Cell division

• Explain why crossing over between two different genes, located on a pair of homologous chromosomes, results in a maximum of 50% of gametes with the recombinant alleles.

  Crossing occurs between chromatids

  Only half the chromosomes produced by anaphase II will be recombinant chromosomes

  Crossing over does not always take place so can be less than 50%
• Explain how meiosis results in genetic variation in the gametes.

  Recombination of alleles

  due to independent assortment

  due to crossing over between chromatids- same homologous chromosomes
• Describe the events that take place during metaphase of mitosis.

  Chromosomes line up along equator

  spindle fibres attach to centromere
• Explain how to make a temporary preparation to show the stages of mitosis, after adding the stain to the slide.

  Heat to intensify stain

  Tease the tissue to separate

  apply coverslip to view at high magnification
• Devise a method the student should use to investigate the effect of waterlogged soil on mitosis in root cells.

  Use waterlogged soil and non water logged soil

  use same plant species

  control temperature

  leave for stated time

  take cells from same part of root tips
• Explain why the student had to make sure that an appropriate stain was used.

  To see chromosomes that would not be clearly visible

  stain must attach to chromosomes
• Describe how to prepare a microscope slide of root tissue to show stages of mitosis.

  Use warm acid

  Remove acid using water

  add acetic orcein

  macerate/tease with needle

  use coverslip and squash
• Explain why the centriole fragments and tubulin were each in a buffer

  solution at 35 °C before being used in these experiments.

  buffer can prevent change in pH

  equilibrate because the temperature is at 35

  a change in pH or temp could denature

  H bonds / ionic bonds may be cahnged
• Name the type of chromosome mutation that results in Down's syndrome.

  non disjunction
• Describe an investigation that could be carried out to confirm the length of time of mitosis and cytokinesis for these human cells.

  Use method similar to root tip squash to stain the cells

  count the number of cells in mitosis

  divide the number of cells by mitosis by the total number of the cells counted

  work out as a proportion of 24 hours
• A cell in G1 of interphase contains 68 arbitrary units of DNA.

  Explain how the DNA content and the structure of the chromosomes of this cell will change during the cell cycle.

  DNA content doubles at the end of S phase due to DNA synthesis before mitosis

  chromosomes will consist of pairs of chromatids joined at centromere and during mitosis the chromatids separate and now consist of one DNA molecule which 68 au

  DNA content will return to normal by the end of cytokinesis because chromatids are separated into new daughter cells
• Explain which type of cell division produces gametes in female bees.

  meiosis

  because haploid gametes are produced from diploid cell
• Explain which type of cell division produces gametes in male bees.

  mitosis

  number of chromosomes remains the same
• Explain the significance of meiosis in living organisms

  genetic variation

  random assortment

  because it enables contribution of alleles from two individuals

  because it produces new combination of alleles

  genetic variation allows organisms to adapt to environmental change
• Which of the following increases the number of different alleles in a population?

  Crossing over, gene mutation, independent assortment or random fertilisation

  gene mutation
• Name the type of nuclear division that produces the plantlets.

  Mitosis

  produces a large number of offspring

  rapid

  no other plant needed
• Describe the process of crossing over that occurs during meiosis.

  homologous chromosomes line up

  chiasmata form

  break in DNA occurs

  genetic information exchanged between chromatids
• Explain why cells carry out mitosis.

  growth/repair/asexual reproduction

  needs genetically identical cells/clones
• Describe how translocation occurs.

  part of chromosome breaks off

  this then joins to another non-homologous chromosomes
• Allele

  different version of a gene
• Describe the end result of each of the following stages of this cell cycle. (i) Mitosis

  1. chromatids separated / chromosomes decondensed / eq

  ;

  2. nucleus divided / two nuclei present /eq ;
• Using information from the table and your own knowledge, suggest why the cells produced become smaller after each cell cycle.

  time in G1 or G2 phase / usually a cell spends { several hours / more time / 14 hours } in G1 and G2 phase ;

  2. less protein synthesis / fewer organelles ;

  3. { cytoplasm / organelles / cell membrane } { shared / divided / halved } with each cell division ;
• Describe the processes that take place inside a cell during specialisation.

  genes would be { activated / deactivated } / eq ;

  2. active genes transcribed / mRNA produced ;

  3. translation (of mRNA) to produce proteins / eq ;

  4. idea that proteins { modify cell / determine function of cell } / structure of cell altered permanently ;
• Give two differences between tissues and organs.

  tissuess made of cells and organs made of tissues /eq ;

  2. tissues made of { one type / similar types } of cells AND organs made of different tissues / eq ;

  3.. organs have more functions than tissues ;
• Explain the role of mitosis in the development of roots.

  idea of increasing cell number ;

  2. idea of replacing {damaged / dead } cells

  OR

  idea of repairing (damaged) tissue ;

  3. to produce genetically identical cells ;
• Explain how meiosis can give rise to genetic variation in the gametes produced.

