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What is the definition of a species?
Group of individuals that share many characteristics, able to interbreed under natural conditions to produce fertile offspring
What is the definition of alleles?
Alternative forms of a particular gene
What is the definition of a population?
Group of organism of the same species living together in a particular place at a particular time
What are gene pools
Sum of all alleles in a given population
- Considers the characteristics of the whole population (not individuals)
- consists of genotypes of the individuals capable of reproducing
What are allele frequencies
How often each allele of a gene occurs in the gene pool
What are populations that vary in characteristics likely to have?
Have different frequencies of alleles and therefore different gene pools
What may happen to gene pools over time?
Overtime, frequency of particular alleles in population may change
Why does the frequency of particular alleles in a population change (genepool) and what does it allow?
Chance events (Mutations, Changes in Environment)
- Changes allow population to be compared at different times/ locations
What are mutations?
Permanent change in a gene or chromosome
- May occur suddenly and purely by chance
- Not all are harmful and sometimes leads to adaption to environment
- Offspring shows variation that do not resemble either parent nor family history
What is a mutant
Organism with a characteristic resulting from a mutation
What are the two types of mutations?
Gene Mutation
Chromosomal
What is Gene mutation ( type of mutation )
Changes in a single gene, so traits normally produced by that gene are changed / absent
- Usually occur during DNA replication
eg cystic fibrosis, sickle cell anemia
What are mutagens?
since Mutations can be spontaneous and have no cause
- Some substances can increase rate of mutations
eg Mustard gas, X-rays, radiation
How do mutagens cause mutations?
-Can resemble proteins and be incorporated into DNA
-Can trigger DNA replication errors
-Can cause DNA breakages/ lengthening
- Can block DNA replication / damage DNA structure
- Can chemically react and modify DNA
What is somatic mutation?
When mutation occurs in the somatic (body cells) of a person
- only that individual affected
What is germline mutation?
Mutation occurs in gamete (sex cells - egg/sperm)
- individual not usually affected
- can be passed into offspring
What are effects of mutations?
A gene is a sequence of nucleotides in DNA or RNA that codes for a molecule that has a function ( usually protein / enzyme )
Sequence of 3 bases in DNA is code for amino acids used to build a protein
- Just 1 missing protein can have a huge effect eg Albinism
What is point mutation
A change in one base - > Could alter a protein, have no effect at all or prevent protein from being produced
How can point mutations occur
Can occur when a nucleotide is
- Inserted (new nucleotide is added)
- Substituted ( an existing nucleotide is replaced w a different one)
- Deleted ( a nucleotide is removed from the strand )
* Typically occurs during DNA replication
What are the mutation effects
Missense
Nonsense
Neutral
Silent
What is the mutation effect of missense
Change the amino acid produced = changed protein
What is the mutation effect of nonsense
Changes the sequence to stop = shorter protein → unlikely to function properly
What is the mutation effect of neutral
Changes amino acid, however new amino acid is the same type = no significant change to protein
What is the mutation effect of silent
No change to amino acid = no change to protein
What will some mutations result in
Frameshift mutation
(occurs with insertions or deletions)
What are frameshift mutations
Affects the reading frame → the way which nucleotides are clustered in groups of 3 to make translatable codons
- Affects all the amino acids that occur after them
What does adding or deleting nucleotides do
Affects the groups of all subsequent nucleotides → they will not be translated into the correct protein sequence
What does substitution mutations do
Affect just one amino acid
What are lethal recessives
Recessive genes can be lethal if not masked by dominant allele
- Causes death of embryo / foetus / young child
what does lethal recessives do to the gene pool
Changes the composition of the gene pool
- People who inherit lethal recessives usually die before alleles can be passed onto next generation
- Thus freq. of these alleles in gene pools are reduced
What is an example of a lethal recessive
Tay sachs disease - disorder of lipid metabolism inherited in autosomal recessive pattern.
