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.