Topic 4: Genetic Diversity and Adaptation Flashcards
What is a mutation?
Any change to the quantity or base sequence of the DNA of an organism
What is a gene mutation?
Any change to 1 or more nucleotide bases, or a change in base sequences of DNA
How do some gene mutations cause non-functional proteins?
Any change in DNA triplets changes the amino acid sequence of the polypeptide (primary structure), so the protein folds into its tertiary structure incorrectly, changing its shape, so it doesn’t function normally anymore
What are the different types of DNA mutation?
- Substitution
- Deletion
- Addition
- Chromosome mutations
What effect do substitution mutations have?
A nucleotide in a DNA molecule is replaced with nucleotide with a different base.
Polypeptide will differ by 1 amino acid (if any - degenerate code). The effect depends on the role of the original amino acid (e.g if in active site of an enzyme / important in forming bonds of the tertiary structure).
Doesn’t cause frameshift
What effect do deletion mutations have?
A nucleotide is lost from the normal DNA sequence.
Causes frameshift - all triplets further on in the sequence are read differently, so the amino acid sequence is entirely different, leading to a non-functional protein
What effect do addition mutations have?
A new base is inserted into the DNA sequence where it shouldn’t be.
Causes frameshift - all triplets further on in the sequence are read differently, so the amino acid sequence is entirely different, leading to a non-functional protein
What is a chromosome mutation and what types are there?
Changes in the structure or number of whole chromosomes.
Polyploidy, non-disjunction
What is polyploidy (in reference to chromosome mutations)?
Changes in whole sets of chromosomes, organisms have 3 or more sets of chromosomes rather than 2
What is non-disjunction (in reference to chromosome mutations)?
Changes in the number of individual chromosomes, sometimes individual homologous pairs fail to separate in meiosis, and the gamete will have 1 more or fewer chromosomes
What is a mutagen? Give some examples
An agent increasing the rate of gene mutations
E.g some chemicals, carcinogens, ionising radiation (e.g UV)
What are the differences between mitosis and meiosis?
Mitosis produces 2 genetically identical daughter cells with the same number of chromosomes as the parent
Meiosis produces 4 genetically different daughter cells with half the number of chromosomes as the parent
Why is meiosis important?
Creates gametes with a haploid number (1 complete set of chromosomes).
In sexual reproduction gametes combine, producing a cell with double their chromosome number - if meiosis didn’t produce haploid gametes, the number of chromosomes would double each generation.
Also brings about genetic variation, leading to adaptations increasing survival chances
Describe the process of meiosis
Interphase - homologous pairs of chromosomes unravel and replicate
Meiosis division 1:
Prophase 1 - chromosomes condense, homologous chromosomes pair up, crossing over occurs, spindle fibres form, nuclear envelope breaks down
Metaphase 1 - Homologous chromosomes line up in pairs on random sides of the equator (independent segregation)
Anaphase 1/Telophase 1 - 1 chromosome from each homologous pair ends up in each cell after division
Meiosis division 2:
Each daughter cell ends up with a different set of sister chromatids
Describe the process of crossing over in meiosis
Chromatids of each chromosome in a homologous pair twist around each other (cross over)
Tensions are created and portions of chromatids break off
Recombination - the broken portions re-join the chromatids of the partner chromosome - equivalent portions exchanged
Creates different portions of alleles
Describe the process of independent segregation in meiosis
After DNA has replicated, it is random which side of the equator the maternal/paternal chromosomes from each homologous pair lie.
It is random which combination of these sets each daughter cell gets
How would you calculate the number of combinations of chromosomes in daughter cells after meiosis?
Why is it not accurate?
2^n
(where n is the number of pairs of homologous chromosomes)
Only takes into account independent segregation, crossing over increases this number further
How would you calculate the number of combinations of chromosomes in a zygote after random fertilisation?
(2^n)^2
(where n is the number of homologous pairs)
What is non-disjunction in meiosis?
What are the different types?
Where homologous pairs / chromatids don’t separate properly
Polyploidy, aneuploidy
What is polyploidy in meiosis and what are the different outcomes after cell division?
