Chapter 9 - Genetic Diversity Flashcards
Gene mutation
What is it?
A gene mutation is a change in the DNA base sequence.
/ change in the base sequence of a chromosome
How do mutations occur?
Gene utations occur spontaneously during DNA replication if DNA is misread during DNA replication.
Factors that increase risk of mutation?
Mutagenic agents/mutagens
Ultraviolet or ionising radiation, chemicals and viruses can increase the rate of mutations.
What are the two types of mutation?
Base substitution and base deletion
What is substiution?
Type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
A substitution mutation is when a base is swapped out for a different one.
This mutation may change the amino acid sequence coded for by the gene.
One amino acid is changed
What is deletion?
Why is it more harmful?
1.Deletion - One base is removed from the sequence - a nucleotide is lost from the DNA sequence
2. Deletions are much more likely to change the amino acid sequence because they cause a frameshift which means the enitre amino acid sequence will be different, not just one.
Why might a mutation not lead to a change in the amino acid sequence?
- Substiution mutation may lead to new triplet coding for the same amino acid as the original triplet because genetic code is degenerate and mutation will have no effect
- Mutation may occur in a non coding region (intron)
What are chromosome mutations?
changes to the structure or number of whole chromosomes
What are the two types of chromosome mutations?
- Polyploidy- Changes in whole sets of chromosomes
- Non-disjunction - changes in the number of indivudal chromosomes
What is polyploidy chromosome mutation?
Where is this mostly found?
When organsims have three or more sets of chromosomes rather than 2. when organisms have more than two sets of chromosomes,
Occurs mostly in plants
What is non-disjunction chromosome mutation and how does it occur?
This occurs when homologous chromosomes fail to separate in meiosis which results in gametes having either one more or one fewer chromosome, and leads to individuals with extra or missing chromosomes in all cells.
E.g Down’s syndrome
Diploid and haploid cells
What is the difference?
Haploid cells have half the amount of chromosomes than diploids
Diploid cells have a full set of chromosomes
Gamtes - sex cells are haploid cells and have 23 chromosomes
Diploid number in humans is 46
Two haploid gametes sperm and egg fuse at fertilisation and form the diploid number
What does meiosis produce?
4 daughter cells each with half the number of chromosomes as the parent cell - Meiosis produces haploid cells from diploid cells
Meiosis involves two divisions(Meiosis 1 and Meiosis 2)
Overview of what happens in each division?
Meiosis I - Homologous chromosomes are separated into different cells
Meiosis II - Chromatids move apart and are separated into four cells
Meiosis I - Prophase I, metaphase I, anaphase I, and telophase I.
Meiosis II - Prophase II, metaphase II, anaphase II, and telophase II.
Meiosis 1 - Describe process
Remember that cells have already replicated their DNA during interphase and each chromosome is made up of two identical sister chromatids.
Homologous chromosomes pair up and thier chroamtids wrap around each other.
Equivalent portions of these chromatids may be exh=changed in crossing over
Chromosomes line up along the equator of the cell in their homologous pairs (so in humans, 23 pairs line up).
Each chromosome attaches to the spindle by their centromere.
Homologous chromosome pairs are separated and pulled to opposite poles of the cell (chromatids stay joined together).
Cell divides into two, one chromosome from each pair into one of the two daughter cells
Meiosis 2 -Describe process
The sister chromatids move apart and at the end of meiosis four cells have been formed, each containing 23 chromosomes in humans
Compare Mitosis and Meiosis
How do they differ?
Mitosis produces 2 daughter cells genetically identical, Meiosis produces 4 genetically different daughter cells
Mitosis 1 division, Meiosis 2 divisions
Functions: Mitosis growth and repair, Meiosis production of gametes
Meiosis and genetic variation
Two main roles of meiosis?
Production of haploid gametes - This allows sexual reproduction to take place.
Creates genetic variation - This increases diversity, allowing natural selection to take place.
What are the two events within mitosis that lead to genetic variation?
Crossing over (or recombination)
Independent segregation (or random assortment)
Independent segregation occurs During metaphase I
What is it ?
How does it increase genetic variation?
It occurs when homologous pairs of chromosomes line up randomly
whether the paternal or maternal chromosomes appears on the left or right is completely random. As a result, which chromosomes end up in each daughter cell is also random.
Independent segregation increases genetic variation by producing gametes with different combinations of alleles
Relationship between pairs of homologous chromosomes and possible chromosome combinations
Formula for possible chromosome combinations?
As pairs of homologous chromosomes increases, the possible chromosome combinations increases too
2^n where n is the number of pairs of homologous chromosomes
So an organism with 4 homologous pairs can produce 2^4 or 16 possible diff combinations of chromosomes in its daughter cells as a result of meiosis
Crossing over when does it occur?
