Chapter 9 - Genetic diversity and mutation Flashcards
What is a gene mutation?
Any change to the quantity or the base sequence of the DNA of an organism
What is a substitution mutation?
When one base is substituted for another
Why are mutations bad?
The order of DNA bases in a gene determines the amino acid sequence which codes for a certain protein. If this is altered, it may lead to the protein lacking characteristics that are important for its role
What is a deletion mutation?
One base is deleted from the DNA sequence
Why do deletion mutations normally cause a large change to the organism?
Every base after the one that has been deleted experiences a frameshift and moves one to the left. Because bases are read in triplets, the codons aren’t read correctly
What will adding a base result in?
A frame-shift to the right
What is a silent mutation?
Because the DNA code is degenerate, a substitution mutation may result in the same amino acid being coded for and so have no effect on the organism
What is a mutagenic agent?
Something that increases the likelihood of a mutation, e.g. UV radiation and ionising radiation
What are chromosome mutations?
Spontaneous changes in the number of or structure of whole chromosomes
What are the two forms of chromosome mutations?
Changes in whole sets of chromosomes
Changes in the number of individual chromosomes
When will changes to whole sets of chromosomes occur?
When organisms have three or more sets of chromosomes rather than the normal two
What is a change to a whole set of chromosomes called?
Polyploidy and it occurs mainly in plants
What is non-disjunction?
When homologous pairs of chromosomes fail to separate during meiosis. This leads to changes in the number of individual chromosomes and sometimes syndromes like Down’s
What causes Down’s syndrome?
An extra copy of chromosome 21
How many daughter cells are produced from meiosis?
4
How many divisions take place during meiosis?
2
*What is a haploid number?
The number of chromosomes gametes have - half of the normal amount
*What is the diploid number?
The full set of 46 chromosomes
*What happens during the first division of meiosis?
Homologous chromosomes pair up and their chromatids wrap around each other
One chromosome from each pair goes into one of the daughter cells
*What happens during the second division of mitosis?
The centromere is divided and the sister chromatids move to each end of the cell
*What is a gene?
A length of DNA that codes for a polypeptide
*What is the locus?
The position of a gene on a chromosome or DNA molecule
*What is an allele?
Different forms of a gene
*What are homologous chromosomes?
One maternal and one paternal chromosome which have the same gene loci
*Are daughter cells genetically different in meiosis or mitosis?
Meiosis
*Process of meiosis
DNA unravels and replicates so there are 2 copies of each chromosome
The DNA forms two chromosomes, made from 2 chromatids joined at the centromere
First division - chromosomes form homologous pairs which separate, halving the chromosome number
Second division -the centromere divides to separate the sister chromatids making up each chromosome
Four genetically different, haploid cells are produced
*What is independent segregation?
Each homologous pair of chromosomes comprises of one paternal and one maternal chromosome
when the pair separates, it is completely random which chromosomes end up in which daughter cell
The four daughter cells will have completely different combinations of maternal and paternal chromosomes
When does independent segregation occur?
When the homologous pairs are separated during meiosis I
What is the diploid number?
The full number of chromosomes
What is the haploid number?
Half the number of chromosomes
During which process will homologous chromosomes pair and separate?
Meiosis
What is another, less obvious way in which genetic variation is induced?
Each haploid gamete has a very different genetic makeup and when two of these fuse randomly at fertilisation, variety is seen in the offspring
What is crossing over of chromatids?
When homologous pairs of chromosomes pair up, the chromatids twist around each other and parts of the chromatids break off due to the tension
These broken parts of chromatids usually rejoin with the chromatids of its homologous partner
This creates genetic variation in the maternal and paternal chromosomes
The chromosomes have the same genes but have a different combination of alleles
What is chiasmata?
The point at which the chromosomes snap and swap alleles
What is recombination?
When the broken off sections of chromatids join with another chromatid
What is the formula for the possible number of chromosomes in each daughter cell?
2^n, where n is the number of pairs of homologous chromosomes
What is the formula for the number of chromosomes in each daughter cell, taking into account the random pairing of gametes?
(2^n)^2
What is an allele?
A different form of a gene
What is genetic diversity?
The total number of different 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
When is genetic diversity reduced?
When a population has fewer alleles
Process of natural selection
There will be a gene pool containing a wide variety of alleles in any population
Random mutations may result in a new allele
In certain environments, this could give the possessor an advantage
They will be better adapted to survive competition
They will live longer and grow more healthily, meaning they have a higher chance of producing offspring
As these new individuals are likely to have the new allele, they are more likely to survive and reproduce successfully too
Over time, the proportion of the population with this allele will increase
What is a genetic bottleneck?
An event that causes a big reduction in a population
What effect does a genetic bottleneck have?
It reduces the number of alleles in the gene pool and so reduces genetic diversity
What is the founder effect?
Just a few organisms from a population start a new colony with a very small gene pool
Why might the founder effect lead to an increase in genetic diseases?
The frequency of each allele in the new colony may be different from that of the original colony - e.g. rare alleles could become more common
Why could the founder effect occur?
Migration leads to geographical separation
What is directional selection?
The population mean will move towards one extreme
What is stabilising selection?
The population mean will move more towards the average
What is disruptive selection?
The population will have two extremes that have large groups and less in the average
What will disruptive selection lead to?
Speciation
What does directional selection do?
Changes the characteristics of a population
What does stabilising selection do?
Maintains the characteristics of a population
How does directional selection work, for example in bacteria?
Some individuals will fall either left or right of the mean and possess a phenotype which is well suited to the conditions - e.g. the ability to make the enzyme penicillinase which breaks down penicillin
These will be more like to survive and breed
The bacteria that survive divide by binary fission to build a small population that was resistant
This population was more likely to survive and breed in the presence of penicillin
The number of the resistant population increased and the frequency of the advantageous allele did too
This lead to the distribution curve shifted to the population having a greater resistance to penicillin
How does stabilising selection work, for example, with babies’ birth weights?
Humans have a range of birth weights
Very small babies are less likely to survive because they can’t maintain their body temperature
Very large babies are less likely to survive because they are difficult to birth
Conditions are more favourable for middle-sized babies, so the weight of human babies tends to shift towards the middle
With what type of selection is the environment not changing?
Stabilising
What are the three types of adaptations?
Behavourial
Physiological
Anatomical
What are behavioural adaptations?
Ways that an organism acts that increase its chance of survival and reproduction, e.g. swallows migrating to Africa to avoid food shortages in the UK in winter
What are physiological adaptations?
Processes inside an organism’s body that increase its chance of survival and reproduction, e.g. brown bears lowering their metabolism when they hibernate which conserves energy so they don’t have to hunt for food
What are anatomical adaptations?
Structural features of an organism’s body which increase its chance of survival and reproduction, e.g. whales having a thick layer of blubber to keep them warm in the sea
Why are some breeds of animals sterile?
They have an odd number of chromosomes so they can’t pair during meiosis to produce gametes
