topic 4B: diversity and selection Flashcards by sehansa panditharatne

What is a gene mutation? + examples

-a change in the BASE SEQUENCE OF DNA (on chromosomes)
-can arise SPONTANEOUSLY during DNA R REPLICATION (interphase)

e.g. base deletion or substitution

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What is a mutagenic agent?

a factor that INCREASES RATE of gene mutation, eg. ultraviolet (UV) light or alpha particles

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

Changes sequence of BASE TRIPLETS in DNA (in a gene) so changes sequence of CODONS on mRNA
So changes sequence of AMINO ACIDS in the polypeptide
So changes POSITION of HYDROGEN / IONIC / DISULPHIDE BONDS (between amino acids)
So changes protein TERTIARY structure (shape) of protein
ENZYMES - ACTIVE SITE changes shape so substrate can’t bind, ENZYME SUBSTRATE COMPLEX can’t form

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Explain the possible effects of a substitution mutation

DNA base / nucleotide (pair) REPLACED by a different base / nucleotide (pair)
This changes ONE TRIPLET so changes one mRNA CODON
So ONE AMINO ACID in polypeptide changes
○ TERTIARY structure may change if POSITION of hydrogen / ionic / disulphide bonds change OR amino acid DOESNT change
○ Due to DEGENERATE nature of genetic code (triplet could code for same amino acid) OR if mutation is in an INTRON so removed during SPLICING

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Explain the possible effects of a deletion mutation

One nucleotide / base (pair) REMOVED from DNA sequence
Changes sequence of DNA TRIPLETS FROM POINT OF MUTATION (frameshift)
Changes sequence of mRNA CODONS after point of mutation
Changes sequence of AMINO ACIDS in primary structure of polypeptide
Changes position of HYDROGEN / IONIC / DISULPHIDE BONDS in tertiary structure of protein
Changes TERTIARY STRUCTURE / shape of protein

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Describe features of homologous chromosomes

Same LENGTH, same GENES at same loci, but may have DIFFERENT ALLELES

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Describe the difference between diploid and haploid cells

-DIPLOID - has 2 complete sets of chromosomes, represented as 2n
-HAPLOID - has a SINGLE set of unpaired chromosomes, represented as n

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Describe how a cell divides by meiosis

In INTERPHASE, DNA REPLICATES, to form 2 COPIES of each chromosome (sister chromatids), joined by a centromere

Meiosis I (first nuclear division) separates HOMOLOGOUS CHROMOSOMES
○ Chromosomes arrange into HOMOLOGOUS PAIRS
○ CROSSING OVER between homologous chromosomes
○ INDEPENDENT SEGREGATION of homologous chromosomes
Meiosis II (second nuclear division) separates CHROMATIDS

-Outcome = 4 GENETICALLY VARIED daughter cells
-Daughter cells are normally HAPLOID (if
diploid parent cell)

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Explain why the number of chromosomes is halved during meiosis

Homologous chromosomes are separated during meiosis I (first division)

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Explain how crossing over creates genetic variation

-HOMOLOGOUS PAIRS of chromosomes associate / form a BIVALENT
-CHIASMATA form (point of contact between (non-sister) chromatids)
-ALLELES / (equal) lengths of (non-sister) chromatids EXCHANGED between chromosomes
-Creating NEW COMBINATIONS of (maternal & paternal) ALLELES on chromosomes

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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

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

-RANDOM FERTILIZATION / FUSION OF GAMETES
-Creating new ALLELE COMBINATIONS / new maternal and paternal CHROMOSOME COMBINATIONS

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Explain the different outcomes of mitosis and meiosis

MITOSIS produces 2 DAUGHTER CELLS, whereas MEIOSIS produces 4 DAUGHTER CELLS
-As 1 DIVISION in mitosis, whereas 2 DIVISIONS in meiosis
MITOSIS MAINTAINS the chromosome number (eg. diploid → diploid or haploid → haploid) whereas MEIOSIS HALVES the chromosome number (eg. diploid → haploid)
-As HOMOLOGOUS CHROMOSOMES SEPARATE in meiosis but not mitosis
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

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Explain the importance of meiosis

-TWO 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

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

-MITOSIS occurs between stages where chromosome number is MAINTAINES
E.g. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)
-MEOSIS occurs between stages where chromosome number HALVES
○ Eg. diploid (2n) → haploid (n)

