IBDP>CORE TOPICS>TOPIC 4 GENETICS>4.2 MEIOSIS
Virtual karyotype
http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html
http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html
Explain how reduction division results in genetic variety. 8marks
Reduction division occurs during meiosis;
Homologous chromosomes pair up (in middle of cell);
Each chromosome in homologous pair came from maternal or paternal parent; maternal and paternal chromosome in each pair is randomly oriented to either side of cell;
Homologous chromosomes separate / each moves to opposite pole/ side of cell;
Cell undergoes first division/meiosis I/cytokinesis; chromosomes separate again; two cells from first division undergo second division/meiosis II/cytokinesis; one cell has given rise to four cells; diploid number/2n becomes haploid number/n; haploid cell contains only one chromosome from each original homologous pair; different haploid cells form because of random orientation during meiosis are basis for first variety; mixture of maternal and paternal chromosomes in any haploid cell is different;
Accept any of the above points if clearly explained in a diagram
Homologous chromosomes pair up (in middle of cell);
Each chromosome in homologous pair came from maternal or paternal parent; maternal and paternal chromosome in each pair is randomly oriented to either side of cell;
Homologous chromosomes separate / each moves to opposite pole/ side of cell;
Cell undergoes first division/meiosis I/cytokinesis; chromosomes separate again; two cells from first division undergo second division/meiosis II/cytokinesis; one cell has given rise to four cells; diploid number/2n becomes haploid number/n; haploid cell contains only one chromosome from each original homologous pair; different haploid cells form because of random orientation during meiosis are basis for first variety; mixture of maternal and paternal chromosomes in any haploid cell is different;
Accept any of the above points if clearly explained in a diagram
Outline how the process of meiosis can lead to Down’s Syndrome 4marks
Accept any of the points below if clearly drawn and correctly labelled in a diagram.
In metaphase homologous chromosome align in the centre of cell;
When these homologues are separating; one pair doesn’t separate/ non-disjunction;
In telophase cells divide into two; cells have either one more/one less chromosome;
This can also occur in second division of meiosis; sister chromatids fail to separate;
Fertilisation with one gamete / sperm / egg carrying extra chromosome; This results in Down’s syndrome that is trisomy of chromosome 21
In metaphase homologous chromosome align in the centre of cell;
When these homologues are separating; one pair doesn’t separate/ non-disjunction;
In telophase cells divide into two; cells have either one more/one less chromosome;
This can also occur in second division of meiosis; sister chromatids fail to separate;
Fertilisation with one gamete / sperm / egg carrying extra chromosome; This results in Down’s syndrome that is trisomy of chromosome 21
Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. 5 marks
application of karyotyping: [2 max]
Helps to find gender / test for Down’s syndrome / other chromosome abnormality;
Identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted;
XX=female and XY =male / third chromosome 21 indicates Down’s syndrome / other chromosome abnormality (e.g. Klinefelter’s syndrome) can be identified.
obtaining chromosomes: [3 max]
Fetal cells are obtained from amniotic fluid, the process amniocentesis / other named source;
White blood cells obtained;cells encouraged to divide ; cells accumulated / blocked in metaphase;
Prepare slide / chromosomes examined.
Helps to find gender / test for Down’s syndrome / other chromosome abnormality;
Identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted;
XX=female and XY =male / third chromosome 21 indicates Down’s syndrome / other chromosome abnormality (e.g. Klinefelter’s syndrome) can be identified.
obtaining chromosomes: [3 max]
Fetal cells are obtained from amniotic fluid, the process amniocentesis / other named source;
White blood cells obtained;cells encouraged to divide ; cells accumulated / blocked in metaphase;
Prepare slide / chromosomes examined.