Nucleotides and nucleic acids (2.1.3)
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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:
|2.1.3 Nucleotides and nucleic acids|
|(a)||the structure of a nucleotide as the monomer from which nucleic acids are made||To include the differences between RNA and DNA nucleotides, the identification of the purines and pyrimidines and the type of pentose sugar.
An opportunity to use computer modelling to investigate nucleic acid structure.
|(b)||the synthesis and breakdown of polynucleotides by the formation and breakage of phosphodiester bonds|
|(c)||the structure of ADP and ATP as phosphorylated nucleotides||Comprising a pentose sugar (ribose), a nitrogenous base (adenine) and inorganic phosphates.|
|(d)||(i) the structure of DNA (deoxyribonucleic acid)
(ii) practical investigations into the purification of DNA by precipitation
|To include how hydrogen bonding between complementary base pairs (A to T, G to C) on two antiparallel DNA polynucleotides leads to the formation of a DNA molecule, and how the twisting of DNA produces its ‘double-helix’ shape.
|(e)||semi-conservative DNA replication||To include the roles of the enzymes helicase and DNA polymerase, the importance of replication in conserving genetic information with accuracy and the occurrence of random, spontaneous mutations.
|(f)||the nature of the genetic code||To include the triplet, non-overlapping, degenerate and universal nature of the code and how a gene determines the sequence of amino acids in a polypeptide (the primary structure of a protein).|
|(g)||transcription and translation of genes resulting in the synthesis of polypeptides.||To include, the roles of RNA polymerase, messenger (m)RNA, transfer (t)RNA, ribosomal (r)RNA.
2.1.3(a) – (d) build up to an understanding of the double helix structure of DNA. Students need to gain confident familiarity with the different parts and bonds of the DNA molecule in order to cope with comprehending the replication (e) and protein synthesis (f–g) topics that follow, as well as later sections of the syllabus (see conceptual links in Thinking Conceptually) that build on this knowledge.
While a grasp of the significance of the linear coding nature of the base-pair sequence supported by the sugar-phosphate backbones is the first goal, it is important that students are trained to be precise in using the biochemical vocabulary to describe the DNA molecule (see student misconceptions in Thinking Conceptually).
Use of videos, 3D animations and models should be backed up with student labelling of 2D diagrams and written tasks which encourage the correct use and spelling of the key terms. Where internet-based learning resources go into more detail than textbooks, the syllabus and OCR legacy past questions and mark schemes provide a guide to the level of detail and accuracy students should attain.
This worksheet gives a tried and tested protocol for extracting DNA from onions in a school laboratory. Ideally students will carry out the whole procedure themselves but if time is short the onion filtrate can be pre-prepared and students can just perform the final extraction step.
High-quality graphics and an easy to follow soundtrack makes this a useful teaching tool. The level of detail is greater than that required by the syllabus so students should be guided to concentrate on absorbing the basics specified on the syllabus rather than the extra details. This animation and other high-quality video clips from this company are available through YouTube. If your school network does not allow access to YouTube, there are instructions at the following link for downloading and incorporating videos into PowerPoints.
For a complete list of other molecular biology and biology animations from this source see link 'Other biology animations'.
Students type in the matching mRNA sequence and then act as tRNAs by matching mRNA codons with the correct amino acids, using a table of the genetic code.
Other learning resources for this module can be accessed via the link 'Molecules of inheritance'.
This is simply a sheet listing words that students should be able to define and relate to each other when they have studied this module. It can be used in various ways:
- use at the start of the topic for students to assess their initial familiarity or to tick off as work progresses
- colour-code or join groups of words judged by students to belong together
- categorise the words in a table or mind diagram as molecules, processes, monomers, polymers, organelles, enzymes, bonds etc.
- enclose words related to the same topic or level of organisation within lines of different colours to make a Venn diagram
- cut out the words and rearrange them into groups according to different criteria e.g. order of size of molecules or cell structures, order of use of each component in protein synthesis
- define the words
- use the words to play hangman or a ‘pictionary’ style game
- match the words to the syllabus outcomes (a-g)
- use at end of topic to write a summary of topic 2.1.3 using each word in the correct context at least once.
Approaches to teaching the content
As a fundamental underpinning of Biology, DNA structure needs to be taught rigorously and the significance of its three functions made clear:
- coding biologically useful information via transcription and therefore acting as a blueprint for building cells and bodies
- ensuring continuity within a multicellular organism and between generations through semi-conservative replication
- allowing evolution to occur by generating variation through mutation.
