Topic 4 - Diversity and Selection

Question 1

what is the diploid number?

Answer 1

• full number of chromosomes (2n)
• each cell contains 2 of each chromosome (a pair) - one from the mum and one from the dad
• diploid number in humans = 46 (23 homologous pairs)

Question 2

what are gametes?

Answer 2

• sperm cells in males
• eggs in females
• have haploid number of chromosomes (only contain one copy of each chromosome in homologous pair)
• haploid no. in humans = 23

Question 3

what happens to gametes in sexual reproduction?

Answer 3

• fuse together at fertilisation to form a zygote
• which, divides and develops into a new organisms

Question 4

what happens during fertilisation?

Answer 4

• at fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes
• half of these are from the father (the sperm), and half are from the mother (the egg)
• during sexual reproduction, any sperm can fertilise any egg - random
• random fertilisation produces zygotes with diff. combinations of chromosomes to both parents
• this mixing of genetical material in sexual reproduction increases genetic diversity within a species

Question 5

describe meiosis

Answer 5

• type of cell division
• takes place in reproductive organs of multicellular, eukaryotic organisms
• original cell = diploid which then splits to form haploid

Question 6

describe the process of meiosis

Answer 6

1. before meiosis starts, DNA unravels and replicates so 2 copies of each chromosomes, called chromatids
2. DNA condenses to form double armed chromosomes, each made from 2 sister chromatids. the sister chromatids are joined via the centromere
3. meiosis I (first division) - chromosomes arrange themselves in homologous pairs
4. these homologous pairs are then separated, halving the chromosome number
5. meiosis II (second division) - the pairs of sister chromatids that make up each chromosome are separated (centromere is divided)
6. four haploid cells that are genetically diff. from each other are produced

Question 7

what are the 2 ways of creating genetic variation in gametes?

Answer 7

1. crossing over of chromatids
2. independent segregation of chromosomes

Question 8

describe how crossing over of chromatids creates genetic variation in gametes

Answer 8

• during meiosis I, homologous chromosomes come together and pair up
• chromatids twist around each other and bits of chromatids swap over
• chromatids still contain the same genes but now have a diff. combination of alleles
• the crossing over of chromosomes in meiosis I means that each of the 4 daughter cells formed from meiosis II contain chromatids with diff. alleles

Question 9

describe how independent segregation of chromosomes creates genetic variation in gametes

Answer 9

• each homologous pair of chromosomes in your cells is made up of one chromosome from your mum (maternal) and one chromosome from your dad (paternal)
• when the homologous pairs are separated in meiosis I, it’s completely random which chromosome from each pair ends up in which daughter cell
• so 4 daughter cells produced by meiosis have completely diff. combinations of those maternal and paternal chromosomes - called independent segregation (separation) of chromosomes
• this shuffling of chromosomes leads to genetic variation in any potential offspring

Question 10

what are the differences between mitosis and meiosis?

Answer 10

mitosis
- produces cells with the same no. of chromosomes as the parent cell
- daughter cells are genetically identical to each other and to the parent cell
- produces 2 daughter cells
meiosis
- produces cells with half the no. of chromosomes as the parent cell
- daughter cells are genetically diff. from one another and the parent cell
- produces 4 daughter cells

Question 11

describe gene mutations

Answer 11

• involve a change in DNA base sequence of chromosomes
  types of errors:
• substitution - one base substituted with another
• deletion - one base is deleted
• the order of DNA bases in a gene determines the order of amino acids in a particular protein
• if a mutation occurs in a gene, the sequence of amino acids its codes for (and the protein formed) could be altered

Question 12

what are some of the effects of mutations?

Answer 12

• degenerate nature of genetic code means some amino acids are coded for by >1 DNA triplet
• meaning, not all substitution mutations will result in a change to the amino acid sequence of the protein - some substitutions will still code for the same amino acid
• deletions definitely change the amino acid sequence as it changes the no. of bases present, causing a shift in all the base triplets after it (frame shift)

Question 13

describe mutagenic agents and give examples

Answer 13

• things that cause an increase in the rate of mutations
• e.g. UV radiation, ionising radiation, some chemicals and some viruses
• increase the probability of a mutation occuring

Question 14

describe chromosome mutations

Answer 14

• when meiosis goes wrong, and cells produced contain variations in the no. of whole chromosomes or parts of chromosomes = chromosome mutation and is caused by errors in meiosis
• chromosome mutations lead to inherited conditions because the errors are present in the gametes (hereditary cells)
• one type of chromosome mutation = chromosome non-disjunction (failure of the chromosomes to separate properly
  e.g. Down’s syndrome

Question 15

definition of genetic diversity

Answer 15

• number of diff. alleles in a species or population
• lots of alleles = large variety of diff. characteristics = high genetic diversity
• allows natural selection to occur because some alleles are more advantageous than others

Question 16

why is genetic diversity important?

