unit 4

Question 1

state how the production of pollen grains has enabled flowering plants to adapt to terrestrial life [3]

Answer 1

• pollen grains carry the male gametes to the stigma and are able to deliver the male gametes to the ovule via the pollen tube
• the male gametes do not therefore need to be motile or require water and are therefore adapted to the terrestrial life
• pollen grains have become adapted to be transported by wind or insects so that they can be moved long distances without the need for water
• one of these adaptation is a thick excine layer, which is sculpted to allow the pollen grain to be blown in the wind or stick to insects
• the thick excine layer also reduces the risk of the pollen grain becoming dehydrated

Question 2

questions related to ‘evolution’

Answer 2

• evolution happens when selection pressures happen to a population in a particular environment
• as there is variation in any population of organisms, some of them will have a selective advantage over others
• some of the variation in the population may also be caused by mutations
• those members of the population with the selective advantage are more likely to survive to sexual maturity and therefore breed successfully
• they will pass on their beneficial alleles, which will increase the allele frequency of this beneficial allele which will lead over generations to the vast majority of the population of this organism possessing the beneficial allele and therefore the beneficial phenotype

Question 3

questions related to ‘3n infertility’

Answer 3

• 3 of each type of chromosome would mean that one of the chromosomes would be unpaired
• this would mean that homologous pairs of chromosomes could not pair up at prophase I of meiosis so meiosis could not take place and therefore no gametes would be produced

Question 4

questions related to ‘importance of meiosis’

Answer 4

• meiosis produces haploid gametes that allow the diploid state to be restored at fertilisation
• meiosis also produces genetically different gametes

Question 5

questions related to ‘incomplete linkage’

Answer 5

• genes are far apart on the same chromosome and therefore if crossing over occurs the genes can therefore be separated and appear in different gametes even though they were originally on the same chromosome
• four different gametes will be produced but not in equal numbers, a small number will be recombinants and a large number will be parental types
• the recombinants will be equal in number as will the parental types

Question 6

questions related to ‘complete linkage’

Answer 6

• genes are close together on the same chromosome and therefore even if crossing over occurs the genes will normally not be separated and most will therefore appear in the same gametes
• four different gametes will be produced but not in equal numbers, a very small proportion will be recombinants and a very large proportion will be parental types
• the recombinants will be equal in number as will the parental types

Question 7

questions related to ‘gene therapy’

Answer 7

• replacing a faulty allele with a normal allele

Question 8

questions related to ‘cystic fibrosis is caused by a mutation of the gene producing the protein CFTR. explain how the presence of this altered protein results in the production of thick, sticky mucus and how this accounts for the respiratory symptoms of the disease’ [4]

Answer 8

• CFTR is a carrier protein which normally allows the active transport of chloride ions out of the lung cells and into the mucus
• the mutation of the gene producing the CFTR means that it does not carry out its normal function which means that chloride ions remain within the cell
• this means that water is retained within the cell and far less water osmoses into the mucus surrounding the cell
• the mucus is therefore very thick and difficult to remove from the lungs
• as the thick mucus cannot be easily removed it can harbour microorganisms and the likelihood of infection increases
• more respiratory infections are a symptom of cystic fibrosis
• the thick mucus also narrows bronchioles which will reduce airflow which leads to another symptom of cystic fibrosis: breathing difficulties
• finally, the thick mucus reduces the surface area of the gas exchange surfaces and therefore insufficient oxygen is absorbed which would again lead to the breathing difficulties symptoms of cystic fibrosis

Question 9

questions related to ‘the chromosome number of the dog is 2n 78 and the European red fox is 2n 38. rare cases of mating between dogs and foxes have been recorded (resulting in an animal called a dox) but the offspring are all sterile. give reasons for the sterility’ [4]

Answer 9

• the dox would be unable to form gametes as meiosis could not take place because the chromosomes would not be homologous and therefore homologous pairs (bivalents) could not form during prophase I

Question 10

questions related to ‘genetic engineering involves the introduction of DNA from one species into another’

Answer 10

• the required ‘human’ gene is isolated
• a common method of achieving this is to isolate the ‘human’ mRNA from ‘human’ cells that have the required gene switched on
• a reverse transcripase enzyme is then used to make a ‘human’ DNA strand (of the required gene) from the ‘human’ mRNA molecule
• another enzyme DNA polymerase is then used to make this ‘human’ DNA stand a double stranded ‘human’ DNA molecule
• the ‘human’ DNA molecule is then treated with a restriction endonuclease enzyme so that it forms sticky ends at both ends of the ‘human’ DNA molecule
• plasmids are obtained from bacterial cells and are treated with the SAME restriction endonuclease enzyme as before
• this breaks open the loop of ‘bacterial’ DNA and produces sticky ends on both ends of the bacterial DNA molecule which have complementary bases to those on the sticky ends of the human DNA molecule
• the bacterial DNA and human DNA are mixed together and combine due to the complementary base pairing of the sticky ends
• this reaction is catalysed by the enzyme DNAligase
• this DNA is now called recombinant DNA as it has DNA from two different species in it and this is placed back in the bacterium
• the bacterium will then produce the product of the original human gene isolated
• markers such as antibiotic resistance can be added to the recombinant DNA to make it easier to identify, isolate and grow only those bacteria that contain the recombinant DNA

Question 11

questions related to ‘sperm entering the ovum’ [2]

Answer 11

• the acrosome contains enzymes which breakdown the zona pellucida

Question 12

questions related to ‘animal cloning’

