B9- Genetic Variation and Adaptations Flashcards
What is gene mutation?
- A change in thebase sequence of DNA(on chromosomes)
- Can arisespontaneouslyduringDNA replication(interphase)
What is a mutagenic agent?
A factor thatincreases rateof gene mutation, eg. ultraviolet (UV) light or alpha particles.
How can mutations in the DNA base sequence occur?
- Deletion of nucleotides
- Insertion of extra nucleotides
- Both types of mutations change the groups of triplet bases - also called a frameshift mutation
Explain how a mutation can lead to the production of a non-functional protein or enzyme
- Changes sequence ofbase tripletsinDNA(in a gene) so changes sequence ofcodonsonmRNA
- So changes sequence ofamino acidsin the polypeptide
- So changespositionofhydrogen / ionic / disulphide bonds(between amino acids)
- So changes proteintertiarystructure (shape) of protein
- Enzymesactive sitechanges shape so substrate can’t bind,enzyme-substrate complexcan’t form
Explain the possible effects of a substitution mutation
- Base / nucleotide in DNAreplacedby a different base / nucleotide
- This changesone tripletso changes onemRNA codon
- Soone amino acidin polypeptide changes
- Tertiarystructure may change ifpositionof hydrogen / ionic /disulphide bonds change ORamino aciddoesn’tchange due todegeneratenature of genetic code (triplet could code for same amino acid)ORif mutation is in anintron
Explain the possible effects of a deletion mutation
- One nucleotide / baseremovedfrom DNA sequence
- Changes sequence ofDNA triplets from point of mutation(frameshift)
- Changes sequence ofmRNA codonsafter point of mutation
- Changes sequence ofamino acidsin primary structure of polypeptide
- Changes position ofhydrogen / ionic / disulphide bondsin tertiary
structure of protein - Changestertiary structure/ shape of protein
Describe features of homologous chromosomes?
Samelength, samegenesat same loci, but may havedifferent alleles.
Describe the difference between diploid and haploid cells
- Diploidhas2complete sets of chromosomes, represented as 2n
- Haploidhas asingleset of unpaired chromosomes, represented as n
Describe how a cell divides by meiosis
- Meiosis I(first nuclear division) separateshomologous chromosomes
◦ ● Chromosomes arrange intohomologous pairs
◦ ● Crossing overbetween homologous chromosomes
◦ ● Independent segregationof homologous chromosomes - Meiosis II(second nuclear division) separateschromatids Outcome =4 genetically varieddaughter cells
Explain how crossing over creates genetic variation
- Homologous pairsof chromosomes associate / form abivalent
- Chiasmataform (point of contact between (non-sister) chromatids)
- Alleles/ (equal) lengths of (non-sister) chromatidsexchangedbetween chromosomes
- Creatingnew combinationsof (maternal & paternal)alleleson chromosomes
Explain how independent segregation creates genetic variation
- Homologous pairsrandomlyalign atequator→so random which chromosome from each pair goes into each daughter cell
- Creatingdifferent combinationsof 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 newallele combinations/ new maternal and paternalchromosome combinations
Explain the different outcomes of mitosis and meiosis
- Mitosisproduces2 daughter cells, whereasmeiosisproduces4 daughter cells. As1 divisionin mitosis, whereas2 divisionsin meiosis
- Mitosis maintainsthe chromosome number (eg.diploid→diploid orhaploid→haploid) whereasmeiosis halvesthe chromosome number (eg.diploid→haploid). Ashomologous chromosomes separatein meiosis but not mitosis.
- Mitosisproducesgenetically identicaldaughter cells, whereasmeiosisproduces genetically varieddaughter cells. Ascrossing overandindependent segregationhappen in meiosis but not mitosis
Explain the importance of meiosis
- Two divisionscreateshaploidgametes (halves number of chromosomes)
- Sodiploidnumber is restored atfertilisation→chromosome number maintainedbetween generations
- Independent segregationandcrossing overcreatesgenetic variation
How can you recognise where meiosis and mitosis occur in a life cycle?
- Mitosisoccurs between stages where chromosome number ismaintained(eg. diploid (2^n)→diploid (2^n) OR haploid (n)→haploid (n)
- Meiosisoccurs between stages where chromosome numberhalves(eg. diploid (2n)→haploid (n))9
- Human the diploid number is 46= 2^23
Describe how mutations in the number of chromosomes arise
- Spontaneouslybychromosome non-disjunctionduringmeiosis
- Homologous chromosomes(meiosis I) orsister chromatids(meiosis II)fail to separateduring meiosis
- So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)
What is genetic diversity?
Number of differentallelesof genes in apopulation
What are alleles and how do they arise?
- Variationsof a particulargene(same locus)→different DNAbase sequence
- Arise bymutation
What is a population?
A group ofinterbreedingindividuals of thesame species.
Explain the importance of genetic diversity
- Enablesnatural selectionto occur
- As in certain environments, a newalleleof a gene mightbenefitits possessor
- By resulting in a change in thepolypeptide(protein) coded for thatpositivelychanges itsproperties
- Giving possessor aselective advantage(increased chances of survival and reproductive success)
What is evolution?
- Changeinallele frequency(how common an allele is) overmany generationsin a population
- Occurring through the process ofnatural selection
Adaptationandselectionare major factors in evolution and contribute to thediversityof living organisms.
Describe 3 types of adaptations
- Anatomicalstructural / physical features that increase chance of survival
- Physiologicalprocesses / chemical reactions that increase chance of survival
- Behaviouralways in which an organism acts that increase chance of survival
What is directional selection
Extreme variation of a trait is favoured increased frequency of that allele so distribution curve shifts towards extreme as trait becomes more common
What is stabilising selection?
When common trait increased in frequency as extreme traits are less frequent
