9.1 - 9.4 Genetic diversity Flashcards
Define mutation
A change in the structure or quantity of an organisms DNA
Define gene mutation
A change to one or more nucleotide bases
Define chromosome mutation
A change in the structure or number of chromosomes
Define polyploidy
Changes in whole sets of chromosomes
When are gene mutations most likely to arise?
During DNA replication
What are the six type of gene mutations?
- Substitution - replace a base
- Addition - add a base
- Deletion - remove a base
- Inversion - flip a sequence of bases so they are read backwards
- Translocation - move a sequence of bases from one place to another
- Duplication - repetition of a sequence of bases
Explain how a substitution of one base could result in a non-functional protein
- Base triplet on DNA is different
- Different codon on mRNA following transcription
- Different tRNA binds to mRNA during transcription
- Different amino acid is incorporated into polypeptide chain
- Hydrogen/ionic/disulphide bonds form in different places
- The tertiary structure of the protein is changed so it is non-functional
Define mutagenic agent
Something that can increase the rate of gene mutation
Give examples of a mutagenic agent
High energy radiation:
- Can ionise chemicals in DNA and change the overall structure
- e.g X-rays, gamma rays and UV radiation
High energy particles:
- Can ionise chemicals in DNA and change the overall structure
- e.g alpha or beta particles
Chemicals:
- Can alter bases
- e.g benzene, mustard gas, tobacco tar, pesticides
Define non-disjunction
When a chromosome fails to separate into to two chromatids during anaphase. This leaves one daughter cell with an extra chromosome.
What is meiosis?
Nuclear division that produces 4 genetically different daughter cells. The cells produced are gametes (sex cells).
What is the importance of meiosis?
- Chromosome number is halved (produces haploid daughter cells) - So when gametes fuse they maintain the correct number of chromosomes
- Genetic variation is introduced (all gametes genetically unique) - Enables natural selection
What are the two ways meiosis introduces genetic variation?
- Crossing over during meiosis 1
- Independent segregation in meiosis 1 and 2
Define independent segregation
When homologous chromosomes line up randomly during meiosis 1 meaning the daughter cells receive a random combination of maternal and paternal chromosomes
What is the formula for the number of possible combinations during independent segregation?
2ⁿ
n = The number of homologous pairs
What is crossing over?
- Homologous chromosomes pair up (to form a bivalent)
- Equal lengths of chromatids are exchanged between non-sister chromatids at points called chiasmata
- This creates new combination of alleles called recombinations
Define genetic diversity
The number of different alleles of genes in a population
Describe how genetic diversity enables natural selection
- Greater genetic diversity
- Greater variety of alleles in gene pool
- Greater variety of phenotypes in a population
- More likely that some members of the population would survive an environmental change
- Enables natural selection
Describe the process of natural selection
- There is genetic variation in a population and random genetic mutations may create new alleles
- Some alleles have advantageous effects due to selection pressure
- Individuals with these characteristics have increased reproductive success, therefore allele more likely to be passed on to next generation
- Frequency of advantageous allele increases
Define selection pressure
A factor that limits the survival of a population and results in evolutionary changes
Give 4 examples of selection pressures
- Food availability
- Harsh abiotic conditions
- Predators
- Disease
Describe a selective advantage for the selection pressure of predators
Adaptation to not be seen or to be able to escape being eaten
Describe a selective advantage for the selection pressure of food availability
Adaptation to eat the available food, produce the correct enzymes
Describe a selective advantage for the selection pressure of disease
Resistance to the disease or ability to survive the disease
Describe a selective advantage for the selection pressure of harsh abiotic conditions
Adaptation to survive condition
Describe anatomical adaptations
Improvements in physical characteristics that allow an organism to be more competitive
Describe physiological adaptations
Improvements in biological processes that allow an organism to be more competitive
Describe behavioural adaptations
Improvements in behaviour that allow an organism to be more competitive
Describe stabilising selection
This is a type of natural selection in which genetic diversity decreases as the population stabilises on a particular trait value. Extreme phenotypes are selected against; it favours the norm (the mean). This is the most common mechanism of action for natural selection.
Organisms with mean phenotype more likely to survive and reproduce
Describe disruptive selection
When natural selection selects for two or more extreme phenotypes that each have specific advantages.
Organisms at both extremes more likely to survive and reproduce
Describe directional selection
This occurs when natural selection favours a single phenotype and therefore allele frequency continuously shifts in one direction.
Organisms at one extreme more likely to survive and reproduce
To do:
- More about meiosis
- Required practical 6 and aseptic technique
Describe what happens to chromosomes in meiosis
- Chromosomes condense
- Chromosomes associate in homologous pairs / form bivalents
- Crossing-over
- Join to spindle fibres
- At middle of cell
- Join via centromere
- Homologous chromosomes move to opposite poles
- (Pairs of) chromatids separated in 2 nd division
Meiosis results in genetic variation in the gametes which leads to variation in the offspring formed by sexual reproduction. Describe how meiosis causes this variation and explain the advantage of variation to the species
- Crossing-over
- Independent segregation of (homologous) chromosomes in meiosis I
- Independent segregation of chromatids in meiosis II
- Different adaptations
- Some survive
- To reproduce
- Pass on allele
- Allows for changing environment
