organelles
-Anaerobic, facultative aerobic metabolism
-Loop of DNA in cytoplasm
-Reproduction by binary fission, some asexual, some parasexual
Eukaryotes
-Protists, fungi, plants, animals
-Bigger
-Mainly multicellular with tissues and organs
-Cell walls made of cellulose or chitin, none in animals
-Flagella or cilia with microtubules
-Membrane bound chloroplasts and mitochondria
-Aerobic metabolism
-DNA in chromosomes in membrane bound nucleus
-Reproduction by mitosis or meiosis, mostly sexual
Origin and early history of planet Earth
4,600 MA
-Earth formed by the gravitational accumulation of dust and larger objects.
-The mass melts and begins to differentiate into the core, mantle and crust.
-Water vapour and various gases are outgassed but do not accumulate due to the great heat and continual bombardment as new material is accumulated.
-The Moon forms during a major collision.
3,750 MA
-Age of the oldest rocks on Earth
-Earth has cooled to the extent that a crust begins to solidify.
-Temperatures fall, oceans and atmosphere can begin to condense out.
> 3,800 MA
-Progress retarded by continued bombardment of large objects.
-Released energy is sufficient to boil off the oceans and atmosphere (along with any prebiotic organic compounds)
< 3,800 MA
-Meteorite bombardment decreases in intensity and the planet cools below a threshold that allows oceans and atmosphere to condense out, ocean and atmosphere permanent
-Organic compounds begin to be synthesised and accumulate, conditions for life begin to develop
By 3,800 MA
-Conditions on planet earth suitable for life to have originated
3,500 MA
-The earliest fossil evidence for life on earth
Origin of life
Panspermia is the hypothesis that life exists throughout the Universe, distributed by meteoroids, asteroids, comets, planetoids, and also by spacecraft carrying unintended contamination by microorganisms
Approaches to solving the origin of life
-Analyse living prokaryotes and attempt to reconstruct their common ancestor
-Compare duplicated genes potentially enabling us to reach back beyond that ancestor and estimate some of the earliest components of genetic machinery
-Reconstruct conditions that existed on earth in these remote times and simulate these experimentally
Prokaryotes believed to have originated before eukaryotes
-Appear earlier in fossil record
-Similar in every aspect
-Evidence that eukaryotes evolved from prokaryotes
Prokaryotes and eukaryotes similarities
-Method of transmitting information in triplet form in DNA and translating it into proteins through RNA
-In living organisms all amino acids are laevo-rotatory and in nucleic acids all the sugars are dextro-rotatory
Chemicals produced by simulating conditions on primitive earth
-Amino acids
-Purines/pyrimidines
-Sugars
-Porphyrins
Life most likely evolved through basic chemistry on earth
Early Metabolic pathways
Early RNA-based life would have survived using the chemicals in the primordial soup, having to develop metabolic pathways when this ran out
Chemoautotrophs (energy from oxidising inorganic substance, C from CO2)
Chemoheterotrophs (energy and C from consuming organic compounds)
Photoautotrophs (energy from light, C from CO2)
Photoheterotrophs (energy from light, C from consuming organic material)
This requires synthesis of cytochromes (oxygen metabolism) and porphyrins and chlorophyll forerunners
Obligate anaerobes (poisoned by O2)
Aerotolerant organisms (cannot use O2 for growth but tolerates it)
Facultative anaerobes (use O2 if present)
Obligate aerobes (cannot live without O2)
The origin of eukaryotes
~2 billion years ago some acritarchs large enough to suggest they were eukaryotes
Multiple symbiotic events theory
-Bacterial cell engulfs purple bacteria to deal with O2, purple bacteria becomes mitochondria
-Chloroplasts were cyanobacteria swallowed by cell to use abilities to photosynthesise
-Flagella and cilia were spirochaete bacteria, has own RNA and similar structures
-Mitosis centriole spindles similar to tubules in spirochaete bacteria
Origin of eukaryotes is coincident with the atmosphere becoming aerobic
The Cambrian explosion
A period of rapid diversification and evolution of life
Resulted in the origin of many major animal groups that exist today
Environmental cause e.g. increased oxygen levels and the emergence of new ecological niches
Ecological cause e.g. hard parts providing protection and new opportunities for feeding and locomotion
Late precambrian
-Ediacaran animals inhibit sea floor
-Small triploblastic animals present
-Ediacaran animals unprotected, no predators
Middle-early cambrian
-Triploblastic predators evolve with teeth
-Most ediacara extinct
-Other multicellular animals develop armour
Late cambrian
-Predators develop eyes
-Multicellular animals develop better armour
Abundant evidence for evolution
Artificial selection - variation in species generated by human driven selection
E.g. pigeons, dogs, crops
Common adaptive responses of organisms in different places
Fossil evidence, contained within sedimentary rock
E.g. horses, bears
Homologous characters - traits that are inherited from a common ancestor
Vestigial characters - extant organisms have structures that serve no function providing evidence of evolutionary change
Universal homologies - e.g. all living organisms use DNA as their genetic material, and share many of the same genes and metabolic pathways
Gene selection as the engine of Natural Selection
Kin selection explains altruistic behaviour
Altruistic behaviour increases survival and reproduction of other individuals (kin who possess the same genes)
Therefore altruistic genes increase the rate of spread of themselves via relatives
Such actions evolve if r x b > c
Relatedness is the proportion of genes shared because of common ancestry
Selfish gene is a gene considered primarily as an element that tends to replicate itself in a population, whether or not it has a direct effect on the organism that carries it
An altruistic gene that is linked to an obvious phenotype will spread if possessors are altruistic towards each other e.g. green beard genes, does not required relatedness
Why is evolution important?
