Topic 4

Question 1

Compare and contrast DNA in eukaryotic cells with DNA in prokaryotic cells

Answer 1

Similarities
Nucleotide structure is identical - deoxyribose attached to phosphate and a base
• Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds
• DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes
• Short, circular, not associated with proteins

Differnces
Eukaryotic DNA is longer
Eukaryotic DNA is linear, prokaryotic DNA is circular
Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not Eukaryotic DNA contain introns, prokaryotic DNA does not

Question 2

What is a chromosome

Answer 2

Long, linear DNA + its associated histone proteins
• In the nucleus of eukaryotic cells

Question 3

What is a gene

Answer 3

sequence of DNA (nucleotide) bases that codes for:
• The amino acid sequence of a polypeptide
• Or a functional RNA (eg. ribosomal RNA

Question 4

What is a locus

Answer 4

Fixed position a gene occupies on a particular DNA molecule

Question 5

Describe the nature of the genetic code

Answer 5

Triplet code A sequence of 3 DNA bases, called a triplet, codes for a specific amino acid
Universal The same base triplets code for the same amino acids in all organisms
Non-overlapping Each base is part of only one triplet so each triplet is read as a discrete unit
Degenerate An amino acid can be coded for by more than one base triplet

Question 6

What are ‘non-coding base sequences’ and where are they found?

Answer 6

Non-coding base sequence - DNA that does not code for amino acid sequences / polypeptides:
Between genes - eg. non-coding multiple repeats
2. Within genes - introns

Question 7

What are introns and exons?

Answer 7

Exon
Base sequence of a gene coding for amino acid sequences (in a polypeptide)
Intron
Base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells

Question 8

Define ‘genome’ and ‘proteome’

Answer 8

Genome
The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)
Proteome
The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Question 9

Compare and contrast the structure of tRNA and mRNA

Answer 9

Comparison (similarities):
• Both single polynucleotide strand

Contrast (differences):
• tRNA is folded into a ‘clover leaf shape”, whereas mRNA is linear / straight
• tRNA has hydrogen bonds between paired bases, mRNA doesn’t
• tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides)
• tRNA has an anticodon, mRNA has codons
ERNA has an amino acid binding site, mRNA doesn’t

Question 10

Describe the role of ATP, tRNA and ribosomes in translation

Answer 10

ATP
• Hydrolysis of ATP to ADP + Pi releases energy
• So amino acids join to tRNAs and peptide bonds form between amino acids
tRNA
Attaches to / transports a specific amino acid, in relation to its anticodon
ERNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds
• 2 tRNAs bring amino acids together so peptide bond can form
Ribosomes
• mRNA binds to ribosome, with space for 2 codons
Allows tRNA with anticodons to bind
Catalyses formation of peptide bond between amino acids (held by tRNA molecules)
Moves along (mRNA to the next codon) / translocation

Question 11

What is a gene mutation?

Answer 11

• A change in the base sequence of DNA (on chromosomes)
• Can arise spontaneously during DNA replication (interphase)

Question 12

What is a mutagenic agent?

Answer 12

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

Question 13

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

Answer 13

1. Changes sequence of base triplets 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

Question 14

Explain the possible effects of a substitution mutation

Answer 14

DNA base / nucleotide (pair) replaced by a different base / nucleotide (pair)
2. 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 doesn’t 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

Question 15

Explain the possible effects of a deletion mutation

Answer 15

One nucleotide / base (pair) removed from DNA sequence
2. Changes sequence of DNA triplets from point of mutation (frameshift)
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

Question 16

Describe the difference between diploid and haploid cells

Answer 16

Diploid - has 2 complete sets of chromosomes, represented as 2n
• Haploid - has a single set of unpaired chromosomes, represented as n

Question 17

Describe features of homologous chromosomes

Answer 17

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

Question 18

Describe how a cell divides by meiosis

Answer 18

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

Question 19

Explain why the number of chromosomes is halved during meiosis

Answer 19

Homologous chromosomes are separated during meiosis i

Question 20

Explain how crossing over creates genetic variation

Answer 20

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

Question 21

Explain how independent segregation creates genetic variation

Answer 21

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

Question 22

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

Answer 22

Random fertilisation / fusion of gametes
• Creating new allele combinations / new maternal and paternal chromosome combinations