- Causes fatty material to build up in the nervous system, destroying the neurons
What are conditional lethal recessives
Some lethal recessive mutations only cause death under certain conditions
- Only cause death if individual is exposed to certain environmental factors
What is an example of a conditional lethal recessive
Favism - Person will develop haemolytic anaemia (abormal breakdown of erythrocytes) but only if they eat fava beans
how are recessives maintained
, a mutated recessive allele is only expressed when two is present (rr)
Thus neutral recessive alleles and even harmful recessive alleles may continue to exist in the population as heterozygotes (carriers)
What is chromosomal mutations ( type of mutation )
All or part of a chromosome is affected
- Often occurs during meiosis / cell division, less often in mitosis
eg down syndrome
How do chromosomal mutations occur
Deletion - part of a chromosome is lost
Duplication - section of chromosome occurs twice, Can occur if part of chromatid breaks off and then joins onto wrong chromatid
Inversion - breaks occur in chromosome, broken piece joins back in but wrong way around
Translocation - part of chromosome breaks off and is rejoined to wrong chromosome
Non-disjunction - during meiosis , chromosome pair does not separate and one daughter cell has an extra chromosome and the other has one less
What may chromosomal mutations be
May be unique but characteristic disorders may occur
What is monosomy (chromosomal mutation)
When an individual is missing a chromosome
Two types - Partial and Total Monosomy
What is partial monosomy
Part of a chromosome is missing
What is total monosomy
If autosome (non-sex chromosome) is totally missing → severe malformations/ miscarriage
What is trisomy (chromosomal mutations)
Conditions caused by an extra chromosome
eg Down syndrome (trisomy 21) → has 3 chromosome 21 not 2
What is the differences between Gene and Chromosomal Mutations (gene)
Gene mutations are more common than chromosomal
Gene changes are common errors during DNA replication → results in substitutions, insertions, deletions and frameshifts
What is the differences between Gene and Chromosomal Mutations (chromosomal)
While chromosomal changes tend to occur during cell division - mitosis and particularly meiosis in crossing over
What is the similarity between Gene and Chromosomal Mutations
Both can result from DNA replication / Cell division and exposure to mutagens
What are the evolutionary mechanisms that can affect the frequency of alleles in a gene pool
Mutation
Differing selection Pressures
Random genetic drift, including the founder effect
Changes in gene flow between adjoining groups
What is the definition of variation?
The natural differences that are present in a given species
What is variation caused by?
Random Assortment
Crossing over
Non-disjunction
Random fertilisation
Mutations
What is random assortment (cause of variation)
The chromosome given to each daughter cell is meiosis is random
- Random assortment of chromosomes during meiosis results in gametes (large number of possible combinations)
What is crossing over (cause of variation)
The genes exchanged between parent chromatids in meiosis is random
- Crossing over of chromatids during meiosis may result in chromatid being broken off and attaching to a different chromatid- results in changed sequence / recomnination
What is non disjunction (cause of variation)
Chromosomes may not seperate correctly during meiosis
- Incorrect number of chromosomes divided into gametes
What is random fertilisation (cause of variation)
the genes / alleles the egg and sperm contain are random
- Large number of different sperm / eggs, any sperm can fertilise an egg - large combination of alleles
What is mutation (cause of variation)
can result in new characteristics / alleles and considered most important source of variation
- Permanent changes in DNA - introduce new / different alleles into gene pool
Who is Charles Darwin
Biologist
- Founded the theory of evolution through natural selection
→ Collected evidence of similarities and different between animals in different places and different times until its conclusion
saw that Species change over time → evolution
What is evolution
A gradual change in the characteristics of a species
What were Darwin’s three main observations
Variation
Birth rate
Nature’s balance
What was Darwins observation of Variation
differences in members of the same species
What was Darwins observation of birth rate
Living organisms reproduce at a faster rate than which their food supply and other resources would increase → overcrowding
What was Darwins observation of nature’s balance
Although birth rate was high, each species tended to maintain its numbers at a relatively constant level as equal numbers would die
What was Darwin’s conclusions from his observations
There is a struggle for existence : due to high birth rate and limited resources
- Survival of the fittest as those with characteristics best suited to their environment were more likely to survive and pass on those characteristics to offspring. Those with the least favourable characteristics likely died and never passed those characteristics.
