Question
Fig. 1.1 shows a cell of a female fruit fly, Drosophila melanogaster, during a stage of mitosis.
(a) (i) Name the stage of mitosis shown in Fig. 1.1.[1]
(ii) Shade a pair of homologous chromosomes. [1]
(iii) Name the structure labelled W and state its function.[2]
(b) State what happens to structure X and to structure Y between the stage shown in Fig. 1.1 and the end of cell division.[3] [Total: 7]
Answer/Explanation
Ans:
1 (a) (i) prophase ;
R prophase I
(ii) two homologous chromosomes shaded ;
(iii) centriole ; A centrosome/ microtubule organising centre/ MTOC
one from
produces spindle/ produces spindle fibres ;
produce/ organises, microtubules ;
disassembles /AW, spindle/ spindle fibres / microtubules ;
A one e.g. of role of, spindle fibres / microtubules if a link to centriole has been made allow if centriole incorrectly named or if not given
(b) max 2 if no attempt made at both X and Y
X / cell surface membrane
1 forms a (cleavage) furrow ; A ‘pinches in’/ constricts /AW
2 ref. fusion ;
3 to divide cell into two ; A idea of formation of two (separate) cells linked to behaviour of (cell surface) membrane;
4 ref. to cytokinesis / contractile ring ;
Y/ nuclear envelope
5 disassembles / breaks down/AW ;
6 during prophase/ by end of prophase/before metaphase ;
A by the end of prometaphase
7 re-forms /AW, during telophase (from ER) ;
Question
(a) A student cut thin sections of a root tip of Allium cepa and stained them to show chromosomes. A photomicrograph of part of one of these sections is shown in Fig. 4.1.
Table 4.1 shows the behaviour of chromosomes and the changes that occur to the nuclear envelope during a mitotic cell cycle in the root tip of A. cepa.
Complete Table 4.1.[5]
(b) Explain why the growth of roots, such as those of A. cepa, involves mitosis and not meiosis.[3]
(c) State two processes, other than growth, in which mitosis is involved.[2]
1
2
[Total: 10]
Answer/Explanation
Ans:
4 (a) one mark for correct cells in column 2 ;
(b) mitosis
needs number of chromosomes to remain constant/diploid ;
needs all daughter cells to be genetically identical/have no genetic variation ; A clones needs genetic stability ;
meiosis
halves the number of chromosomes/diploid → haploid ; A undergoes a reduction division
daughter cells are all genetically different ; accept once only
produces genetic variation ; accept once only
involved in sexual reproduction (in flowering plants) not growth ; A production of gametes
idea that cells that are genetically different will not function together in tissues ; ora
(c) asexual reproduction/vegetative propagation ;
(tissue) repair ; R cell repair
(cell/tissue) replacement ;
AVP ; e.g. clonal expansion/part of gametogenesis/spores in fungi
Question
A student investigated growth in the roots of broad bean, Vicia faba. The student cut sections of the root tip of this plant and viewed them with a light microscope.
Fig. 1.1 is a photomicrograph of one of the sections. The cell labelled D is in interphase.
(a) Complete the table below by: [5]
- naming the stages of mitosis in the correct sequence following interphase
- identifying one example from the cells labelled A to H that is in each stage of mitosis that you have named.
(b) In animal cells, centrioles are responsible for assembling microtubules to make the spindle at the beginning of mitosis.
Describe the role of the spindle during mitosis.[2]
(c) State two roles of mitosis in plants and animals other than growth.[2]
1
2
(d) V. faba is a legume. Roots of legumes often have swellings at intervals known as nodules. Cells within the nodules contain nitrogen-fixing bacteria.
(i) Explain the role of nitrogen fixation in the nitrogen cycle.[2]
(ii) Farmers in some parts of the world grow legume crops together with cereal crops in the same field. This is known as intercropping.
Explain how intercropping results in an increase in the yield of the cereals when the legumes die.[3] [Total: 14]
Answer/Explanation
Ans:
1 (a) one mark for the stages of the cell cycle in the correct sequence
one mark for correct matching of each stage with a cell
(b) microtubules / spindle (fibres), attach to centromere/ kinetochore
(of chromosome during prophase) ; I metaphase
arranging/aligning/ orienting/AW, chromosomes at the equator/
metaphase plate ; R centre
fibres, shorten/ contract/retract ; A microtubules disassemble/AW
move/ pull, (sister) chromatids /(daughter) chromosomes, to opposite poles/ centrioles ;
idea that equal number of chromosomes in each daughter, nucleus / cell ;
(c) maintaining number of chromosomes ;
ensuring genetic stability / maintaining genetically identical cells /AW ;
asexual reproduction ; A vegetative reproduction/ cloning
cloning/ clonal expansion, of (named) lymphocytes ; A B/T cells
replacement of (worn out/ dead/damaged) cells ;
regeneration, of (named) tissues /organs ;
(wound) repair (of tissues) ; R repair of cells
ref. to production of gametes ;
e.g. mitosis in gametogenesis / gamete production in plants
R ‘copying of cells’
(d) (i) accept biological N fixation or Haber-Bosch process for mp1
1 either
converts, (inorganic) nitrogen/ dinitrogen/N2, into organic nitrogen/
ammonia/NH3 / ammonium/NH4+ ; R if nitrate given
or
lightning converts, nitrogen/ ammonia/NH3 / ammonium/NH4+, into, nitrite/ nitrate (ions) ;
2 reduces nitrogen/ breaks triple bond ;
3 makes (fixed) nitrogen available to, legumes / other organisms /
community / AW ; A ref. to amino acids / proteins
not to be awarded if it follows nitrification
4 increase soil fertility ;
5 balances the loss of fixed nitrogen in, denitrification/ ocean deposits ;
(ii) 1 idea of decay /decomposition ;
e.g. breakdown by, (saprophytic) bacteria/ fungi
2 legumes eaten by, detritivores ; A named detritivores
3 decomposers produce proteases ;
4 to, hydrolyse/ convert/ change/AW, protein to amino acids ;
5 amino acids are deaminated ;
6 (amino acids) to, ammonia/NH3 / ammonium (ions)/NH4+ ;
7 nitrifying bacteria/Nitrosomonas, convert ammonia to nitrite (ions);
8 nitrifying bacteria/Nitrobacter, convert nitrite to nitrate (ions) ;
if mp7 or mp8 not awarded allow one mark for the following as mp9
9 (named) nitrifying bacteria convert, ammonia/ ammonium, to nitrate (ions) ;
mp10 only to be awarded following nitrification
10 nitrate (ions) used for making, amino acids / proteins (hence increase in growth of cereals) ;