Question 23

Explain the different outcomes of mitosis and meiosis

Answer 23

Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells
• As l division in mitosis, whereas 2 divisions in meiosis
2. 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
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

Question 24

Explain the importance of meiosis

Answer 24

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

Question 25

Describe how mutations in the number of chromosomes arise

Answer 25

Spontaneously by chromosome non-disjunction during meiosis • Homologous chromosomes (meiosis l) or sister chromatids (meiosis il) fail to separate during meiosis So some gametes have an extra copy (n+l) of a particular chromosome and others have none (n-1)

Question 26

Suggest how the number of possible combinations of chromosomes in daughter cells following meiosis can be calculated and Suggest how the number of possible combinations of chromosomes following random fertilisation of two gametes can be calculated

Answer 26

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

Question 27

What is genetic diversity?

Answer 27

Number of different alleles of genes in a population

Question 28

What are alleles and how do they arise?

Answer 28

Variations of a particular gene (same locus) - different DNA base sequence • Arise by mutation

Question 29

What is a population?

Answer 29

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

Question 30

Explain the importance of genetic diversity

Answer 30

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)

Question 31

What is evolution?

Answer 31

Change in allele frequency over many generations within a population which occurs through the process of natural selection

Question 32

Explain the principles of natural selection in the evolution of populations

Answer 32

1. Mutation Random gene mutations can result in (named] new alleles of a gene 2. Advantage In certain (named] environments, the new allele might benefit its possessor [explain why] → organism has a selective advantage 3. Reproduction Possessors are more likely to survive and have increased reproductive success 4. Inheritance Advantageous allele is inherited by members of the next generation (offspring) 5. Allele frequency Over many generations, [named) allele increases in frequency in the population

Question 33

Describe 3 types of adaptations

Answer 33

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

Question 34

Explain two types of selection, with examples

Answer 34

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 Stabilising selection Example Human birth weight Key feature Organisms with an average / modal variation of a trait eg. babies with an average weight Enviroment Usually stable Effect on population over generations • Increased frequency of organisms with / alleles for average trait • Normal distribution curve similar, less variation around the mean

Question 35

Explain examples of aseptic techniques that could be used

Answer 35

• 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

Question 36

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

Answer 36

1. Prepare area using aseptic techniques (as above) Use a sterile pipette to transfer bacteria from broth to agar plate using aseptic techniques (as above) Use a sterile spreader to evenly spread bacteria over agar plate 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 25°C for 48 hours 6. Measure diameter of inhibition zone around each disc and calculate area using pi r squared

Question 37

Explain the presence and absence of clear zones

Answer 37

Clear zones → antimicrobial diffuses out of disc into agar, killing / inhibiting growth of bacteria • Larger clear zones - more bacteria killed → more effective antimicrobia! 2. No clear zones → if antibiotic used, bacteria may be resistant or antibiotic may not be effective against that specific bacteria

Question 38

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

Answer 38

Categorical data → bar chart, eg. • X axis type of antimicrobial • Y axis area of zone of inhibition / mm' • Continuous data → line graph joined by a line of best fit, eg. • X axis concentration of antibiotic / ugmL • Y axis area of zone of inhibition / mm'

Question 39

What is a species?

Answer 39

group of organisms that can (interbreed to) produce fertile offspring

Question 40

Suggest why 2 different species are unable to produce fertile offspring

Answer 40

• Different species have different chromosome numbers → offspring may have odd chromosome number • So homologous pairs cannot form → meiosis cannot occur to produce gametes

Question 41

Explain why courtship behaviour is a necessary precursor to successful mating

Answer 41

Allows recognition of members of same species - so fertile offspring produced Allows recognition / attraction of opposite sex Stimulates / synchronises mating / production / release of gametes Indicates sexual maturity / fertility Establishes a pair bond to raise young

Question 42

Describe a phylogenetic classification system

Answer 42

Species (attempted to be) arranged into groups, called taxa, based on their evolutionary origins (common ancestors) and relationships • Uses a hierarchy: • Smaller groups are placed within larger groups • No overlap between groups

Question 43

How is each species universally identified?

