4- Genetic diversity, meiosis, adaptation, RP6

What is a gene mutation?

• A change in the base sequence of DNA (on chromosomes)

• can arise spontaneously during DNA replication (interphase)

What is a mutagenic agent?

A factor that increases rate of gene mutation, e.g. ultraviolet (UV) light or alpha particles

Explain how a mutation can lead to the production of a non-functional protein or enzyme

1. Changes sequence of base triplet in DNA (in a gene) so changes sequence of codons on mRNA

2. so changes sequence of amino acids in the polypeptide

3. so changes position of hydrogen/ ionic/ disulphide bonds (between amino acids)

4. so changes protein tertiary structure (shape) of protein

5. Enzymes- active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Explain the possible effects of a substitution mutation

1. Base/ nucleotide in DNA replaced by a different base/ nucleotide

2. This changes one triplet so changes one mRNA codon

3. so 1 amino acid in polypeptide changes

   • tertiary structure may change if position of hydrogen/ ionic/ disulphide bonds change

     OR amino acid doesn’t change

   • due to degenerate nature of genetic code (triplet could code for same amino acid) OR if mutation is in an intron

Explain the possible effects of a deletion mutation

1. One nucleotide/ base removed from DNA sequence

2. Changes sequence of DNA triplets from point of mutation (frameshift)

3. Changes sequence of mRNA codons after point of mutation

4. Changes sequence of amino acids in primary structure of polypeptide

5. Changes position of hydrogen/ ionic/ disulphide bonds in tertiary structure of protein

6. Changes tertiary structure/ shape of protein

Describe features of homologous chromosomes

Same length, same genes at same loci, but may have different alleles

Describe diploid cells

has 2 complete sets of chromosomes, represented by 2n

Describe haploid cells

has a single set of unpaired chromosomes, represented as n

Describe how a cell divides by meiosis

In interphase, DNA replicates= 2 copies of each chromosome (sister chromatids), joined by a centromere

1. MEIOSIS I (first nuclear division) separates homologous chromosomes

   • chromosomes arrange into homologous pairs

   • crossing over between homologous chromosomes

   • independent segregation of homologous chromosomes

2. MEIOSIS II (second nuclear division) separates chromatids

What is the outcome of meiosis?

4 genetically varied daughter cells

• daughter cells are normally haploid (if diploid parent cell)

Draw a diagram to show the chromosome content of cells during meiosis

Explain why the number of chromosomes is halved during meiosis

homologous chromosomes are separated during meiosis I (first division)

Explain how crossing over creates genetic variation

• Homologous pairs of chromosomes associate/ form a bivalent

• Chiasmata from (point of contact between (non-sister) chromatids)

• Alleles/ (equal) lengths of (non-sisters) chromatids exchanged between chromosomes

• Creating new combinations of (maternal & paternal) alleles on chromosomes

Explain how independent segregation creates genetic variation

• Homologous pairs randomly align at equator= so random which chromosome from each pair goes into each daughter cell

• Creating different combinations of maternal & paternal chromosomes/ alleles in daughter cells

Other than mutation and meiosis, explain how genetic variation within a species is increased

• Random fertilisation/ fusion of gametes

• creating new allele combinations/ new maternal and paternal chromosome combinations

Explain the different outcomes of mitosis and meiosis

1. Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells

   • as 1 division in mitosis, whereas 2 division in meiosis

2. Mitosis maintains the chromosome number, whereas meiosis halves the chromosome number

   • as homologous chromosomes separate in meiosis but not mitosis

3. Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically varied daughter cells

   • as crossing over and independent segregation happen in meiosis but not mitosis

Explain the importance of meiosis

• 2 divisions creates haploid gametes (halves number of chromosomes)

• so diploid number is restored at fertilisation= chromosome number maintained between generations

• independent segregation and crossing over creates genetic variation

How can you recognise where meiosis and mitosis occur in a life cycle?

• Mitosis occurs between stages where chromosome number is maintained

• Meiosis occurs between stages where chromosome number halves

Describe how mutations in the number of chromosomes arise

• Spontaneously by chromosome non- disjunction during meiosis

• Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis

• so some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)

Suggest how the number of possible combinations of chromosomes in daughter cells following meiosis can be calculated

2^n where n= number of pairs of homologous chromosomes (half the diploid number)

Suggest how the number of possible combinations of chromosomes following random fertilisation of 2 gametes can be calculated

(2^n)² where n= number of pairs of homologous chromosomes (half the diploid number)

What is genetic diversity?

Number of different alleles of genes in a population

What are alleles and how do they arise?

• Variations of a particular gene (same locus)= different DNA base sequence

• Arise by mutation

What is a population?

A group of interbreeding individuals of the same species

Explain the importance of genetic diversity

• Enables natural selection to occur

• as in certain environments, a new allele of a gene might benefit its possessor

• by resulting in a change in the polypeptide (protein) coded for that positively changes its properties

• giving possessor a selective advantage (increased chances of survival and reproductive success)

What is evolution?

