Biology - Chapter 9: Genetic Diversity and Adaptation

Mutation

Any change to the quantity or the base sequence of the DNA of an organism

When do gene mutations occur?

During DNA replication

How might you disinfect a glass bottle so it is sterile?

Heat using an autoclave / steam

6 types of mutation

- Substitution
- Deletion
- Addition
- Duplication
- Inversion
- Translocation

What happens during a substitution mutation?

Replacement of a base by a different base

What happens during a deletion mutation?

One or more bases are removed

What happens during an addition mutation?

One or more bases are added

What happens during a duplication mutation?

One or more bases are repeated

What happens during an inversion mutation?

A sequence of bases is reversed

What happens during a translocation mutation?

A sequence of bases is moved from one location in the genome to another, either within the same chromosome or to a different chromosome

Why might a mutation (e.g. substitution) have no effect?

The new triplet of bases codes for the same amino acid sequence as before (because the genetic code is degenerate)

Chromosome mutations

Any changes in the structure or number of whole chromosomes

What are the two types of chromosome mutations?

1. Changes in the whole sets of chromosomes (having 3+ sets of chromosomes instead of 2 - polyploidy)
2. Non-disjunction: changes in the number of individual chromosomes (gametes have +/- 1 chromosome when homologous pairs of chromosomes do not separate in meiosis)

Why does addition, duplication and deletion mutation almost always change the amino acid sequence of a polypeptide?

They all change the number of bases in the DNA code, resulting in a frameshift

What are the two ways cell division occurs?

Mitosis (produces 2 daughter cells with same number of chromosomes as the parent cell and as each other)
Meiosis (produces 4 daughter cells with half the number of chromosomes as the parent cell)

Why does each gamete have half the number of chromosomes (haploid)?

To maintain a constant number of chromosomes in the adults of a species

What is the haploid number of chromosomes in humans?

23

What happens at fertilisation?

Two haploid gametes fuse, restoring the diploid number of chromosomes and producing new allele combinations

Describe the process of meiosis (9)

First division (meiosis I):
1. Chromosomes condense (shorten and thicken)
2. They associate in homologous pairs, forming bivalents
3. Crossing over between homologous chromosomes / chiasma forms - this produces a new combination of alleles due to alleles being exchanged
4. Independent assortment: chromosomes also separate at random
5. Spindle fibres attach to centromere
6. They then move to the equator of the cell
7. Homologous chromosomes separate, move to opposite poles

Second division (meiosis II):
8. Pairs of chromatids separated in second division
9. This produces 4 haploid cells

2 ways meiosis brings about genetic variation

1. Independent assortment of homologous chromosomes
2. New combinations of maternal and paternal alleles by crossing over (alleles exchanged between homologous chromosomes)

Homologous chromosomes

A pair of chromosomes, one maternal and one paternal, that have the same gene loci and so determine the same features

What happens during the independent segregation of homologous chromosomes? (2)

1. Homologous chromosomes pair up
2. Maternal and paternal chromosomes are re-shuffled in any combination

Formula to calculate the number of possible combinations of chromosomes for each daughter cell produced by meiosis

2^n (where n = the number of pairs of homologous chromosomes)

Formula to calculate the number of possible combinations of chromosomes when two gametes fuse during fertilisation

(2^n)^2 (where n = the number of pairs of homologous chromosomes)

When calculating the number of pairs of homologous chromosomes, what are you assuming?

The chromosomes stay intact throughout meiosis - does not occur due to crossing over of chromatids, so actually there are more possible combinations of chromosomes in gametes

Mutagenic agent

A factor that increases the rate of mutations

2 ways the genetic variation of a species is increased

Mutation and the random fusion of gametes (random fertilisation)

Explain the importance of meiosis in the life cycles of organisms which reproduce sexually (3)

1. Meiosis halves chromosome number (from diploid number to haploid number)
2. When gametes fuse, diploid number is restored (number of chromosomes constant)
3. Introduces genetic variation due to independent assortment and crossing over

Bivalent

The pair of homologous chromosomes

Describe independent assortment

Different combinations of maternal and paternal chromosomes

Meiosis I vs Meiosis II

Meiosis I: chromosomes move, independent assortment of homologous chromosomes
Meiosis II: chromatids move, independent assortment of chromatids

Explain two ways in which the behaviour of chromosomes during meiosis produces genetic variation in gametes.

1. Crossing over - different combinations of alleles / linkage groups changed
2. Independent assortment - different combinations of maternal and paternal chromosomes

Why does the chromosome number reduce when cells divide by meiosis?

So that during fertilisation when the gametes fuse, the original diploid number will be restored

Species

A group of similar organisms that can breed together to produce fertile offspring

What do all members of the same species have?

The same genes (but different combination of alleles)

Genetic diversity

The total number of different alleles in a population - it enables natural selection to occur

Population

A group of individuals of the same species that live in the same place and can interbreed

What happens as genetic diversity increases and why?

The more likely that some individuals in a population will survive an environmental change as there would be a wider range of alleles, so a wider range of characteristics and greater probability someone will have advantageous characteristic

Allele frequency

How frequently a particular allele appears in a population

Gene pool

The total number of alleles in a particular population at a specific time

What happens during natural selection? (6)

- Change in environment / random mutation occurs
- Due to mutation and different alleles, some individuals have more advantageous allele
- These individuals would be better adapted for environment so would be more likely to survive when competing against others; selective advantage
- So they're more likely to reproduce and pass on advantageous alleles to offspring
- Over many generations, number of individuals with advantageous allele increases and allele frequency of the advantageous allele increases (while non-advantageous allele frequency decreases)
- This is an example of directional selection

Selection

The process by which organisms that are better adapted to their environment tend to survive and breed, while those that are less well adapted tend not to.

