Chapter 9 - Gene Diversity And Adaptation Flashcards
What is a mutation
Any change to the quantity or the base sequence of the DNA of an organism
What is a gene mutation
A change in the sequence of bases that results in the formation of a new allele
3 types of gene mutations
- base deletion (known as point mutation as it occurs at a single point)
- base substitution (also known as point mutation as it occurs at a single point)
- base insertion
When does gene mutation occur
- spontaneously during dna replication
What is a chromosome mutation
Any changes in the structure or number of whole chromosomes
When do chromosome mutations occur
During the process of meiosis
Two forms of chromosome mutations and how the occur
Polyploidy - This is when organisms have more than two sets of chromosomes, and is mostly seen in plants.
Non-disjunction - This occurs when homologous chromosomes fail to separate in meiosis, resulting in gametes with extra or missing chromosomes, and leads to individuals with extra or missing chromosomes in all cells.
What happens during base substitution
- one base is replaced by another
Why might base substitution NOT have an effect
Due to degenerate coding as most amino acids have more than one codon that codes for the same amino acid so there will be no change in the polypeptide produced and so the mutation will have no effect
Why might base substitution HAVE an effect
The polypeptide produced will differ in a single amino acid which may form different bonds in different locations that would effect the tertiary structure of the final protein, causing the protein to be a different shape and therefore not function properly
What happens during base deletion
A base is removed from the sequence
Why can base deletion have a major effect
One deleted nucleotide causes all triplets in a sequence to be read differently as each has been shifted to the left by one base, which is called frame shift mutation to the left so this will change the amino acid sequence of the encoded polypeptide, therefore changing the primary structure of the protein causing bonds to form in different places in the tertiary structure which will cause the proteins structure and function to change
3 types of base substitution and what they do
Nonsense mutation - changes triplet codon to stop codon
Misense mutation - changes triplet codon to code for another amino acid
Silent mutation: when a different triplet codon codes for the same amino acid due to degenerate coding (or any change that does not have an effect)
What is produced in meiosis
- Daughter cells that are genetically DIFFERENT from each other
- 4 haploid daughter cells from a single diploid parent cell
What are haploid cells and diploid cells
Cells that contain only one set of chromosomes - haploid
Cells that contains two sets of chromosomes - diploid
Two mechanisms that introduce variation of cells in meiosis and when they occur
- Independent segregation of homologous chromosomes
- Crossing over between homologous chromosomes
- both happen during the first division of meiosis
What are homologous chromosomes
Pair of chromosomes with the same gene sequence, loci, centromere location and chromosomal length but different alleles (one chromosome comes from mum the other from dad)
What happens in independent segregation
1) in metaphase 1 the homologous pairs of chromosomes line up opposite each other at the equator of the cell
2) it is random which side of the equator the paternal and the maternal chromosomes from each homologous pair lie
3) these pairs are separated so one of each homologous pair ends up in the daughter cell
4) this creates a large number of possible combinations of chromosomes in the daughter cells produced
How can you calculate the number of possible combinations of chromosomes in the daughter cell produced in independent segregation
2^n
n = number of homologous pairs/ pairs of chromosomes
What happens during crossing over
Steps of crossing over:
1) During prophase I, the homologous chromosomes condense and come together to form bivalents (pair up)
2) The chromatids of each chromosome then twist around one another, forming a chiasmata.
3) When the chromosomes are separated during anaphase I, the chromatids break at the chiasmata and then reconnect to a chromatid from the other homologous chromosome, this creates a recombinant chromatid
4) This swaps alleles between the homologous chromosomes to produce different combinations of alleles on each chromosome.
