Topic 4B - Diversity, Classification And Variation

Question 1

How is DNA from one generation passed onto the next?

Answer 1

Gametes are the sperm cells in males and egg cells in females. They join together at fertilisation to form a zygote, which divides and develops into a new organism. Normal body cells have 2n, each cell contains two of each chromosome (one from the mum and one from the dad). Gametes have n. At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. During sexual reproduction, random fertilisation produces zygotes with different combinations of chromosomes to both parents. The mixing of genetic material increases genetic diversity within a species.

Question 2

What is the importance of meiosis?

Answer 2

A diploid number of chromosomes will be restored when haploid gametes fuse. Genetic variation.

Question 3

What happens before and during meiosis?

Answer 3

Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome. The DNA condenses to form double-armed chromosomes, each made from two sister chromatids. Meiosis 1 occurs and the chromosomes arrange themselves into homologous pairs, which are separated. Meiosis 2 is when the pairs of sister chromatids are separated (the centromere is divided). Four haploid cells are produced.

Question 4

What is crossing over?

Answer 4

In meiosis 1, homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of chromatid swap over, so the chromatids have the same genes but different combinations of alleles.

Question 5

What are the two processes that produce genetically different cells in meiosis?

Answer 5

Crossing over of chromatids in meiosis 1 means that each of the four daughter cells contain chromatids with different alleles. Independent segregation of chromosomes - each homologous pair is made up of one chromosome from your mum and one from dad. When the homologous pairs are separated in meiosis 1, it’s completely random which chromosome from each pair ends up in which daughter cell. So the four daughter cells have completely different combinations of maternal and paternal chromosomes.

Question 6

What are the differences between meiosis and mitosis?

Answer 6

Mitosis produces cells with the same number of chromosomes as the parent cell, but meiosis produces cells with half the number of chromosomes. In mitosis, daughter cells are genetically identical to each other and to the parent cell, but in meiosis the daughter cells are genetically identical. Mitosis produces 2 daughter cells, meiosis produces 4. Mitosis has one division but meiosis has two divisions. Mitosis has genetically identical, meiosis genetically varied.

Question 7

What causes chromosome mutations?

Answer 7

When meiosis works properly, all four daughter cells will end up with 23 whole chromosomes - one from each homologous pair. But sometimes meiosis goes wrong and the cells contain variations in the number of whole chromosomes or parts of chromosomes. This is called chromosome mutations and is caused by errors during meiosis. Chromosome mutations lead to inherited conditions because the errors are present in the gametes. One type of chromosome mutation is called non-disjunction - it’s a failure of the chromosomes to separate properly.

Question 8

What causes Down’s syndrome?

Answer 8

A person having an extra copy of chromosome 21 (or part of chromosomes 21).
Non-disjunction means that chromosome 21 fails to separate properly during meiosis, either in meiosis 1 or 2, so one cell gets an extra copy of 21 and another gets none. When the gamete with the extra copy fuses to another gamete at fertilisation, the resulting zygote will have three copies of chromosome 21.

Question 9

What are mutations?

Answer 9

Change in the DNA base sequence of chromosomes. E.g substitution - one base is substituted with another, or deletion. If a mutation occurs in a gene, the sequence of amino acids it codes for could be altered.

Question 10

Why don’t all mutations affect the order of amino acids?

Answer 10

The degenerate nature of the genetic code means that some amino acids are coded more than one DNA triplet. This means that not all substitution mutations will result in a change to the amino acid sequence of the protein. Deletion will always lead to changes in amino acids, because it will change the number of bases present, which will cause a shift in all the base triplets after it.

Question 11

What are mutagenic agents?

Answer 11

Cause an increase in rate of mutations, such as ultraviolet radiation, ionising radiation, some chemical and some viruses.

Question 12

What causes genetic diversity?

Answer 12

Genetic diversity is the number of different alleles of genes in a species or population. It is increased by mutations in the DNA - forming new alleles. Different alleles being introduced into a population when individuals from another population migrate into them and reproduce. This is known as gene flow.
Genetic diversity is what allows natural selection to occur.

Question 13

What reduces genetic diversity?

Answer 13

A genetic bottleneck is an event that causes a big reduction in a population e.g when a large number of organisms within a population die before reproducing. This reduces the number of different alleles in the gene pool and so reduces genetic diversity. The survivors reproduce and a larger population is created from a few individuals.

Question 14

What is natural selection?

Answer 14

Randomly-occurring mutations sometimes result in a new allele being formed. Some mutations can produce alleles that are beneficial to an organism, helping the organism to survive in certain environments. Individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes. This means a greater proportion of the next generation inherits the beneficial allele. So they are more likely to survive, reproduce and pass on their genes. So the frequency of the beneficial allele increases from generation to generation. Over generations this leads to evolution as the advantageous alleles become more common in the population. Evolution is the gradual change of species over time. It has led to the huge diversity of living organisms on Earth.

Question 15

How does natural selection lead to populations becoming better adapted?