Answer 43

A binomial consisting of the name of its genus and species,

Question 44

Suggest an advantage of binomial naming

Answer 44

Universal so no confusion as many organisms have more than one common name

Question 45

How can phylogenetic trees be interpreted?

Answer 45

Branch point = common ancestor Branch = evolutionary path If two species have a more recent common ancestor, they are more closely related

Question 46

Describe two advances that have helped to clarify evolutionary relationships between organisms

Answer 46

Advances in genome sequencing → allowing comparison of DNA base sequences • More differences in DNA base sequences - more distantly related / earlier common ancestor • As mutations (change in DNA base sequences) build up over time 2. Advances in immunology → allowing comparison of protein tertiary structure (eg. albumin) • Higher amount of protein from one species binds to antibody against the same protein from another species - more closely related / more recent common ancestor • As indicates a similar amino acid sequence and tertiary structure • So less time for mutations to build up

Question 47

What is biodiversity?

Answer 47

Variety of living organisms (species, genetic and ecosystem diversity) • Can relate to a range of habitats, from a small local habitat to the Earth

Question 48

What is a community?

Answer 48

All populations of different species that live in an area

Question 49

What is species richness?

Answer 49

A measure of the number of different species in a community

Question 50

What does an index of diversity do?

Answer 50

Describes the relationship between: 1. The number of species in a community (species richness) 2. The number of individuals in each species (population size)

Question 51

Suggest why index of diversity is more useful than species richness

Answer 51

• Also takes into account number of individuals in each species So takes into account that some species may be present in small or high numbers

Question 52

Explain how some farming techniques reduce biodiversity

Answer 52

Monocultre Removal of woodland and hedgerows Reduces variety of plant species So fewer habitats and niches And less variety of food sources Use of pesticides to kill pests Predator population of pest decreases

Question 53

Explain the balance between conservation and farming

Answer 53

Conservation required to increase biodiversity But when implemented on farms, yields can be reduced, reducing profit / income for farmers Eg. by reducing land area for crop growth, increasing competition, increasing pest population • To offset loss, financial incentives / grants are offered

Question 54

Give examples of how biodiversity can be increased in areas of agriculture

Answer 54

Reintroduction of field margins and hedgerows (where farmers only grow one type of crop) • Reduce use of pesticides • Growing different crops in the same area (intercropping) • Using crop rotation of nitrogen fixing crops instead of fertilisers

Question 55

How can genetic diversity within or between species be measured?

Answer 55

• Comparing frequency of measurable or observable characteristics Comparing base sequence of DNA Comparing base sequence of mRNA Comparing amino acid sequence of a specific protein encoded by DNA and mRNA

Question 56

Explain how comparing DNA, mRNA and amino acid sequences can indicate relationships between organisms within a species and between species

Answer 56

More differences in sequences → more distantly related / earlier common ancestor As mutations (change in DNA base sequences) build up over time More mutations cause more changes in amino acid sequences

Question 57

Explain the change in methods of investigating genetic diversity over time

Answer 57

Early estimates made by inferring DNA differences from measurable or observable characteristics • Many coded for by more than one gene - difficult to distinguish one from another • Many influenced by environment → differences due to environment not genes • Gene technologies allowed this to be replaced by direct investigation of DNA sequences

Question 58

Explain how data should be collected when investigating variation within a species quantitatively

Answer 58

Collect data from random samples → to remove bias • Use a grid / divide area into squares • Use a random number generator to obtain random coordinates Use same method of measurement each time • Use a large sample size / measure a large number of organisms → so representative of population Calculate a running mean and sample until number becomes (fairly) constant • (Where applicable) ensure sampling is ethical → must not harm organism