Cell division, cell diversity and cellular organisation (2.1.6)
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Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:
- Content: A clear outline of the content covered by the delivery guide
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
Content (from AS and A-level)
The content from the specification that is covered by this delivery guide is:
|2.1.6 Cell division, cell diversity and cellular organisation|
|(a)||the cell cycle||To include the processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically identical cells.
|(b)||how the cell cycle is regulated||To include an outline of the use of checkpoints to control the cycle|
|(c)||the main stages of mitosis||To include the changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell membrane.
|(d)||sections of plant tissue showing the cell cycle and stages of mitosis||To include the examination of stained sections and squashes of plant tissue and the production of labelled diagrams to show the stages observed.
|(e)||the significance of mitosis in life cycles||To include growth, tissue repair and asexual reproduction in plants, animals and fungi.
|(f)||the significance of meiosis in life cycles||To include the production of haploid cells and genetic variation by independent assortment and crossing over.
|(g)||the main stages of meiosis||To include interphase, prophase 1, metaphase 1, anaphase 1, telophase 1, prophase 2, metaphase 2, anaphase 2, telophase 2 (no details of the names of the stages within prophase 1 are required) and the term homologous chromosomes.
|(h)||how cells of multicellular organisms are specialised for particular functions||To include erythrocytes, neutrophils, squamous and ciliated epithelial cells, sperm cells, palisade cells, root hair cells and guard cells.|
|(i)||the organisation of cells into tissues, organs and organ systems||To include squamous and ciliated epithelia, cartilage, muscle, xylem and phloem as examples of tissues.|
|(j)||the features and differentiation of stem cells||To include stem cells as a renewing source of undifferentiated cells.|
|(k)||the production of erythrocytes and neutrophils derived from stem cells in bone marrow|
|(l)||the production of xylem vessels and phloem sieve tubes from meristems|
|(m)||the potential uses of stem cells in research and medicine.||To include the repair of damaged tissues, the treatment of neurological conditions such as Alzheimer’s and Parkinson’s, and research into developmental biology.
HSW2, HSW5, HSW6, HSW7, HSW9, HSW10, HSW11, HSW12
This section lists resources for teaching the main content of the learning outcomes in 2.1.6 Cell division, cell diversity and cellular organisation. Practical laboratory work and learning activities are listed in later sections, as are a variety of contexts in which to explore ideas further.
Approaches to teaching the content
Four conceptual sub-sections are apparent in 2.1.6:
- Learning outcomes (a) to (e) introduce mitosis
- Outcomes (f) and (g) outline meiosis
- Specialisation of cells and organisation into tissues and organs is covered in (h) and (i)
- Stem cells link learning outcomes (j) to (m).
The topics mitosis, meiosis, cell specialisation and stem cells could be taught in the order given, with the advantage that students tend to prefer predictability when keeping track of their learning against the syllabus. The two types of nuclear division with the focus on chromosome movement and sub-cellular activity are dealt with together, and then later the products of division are considered, i.e. what happens to the new cells formed by cell division. The various end-products (types of cells, and the way they are organised into tissues) are considered before looking at the intervening stages, the journey from undifferentiated stem cells produced by mitosis to the formation of the differentiated end-product.
An alternative approach is to link stem cells – mitosis – specialised cells in that order. This starts with the idea that stem cells are undifferentiated and self-renewing and explains first, how they renew (mitosis) and secondly, how they differentiate and into what (specialised cells). This concludes with the teaching of meiosis. As students often confuse mitosis and meiosis there is some merit in dealing with them separately, allowing understanding of mitosis to be reinforced first, and giving an opportunity for revision or testing of mitosis at the start of the meiosis topic.
Common misconceptions or difficulties students may have
Many students have difficulty imagining events on the small-scale level of cells, even in this case where ‘the dance of chromosomes’ is visible under a light microscope and can be filmed and sped up to provide a live window into cell activity (e.g. the pig cell mitosis footage referenced in the context section below). Modelling the nuclear division process with pipe cleaners or icing the stages onto cupcakes may help.
The terms chromosome, chromatid and chromatin may be confused and need to be carefully taught so students both recognise the specific import of these terms when used in questions, and that they select the correct term when writing or labeling a diagram. The distinction between nuclear division and cell division by cytokinesis is also worth highlighting.
Students often have difficulty relating the stylised diagrams in a textbook or examination question to the appearance of real slides of cell division, or photographs of the process imaged with various types of microscope. They also frequently have difficulty slotting their understanding of mitosis into the context of the growth and development of whole organisms and of meiosis to the ideas of independent assortment and linkage of genes in Mendelian crosses.
The relationship of packaged chromosomes to students’ knowledge of the linear DNA molecules of interphase also needs to be carefully explained. A common mistake to watch out for is students thinking mitosis occurs in prokaryotic cells.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
To enhance synoptic understanding, links between section 2.1.6 and other topics can be highlighted at the appropriate time:
- 2.1.1(a) microscopy as a pre-requisite for the discovery of mitosis
- 2.1.1(g) eukaryotic cell structure – the nucleus, centrioles, microtubules
- 2.1 3(d) – (g) structure and function of DNA
- 2.1.4(a) enzymes controlling the cell cycle, e.g. CDK enzymes
- 3.1.1(c) and (h) exchange epithelia as specialised tissues
- 3.1.2 the transport system (blood) links to haemopoietic stem cells 2.1.6(k)
- 3.1.3(b) xylem and phloem as specialised transport tissues
- 4.1.1(e) neutrophils as specialised immune system cells
- 4.2.2(f) meiosis relates to genetic variation in evolution
- 5.1.4(f) stem cell treatment of Type 1 diabetes (see links in context section)
- 5.1.5(h) the brain – link to stem cell research into degenerative brain diseases
- 5.2.2(a) movement of chromosomes in cell division is an example of why energy from respiration is needed
- 6.1.1(c) embryonic stem cell research into development relates to Hox genes
- 6.1.1(d) mitosis and apoptosis controlling body development
- 6.1.2(a) meiosis and genetic variation in sexually reproducing species
- 6.1.2(b) Mendelian genetics including dihybrid inheritance and linkage of genes
- 6.2.1(a)-(d) mitosis and cloning.