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Describe how mutations in the number of chromosomes arise

-SPONTANEOUSLY by CHROMOSOMES NON DISJUNCTION during MEIOSIS
-HOMOLOGOUS CHROMSOMES (meiosis I) or SISTER CHROMATIDS (meiosis II) FAIL TO SEPERATE 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

2n 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 two gametes can be calculated

(2n)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 organisms of the same species in a particular space at a particular time
● That can (potentially) interbreed (to produce fertile offspring)

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
● Occurring through the process of natural selection

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

Explain the principles of natural selection in the evolution of populations (MARIA)

Mutation: Random gene mutations can result in [named] new alleles of a gene
Advantage: In certain [named] environments, the new allele might benefit its possessor
[explain why] → organism has a selective advantage
Reproduction: Possessors are more likely to survive and have increased reproductive success
Inheritance: Advantageous allele is inherited by members of the next generation (offspring)
Allele frequency: Over many generations, [named] allele increases in frequency in the population

Describe 3 types of adaptations

NATURAL SELECTION results in species that are better adapted to their environment: ● ANATOMICAL - structural / physical features that increase chance of survival ● PHYSIOLOGICAL - processes / chemical reactions that increase chance of survival ● BEHAVIOURAL - ways in which an organism acts that increase chance of survival

Explain two types of selection, with examples - DIRECTIONAL SELECTION

EXAMPLE: Antibiotic resistance in bacteria KEY FEATURE - who has a selective advantage? Organisms with an extreme variation of a trait eg. bacteria with high level of resistance to a particular antibiotic ENVIRONMENT: Often a change, eg. 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 two types of selection, with examples - STABILISING SELECTION

EXAMPLE: Human birth weight KEY FEATURE: WHO HAS A SELECTIVE ADVANTAGE? Organisms with an average / modal variation of a trait eg. babies with an average weight ENVIRONMENT: 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

REQUIRED PRACTICAL 6: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.

Explain examples of aseptic techniques that could be used

● Wash hands with SOAP AND DISINFECT surfaces → KILL microbes and prevent CONTAMINATION ●STERILISE PIPPETTE AND SPREADER AND BOIL AGAR growth medium → KILL microbes and prevent CONTAMINATION ● FLAME NECK of bottle of bacteria → KILL microbes and prevent CONTAMINATION ● BUNSEN BURNER close → UPWARD CURRENT of air draws air-borne microbes away to prevent CONTAMINATION ● Lift lid of petri dish SLIGHTLY and minimise opening → PREVENT ENTRY of microbes and CONTAMINATION

Describe a method to investigate the effect of antimicrobial substances (eg. 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 (as above) 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 and 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 25oC for 48 HOURS 6. Measure DIAMETER of INHIBITION ZONE around each disc and calculate AREA using πr2

Explain why it is important to maintain a pure culture of bacteria. (1)

● Bacteria may OUTCOMPETE bacteria being investigated ● Or could be harmful to humans / pathogenic

Explain why the lid is held with 2 pieces of tape and not sealed completely. (2)

● Allows OXYGEN in preventing growth of ANAEROBIC bacteria ● More likely to be PATHOGENIC AND HARMFUL TO HUMANS

Explain why a paper disc with water / no antimicrobial agent is used. (2)

● Act as a CONTROL ● Ensuring antimicrobial prevented growth, NOT PAPER DISC

Explain why petri dishes are incubated upside down. (1)

● CONDENSATION drips onto lid rather than surface of agar

Explain how zones of inhibitions can be measured if irregular. (1)

● REPEAT readings in different POSITIONS, calculate a MEAN

Explain why a higher antimicrobial concentration isn’t used. (1)

● More bacteria killed so clear zones may OVERLAP

Explain why bacteria should be incubated at 25oC or less in a school laboratory. (1)

● Below HUMAN BODY TEMP to prevent growth of PATHOGENS

Explain the presence and absence of clear zones

1. CLEAR ZONES → antimicrobial DIFFUSES out of disc into agar, KILLING AND 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

Describe how data about the effect of antimicrobial substances can be presented as a graph

● CATEGORICAL data → BAR CHART, eg. ○ X axis type of antimicrobial ○ Y axis area of zone of inhibition / mm3 ● CONTINUOUS data → LINE GRAPH joined by a line of best fit, eg. ○ X axis concentration of antibiotic / μgmL -1 ○ Y axis area of zone of inhibition / mm3