• change in allele frequency (how common an allele is) over many generations in a population

• occuring through the process of natural selection

Adaptation and selection are major factors in evolution and contribute to the diversity of living organisms

What are the main principles of natural selection in the evolution of populations

1. Mutation

2. Advantage

3. Reproductive success

4. Inheritance

5. Allele frequency

Explain the principle mutation in natural selection in the evolution of populations

random gene mutations can result in (named) new alleles of a gene

Explain the principle advantage in natural selection in the evolution of populations

in certain (named) environments, the new allele might benefit its possessor (explain why)= organism has a selective advantage

Explain the principle reproductive success in natural selection in the evolution of populations

possessors are more likely to survive and have increased reproductive success

Explain the principle inheritance in natural selection in the evolution of populations

advantageous allele is inherited by members of the next generation (offspring)

Explain the principle allele frequency in natural selection in the evolution of populations

over many generations, (named) allele increases in frequency in the population

What are the 3 types of adaptations?

• anatomical

• physiological

• behavioural

Describe anatomical adaptations

structural/ physical features that increase chance of survival

Describe physiological adaptations

processes/ chemical reactions that increase chance of survival

Describe behavioural adaptations

ways in which an organism acts that increase chance of survival

What are the 2 types of selection?

• directional

• stabilising

Explain directional selection, with an example

• EXAMPLE= Antibiotic resistance in bacteria

• KEY FEATURE- WHO HAS A SELECTIVE ADVANTAGE?= organisms with an extreme variation of a trait e.g. bacteria with high level of resistance to a particular antibiotic

• CHANGE IN ENVIRONMENT= yes, usually e.g. antibiotic introduced

• EFFECT ON POPULATION OVER MANY GENERATIONS= increased frequency of organisms with/ alleles for extreme trait. Normal distribution curve shifts towards extreme trait

Explain stabilising selection, with an example

• EXAMPLE= humans birth weight

• KEY FEATURE- WHO HAS A SELECTIVE ADVANTAGE?= organisms with an average/ modal variation of a trait e.g. babies with an average weight

• CHANGE IN ENVIRONMENT= no, usually stable

• EFFECT ON POPULATION OVER MANY GENERATIONS= increased frequency of organisms with/ alleles for average trait. Normal distribution curve similar, less variation around the mean

RP6- What is RP6?

Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.

RP6- Explain examples of aseptic techniques that could be used

• Wash hands with soap/ disinfect surfaces= kill microbes/ prevent contamination

• Sterilise pipette/ spreader/ boil agar growth medium= kill microbes/ prevent contamination

• Flame neck of bottle of bacteria= kill microbes/ prevent contamination

• Bunsen burner close= upward current of air draws air-borne microbes away to prevent contamination

• Lift lid of petri dish slightly/ minimise opening= prevent entry of microbes/ contamination

RP6- Describe a method to investigate the effect of antimicrobial substances (e.g. antibiotics, disinfectants, antiseptics) on microbial growth

1. Prepare area using aseptic techniques (as above)

2. Use a sterile pipette to transfer bacteria from broth to agar plate using aseptic techniques

3. Use a sterile spreader to evenly spread bacteria over agar plate

4. Use sterile forceps to place same size discs that have been soaked in different types/ concentrations of antimicrobials for same length of time, onto agar plate (at equal distances)

5. Lightly tape lid onto plate (not fully sealed), invert and incubate at 25C for 48 hours

6. Measure diameter of inhibition zone around each disc and calculate area using pi r²

RP6- Why is it important to maintain a pure culture of bacteria?

• Bacteria may out-compete bacteria being investigated

• or could be harmful to humans/ pathogenic

RP6- Why hold lid with 2 pieces of tape instead of sealing it completely?

• Allows oxygen in preventing growth of anaerobic bacteria

• which are more likely to be pathogenic/ harmful to humans

RP6- Why use a paper disc with water/ no antimicrobial agent?

• Act as a control

• ensuring antimicrobial prevented growth, not paper disc

RP6- Why incubate upside down?

Condensation drips onto lid rather than surface of agar

RP6- What if inhibition zones are irregular?

Repeat readings in different positions, calculate mean

RP6- Why not use higher antimicrobial conc.?

More bacterial killed so clear zones may overlap

RP6-Why incubate at 25C or less?

Below human body temp to prevent growth of pathogens

RP6- Describe how data about the effect of antimicrobial substances can b presented as a graph

• Categorical data= bar chart (x axis type of antimicrobial, y axis area of zone of inhibition/ mm³)

• Continuous data= line graph joined by a line of best fit (x axis conc of antibiotic/ ugmL^-1, y axis area of zone of inhibition/mm³)

RP6- Explain the presence and absence of clear zones

1. Clear zones= antimicrobial diffuses out of disc into agar, killing/ inhibiting growth of bacteria

   • larger clear zones= more bacteria killed= more effective antimicrobial

2. No clear zones= if antibiotic used, bacteria may be resistant or antibiotic may not be effective against that specific bacteria