1.6 Cell division
Nature of science:
Serendipity and scientific discoveries—the discovery of
cyclins was accidental. (1.4)
Understandings:
Mitosis is division of the nucleus into two genetically
identical daughter nuclei.
Chromosomes condense by supercoiling during mitosis.
Cytokinesis occurs after mitosis and is different in plant
and animal cells.
Interphase is a very active phase of the cell cycle with
many processes occurring in the nucleus and cytoplasm.
Cyclins are involved in the control of the cell cycle.
Mutagens, oncogenes and metastasis are involved in the
development of primary and secondary tumours.
Applications and skills:
Application: The correlation between smoking and incidence
of cancers.
Skill: Identification of phases of mitosis in cells viewed
with a microscope or in a micrograph.
Skill: Determination of a mitotic index from a micrograph.
Guidance:
The sequence of events in the four phases of mitosis should
be known.
Preparation of temporary mounts of root squashes is
recommended but phases in mitosis can also be viewed using permanent slides.
To avoid confusion in terminology, teachers are encouraged
to refer to the two parts of a chromosome as sister chromatids, while they are
attached to each other by a centromere in the early stages of mitosis. From
anaphase onwards, when sister chromatids have separated to form individual
structures, they should be referred to as chromosomes.
International-mindedness:
Biologists in laboratories throughout the world are
researching into the causes and treatment of cancer.
Theory of knowledge:
A number of scientific discoveries are claimed to be
incidental or serendipitous. To what extent might some of these scientific
discoveries be the result of intuition rather than luck?
Utilization:
The mitotic index is an important prognostic tool for predicting
the response of cancer cells to chemotherapy.
Aims:
Aim 8: The tobacco industry could be discussed. Suppression
of the results of research by tobacco companies into the health effects of
smoking tobacco was unethical. Smoking causes considerable social harm, but,
with the exception of laws on production and supply in Bhutan, has never been
made illegal.
3.3 Meiosis
Nature of science:
Making careful observations—meiosis was discovered by
microscope examination of dividing germ-line cells. (1.8)
Understandings:
One diploid nucleus divides by meiosis to produce four
haploid nuclei.
The halving of the chromosome number allows a sexual life
cycle with fusion of gametes.
DNA is replicated before meiosis so that all chromosomes
consist of two sister chromatids.
The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation.
Orientation of pairs of homologous chromosomes prior to
separation is random.
Separation of pairs of homologous chromosomes in the first
division of meiosis halves the chromosome number.
Crossing over and random orientation promotes genetic
variation.
Fusion of gametes from different parents promotes genetic
variation.
Applications and skills:
Application: Non-disjunction can cause Down syndrome and
other chromosome abnormalities.
Application: Studies showing age of parents influences
chances of non-disjunction.
Application: Description of methods used to obtain cells for
karyotype analysis e.g. chorionic villus sampling and amniocentesis and the
associated risks.
Skill: Drawing diagrams to show the stages of meiosis
resulting in the formation of four haploid cells.
Guidance:
Preparation of microscope slides showing meiosis is
challenging and permanent slides should be available in case no cells in
meiosis are visible in temporary mounts.
Drawings of the stages of meiosis do not need to include
chiasmata.
The process of chiasmata formation need not be explained.
Theory of knowledge:
In 1922 the number of chromosomes counted in a human cell
was 48. This remained the established number for 30 years, even though a review
of photographic evidence from the time clearly showed that there were 46. For
what reasons do existing beliefs carry a certain inertia?
Utilization:
An understanding of karyotypes has allowed diagnoses to be
made for the purposes of genetic counselling.
Aims:
Aim 8: Pre-natal screening for chromosome abnormalities
gives an indication of the sex of the fetus and raises ethical issues over
selective abortion of female fetuses in some countries.
Essential idea: The inheritance of genes follows patterns.
10.1 Meiosis
Nature of science:
Making careful observations—careful observation and record
keeping turned up anomalous data that Mendel’s law of independent assortment
could not account for. Thomas Hunt Morgan developed the notion of linked genes
to account for the anomalies. (1.8)
Understandings:
Chromosomes replicate in interphase before meiosis.
Crossing over is the exchange of DNA material between
non-sister homologous chromatids.
Crossing over produces new combinations of alleles on the
chromosomes of the haploid cells.
Chiasmata formation between non-sister chromatids can result
in an exchange of alleles.
Homologous chromosomes separate in meiosis I.
Sister chromatids separate in meiosis II.
Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.
Applications and skills:
Skill: Drawing diagrams to show chiasmata formed by crossing
over.
Guidance:
Diagrams of chiasmata should show sister chromatids still
closely aligned, except at the point where crossing over occurred and a chiasma
was formed.
Aims:
Aim 6: Staining of lily anthers or other tissue containing
germ-line cells and microscope examination to observe cells in meiosis are
possible activities.
Essential idea: Genes may be linked or unlinked and are
inherited accordingly.