Answer 15

Behavioural adaptations - possums play dead if they’re threatened.
Physiological adaptations - brown bears hibernate over winter so they lower their rate of metabolism (chemical stuff)
Anatomical adaptations - structural features that increase its chance of survival e.g whales have a thick layer of blubber.

Question 16

What is directional selection?

Answer 16

Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce. This could be in response to an environmental change. Bacteria evolving antibiotic resistance is an example. Some individuals in a population have alleles that give them resistance to an antibiotic. The population is exposed to the antibiotic, killing bacteria without the resistant allele. The resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring. After some time, most organisms in the population will carry the antibiotic resistance allele.

Question 17

What is the founder effect?

Answer 17

It is a type of genetic bottleneck that describes what happens when just a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool. The frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population - for example, an allele that was rare in the initial population might be more common in the new colony. This may lead to a higher incidence of genetic disease. The founder effect can occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason, such as religion.

Question 18

What is stabilising selection?

Answer 18

Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn’t changing, and it reduces the range of possible characteristics. E.g human birth weight.

Humans have a range of birth weights. Very small babies are less likely to survive - party because they find it hard to maintain their body temperature. Giving birth to large babies can be difficult, so large babies are less likely to survive too. Conditions are most favourable for medium-sized babies - so weight of human babies tends to shift towards the middle of the range.

Question 19

How do you test the effects of antibiotics using agar plates?

Answer 19

The bacteria you will use are likely to have been grown in a liquid broth (a mixture of distilled water, bacterial culture and nutrients). Use a sterile pipette to transfer the bacteria from the broth to an agar plate (a Petri dish containing agar jelly). Spread the bacteria over the plate using a sterile plastic spreader. Use sterile forceps to place paper discs soaked with different antibiotics spaced apart on the plate. Make sure you add a negative control disc soaked only in sterile water. Lightly tape a lid on, invert, and incubate the plate at about 25°C (to prevent condensation) for 48 hours. This allows the bacteria to grow (forming a lawn). Anywhere the bacteria can’t grow can be seen as a clear patch in the lawn of bacteria, the zone of inhibition. The size of the inhibition zone tells you how well an antibiotic works. The larger the zone, the more bacteria were inhibited from growing.

Question 20

What are aseptic techniques and why are they necessary?

Answer 20

They prevent contamination of cultures by unwanted organisms, which is important because contamination can affect the growth of the microorganism that you’re working with. Contamination with disease-causing microbes could make you ill. Regularly disinfect work surfaces to minimise contamination. Don’t put any utensils on the work surface. Contaminated utensils should be placed in a beaker of disinfectant. Use sterile equipment and discard safely after use e.g glassware can be sterilised before and after use in an autoclave (which steams equipment at a high pressure). Pre-sterilised plastics instruments are used once, then discarded. Work near a Bunsen flame. Hot air rises, so any microbes in the air should be drawn away from your culture. Minimise the time spent with the lid off the agar plate, to reduce the chance of airborne microorganisms contaminating the culture. Briefly flame the neck of the glass container of broth just after it’s opened and just before it’s closed - this causes air to move out of the container, preventing unwanted organisms from falling in. Wash hands thoroughly before and after handling cultures.

Question 21

What is phylogeny?

Answer 21

The study of the evolutionary history of groups of organisms. Phylogeny tells us who’s related to whom and how closely related they are. All organisms have evolved from shared common ancestors. This can be shown on a phylogenetic tree. Closely related species diverged away from each other most recently. E.g humans and chimpanzees are closely related, as they diverged very recently and their branches are close together.

Question 22

What is taxonomy? Classification?

Answer 22

Naming organisms and organising them into groups, which makes it easier to identify and study them. Scientists now take into account phylogeny when classifying organisms, and group organisms according to their evolutionary relationships. There are eight levels of groups, each group is taxon, many are taxa. The groups are arranged in a hierarchy, with the largest groups at the top and the smallest at the bottom. Organisms can only belong to one group at each level in the hierarchy- there’s no overlap. Organisms are first sorted into three large groups (or taxa) called domains - the eukarya, bacteria and archaea. Related organisms in a domain are then sorted into slightly smaller groups called kingdoms. Domain, Kingdom, phylum, class, order, family, genus, species. As you move down the hierarchy, there are more groups at each level but fewer organisms in each group. The organisms in each group also become more closely related. Species only contain one type of organism - a group of similar organisms able to reproduce to give fertile offspring. Scientists constantly update classification systems because of discoveries about new species and new evidence about known organisms e.g DNA sequence data.

Question 23

What is the binomial naming system?

Answer 23

The first part is the genus and has a capital letter. The second part is the species and begins with a lower case.

Question 24

What is the importance of courtship behaviour?

Answer 24

Is carried out by organisms to attract a mate of the right species. It can be simple e.g releasing chemicals or complex like a series of displays. Courtship behaviour is species specific, to prevent interbreeding and making reproduction more successful. Courtship behaviour can be used to classify organisms. The more closely related, the more similar their courtship behaviour. E.g
Fireflies give off pulses of light. The pattern of flashes is specific to each species. Crickets make sounds that are similar to morse code, the code being different for different species. Male peacocks show off their colourful tails. Make butterflies use chemicals to attract females.