What must natural selection have to occur
Variation
Selective agent : the environmental factor acting on the population that will favour particular characteristics
What is natural selection
Viewed as selection of those alleles in a population that give an organism a greater survival advantage
- where the environment is gradually changing, characteristics that enhance survival allow succeeding generations to gradually adapt to it
What are the principles of evolution through natural selection
1. There is a variation of characteristics within a species.
2. More offspring of a species are produced than can possibly survive to maturity.
3. Due to excessive birth rate, and limited resources, there is a struggle for existence - competition for survival.
4. The individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of the fittest.
5. Favourable characteristics (those with survival value) are passed on to the next generation
6. In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases -> evolution.
What is the heterozygote advantage
Alleles with a negative effect r expected to disappear from the population over time.
- In some cases the allele frequency for a negative allele is much higher in a population than expected because heterozygote advantage
def :Individuals heterozygous for a trait exist at a higher frequency in a population than expected, due to providing some survival advantage, in certain environment
What is sickle cell anemia?
Recessive point mutation ( one amino acid change ) in allele for one of the beta chains in haemoglobin
- Causes blood cells to distort (sickle shaped) and die early, leading to anaemia and blockages in blood vessels in homozygous recessives → early death
Leads to expectation that allele to decrease in frequency
Where is sickle cell anaemia most common
Most common in areas where malaria is endemic and places where malaria isnt → sickle cell anaemia is rare.
Why is sickle cell anaemia most common in areas where malaria is endemic
Malaria is the selective agent : favours individuals with sickle cell trait, (Rr) as they are resistant to malaria (for no. of reasons)
- Also don’t suffer the full effects of sickle cell anaemia (single dominant allele compensates unless oxygen is low)
- Hence heterozygote advantage - advantage to having one copy of allele only
What is random genetic drift?
Random, non - directional change in allele frequency between generations
- Occurs purely by chance
- Most common and occurs faster in small populations
- Compared to natural selection in which population size has no effect
what is the founder effect?
A sub type of genetic drift
- Occurs when a small group moves away from its homeland to a totally new area and establishes a population, which later expands
What does the founder effect lead to
Decreased genetic variation / decrease alleles in gene pool
Different allele frequencies to original population ( not representative of original population )
What is the bottle neck effect
A sub - type of genetic drift
An event such as a natural disaster , suddenly and severely reduces the size o the population
- Allele frequencies of the new, smaller population may be different / not representative to the population before the event
What is migration / gene flow?
The movement of genetic material from one population to another
- Can result from the movement of a single individual
- If the population does’t contain the allele, then it is likely the allele will increase in frequency
What are barriers to gene flow?
