Biology - GENETIC DIVERSITY AND ADAPTATION

What is a mutation?

A change in a DNA base sequence.

How many bases in a codon?

3

What base replaces thymine in RNA?

uracil

What molecule are anti-codons on?

tRNA

Why is the genetic code degenerate?

a single AA may be coded for by more than one codon

What is a substitution mutation?

a substitute base replaces one base with another

What is a deletion mutation?

bases are taken away

Which type of mutation is most likely to result in no amino acid change and why?

Substitution - the number of bases stay the same and after the substitution all other codons stay the same. New triplet of bases may code for same amino acid.

Which type of mutation can result in large changes to the amino acids that are coded for?

Deletions/insertions

Explain what a frameshift mutation does

Frameshifts occur when there is an insertion/deletion and cause the base triplets to change along the entire polypeptide chain after the mutation - different AA’s made so different protein produced.

What are mutagenic agents ?

A chemical or physical agent that can increase the rate of mutation (eg ionising radiation , carcinogens - asbestos etc)

Explain a positive effect of mutations

may provide a survival advantage giving the organism a better chance of surviving and reproducing

What 2 syndromes are the result of non-disjunction of the X and Y chromosomes?

Down Syndrome

Turner’s syndrome

What is non-disjunction?

The failure of the chromosomes to separate producing daughter cells with abnormal numbers of chromosomes.

What is a spontaneous mutation?

random mutations in DNA

What cells does meiosis produce?

4 genetically different haploid daughter cells

What are gametes?

an organisms reproductive cells - haploid and each cell carries only 1 copy of each chromosome eg egg/sperm cell

How does meiosis produce cells that are genetically different?

crossing over and independent assortment

Describe a haploid nucleus

has a single set of chromosomes

Describe a diploid nucleus

has paired chromosomes , one from each parent

Define genetic variation

the differences in the DNA base sequences between individuals of the same species, resulting in distinct alleles and genotypes

Describe independent assortment/segregation

the random alignment and separation of homologous chromosomes in meiosis I

What is an allele?

a different version of the same gene located at the same position on the chromosome

What splits in meiosis I ?

homologous chromosomes

When does independent assortment of chromosomes occur in meiosis?

metaphase I

What happens in anaphase II ?

sister chromatids separate and are pulled to opposite poles of the cell by spindle fibres

How many chromatids does a daughter chromosome have just after they have been split in meiosis II ?

1

Give 3 differences between mitosis and meiosis

Meiosis:2 divisions, 4cells ; genetically different daughter cells ; homologous chromosomes randomly segregated during meiosis I ; crossing over

Mitosis: 1 division , 2 genetically identical daughter cells ; no independent segregation ; no crossing over

What is a chromatid ?

one section of DNA on a homologous chromosome. 2 chromatids make up a chromosome

what happens in the first meiotic division

pairs of homologous chromosomes separated to halve the chromosome number (diploid to haploid)

what are homologous chromosomes?

chromosomes that have the same structural features and have the same loci

what happens in meiosis 2 ?

the 2 haploid cells undergo a second division and sister chromatids separate and are pulled to opposite poles of the cell

In which stage of meiosis does crossing over occur ?

prophase I

What is a chiasma ?

point where chromosomes cross over during crossing over

What are swapped during crossing over ?

sections of DNA are swapped between homologous chromosomes

why can’t non homologous pairs of chromosomes swap over ?

they don’t have the same genes in the same order

How does crossing over increase genetic variation?

leads to exchange of alleles between homologous chromosomes - this recombines alleles that wouldn’t normally be found on the same chromosome

Explain the crossing over process

Chromatids of each pair of homologous chromosomes become twisted around each other. Sections of the chromatids break off and rejoin with the chromatid on the opposite homologous chromosome.

What are the 3 causes of genetic variation that occur during meiosis?

crossing over

independent assortment

random fusion of gametes after meiosis

How can you identify where meiosis has occurred in a lifecycle?

chromosome number has halved (2n → n) OR if a diploid individual is producing haploid cells

Give an example of a spore producing organism

fungi

Name 2 gametes apart from sperm and eggs

pollen

spores

What is a spore ?