Changes in whole sets of chromosomes
Non-disjunction in meiosis 1:
- Homologous pairs don’t separate, 1 daughter cell receives all chromosomes, other receives nothing
Normal meiosis 2:
- 2 daughter cells receive 2 sets of chromatids (2n), 2 daughter cells receive 0
OR
Normal meiosis 1:
- Each daughter cell receives 1 set of chromosomes
Non-disjunction in meiosis 2:
- I cell splits normally, forming 2 normal daughter cells (n). The other doesn’t split sister chromatids, so 1 daughter cell receives nothing, 1 receives 2 sets of chromatids (2n)
What is aneuploidy in meiosis and what are the different outcomes after cell division?
Changes in the number of individual chromosomes
Non-disjunction in meiosis 1:
- 1 homologous pair doesn’t separate, 1 daughter cell gets (2n+2), the other gets (2n-2)
Normal meiosis 2:
- Chromatids split normally, 2 daughter cells receive (n+1), the other 2 receive (n-1)
OR
Normal meiosis 1:
- Homologous pairs split normally, each daughter cell gets (2n)
Non-disjunction in meiosis 2:
- 1 cell splits normally, 2 daughter cells get (n)
- 1 chromosome in the other cell fails to split, 1 daughter cell gets (n-1), the other gets (n+1)
What is genetic diversity?
The total number (variety of) alleles in a population
What is a population?
A group of individuals of the same species that live in the same place and can interbreed
What is a species?
A group of organisms that can interbreed to produce fertile offspring
Why is genetic diversity a good thing?
Greater genetic diversity means the population is more likely to survive an environmental change because:
- Wider range of alleles + characteristics means there is a greater probability of an individual having a characteristic suited to the new environment = a factor enabling natural selection
How does reproductive success affect allele frequency? (natural selection)
- Organisms produce more offspring than can be supported by the environment (food, water etc) but most populations remain constant
- Because of intraspecific competition there is a gene pool within a population
- Random mutations mean some individuals possess combinations of alleles making them more likely to survive in competition
- These individuals more likely to obtain resources + live longer = better chance of breeding, passing on advantageous alleles
- Over many generations, number of individuals with advantageous alleles increases at expense to other individuals
- Frequency of advantageous alleles in the population increases and vice versa
What is selection?
The process by which organisms better adapted to their environment tend to survive and reproduce, while those less well adapted tend not to
What is the distribution of alleles in a population and why?
A normal distribution curve
Because:
- Most characteristics are influenced by multiple genes (polygenes)
- These are more influenced by the environment than single-gene characteristics
- The environmental effect on polygenes produces individuals in a population that vary about the mean
What is directional selection?
Give an example
Selection favours individuals varying in one direction from the mean of the population. Changes the characteristics of the population
e.g antibiotic resistance
Describe how antibiotic resistance comes about
- Resistance naturally occurs in a population due to chance mutation (new alleles)
- Allele codes for an enzyme complementary to the antibiotic = mutant bacteria synthesises enzyme that breaks down antibiotic before it can be killed
- When antibiotic is applied susceptible bacteria are killed
- Bacteria with mutant allele survive + reproduce by binary fission, passing on the allele + their population increases in proportion to non-resistant bacteria (they are ‘selected for’)
- Frequency of the resistance allele increases + the normal distribution curve shifts in the direction of greater resistance
Describe a graph displaying directional selection
Number of individuals on the y axis, the characteristic on the x axis.
The curve will stay exactly the same shape, but be moved in one direction from the mean
What is stabilising selection? Give an example
Selection favours average individuals - eliminates phenotypes at the extremes and so reduces variation around the optimum modal value.
E.g human birth weights (very heavy + very light babies are much more likely to die)
Describe a graph displaying stabilising selection
Number of individuals on the y axis, the characteristic on the x axis.
The original graph has the normal distribution curve. The graph after selection has a much narrower, taller curve.
What are the different types of adaptation?
- Anatomical (physical features)
- Physiological (biochemical processes)
- Behavioural