Comparison of meiosis to mitosis
Meiosis
- two nuclear divisions
- produces haploid cells
- introduces genetic variation
Mitosis
- one nuclear division
- produces diploid cells
- creates genetically identical cells
How to identify meiosis in a cell cycle
- it’s the point of the cell cycle where the cells are turning from a diploid cell (2n) to a haploid cell (n)
How to calculate the possible combinations of genetically different zygotes that are possible from two parents as a result of independence segregation
(2^n)^2
N = number of chromosomes pairs
Stages of meiosis - meiosis 1
1) before meiosis starts the cell would have gone through interphase where the cell copies its chromosomes and the organelles
2) then the cell enters meiosis 1- Prophase 1 occurs - The chromosomes condense and homologous chromosomes pair up. Centrioles migrate to opposite poles of the cell where each centriole starts forming spindle fibres. The nucleolus disappears and the nuclear envelope starts to break down, leaving the chromosomes free in the cytoplasm. (Crossing over takes place)
3) Metaphase 1- the pair of homologous chromosomes line up at the equator of the cell and are each attached to the spindle fibres (independent segregation takes place)
4) Anaphase 1 - the spindle fibres shorten and the homologous chromosomes move to opposite poles (crossing over ends)
5) telophase 1 - the nuclear envelope starts to form around each set of chromosomes and the nucleolus reform and the chromosomes uncoil back to their chromatin state and at this point the cell undergoes cytokinesis dividing into two haploid cells
6) then the cells enter meiosis 2
Stages of meiosis - meiosis 2
1) Prophase 2 - The chromosomes condense and are now visible under a microscope. Centrioles migrate to opposite poles of the cell where each centriole starts forming spindle fibres. The nucleolus disappears and the nuclear envelope starts to break down.
2) Metaphase 2 - the chromosomes are lined up on the equator of the cell and are attached to spindle fibres
3) Anaphase 2 - the centromere divides and the spindle fibres shorten, the separate chromatids and are pulled to opposite poles of the cell
4) Telophase 2 - The chromatids reach the opposite poles of the cell where they uncoil to become long and thin again. A nuclear envelope forms around each set of chromosomes and the nucleolus starts to reform. Cytokinesis then occurs and divides the cytoplasm to form 4 genetically different haploid cells
What is selection
The process by which organisms that are better adapted to the environment survive and breed
What is genetic diversity
The number of different alleles in a population
What is a population
A group of organisms of the same species that live in the same place and can interbreed
When does natural selection occur
If there is a genetic diversity within a population
What is natural selection
The process that leads to evolution in populations
What is evolution
The change in allele frequency over many generations in a population
Different types of adaptions names
Anatomical - structural features
Physiological - chemical reactions in the body
Behavioural - how organisms behave to increase its chance of survival
Process of natural selection
1) within any generation there will be a gene pool containing a different alleles
2) RANDOM MUTATIONS OF ALLELES within the gene pool results in new alleles to be formed
3) in certain environments the new allele may give the organism an advantage over the other individuals in the population, so will be better adapted and more likely to survive in competitions with others
4) these organisms will be more likely to obtain resources and have a higher chance of surviving and breeding successfully, passing on their genes to their offspring
5) over time the frequency of the new advantageous allele will increase whilst the non advantageous allele will decrease
Types of natural selection
- directional selection
- stabilising selection
What is directional selection
When a single phenotype to a particular environment is favoured causing the allele frequency to shift away from the mean
When does directional selection occur
When there is a change in the environment
What is stabilising selection
When the individuals with the phenotype closest to the mean are favoured when the environmental conditions remain stable
What happens in stabilising selection
Individuals with extreme traits are less likely to survive so the individuals with phenotype closer to the mean will pass on their genes causing their to be a higher frequency of individuals with alleles closer to the mean
Environmental factors that exert selection pressure (determine which individuals will do best at surviving or reproducing)
- availability of resources
- abiotic factors (non living)
- biotic factors
What is the founder effect
when a subset of a large population is used to establish a new colony, which causes the genetic variation of the new colony to decrease
What is genetic drift
Variations in allele frequencies in small populations due to chance
What is genetic bottleneck effect
Is an extreme example of genetic drift when the size of a population is severely reduced
Advantages of greater genetic diversity
The greater genetic diversity the greater the chance an organism will survive an environmental change as the more alleles the greater probability that some individuals have alleles better suited for the environment
Apart from mutation explain how one other genetic variation within a species is increased
- Random fusion of gametes
- produces new alleles combinations
What happens during base insertion
One base is added into the sequence
