Question
Outline what occurs in cells in the first division of meiosis. [5]
▶️Answer/Explanation
Ans
a.halves the chromosome number/produces haploid cells;
b.at start of meiosis each chromosome consists of two sister chromatids attachedby a centromere;
c.prophase (I): pairing of homologous chromosomes;
d.crossing over occurs;
e.chromosomes condense by supercoiling;
f.metaphase (I): pairs of homologous chromosomes/bivalents move to equator ofspindle
OR
metaphase (I): orientation of pairs of homologous chromosomes (prior toseparation) is random;
g.anaphase (I): centromeres do not divide
OR
anaphase (I): spindle fiber pulls chromosome/whole centromere with two sisterchromatids to opposite poles;
h.telophase (I): arrival of centromere with sister chromatids at opposite poles;
Question
Meiosis in humans produces cells that participate in fertilization. Outline the processes involved in meiosis.
Following fertilization, cells in the developing embryo differentiate. Outline a technique for cloning using differentiated animal cells.
Discuss ethical issues of therapeutic cloning in humans.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. meiosis reduces a diploid cell into (four) haploid cell(s);
b. (during prophase I) homologous chromosomes pair up/synapsis;
c. chromatids (break and) recombine / crossing over
d. (metaphase I) (homologous chromosomes) at the equator of the spindle / middle of cell;
e. (anaphase I) (homologous) chromosomes separate and move to opposite poles;
f. (telophase I) chromosomes reach poles and unwind WTTE;
g. (prophase II) chromosomes (condense and) become visible, new spindles form;
h. (metaphase II) chromosomes line up at the centre of the cells/equator;
i. (anaphase II) sister chromatids separate;
j. (telophase II) chromatids reach the poles and unwind;
Remember, up to TWO “quality of construction” marks per essay.
a. differentiated/somatic/diploid cells taken from donor animal/sheep udder;
b. (diploid) nucleus from donor cells removed;
c. ova/eggs cells removed from (donor) animal/female sheep;
d. (haploid) nucleus removed from eggs/ova;
e. (diploid/donor’s) nucleus is fused with/inserted into egg/ovum (to form zygote);
f. embryo (from cell with donor nucleus and egg from surrogate) implanted in uterus of surrogate mother;
g. normal pregnancy and birth is completed;
h. offspring is a genetic copy/clone of the donor mother/diploid nucleus WTTE;
Remember, up to TWO “quality of construction” marks per essay.
a. therapeutic cloning involves producing embryos from which embryonic stem cells can be harvested for medical use;
argument in favour:
b. (to many people) any procedure that reduces pain and suffering is ethically/morally justified;
c. stem cells can be used to replace organs/tissues that have been lost/damaged in a patient;
d. (thus) pain and suffering can be reduced/lives can be saved/life quality improved;
e. cells can be removed from embryos that have stopped developing and would have died anyway;
f. cells are removed at a stage when no pain can be felt by the embryo;
g. use embryos from IVF that would otherwise be destroyed;
Accept up to one additional reasonable argument in favour.
argument against:
h. embryonic stem cells are no longer needed as adult stem cells can be used without causing loss of life;
i. there is danger of embryonic stem cells developing into tumour cells/harmful effects are not yet known;
j. every human embryo is a potential human with the right to development;
k. more embryos may be produced than can be used and so some would be killed;
l. (to many people) any procedure that harms a life/kills is unethical/morally wrong;
Accept up to one additional reasonable argument against.
To award [8] at least one pro and one con must be addressed.
Question
State three processes occurring in a cell during interphase of the cell cycle but not in mitosis.
1. …………………………………………………………..
2. …………………………………………………………..
3. …………………………………………………………..
Explain how sexual reproduction can allow evolution to occur.
▶️/Explanation
Markscheme
a. growth (of cells);
b. transcription/protein synthesis/translation;
c. DNA replication / genetic material copied;
d. production of organelles/mitochondria/chloroplasts;
e. named normal activity of cell (eg active transport, movement, secretion);
NB Do not accept G1, S, G2 unless linked to correct process.
a. sexual reproduction promotes variation in species;
b. independent assortment of genes / random orientation of chromosomes in metaphase/meiosis;
c. crossing-over provides new combinations of alleles;
d. production of great variety of gametes (by meiosis) / different combinations of chromosomes in gametes;
e. (random) combination of gametes from both parents (in fertilization);
f. (genetic) variation allows natural selection which leads to evolution;
Question
The following sequence of pictures, made using an electronic imaging technique, shows a cell undergoing division.
State the stage of mitosis typified by image II.
List two processes that involve mitosis.
State the process that results in tumour (cancer) formation or development.
Explain, using one example, how non-disjunction in meiosis can lead to changes in chromosome number.
▶️Answer/Explanation
Markscheme
anaphase
a. growth (through increasing cell number);
b. embryonic development;
c. tissue production/repair;
d. (asexual) reproduction;
uncontrolled mitosis/cell division
a. pair of homologous chromosomes moves in same direction/does not separate during anaphase I / chromatids move in same direction/do not separate during anaphase II;
b. leaving a cell with an (some) extra chromosome(s)/missing chromosome(s);
c. an example; (e.g. Down syndrome where there is an extra chromosome number 21);
Question
The diagram below shows the female reproductive system.
Label the diagram above with the letter U to show the uterus.
Outline the role of luteinizing hormone (LH) after ovulation.
Explain how sexual reproduction can lead to variation in a species.
▶️Answer/Explanation
Markscheme
letter U marked/labelled on uterus
Accept in lumen or on wall.
formation of the corpus luteum
allows characteristics from both parents to appear in offspring;
crossing over (during prophase 1) changes chromosome composition;
produces gametes which are all different;
random chance of which sperm fertilizes ovum;
greater variation (resulting from sexual reproduction) favours survival of species through natural selection;
Accept independent assortment during meiosis from AHL.