U2)KA3) VARIATIONA + SEXUAL REPRODUCTION-

Question 1

costs of sexual reproduction

Answer 1

• males are unable to produce offspring- only half the population (females) can
• only half of each parents genome is passed onto offspring , disrupting successful parental genome

Question 2

costs of asexual reproduction

Answer 2

• genetic variation of population is low

Question 3

how does the positives of sexual reproduction outweigh the negatives of having males

Answer 3

• advantage is that there is increase in genetic variation
• genetic variation provides the raw materials required for adaptation , giving sexually reproducing organisms a better chance of survival under changing selection pressures

Question 4

how can the red queen hypothesis explain the persistence of sexual reproduction

Answer 4

-as co evolutionary interactions between parasites and hosts may select for sexually reproducing hosts

If hosts reproduce sexually, the genetic variability in their offspring reduces the chances
that all will be susceptible to infection by parasites

Hosts better able to resist and tolerate
parasitism have greater fitness. Parasites
better able to feed, reproduce and find new
hosts have greater fitness.

• hosts that are better able to resist/ tolerate parasitism have greater fitness , so this trait increases in subsequent generations. If hosts reproduce sexually the genetic variability of their offspring reduces the chances that they will be susceptible to to infection by parasites.
• parasites better able to feed , reproduce and find new hosts have greater fitness. if parasites reproduce sexually , the genetic variation in their offspring increases the chances of some of the offspring having improved ability to exploit their hosts

Question 5

what does asexual reproduction involve

- how / when is it successful

Answer 5

• involves the production of genetically identical offspring by a single parent

Question 6

advantages of asexual reproduction

Answer 6

• whole genomes are passed from parent to offspring, just one parent can produce daughter cells and establish a colony of unlimited size overtime. maintaining the genome is an advantage , particularly on very narrow , stable niches or when re colonising disturbed habitats
• offspring can be reproduced more often and in larger numbers than with sexual reproduction

Question 7

disadvantages of asexual reproduction

Answer 7

• asexually reproducing populations are not able to adapt easily to changes in their habitat because thier genetically identical offspring do not provide the variation required for natural selection
• however mutations , although rare, do arise and provide some variation and enable some natural selection and evolution

Question 8

examples of asexual reproduction

Answer 8

• vegetative cloning in plants

- parthenogenesis in animals and lower plants

Question 9

vegetative cloning in plants

Answer 9

• eukaryotes usually use sexual reproduction
• flowering plants can use sexual and asexual
• asexual : vegetative cloning has evolved in flowering plants : including bulbs ( daffidols) , corms , yubers (potato) . stolons (strawberry) rhizomes (ginger)

Question 10

what is parthenogenesis

- give an example

Answer 10

• reproduction from a female gamete without fertilisation
• stick insect and aphids are examples of insects that produce multiple female clones, but can also produce males by deleting a sex chromosome

Question 11

• where is parthenogenesis more commonly found

Answer 11

• parthenogenesis is more commonly found in areas with lower parasitism.
• these include cooler climates that are disadvantageous parasites and regions where there is low parasite density and diversity . in these areas the RQs hypothesis is being run more slowly , so there is a lower selection pressure on producing offspring with genetic variation

Question 12

both sexual and asexual reproduction are examples pf ______ gene transfer
- what is this

Answer 12

vertical

- where parent individuals of one generation pass copies of their genes on to offspring that form the next generation

Question 13

what is horizontal gene transfer

Answer 13

when genes are transferred across from an individual to others in the same generation. This does not occur due to reproduction.

Question 14

organisms that reproduce mostly by asexual reproduction often have mechanisms for _____ gene transfer between individuals within a generation

Answer 14

horizontal

Question 15

give an example of organisms that exchange genetic material horizontally

Answer 15

• prokaryotes can exchange genetic material horizontally eg such as the transfer of plasmids between individual bacteria in a population.
• although yeast is eukaryotic , it is unusual in having plasmids that can be transferred between individuals

Question 16

prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use ____ _____

Answer 16

vertical transfer

eg -

Question 17

what is meiosis

Answer 17

• meiosis is the division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte

Question 18

body cells

- diploid and haploid

Answer 18

• all body cells are diploid (have 2 sets of chromosomes)
• gametes are haploid (sex cells) carry only one set of chromosomes
• diploid number for humans is 46- two sets of 23

Question 19

in diploid cells , chromosomes typically appear as ______ ______
what is similar and different about these ?

Answer 19

• homologous chromosomes
• the chromosome in each set have a matching homologous chromosome - that is the same size , has same centromere position , has the same genes at the same loci
• though the genes are the same on the homologous chromosome , the alleles may be different as each chromosome in the pair has been inherited from a different parent

Question 20

the production of haploid gametes starts with the cell division called ____
- when does fertilisation occur

Answer 20

meiosis.

- when the haploid gametes (formed by meiosis ) fuse together to form a new diploid nucleus

Question 21

how does combining haploid gametes increase variation

Answer 21

• the combining of haploid genomes from two different individuals produces a new combination of alleles in the offspring , so variation is increase

Question 22

meiosis I

Answer 22

1) at the start of meiosis I , chromosomes have already replicated , condensed and become visible. Each chromosome consist of two genetically identical sister chromatids attached at the centromeres
2) homologous chromosomes pair up , so they are aligned gene by gene, forming a bivalent (pair of homologous chromosomes)
3) the homologous chromosomes undergo random crossing over at points called chiasmata between non sister chromatids , and this allows the exchange of sections of DNA and recombination of alleles of linked genes
4) nuclear membrane breaks down so that microtubules from centrosome can connect with centromere. microtubules form spindle fibres aligning the chromosomes at the equator. orientation of the pairs of homologous chromosomes at the equator is random , irrespective of the parental origin: this is described as independent assortment
5) microtubules of spindle fibres shorten and chromosomes of each homologous pair are separated and move towards opposite poles
6) nuclear membrane forms around chromosomes, and cytokinesis occurs forming two daughter cells

Question 23

meiosis II

Answer 23

• there are now two cells- each one is haploid , with one copy of each homologous chromosome. the sister chromatids are no longer identical due to the crossing over
  1) the nuclear membrane breaks down (of each cell) centrosomes send out microtubules again to bind to centromeres of each sister chromatid. spindle fibres align the chromosomes across the equator of the cell
  2) microtubules shorten and pull the sister chromatids to opposite ends of the cell. the sister chromatids are now called chromosomes
  3) new chromosomes group in each end of the cell and a nucleur membrane forms around them , cytokensis then seperates the cells
  4) at the end of meiosis II, four genetically different haploid cells have been produced

Question 24

what are the two different processes in meiosis that produce cells that are genetically variable and when do they occur

Answer 24

• crossing over , in meiosis I , after homolgous chromosomes pair up gene by gene forming a bivalent
• independent assortment - in meiosis I , when homologous chromosomes are aligned at the equator

Question 25

crossing over

- what are linked genes

Answer 25

• random crossing occurs at random points called chiasmata - chiasmata can only form between chromatids of two homolgous chromosomes ( never between the chromatids of one the same)
• linked genes are those on the same chromosome and crossing over can result in new combinations of the alleles of these genes
• crossing over of DNA