Cellular control (6.1.1)
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Content (from A-level)
|6.1.1 Cellular control|
|(a)||types of gene mutations and their possible effects on protein production and function||To include substitution, insertion or deletion of one or more nucleotides
the possible effects of these gene mutations (i.e. beneficial, neutral or harmful).
|(b)||the regulatory mechanisms that control gene expression at the transcriptional level, post-transcriptional level and post-translational level||To include control at the,
• transcriptional level: lac operon, and transcription factors in eukaryotes
• post-transcriptional level: the editing of primary mRNA and the removal of introns to produce mature mRNA
• post-translational level: the activation of proteins by cyclic AMP.
|(c)||the genetic control of the development of body plans in different organisms||Homeobox gene sequences in plants, animals and fungi are similar and highly conserved
the role of Hox genes in controlling body plan development.
|(d)||the importance of mitosis and apoptosis as mechanisms controlling the development of body form.||To include an appreciation that the genes which regulate the cell cycle and apoptosis are able to respond to internal and external cell stimuli e.g. stress.|
Content (from AS and A-level)
6.1.1 Cellular control
The basic definitions of insertion, deletion and substitution gene mutations can be reinforced with interactive activities like the two listed below. Students can then be introduced to a variety of mutations with beneficial, neutral and harmful effects on cell functioning and the organisms’ phenotype in the context-based scenarios listed at the end of this guide.
Control of gene expression encompasses regulation of transcription, mRNA splicing and protein activation. It is important that students realise the significance of this control for differentiation of cells in multicellular animals or plant, and also that different genes are switched on and off in the course of development over time. Embryonic development is controlled to a large degree by the homeobox genes which code for transcription factors that switch on and off suites of other genes. This triggers different outcomes for the various segments of arthropods like the fruit fly and for different areas of vertebrate embryos.
While students will be familiar with mitosis as the source of new cells for building a body, they may not have previously considered the reverse idea, how morphological development is sculpted by apoptosis removing areas of tissue. A good example is the removal of the webs between the fingers and toes of human embryos.
Students compare the effect of nucleotide substitution, insertion and deletion and the effects of mutations on protein structure. This activity needs Java to run.
Students gain practice in applying mutation rules to mRNA sequences and seeing the effect on protein structure. The screen provides a forum for revision of protein synthesis and the genetic code
There are 8 slides in the step-through animation and a 3-question quiz at the end. The narrated version could be used for class teaching.
This Java simulation allows students to play with the components of the lac operon, adding or removing genes, promoters and lactose molecules to see what happens. A worksheet with instructions and questions is provided.
The worksheet instructs the students on what to do with the animation and provides a format for them to express in writing what they have seen and understood.
This introductory article could be used as the basis of a comprehension exercise, to get students to list the different levels of control or to write a summary. It should also help them to gain an appreciation of the significance of differential gene expression in the tissues of a multicellular organism.
This is a quick, concise narrated animation summarising how gene expression is controlled in eukaryotes, covering transcriptional, post-transcriptional, translational and post-translational control.
This is an excellent introduction to homeotic genes and the control of body plan. It is composed of seven sections with good illustrations. A task or specific questions should ideally be provided to allow students to extract the key fact from each of the seven sections.
This is a more advanced article but a useful summary of the significance of Hox genes for the teacher or interested students.
This essay explains the difference between homeobox genes and Hox genes. In the course of doing so, it provides a useful introduction to what Hox genes do and to the significance of the conservation of the homeobox DNA sequence. The syllabus makes a clear distinction between the two (‘homeobox gene sequences are highly conserved’ and ‘Hox genes control body plan development’).
This is a silent animation accompanied by text which compares and described cell necrosis with apoptosis (programmed cell death).
The topics of mutation, control of gene expression, homeobox genes and apoptosis are linked in that they all contribute to our understanding of how DNA controls the growth, development and functioning of an organism.
Approaches to teaching the content
Interactive games that build understanding mutation and gene regulation work are useful but students do need practice in solving paper-based problems (such as those posed by past exam questions) and in writing summaries and explanations to express the concepts and use the biological terminology correctly. The lac operon Phet simulation worksheet given in the 'Transcription' activity in Curriculum Content is an example of how to bridge this gap.
Common misconceptions or difficulties students may have
- Students often fail to understand that the word ‘mutation’ has a precise definition in Biology and that it refers to change in the DNA molecule (not mRNA or protein).
- Unless triggered by mutagens or radiation, whether by accident or purposefully to create mutants for research, mutation is spontaneous and random and is not an adaptive response to change in environmental conditions, as some students think.