What is genetic recombination by crossing over?
How does this increase genetic variation?
Occurs during prophase one when each chromosome lines up with its homologousnpartner
chromatids from homologous chromosomes twist around each other. during this twisting, tensions are created and portions of the chromatids break off and these broken portions of each chromatid are exchanged - rejoin with the chromatids of its homologous partner
It is this way that new genetic combinations of maternal and paternal alleles are produced (this produces a new combination of alleles in each chromosome. )
Random fertilisation occurs after meiosis
What is it?
How does it increase genetic variation?
As each gamete contains half the normal number of chromosomes (n), the resulting zygote cell contains the diploid number of chromosomes (2n).
Half of these chromosomes are from the father’s sperm cell and half are from the mother’s egg cell.
Gametes from male and female pair at random, combines different alleles and so provides even more variety.
Formula for the different combinations of chromosomes in thee offspring procued as the result of sexual reproduction
(possible chromosome combinations after random fertilisation)
(2^n)^2 where n is the number of pairs of homologous chromosomes
squared to account for the combination of two gametes.
2^n x 2^n
Genetic diversity and adaptation
Define genetic diversity
The total number of different alleles in a population
What is a population?
All of the organisms of a particular species that live in the same place and can interbreed
What is a gene pool?
The total number of alleles in a particular population at a particular time
How does higher genetic diversity affect natural selection?
The greater the genetic diversity, the more likely that some individuals within a population will survive an environmental change
- as organisms with advantageous traits are more likely to survive, reproduce, and pass on their alleles, influencing the gene pool. The more successful an organism is at reproducing, the more likely its alleles will be passed on to the next generation, increasing the allele frequency.
Define allele frequency
The number of times an allele occurs within the gene pool
Describe the process of natural selection
-There is variation in characteristics within a species.
-More genetic variation emerges within a population due to random mutations.
-Individuals with alleles that code for traits that are advantageous for survival are more likely to reproduce.
-These advantageous alleles are passed down to offspring.
-Over time, these beneficial alleles become more common in the population.
-This mechanism leads to populations becoming more adapted to their environment over generations.
What are the 3 types of selection?
(only first 2 are needed )
Directional selection
Stablising selection
(Disruptive selection) not required
Directional selection - What is it and how does the graph shift
Favours extreme phenotypes, favours individuals that vary in one direction from the mean of the population - changes the characteristics of a population
Shifts curve in the direction of the favoured extreme
Define phenotype
A phenotype is the expression of the genotype and its interaction with the environment.
What is stabilising selection?
How does it shift the graph?
Stabilising selection is a type of natural selection in which the intermediate phenotype is favoured. - preserves characteristics of a population
Narrows the curve
Example of directional selection
Antibiotic resistance in bacteria - where the resistant bacteria survive and reproduce.
-Some bacteria develop random mutations that provide resistance to antibiotics.
-When antibiotics are used, only the resistant bacteria survive, while the others die off.
-The resistant bacteria reproduce, passing on resistant alleles to their offspring.
-Over time, the proportion of resistant alleles increases, leading to mostly resistant bacteria.
Example of stabilising selection
Human birth weight, where infants of average weight have higher survival rates
What is an adaptation?
Adaptations are inherited characteristics that enhance an organism’s ability to survive and reproduce in its specific environment.
What are 3 types of adaptations
Anatomical - changes to body structure- Physical structures, both internal and external.
Behavioural - changes to actions- Activities and responses, whether inherited or learned.
Physiological - changes to bodily processes- Internal biological functions.
Examples of anatomical adaptations
Anatomical adaptations are the physical attributes that aid an organism’s survival.
- Shorter ears, thicker fur in arctic foxes compared to foxes in warmer climates
Body coverings - Fur, feathers, scales protect organisms and support their survival.
Camouflage - Organisms with colouration that blends into their environment are less likely to be detected.
Mimicry - Some species imitate the appearance of more dangerous ones for protection.
Teeth - The shape of an animal’s teeth are adapted to its diet.
Examples of behavioural adaptations
Behavioural adaptations are actions that increase an organism’s chances of survival and reproduction.
Hibernation or migration
Defensive responses - For example, opossums play dead and rabbits freeze to avoid detection by predators.
Courtship displays - For example, male scorpions dance to attract mates.
Seasonal actions - For example, migration helps birds access resources year-round and hibernation helps bears conserve energy when food is scarce.
Examples of physiological adaptations
Physiological adaptations are internal processes that give organisms an edge in survival.
venom production
Venom - Used by snakes to immobilise prey and by plants to deter herbivores.
Antibiotics - Produced by bacteria to outcompete rival species.
Water storage - Desert frogs can survive for over a year without a water source by storing water in their bodies.