Question 25

What are some technologies that have been useful to clarifying evolutionary relationships?

Answer 25

Genome sequencing - entire base sequence of an organism’s DNA can be determined. The DNA base sequence of one organism can then be compared to the DNA base sequence of another organism, to see how closely they related they are. Closely related species will have a higher percentage of similarity in their DNA base order. Comparing amino acid sequence - proteins are made of amino acids. The sequence of amino acids in a protein is coded for by the base sequence in DNA. Related organisms have similar DNA sequences and so similar amino acid sequences in their proteins. Immunological comparisons - similar proteins will also bind the same antibodies.

Question 26

How have gene technologies changed the way genetic diversity is assessed?

Answer 26

Early estimates of genetic diversity were made by looking at the frequency of measurable or observable characteristics in a population. Since different alleles determine different characteristics, a wide variety of each characteristic in a population indicates a high number of different alleles - and so a high genetic diversity. However gene technologies have no been developed that allow us to measure genetic diversity directly. For example, different alleles of the same gene will have slightly different DNA base sequences. Comparing the DNA base sequences of the same gene in different organisms in a population allows scientists to find out how many alleles of that gene there are in that population. Different alleles will also produce slightly different mRNA base sequences, and may produce proteins with slightly different amino acid sequences, so these can be compared. These new technologies can all be used to give more accurate estimates of genetic diversity within a population or species. They also allow the genetic diversity of different species to be compared more easily.

Question 27

How can variation occur?

Answer 27

Variation is the differences that exists between individuals. There’s variation between species and within species. Variation can be caused by genetic factors. Different species have different genes, which causes variation between species. Individuals of the same species have the same genes, but different alleles, which causes variation within a species. Variation within a species can also be caused by differences in the environment e.g climate, food, lifestyle. Most variation is caused by a combination.

Question 28

How do you study variation?

Answer 28

You only look at a sample of the population, not the whole thing, as it would be time-consuming or not possible. Samples are used as models for the whole population. The sample but accurately represent the whole population and any pattern observed are tested to make sure they’re not due to chance. Sample should be random, so if you were looking at plant species in a field you could pick random sample sites by dividing the field into a grid and using a random number generator to select coordinates. To ensure any variation observed isn’t just to chance, you have to analyse the results statistically.

Question 29

How do you use the mean?

Answer 29

The mean can be used to tell if there is variation between samples. Most samples will include values either side of the mean, so you end up with a bell-shaped graph - this is called a normal distribution. A normal distribution is symmetrical about the mean.

Question 30

What does standard deviation tell you?

Answer 30

How much the values in a single sample vary. It’s measure of the spread of values about the mean. A large standard deviation means the values in the sample vary a lot.

Question 31

What is biodiversity, habitat, and community?

Answer 31

Biodiversity is the variety of living organisms in area. Habitat is the place where an organism lives. Community is all the populations of different species in a habitat. Areas with a high biodiversity are those with lots of different species.

Question 32

How can biodiversity be considered at different levels?

Answer 32

Local biodiversity - variety of different species living in a small habitat that’s local to you.
Global biodiversity - you could also consider the variety of species on Earth.

Question 33

How can biodiversity be measured?

Answer 33

Species richness is a measure of the number of different species in a community. It can be worked out by taking random samples of a community and counting the number of different species. Species richness is also a simple measure of biodiversity. But the number of different species in a community isn’t the only thing that affects biodiversity. The population sizes of those species do too. Species that are in a community in very small numbers shouldn’t the treated the same as those with bigger. An index of diversity is another way of measuring biodiversity, by taking number of species in a community (species richness) and the abundance of each species (population sizes) into account.

D = N(N-1)/ sum n(n-1)
N is total number of organisms
n is total number of organisms of one species
The higher the number, the more diverse the area is. If all the individuals are of the same species, the index is 1.

Question 34

What are some agricultural practices that reduce biodiversity?

Answer 34

Woodland clearance - this is done to increase the area of farmland. It directly reduces the number of trees and sometimes the number of different tree species. It also destroys habitats, so some species could lose their shelter and food source. This means that species will die or be forced to migrate to another suitable area, further reducing biodiversity.
Hedgerow removal - this is also done to increase the area of farmland by turning lots of small fields into fewer large fields.
Pesticides - chemicals that kill organisms (pests) that feed on crops. Also, any species that feed of the pests will lose a food source.
Herbicides - these are chemicals that kill weeds. Reduces plant diversity and number of organisms that feed on the weeds.
Monoculture - fields with one type of plant reduces biodiversity and support fewer organisms.

There has to be a balance between agriculture and conservation. Conservationists try to protect biodiversity. Some examples of conservation schemes are:
Giving legal protection to endangered species. Creating protected areas such as SSSIS (sites of special scientific interest) and AONBs (areas of outstanding natural beauty). These restrict further development, including agricultural development.
The environment stewardship scheme which encourages farmers to conserve biodiversity, by replanting hedgerows and leaving margins around fields for wild flowers to grow.