Directional selection and what does it do?

A type of selection that favours individuals that vary in one direction from the mean of the population - it changes the characteristics of the population.

Stabilising selection and what does it do?

A type of selection that favours average individuals - it preserves the characteristics of the population.

What type of characteristics are more influenced by the environment: ones determined by multiple genes, or ones determined by only a single gene?

Characteristics determined by multiple genes

Describes what happens during stabilising selection (3).

1. Environmental conditions remain stable
2. So phenotypes closest to the mean are selected for, while those with phenotypes at both extremes are selected against
3. So individuals with phenotypes closer to the mean are more likely to pass their alleles to the next generation, while individuals with phenotypes at the extremes are less likely (on graph, lower number of individuals have extreme phenotypes while most have the mean)

3 types of adaptations

1. Anatomical: involve structural features of an organism's body
2. Physiological: involve processes inside an organism's body
3. Behavioural: involve ways an organism acts

What would happen if a population had a low genetic diversity?

It may not be able to adapt to a change in the environment so the whole population could be wiped out by a single event

How does genetic diversity increase in a population? (2)

- Mutations in the DNA for new alleles
- Different alleles being introduced into a population after migration (gene flow)

Give 5 examples of mutagenic agents

1. High energy radiation (X-rays, UV, gamma)
2. Mustard gas
3. Tar
4. High energy particles
5. Carcinogens

Describe how a change in chromosome number might occur

During meiosis, the homologous chromosomes do not separate - non-disjunction

2 types of factors that result in variation

Genetic and environmental factors

Explain how bivalents form

During DNA replication, two/sister chromatids join by centromere

Does crossing over occur often?

No - it is rare

What happens to genetic diversity if the number of organisms of a species decreases and why?

Reduced genetic diversity - fewer different alleles so reduced gene pool

Why are homologous chromosomes not genetically identical?

They carry different alleles

If there is a mutation in one of the cells, explain why all the cells of the body will have this mutation.

All cells are derived from the zygote by mitosis - mitosis produces genetically identical cells

What does it mean if some organisms have the same genus?

They have the same evolutionary origin / common ancestor

How can you use breeding experiments to determine whether two populations are from the same species?

Breed two organisms together - should produce fertile offspring

Give 4 reasons why a population might show very low levels of genetic diversity

1. Hunting
2. Population might've been very small - genetic bottleneck
3. Population might've started with a small number of individuals
4. Inbreeding

Why is there a time lag before species develop resistance to something? (3)

1. Initially, only few individuals have the favourable allele
2. Individuals with this allele have more offspring (as they are more likely to survive and reproduce)
3. It takes many generations for the favourable allele to become the most common allele of the gene

Why might a species be infertile?

They have an odd number / extra set of chromosomes so the homologous chromosomes do not pair or separate evenly to form haploid cells, meaning that meiosis cannot occur

Gene mutation

A change to the base sequence of DNA, resulting in new alleles forming

How does crossing over occur? (4)

1. Homologous chromosomes form a bivalent
2. Chiasma form
3. Alleles are exchanged between the chromosomes
4. This forms new combinations of alleles

Give some sources of DNA from dead animals (3)

Bone, skin, preserved remains

How does hunting affect genetic diversity? (2)

- It reduces the population size - so few alleles are left
- This then leads to inbreeding.

How does choosing specific crops to grow impact genetic diversity?

Genetic diversity is reduced - alleles have been chosen / rejected based on desired features of crop

Describe what happens during directional selection (4).

1. Environmental conditions change
2. So phenotypes at one extreme of population are selected for (organisms with that phenotype are more likely to survive and breed)
3. The organisms with the advantageous alleles survive and breed, passing on the alleles to the next generation
4. This causes the frequency of the advantageous alleles to increase

Describe and explain 5 different aseptic techniques

1. Washing hands to kill microbes / wearing gloves to prevent contamination
2. Burning Bunsen close by to create upward current of air
3. Disinfect bench to kill microbes and prevent contamination
4. Flame instrument to kill microbes and prevent contamination
5. Lift lid slightly to prevent entry of microbes

How do you produce a serial dilution? (3)

1. Add 1 part culture to 9 parts liquid (sterile liquid/water/nutrient broth)
2. Mix well
3. Repeat to create desired dilution

Why might someone choose to use a 1 in 1000 dilution instead of a 1 in 10 dilution when trying to count the number of bacterial cells?

The count with the 1 in 10 dilution would be unlikely to be accurate as there would be too many cells / the cells may overlap

What type of selection occurs if environmental conditions change?

Directional selection

What type of selection occurs if environmental conditions remain constant?

Stabilising selection

How can differences in the primary structure of proteins provide evidence of phylogenetic relationships between species? (5)

1. Mutations would result in a change to the DNA base sequence, resulting in a change to the amino acid sequence
2. The mutations build up over time
3. So there are more mutations between distantly related species and fewer mutations between more closely related species
4. Distantly related species would have an earlier common ancestor, while closely related species would have a more recent common ancestor

When nondisjunction takes place after meiosis, are any sister chromatids joined by a centromere?

No