Opposite of gene flow
- Geographical barriers
- Sociocultural barriers
Populations are isolated → no interbreeding → seperate gene pools form
What are examples of geographical barriers
e.g Oceans, mountain ranges , large lake systems , deserts , expansive ice sheets
What are examples of sociocultural barriers
economic status , educational background , religion , social position
How does the introduction of genetic diseases occur
If carrying one copy of the affect allele confers a selective advantage → increase allele frequency in the population
e.g sickle cell trait in areas where malaria is prevalent
If the disease is fatal (eg lethal recessive) → decrease allele frequency in the population
- If a fatal disease is introduced into a population who doesn’t have the allele, it may initially increase in frequency before decreasing
What is speciation
The process of producing two species from one, due to long term isolation, different environmental influences and major changes in allele frequencies
What is differential selection
a difference in how traits are favoured in one condition compared to another
selective agents r different
What are the steps to speciation
4 steps
- Variation
- Isolation
- Selection
- Speciation
What is the step of variation in speciation (1st step)
Individuals of a species possess different characteristics
What is the step of isolation in speciation (2nd step)
Populations of the species are isolated without gene flow (due to barriers to gene flow (geo or sociocult) as it prevents interbreeding
What is the step of selection in speciation (3rd step)
Each population is subject to different selective agents
Natural selection acts differently on each population (different characteristics are favourable in each environment and gene pools change) → populations become less and less alike as their allele frequencies start to become different
What is the step of speciation in speciation (4th step)
the populations gene pools change until they become so different that the groups can no longer interbreed (to produce fertile offspring)
start of knowledge for the test
HAHA YAY
What is evolution?
Gradual change in physical characteristics of a species over many generations
What is DNA
Chemical compound that contains our genes → determines types of protein a cell makes
DNA is universal - same in all living things but the sequences is different between species
Allows for comparisons
What happens to DNA when speciation occurs
New species have very similar DNA
- The more closely related two species are, the greater proportion of DNA they share
DNA contains some non-coding sequences (junk dna)
More closely related species have more junk sequences in common
What is DNA hybridisation
Molecular biology technique that measures the degree of genetic similarity between DNA sequences.
- Used to determine the genetics distance between two organism
What is DNA melting
DNA of one organism is labelled, then mixed with the unlabelled DNA to be compared against
- The mixture is heated to allow DNA strands to break apart and then cooled to form renewed hybrid double - stranded DNA
Hybridised sequences with a high degree of similarity will bind more firmly , and require more energy to seperate them → they seperate when heated at higher temps than less sequences
What are Endogenous Retroviruses
Occurs when a viral DNA sequence becomes a part of an organism/s genome
- Retroviruses will enter a cell and copy its RNA sequence into the DNA (reverse transcription)
- Only becomes endogenous if it inserts its DNA into chromosomes of sex cell which can be passed on through generations
- Once this happens , the ERV DNA appears in the same place in the chromosome in every single cell and in all subsequent generations
How much ERVs make up the human genome
8%
- Other primates possess same ERVs in same locations in their genomes (common ancestor)
- More ERVs in common - more closely / recently related
What are Mitochrondrial DNA (mtDNA)
Small Circular molecules (5-10)
- 37 genes → 24 genes code for making tRNA, 13 genes have code for enzymes for cellular respiration
- Most cells have numerous amounts of mitochondria → thus 500-1000 copies of mtDNA molecule
- Alot easier to extract than DNA
Where are mtDNA found
Human eggs / sperm contain mitochondria
- After fertilisation , mitochondria in sperm are destroyed
Nuclear DNA comes from egg / sperm, mtDNa only comes from egg.
What is the rate of Mutation in mtDNA compared to nuclear DNA
mtDNA has a higher rate of mutation than nuclear DNA
Means that human mtDNA slowly diverging from mtDNa from original female ancestor
What correlation is there between number of mutations
More time has passed → more mutations → more difference → less closely related
What does similarities in mtDNA mean in two individuals
Provides estimate of closeness of their relationship through maternal ancestors
used to map relationship between species
what does it mean if mtDNA is identical
Closely related / siblings
What does it mean if mtDNA is different
last common maternal ancestor lived long ago
What are protein sequences
Proteins consist of long chains of amino acids, the sequence of which is determined by the DNA code
- We can compare types of proteins and sequence of amino acids in proteins in different species
Animals of same species have identical amino acid sequences (diff species diff sequences
What does the degree of difference between proteins show between animals
Gives estimate of how much evolution has occurred since they shared a common ancestor → longer the period of time involved, the greater number of amino acids that are different
What is ubiquitous proteins
proteins that appear to be in all species —> usually responsible for essential basic tasks required by all cells