All species are the outcome of evolution
If we identify species more likely to become extinct, it can help us to be proactive in conservation and protect them
Can identify characteristics making species more/less likely to become extinct
Phylogenetic niche conservatism
-species that are closely related are more likely to share similar traits and ecological requirements than distantly related species
-if a particular region contains multiple species that are closely related and share similar ecological requirements, then conserving that region may help protect the diversity of those species
-phylogenetically similar species have low rates of evolution and adaptability
IUCN threat is the measure of how likely populations are to become extinct
Adaptive radiation
The evolution of ecological and phenotypic diversity within a rapidly multiplying lineage
Requires differentiation of a single ancestor into multiple species
Requires variation in morphological traits that allow exploitation of a range of environments
Can be caused by ecological opportunity, a new environment or ecological niche becomes available, providing opportunities for a group of organisms to diversify and specialise
Can be caused by key innovation, a novel trait or adaptation evolves that enables a group of organisms to exploit a new ecological niche. For example, the evolution of wings in birds or the development of flowers in plants
Mass extinction events
End-Ordovician extinction (443 million years ago): Thought to have been caused by a combination of global cooling and glaciation, changes in sea level, and possibly a series of volcanic eruptions. An estimated 85% of marine species went extinct
Late Devonian extinction (359 million years ago): Thought to have been caused by a combination of climate change, oceanic anoxia, and a series of catastrophic events such as asteroid impacts and volcanic eruptions. An estimated 75% of marine species and 20% of plant and animal families went extinct
End-Permian extinction (252 million years ago): Thought to have been caused by massive volcanic eruptions, which triggered rapid climate change, oceanic anoxia, and acidification. An estimated 96% of marine species and 70% of terrestrial vertebrate species went extinct
End-Triassic extinction (201 million years ago): Thought to have been caused by a combination of volcanic activity, climate change, and asteroid impacts. An estimated 80% of marine species and 50% of terrestrial species went extinct.
End-Cretaceous extinction (66 million years ago): Thought to have been caused by a massive asteroid impact causing widespread wildfires, global cooling, and acid rain, leading to the extinction of an estimated 75% of all species on Earth, including the non-avian dinosaurs
Describe examples of genetic variation
Polyploidy - more copies of complete chromosome sets
-Always lethal
Aneuploidy - one set of chromosomes incomplete
-Nullisomy - both members of pair missing (lethal)
-Monosomy - one member of pair missing (lethal)
-Trisomy - one extra chromosome (usually lethal)
—–Trisomy 21 - down’s syndrome
Aneuploidy in sex chromosomes
-Lacking
—–45x - turner’s syndrome (infertile)
—–45y - inviable
-Extra
—–47xyy - minor effects
—–47xxy - minor effects
—–47xxx - minor effects
Translocations - during meiosis, non-homologous chromosomes exchange parts
-Carrier unaffected, usually lethal for offspring
Deletions - part of chromosome missing
-Severity depending on amount missing
Inversions - section of chromosome inverted
-Paracentric - centromere excluded, common, no issues
-Pericentric - centromere included, rare, possible problems
Single Nucleotide Polymorphisms (SNPs) - The most common type of genetic variation and involve a change in a single nucleotide
Describe the different features of the genome XX
Three nucleotides = codon
Codon = encodes specific amino acid
The genetic code is degenerate
20-30,000 genes in the human genome, 1.5% encodes proteins
Nucleotide mutations can occur in coding or non-coding DNA
-Coding region mutations e.g. sickle cell, albinism
—–Substitutions
—–Insertions and deletions (results in different protein)
-Non-coding region mutations
—–Repeat length variation
—–Useful genetic markers
Gametic mutations inherited
Segregation XX
Two members of a gene pair segregate from each other during the formation of gametes. Half of the gametes carry one member of the pair and the other half carry the other member of the pair
Homozygote - individual that has two copies of the same allele
Heterozygote - individual that has two different alleles
Testcross - cross a heterozygote with an individual that is homozygous for the recessive allele
Null hypothesis - no significant difference between two groups or no relationship between two variables in a statistical analysis
Chi-square test - x^2 = sigma (O-E)^2 / E
Chi-squared value is a measure of goodness of fit
To show how Mendelian characters can be detected and understood in pedigrees (family trees) XX
Pedigrees used to infer mode of inheritance and genetic counselling
One gene involved in disease - mendelian/monogenic
Many genes involved in disease - multifactorial
Autosomal dominant disorders (caused by gene on chromosomes 1-22)
-Affects and transmitted by either sex
-Affected has at least 1 affected parent
-Child of affected and unaffected has 50% chance of disease
-E.g. widows peak, achondroplasia (dwarfism)
Autosomal recessive (caused by gene on chromosomes 1-22)
-Effects either sex
-Usually unaffected carrier parents
-Common where inbreeding occurs
-Carriers and non-carriers indistinguishable
-Two carriers mate = offspring have ¼ chance of being affected, ½ chance of being carriers
-E.g. albinism, sickle cell anemia, cystic fibrosis
X-linked dominant (caused by gene on sex chromosomes)
-Affects either sex
-Child of affected female has 50% chance of being affected
-All female and no male offspring of affected males are affected
-Very few examples
X-linked recessive (caused by gene on sex chromosomes)
-Mainly affects males
-0.5 probability of male offspring of female carrier being affected
-Females only affected if father is affected and mother is carrier
-E.g. haemophilia, red-green colourblindness
Y-linked (caused by gene on sex chromosomes)
-Affects only males
-All sons of affected males are affected
-E.g. maleness