This can be done with pipe-cleaners or with modelling clay. Students will need to refer to illustrations outlining the stages of either mitosis or meiosis (on a sheet, the whiteboard or in a textbook). They can either produce tableaux of each stage and display these in sequence or run-though the process moving and manipulating one starting set of chromosomes. It is usual to consider only 2 or 3 pairs of chromosomes. It is important after the working out stage that students have the opportunity to orally describe the process, i.e. to talk though their range of models or to demonstrate the process. This can be in pairs, one pair to another, to the teacher walking round the room or to the whole class. Students usually make errors with terminology in their descriptions or miss out key information, so it is best if the teacher listens to each description. It could be turned into a group learning game like ‘I’m Sorry I Haven’t a Clue’ where if the first student is stopped by students, or the teacher sounding the alarm at a mistake or omission, then the task is passed to the next student.
In the case of meiosis, maternal and paternal homologues need to be different colours, and the different homologous pairs should be distinguishable by being different lengths. So, for example, two blue (paternal) and two red (maternal) pipe-cleaners can be cut into a two-thirds length to represent chromosome 1 and the smaller third length can be chromosome 2. The two long blues are crossed and twisted over each other at one point to represent a centromere, and the same for the short blue, long red and short red. At this stage, prophase 1 in a cell where n=2 can be modelled. Scissors and sticky tape are needed to show crossing over with pipe-cleaners. While modelling clay can be pulled apart and stuck together more easily, students are more prone to going off-task when using it.
This features a useful comparison table, a good section on the significance of the processes and two ‘crash course’ videos which are very engagingly presented:
The delivery is pacey and the graphics are colourful and eye-catching. Viewer comments suggest learners found these videos very helpful.
Theoretical understanding of mitosis should be related to practical investigation, learning outcome 2.1.6(d). The resources in the activities outline the standard protocol for staining chromosomes in the apical meristem of an onion or garlic root tip. They also give practice in the skill of drawing (linking to Module 1 and PAG 1), plus some work of a mathematical nature in calculating the duration of each stage of mitosis from the percentage of cells viewed in each stage (linking to mathematical skills such as M0.3 and M0.4). These figures obtained by practical work could be compared to figures obtained from the time lapse film of mitosis viewed in 3D, as described in more detail in the activities.
Practical work should also include viewing stages of meiosis 2.1.6(g), for example in prepared slides of lily anthers or locust testes. If time allows, students might find it more interesting and challenging to make their own slides, using freshly-killed locusts, or the anthers of chive flowers. The virtual lab for investigating meiosis in lily anthers offers a shortcut for individual study compared to viewing slides directly.
The stem cells topic provides a broad context for students to explore stimulating questions. Resources are listed covering stem cell ethics, stem cells and neurological disorders, regeneration of limbs and organs, treatment for type 1 diabetes and how Superman links to stem cells.
This resource gives details useful to the teacher for running the root tip squash as a class practical. It also supplies sample results for counts of cells at different stages of mitosis.
There is no class worksheet supplied so the one from Marietta College (Mitosis in onion root tip cells) could be used in conjunction with these practical instructions for the teacher and technician.
The technique used shows 3D fluorescent chromosomes moving in a time-lapse film of mitosis in a living cell. Use it as a class resource, after the first showing it could be repeated with students raising their hands or calling out at the points when one recognisable stage is succeeded by the next. The time clock in minutes could then be used for students to note down how long each stage takes and to work out the percentage of time of mitosis spent in each stage.
Depending on when the filming started and ended, the accuracy of this approach could be discussed, and how these figures from live imaging could be validly compared with estimates of how long each stage takes from students’ own work counting cells at each stage in their own root squash or a prepared root tip slide (linking to mathematical skills M0.3, M1.5). The remarkable nature of this footage could also stimulate discussion and revision of microscopy techniques (2.1.1a-f).
This reports Harvard’s breakthrough in treating Type 1 diabetes by stem cell transplantation. Dr. Douglas Melton is the lead researcher. The research was published in Cell on 9th October 2014 and an author’s proof copy of the article is downloadable. Newspaper coverage of the work includes The Guardian and The Telegraph.
There is a useful video ‘What are Stem cells?’ on The Telegraph's page.
Simultaneous publication of the peer-reviewed research, the announcement on the institution’s website and the issuing of press releases to give widespread media coverage provides a context for students to examine HSW7, HSW8 and HSW11.
This review of five examples of regeneration in animals can be used to stimulate discussion of tissue repair in humans and what is possible naturally and in the laboratory. The links in this activity provide more resources to explore this theme. This line of enquiry links learning outcome 2.1.6 (e) with (m).
The last link gives a timeline of breakthroughs in obtaining induced pluripotent stem cells.
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