Mrs H bio 10 Genetic Diversity Flashcards
What is meiosis used by and for?
Used by diploid organisms to produce gametes for sexual reproduction
What does meiosis produce?
4 genetically different daughter cells
What occurs to do with the gametes during sexual reproduction?
Gametes fuse to form a zygote which then divided repeatedly by mitosis to form the adult organism
How many chromosomes would the zygote have us it was diploid?
Double the number of chromosomes
What occurs before meiosis?
The DNA is replicated in S phase (interphase) of the cell cycle. The chromosomes then go through 2 divisions. Each division has a prophase, metaphase, anaphase and telophase
What happens in the first meiotic division?
The homologous chromosomes pair up. Crossing over occurs. By the end of the first division, the homologous pairs are seperated, with one chromosome from each pair going into one of the daughter cells
What happens in the seconds division of meiosis?
The chromatids are separated. At the end of meiosis 2, four cells have usually been formed. In humans each of these cells contains 23 chromosomes
what three processes in meiosis introduce variation between the daughter cells?
crossing over between sister chromatids during prophase one,
independent assortment of homologous chromosomes during metaphase one
both of these result in different combinations of alleles in gametes, thus creating genetic variation
also, random fertilisation
what is crossing over?
during prophase the non-sister chromatids exchange alleles between homologous pairs forming a chiasmata
what is the crossing points of two non-sister chromatids called?
chaismata
how do chromatids swap alleles during crossing over?
entanglement places stress on the DNA molecules and as a result a section of chromatid from one chromosome may break and re-join with the chromatid from the other chromosome
why is the swapping of alleles significant?
it can result in a new combination of alleles on the two chromosomes
what is independent assortment?
the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle
how is there a different combination of chromosomes in daughter cells through metaphase?
during metaphase, the homologous chromosomes attach to the spindle fibres and they line up side by side. it is entirely random which way they attach
why is variation in the gametes important?
produces new combinations of characteristics which means the species is more likely to adapt to changes in the environment
what happens if chromosomes fail to separate during meiosis?
a gamete may end up with two copies of a chromosome
how and when does non-disjunction occur?
improper separation of the chromosomes during anaphase or telophase of meiosis
what would happen to the gamete as a result of chromosome non-disjunction
the gamete would contain one more than the haploid number and would result in a chromosome mutation
what is an example of a disease that results from chromosome non-disjunction?
Down’s syndrome
where does variation exist and why?
between members of the same species (intra-specific) and between species (inter-specific) and may be due to genetic factors or environmental factors
how does genetic diversity occur?
the differences in what genes are present in an organism and the different alleles of a gene
what 4 things cause genetic variation?
-mutation: produces new alleles
-independent assortment- gives new combinations of existing alleles
-crossing over: gives new combinations of existing alleles
-random fusion of gametes: creates zygotes that will have a unique combination of alleles
what is the formula to calculate the no. of combinations of chromosomes after the random fertilisation of two gametes?
(2 to the power of n) squared
n is the haploid number and the square is the no. of gametes
what can mutations do?
-confer a selective advantage
-be neutral
-be disadvantageous
what is natural selection?
process by which organisms that are better adapted to their environment survive and reproduce in greater numbers, resulting in the increase of the frequency of the advantageous allele within a population
describe the process of natural selection
-genetic variation within a population result in differences in phenotype
-environmental factors affect the chance of survival of an organism, therefore acting as a selection pressure
-better adapted individuals more likely to survive and reproduce
-these organisms more likely to pass on their advantageous alleles
-as a result, over many generations, the new allele will increase in frequency in the population
what is a selection pressure?
a factor which determines the spread of an allele within the gene pool
what is a gene pool?
the total sum of all the alleles of all the genes in the population. in large populations, the gene pool will be large (high diversity) and so they have a strong ability to adapt to change
what are the 2 different types of selection?
stabilising and directional
when does stabilising selection occur?
all populations where the environment is stable
what does stabilising selection mean?
there is selective pressure at both ends of distribution
favours the average
tends to eliminate extremes
what does stabilising selection reduce?
variability ( the size of the range within population)
opportunity for evolutionary change
when does directional selection occur?
when an environmental change may produce new selection pressure that favours an extreme characteristic
when conditions change optimum necessary for survival also changes
what does directional selection mean?
mean in population represents optimum for existing conditions some organisms will possess the new optimum. over time selection selection means these will predominate and the mean will shift
how can bacteria become resistant to an antibiotic?
some bactewria can replicate asexually as often as every 20 minutes, which means that their DNA is replicated regularly. As a result there is an increased chance of error during replication leading to mutation which may cause resistant bacteria
how does the whole population of bacteria eventually become antibiotic resistant?
the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead so genes for antibiotic resistance are passed on with much greater frequency and over time the whole population will be resistant