Typically 1 celled reproductive unit capable of giving rise to a new individual without sexual fusion

What are the causes of variation?

Environmental - climate change , food/water availability, light intensity, soil nutrients

Genetic - mutation, meiosis, fertilisation of gametes

How do mutations cause genetic variation?

introducing changes to the DNA sequence resulting in new alleles

How does sexual reproduction cause genetic variation?

meiosis

random fertilisation

in most cases, what causes variation in a population?

due to individuals having different combinations of alleles producing different phenotypes

What is genetic diversity?

the number of different alleles of genes in a population

What is natural selection?

process by which organisms better adapted to their environment survive and reproduce and pass on their advantageous alleles to their offspring

What is a selection pressure?

environmental factors (eg predation, water availability) that influence an organisms ability to survive and reproduce

What is meant by selective advantage?

a favourable allele that increases the chance of an organism surviving and reproducing

Define evolution

the change of the frequency of a gene in the population

Summarise how natural selection drives evolution

• variation in a pop

• due to mutations some individuals have favourable alleles which help them to survive and reproduce

• they pass on these favourable alleles to offspring so frequency of these alleles increases in the pop

• if no interbreeding with other populations occurs, then new species can be formed (speciation)

What is a normal/gaussian distribution?

a frequency distribution with a bell shaped curve

What is directional selection?

favours 1 extreme of the range of characteristics and the other extreme is selected against - shift in population curve. Occurs in response to environmental change.

What is stabilising selection?

favours the mean of the distribution bc the extremes are at a selective disadvantage - frequency of mean phenotype increases

Give an example of stabilising selection

birthweight in humans

Give an example of directional selection

bacterial resistance to antibiotics

moth colour (peppered moths)

What is disruptive selection?

favours both extremes of a distribution, selection occurs against the mean - results in bimodal distribution

What is an extremophile?

an organism, often a microbe, that thrives in conditions considered too extreme for most other life forms eg high temp, high pressure, acidic/alkaline

What is an adaptation?

a genetically inherited characteristic that increases an organisms chance of surviving and reproducing in its specific environment

Why is adapting to live in harsher climates an advantage?

reduces interspecific competition

Give 5 examples of anatomical adaptations

body coverings eg fur

camouflage

sunken stomata

waxy cuticle

coloured petals (to attract pollinators)

Give 5 examples of behavioural adaptations

survival eg playing dead

courtship eg mating dances

seasonal eg hibernation

phototropism

geotropism

Give 5 examples of physiological adaptations

poison production

kidney adaptations (reduce water loss)

CAM photosynthesis (gas exchange only at night to prevent water loss)

seed dormancy in dry/cold conditions

How are plant roots adapted in xerophytes?

shallow but wide area root systems to absorb rainfall , swollen stems to store collected water (succulents)

Define interspecific competition

individuals of different species compete for the same resources in an ecosystem

Define intraspecific competition

competition between individuals of the same species

How is nutrient agar medium sterilised when preparing to grow microorganisms?

boiling or autoclaving which involves heating to high temperatures with high pressure and steam to kill microorganisms so only the microorganisms added to the agar will grow

Give 3 steps you’d take to work aseptically

light bunsen burner on bench - create upwards convection current to carry any microbes in the air away from workspace

flame neck of any bottles containing bacteria before using them to prevent any unwanted bacteria entering the vessel

only lift lid of petri dish slightly and keep lid on whenever possible to prevent unwanted bacteria contaminating the dish

How is the equipment sterilised during the practical ?

flamed using alcohol

Describe the plate spreading technique

move the spreader in a side to side motion to spread the inoculum over the whole surface of the agar

Why don’t you seal the whole lid of the agar plate using sticky tape after inoculation

air needs to get in to allow bacteria to respire otherwise anaerobic pathogenic bacteria may grow

Why do you incubate the agar plate at 25 degrees C

to avoid human pathogenic bacteria multiplying (which would at 37 degrees)

Why do you store the agar plate upside down after inoculation?

so condensation drips on lid and not agar

What is the zone of inhibition?

a circular spot around the spot of an antibiotic where bacteria colonies do not grow