  1. crossing over and { independent/ random} assortment ;

  2. description of crossing over as swapping over sections of { chromatid / DNA } ;

  3. description of independent assortment of maternal and paternal chromosomes ;

  4. consequence described e.g. produces recombinants or new combinations of alleles ;
• Describe what occurs during prophase.

  { chromatin / DNA } condenses / eq ;

  2. chromosomes { condense /become visible /eq } ;

  3. idea of nuclear { membrane

  /envelope } breaking down ;

  4. nucleolus {disappears /eq} ;

  5. reference to centrioles moving to poles or opposite ends of cell ;

  6. reference to formation of spindle

  (fibres) ;
• Describe what happens inside an animal cell as it undergoes the last stage of mitosis and forms two new cells

  1. telophase ;

  2. chromosomes {decondense / eq } ;

  3. spindle (fibres) break down / eq ;

  4. nuclear { membrane / envelope } reforms / eq ;

  5. two nuclei present /eq ;

  6. nucleoli reform / eq ;

  7. each cell will have centrioles ;

  8. idea of cytokinesis ;
• Describe how the student could distinguish between a cell in metaphase and a cell in anaphase.

  Metaphase {chromatids/chromosomes) at the equator of cell

  chromatids attached (to each other / at equator

  centromere complete and spindle fibre complete

  Anaphase Not at the equator

  chromatids separated

  centromere splits and fibres shorten
• Gametes contain mitochondria. Describe the function of mitochondria in sperm cells.

  (they carry out) (aerobic) respiration

  2. provide {ATP / energy / eq} ;

  3. to { move / drive the / eq } { flagellum / tail } / eq ;
• Explain the importance of meiosis in the production of gametes.

  halves the chromosome number / eq ;

  2. to produce a haploid nucleus / eq ;

  3. so that at fertilisation the {full complement / diploid number / eq} of chromosomes is restored /

  4. allows genetic variation (in gametes) / eq

  5. through independent assortment / eq ;

  6. through crossing over / eq ;
• Describe and explain how, in mammals, events following the acrosome reaction

  prevent more than one sperm fertilising an egg. 


  sperm cell fuses with egg cell (membrane)

  2. reference to {cortical granules /lysosomes} ;

  3. (cortical granules) fusing with egg cell (surface)

  4. exocytosis (of cortical granules/lysosomes) ;

  5. idea of contents (of cortical granules) {secreted

  /released into jelly layer / eq} OR cortical reaction ;

  6. thickening of { zona pellucida OR formation of fertilisation membrane ;

  7. reference to change in charge across egg cell

  membrane ;
• Explain how meiosis gives rise to genetic variation in gametes.

  1. reference to both { independent / random } assortment and { crossing-over/chiasma(ta)

  2. independent assortment gives rise to {new / different / eq} combinations of (paternal and maternal) chromosomes ;

  3. crossing over involves swapping of {sections / eq} of {chromatids /chromosomes} ;
• Describe and explain three ways in which a human sperm cell is specialised for its function.

  streamlined ; 2. idea of reduced resistance ;

  3. acrosome containing enzyme

  4. involved in digestion of the zona pellucida

  5. haploid nucleus

  6. allows restoration of {diploid / full complement / 46 / eq} chromosomes at fertilisation ;

  7. mitochondria large number to supply ATP for movement

  9.flagellum for swimming

  11.{markers / receptors} in cell surface membrane ;

  12.to bind to egg cell surface membrane / detect chemicals released by ovum / eq ;
• Describe the process of fertilisation that takes place in flowering plants

  idea that generative nucleus divides to form two male gametes ;

  2. by mitosis ;

  3. pollen tube fuses with embryo sac

  4. reference to double fertilisation ;

  5. (one) male { gamete / nucleus } fuses with egg (cell) nucleus ;

  6. to produce diploid zygote;

  7. other male nucleus fuses with two polar nuclei ;

  8. to produce triploid endosperm ;