- Students do not always see that a shift in the reading frame of a gene (frameshift) only arises from deletion or insertion of bases in a number indivisible by three, and not by substitution.
- Students also get confused by the concept of the degeneracy of the genetic code (61 coding combinations for only 20 amino acids) and may present the argument the wrong way round, i.e. instead of more than one codon coding for the same amino acid they suggest that one codon can code for more than one amino acid.
- The lac operon is generally well understood by students but they do persist in incorrectly referring to the main circular DNA molecule of a prokaryote as a plasmid, and in thinking that this DNA might be single-stranded, which it is not.
- It is important that teachers and students make the link between transcriptional control and homeotic genes as ‘super-regulators’ of transcription of suites of genes during embryonic development.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course.
The mutation and control topics reinforce the teaching of section 2.1.3 (nucleic acids, including protein synthesis and the genetic code). It also helps prepare for the rest of section 6 (genetics, genome manipulation and biotechnology).
In classical genetics mutants are used to identify wild-type genes. Only when an altered gene produced a different phenotype can the existence of the wild type gene be inferred and steps taken to locate it through breeding experiments and cross-over linkage group (chromosome) mapping. Knock-out mutants are routinely used to see how breaking one part of a complex system affects the functioning of the whole organism. This concept can be explored through examining the work of Beadle and Tatum and Paul Nurse, and by looking at the contribution of fruit flies and other genetic model organisms to our understanding of how gene mutations impact on cellular biochemistry.
Some rare human mutations produce phenotypes that in the popular imagination resemble mythical creatures like mermaids, werewolves and giants. Students could be set an assignment using the links in the activities below to investigate whether these actually exist. This context should be explored at the end of the unit as transcriptional control, homeotic genes and apoptosis are all relevant to understanding disorders like hypertrichosis (werewolf syndrome), sirenomelia (mermaid syndrome), auriculocondylar syndrome (a human homeotic mutation) and syndactyly (webbed digits).
A practical context in which to consider the phenotypic effects of homeotic genes is suggested at the end of this section.
This animation explains the Beadle and Tatum experiments that led to the ‘one gene one enzyme’ hypothesis in 1941. It could be shown to stimulate class discussion of how we ‘know’ that a gene is a length of DNA that codes for one polypeptide. Useful related content links are listed beneath the animation.
Whether read individually or by the class as a group, this transcript can be used to reinforce the idea that the study of mutants leads to important discoveries of what genes do. Here the focus on a gene controlling the cell cycle in yeast can be used to show how work on small parts of small organisms is important in the fight against cancer. The Lee and Nurse experiment repairing a yeast cdc2 mutant with a human gene shows how important regulatory genes like cell cycle genes and homeobox genes are highly conserved.
Here students can compare wild-type Drosophila with two mutants in each of four categories: wing shape, body colour, eye colour and head shape. A suitable way to use this resource would be to ask students to draw up a table summarising the name of each mutation, which of the four chromosomes it is located on, whether it is dominant or recessive and the phenotype it produces. The final mutant, antennapedia, leads on to homeobox mutations and the idea that a single gene can have far-reaching effects on the phenotype through controlling a suite of other genes.
This webpage provides a springboard to explore in more depth seven important genetic model organisms: bacteriophage, maize, Arabidopsis, nematode, Drosophila, rodent and human. An open-ended assignment on ‘What model organisms tell us’ or instructions to students to pick three of the organisms and to summarise their contribution to our understanding should help impress upon students how it is the study of mutants that reveals information about ‘normal’ genes and the biochemical pathways they influence.
The 'Human mutants – tv series (YouTube clips)' link can be used to explore some rare mutations in humans. The clips (shown via the YouTube platform) come from the 2003 television series Human Mutants presented by Armand Marie Leroi, professor of evolutionary developmental biology at Imperial College, London. A book related to the series can be found on Amazon, see link - Book - Mutants: On the Form, Varieties and Errors of the Human Body.
Names to search for to find further information are Jesus Chuy Aceves (hypertrichosis) and Milagros Cerron, Tiffany Yorks and Shiloh Pepin (sirenomelia). Students tend to be fascinated by these unusual mutations in humans but sensitivity is needed to instil proper respect amongst students for differences in human appearance and functioning.
A practical context in which to consider homeotic genes is to provide specimens of a prawn and a locust for examination and to ask students to summarise in a table the features of each segment and to consider which segments match across the two organisms, i.e. which have the same Hox genes active. A sample worksheet is provided. The Homeotic Genes and Body Patterns (University of Utah) link given above has a useful diagram to help make sense of this exercise.
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