Answer 12

• animal cloning can be attempted in two different ways
• firstly, embryo cloning can be used
• this is when in vitro fertilisation is used and then the resulting zygote is allowed to divide by mitosis until the 8 cell stage
• at this point the embryo is split into 8 separate cells
• each of these cells are genetically identical and as they are at this stage undifferentiated they should each develop normally into an embryo that can be implanted into a surrogate and develop into the adult organism
• the second form of animal cloning involves nucleus transplant
• in this case the haploid nucleus is removed from the ovum and replaced with a diploid nucleus from the donor animal that is to be cloned
• this ovum is then stimulated to divide forming an embryo that can be implanted into a surrogate and develop into the adult organism
• the adult animal will be a clone of the animal that donated the diploid nucleus

Question 13

• sperm duct (no mark)
• vas deferens (mark)

Question 14

• must get spelling right for ‘prostate’ (not prostrate)

Question 15

• correct spelling of urethra and ureter

Question 16

• correct spelling of adenine, thymine, cytosine and guanine

Question 17

• correct spelling of spermatogenesis
• spermiogenesis (no mark)

Question 18

be specific when discussing ‘spermatocytes’
- which ones ‘primary’ or secondary’

Question 19

• sertoli cells (mark)
• nurse cell (no mark)

Question 20

mitosis and meiosis spelt correctly

Question 21

questions related to making sure an experiment is reliable:
- always state that you will repeat and mention specifically some controlled variables that could be used

Question 22

follow instructions carefully:
- if asked for 5 letters representing stages of meiosis, all letters correct 5 marks, all words correct 4 marks

Question 23

match genotype to phenotype in dihybrid inheritance questions:
a) RR RW WW
b) Red Pink White
c) 1 3 1
(3 marks)

a) RR RW WW
b) pink, white, red
c) 3,1,1
(only 1 mark for top line)

Question 24

draw punnett squares when you have the opportunity, examiners can give you marks for genotypes from these

Question 25

understand the difference between a point mutation and a chromosome mutation

Question 26

- ‘polypeptide’ (no mark) - ‘polypeptide chain’ (mark)

Question 27

- remember to be detailed: - e.g point mutations cause ‘a change in amino acids sequence’ or ‘alters primary structure of protein’ or ‘produces a different polypeptide chain’ (mark) - ‘sequence is wrong’, ‘different protein’, ‘the wrong polypeptide would be produced’ - too vague - (no mark)

Question 28

- ‘bond formed between amino acids’ (no mark) - ‘peptide bond formed between amino acids’ (mark)

Question 29

nucleated eggs must be STIMULATED to divide

Question 30

in evolution: - beneficial genes (no mark) - beneficial alleles (mark)

Question 31

in evolution: - use: • selective pressure • selective advantage • produces a useful phenotype • variation in population - produces a useful feature (no mark)

Question 32

in evolution: - organisms breed together (mark) - organisms reproduce together (mark) - organisms mate together (no mark)

Question 33

- use information from stem of question - e.g information regarding cod and question related to evolution - if you do a general evolution answer rather than make it specific to the cod you will not get full marks

Question 34

- use information from stem of question - e.g table, graph or diagram - quote relevant figures or phrases from the source for a mark point

Question 35

- careful with language with evolution questions - ‘because of the use of nets VERY FEW of the large cod survived’ (mark) - ‘because of the nets none of the large cod survived’ (no mark)

Question 36

- careful with language with evolution questions - ‘mutations appear spontaneously in populations at all times, one of these mutations may confer a selective advantage if there is a particular selection pressure’ (mark) - ‘mutations appear that confer a selective advantage if there is a particular selection pressure’ (no mark) because the answer implies that mutations happen because of the selection pressure

Question 37

- 3n organisms cannot breed: - ‘no pairing of chromosomes can happen’ (no mark) - ‘no pairing of homologous chromosomes can happen’ (mark)

Question 38

- sex chromosomes (no mark) - X and Y chromosome (mark)

Question 39

- if asked about probability of a particular phenotype appearing from a genetic cross answer with % - e.g. 25% or 1 in 4 - NOT a ratio - only give ratios if requested

Question 40

- restriction endonuclease cuts DNA (mark) - restriction endonuclease cuts gene (no mark)

Question 41

- DNA ligase joins section of DNA (mark) - DNA ligase join sticky ends (no mark)

Question 42

- remember a DNA molecule consists of two strands of DNA - use the correct terms - after replication the DNA molecule is made up of an old and a new DNA strand

Question 43

- replication of DNA (mark) - copying/doubling of DNA (no mark)

Question 44

- transcription: - DNA is used as a template to produce a mRNA molecule - DNA is not changed/made/converted into mRNA

Question 45

- reverse transcriptase - mRNA is used as a template to produce a DNA molecule - mRNA is not changed/made/converted into DNA

Question 46

- translation - mRNA leaves the nucleus (no mark) - mRNA leaves the nuclear pore (mark)

Question 47

pollination: - pollen from the ANTHER of one flower to the STIGMA of another

Question 48

- pollen tube travels through the stigma, style and ovary wall before entering the micropyle

Question 49

- acrosome enzymes digest the zona pellucida not the egg’s membrane

Question 50

- gene therapy is replacing a faulty ALLELE with the normal/correct version of that ALLELE

Question 51

- use the words DONOR and RECIPIENT cells when describing the transfer of nuclei from one ovum to another