Answer 16

if a population has a low genetic diversity, it might not be able to adapt to a change in the environment and the whole population can be wiped out by a single event (e.g. disease)

Question 17

how can genetic diversity within a population be increased?

Answer 17

• mutations in DNA = new alleles (some can be advantageous whilst some can lead to problems)
• diff. alleles being introduced into a population when individuals from another population migrate into it and reproduce (this is called gene flow)

Question 18

what are genetic bottlenecks?

Answer 18

• an event that causes a big reduction in the population (e.g. when a large no. of organisms die without reproducing)
• this reduces no. of diff. alleles in a gene pool
• meaning, reduction in genetic diversity
• the survivors reproduce, and a larger population is created from few individuals

Question 19

describe the founder effect

Answer 19

• describes what happens when just a few organisms from a population start a new colony and there are a small no. of diff. alleles in the initial gene pool
• frequency of each allele in the new colony might be very diff. to the frequency of those alleles in the original population
• may lead to a higher incidence of genetic disease
• can occur as a result of migration leading to geographical separation or if a colony is separated from the original population for another reason (e.g. religion)

Question 20

define natural selection

Answer 20

• when an allele codes for a specific characteristic which increases an organism’s chance of survival therefore, increasing the frequency of the population
• the fitter individual, who is better adapted to the environment, survives and passes on the advantageous alleles to future generations

Question 21

how does natural selection work?

Answer 21

1. there’s differential reproductive success in a population - individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial allele), than individuals with less advantageous alleles
2. means a greater proportion of the next generation inherits the beneficial allele
3. therefore, are more likely to survive, reproduce and pass on their genes
4. so the frequency of the beneficial allele in the population increases from generation to generation
5. over generations, this leads to evolution as the advantageous alleles have become more common in the population

Question 22

define evolution

Answer 22

process where frequency of alleles in a gene pool changes over time due to natural selection

Question 23

what are adaptations?

Answer 23

• features that help organisms to survive in their environment
• can be behavioural, physiological or anatomical

Question 24

describe behavioural adaptations

Answer 24

• changes in behaviour which improve an organisms chance of survival
  (e.g. playing dead)

Question 25

describe physiological adaptations

Answer 25

• processes inside an organism’s body which increase its chance of survival
  (e.g. hibernation to lower rate of metabolism over winter to conserve energy so they don’t need to look for food in the months when its scarce - increasing their chance of survival)

Question 26

describe anatomical adaptations

Answer 26

• structural adaptations (external or internal)
• structural features of an organism’s body that increase its chance of survival
  (e.g. whales have blubber (fat) which helps them to keep warm in the cold sea increasing their chance of survival in places where their food is found)

Question 27

what is meant by a niche of a species?

Answer 27

• its role within the environment
• species that share the same niche compete with each other and a better adapted species survive
• the idea that a better adapted species survive is the basis of natural selection

Question 28

name 2 types of natural selection

Answer 28

• directional selection
• stabilising selection

Question 29

describe directional selection

Answer 29

• when environmental conditions change and phenotypes best suited to the new conditions are more likely to survive
• meaning, these individuals breed and produce offspring
• overtime, the mean of the population will move in the direction of individuals

Question 30

describe stabilising selection

Answer 30

• phenotypes with successful characteristics are preserved and those of greater diversity are reduced
• this selection doesn’t occur due to a change in the environment
• if the environment stays the same, the individuals closest to the mean are favoured because they have the alleles that have given them the survival advantage
• furthest from the mean are selected against

Question 31

what are aseptic techniques

Answer 31

• used to prevent contamination of cultures by unwanted microorganisms important becayse contamination can affect the growth of the organsims that you are workng with

Question 32

describe some aseptic techniques

Answer 32

• regularly disinfect work surfaces to minimise contamination
• suer sterile equipment and discard safely after use
• work near a Bunsen flame (hot air rises, so microbes in the fair are drawn away from your plate
• minimise time spent with lid off the gar plate, to reduce the chance of airborne microorganisms contaminating the culture
• briefly flame the neck of the glass container of broth just after its opened and just after its close - causes air to move out of container, preventing unwanted organisms falling in