Common misconceptions or difficulties students may have
There are a number of common misconceptions that surface in student exam answers. These include:
- Misunderstanding that the double helix means that a DNA molecule is composed of two strands. It is common for students to refer to the whole double-stranded molecule as a ‘strand of DNA’
- Using the words base and nucleotide interchangeably and inappropriately, for example, saying that new bases come in and pair up with DNA during replication
- Confusion between transcription and translation, and between replication and transcription
- Misuse of the verb ‘transcribe’, for example, saying the mRNA transcribes the DNA rather than that the DNA is transcribed (passive) into mRNA
- Failing to discriminate between DNA, mRNA and protein sequences
- Misunderstanding that the word mutation refers to a change in the DNA, not in mRNA and protein sequences
- Failing to adequately discriminate between the gene (DNA) and the gene product (protein) in contextual questions.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
A thorough knowledge of this section is a prerequisite for study of cellular control (6.1.1), patterns of inheritance (6.1.2), manipulating genomes (6.1.3) and cloning (6.2.1 a-d). It is also important to relate molecular changes at the DNA level to classification and evolution (4.2.2) as well as to biodiversity (4.2.1a). Knowledge of DNA structure and function should inform work on the cell cycle (2.1.6) also, particularly the relationship between the DNA double helix and whole chromosome structure, and the occurrence of DNA replication at S of interphase in the cell cycle. The relationship of the two daughter DNA molecules to sister chromatids should also be stressed to make the link between the molecular level and whole cell functioning. In section 2.1.1(i) students tend to concentrate on the ribosome and Golgi body, overlooking the role of the nucleus, so the role of DNA and the enzymes involved in transcription should be stressed. For all work on applications of genetics, it is vitally important that students retain a working knowledge of the structure of the DNA molecule and of the link between the base sequences in DNA and mRNA and the amino acid sequence in proteins.
An activity designed to consolidate student knowledge of the structure of DNA. There is now a plethora of different ways of producing DNA models (see the two links), but students are expected to show increasing understanding of the way in which components fit together. Consider spending some time with the nucleotide ‘molecule’ containing Adenine to show how this is a precursor to ADP and ATP (2.1.2c).
The activity is best completed in small groups of 3-4 to enable students to construct a double helix using criteria shown in Learner resource 2.
- Using chosen materials e.g. molecular modelling kits or paper, build at least four nucleotides per student, representing A, T, C and G. They must choose a suitable colour key for different components and maintain this key throughout the activity.
- Once the nucleotides are made, join these to form a single-stranded polynucleotide. (The strand should contain at least 10 nucleotides which can be joined in random sequence to provide some interest in later activities involving translation and transcription).
- Using this strand as a template, single nucleotides can now be added (refer to Learner resource 2 for criteria). These are then joined to form the second strand once the ‘complementary base-pairing rule’ has been observed. More nucleotides may need to be made at this stage dependent on the sequence chosen.
- Finally twist the molecule - how many nucleotides per twist are found in the double helix?
Save this molecule for use in further activities.
This website is a Manitoba government website but has a conservation section with a good worksheet example for model DNA.
Another example for completing DNA models.
The first four resources (in the activities) place understanding DNA structure and function within its historical context. Some students engage more positively with this human interest angle than with the structural formulae and technical vocabulary of the subject. The history of science context also demonstrates that the knowledge in textbooks is hard-won and took years to elucidate. A third advantage of this approach is that it shows how the scientific community operates and the degree of competition and collaboration between researchers.
Many of the applied contexts within which knowledge of DNA is important arise later in the course, though with a linear scheme of assessment there is no reason why key topics of interest should not be cherry-picked from later modules and introduced early to give students an idea of how DNA structure is relevant to modern life. Topics such as DNA fingerprinting for forensics or wildlife forensics, testing for genetic diseases, sequencing a person’s DNA and applications of genetic modification could be taught to reinforce knowledge of DNA structure or the link between genes and proteins. A link to online activities relating to four applications is given (in the activities).
‘Life Story’, also re-named ‘The Race for the Double Helix’ (BBC Horizon, 1987). This film is available on the educational video-sharing service Clickview for those schools that subscribe. Copies also exist online on various video-sharing sites. The drama is an hour long but provides a fascinating context for understanding the structure of DNA and the excitement generated by this discovery.
Learner resource 3, the ‘Life Story Checklist of Scientists’ sheet, lists the main characters to help students follow the story.
Follow on questions on Learner resource 4, the ‘Discovering the Structure of DNA’ worksheet (Year 12 version), focus on the structure of DNA.
This is a 17 minute video on the discovery of the structure of DNA if the BBC Life Story version is considered too long.
This covers key breakthroughs in the study of genetics and inheritance in each decade of the 20th century. Information about personalities and their research will familiarise students with key facts like Chargaff’s ratios of bases and the structure of the DNA molecule.
Before completing this activity, students should have prior knowledge of the history of DNA and some of the key individuals involved in its discovery.
Having constructed a model DNA molecule in the activity ‘Building DNA - advanced version!’, students now have the opportunity to look at the evidence that was available to Watson and Crick in their quest to discover the secrets of the DNA molecule.
Using some of the data from Chargaff’s research, students are encouraged to complete simple mathematical calculations involving ratios (M0.3) and formulate their own conclusions.
Problems to be solved include Finding the Structure (basic DNA molecule structure), Copying the Code (DNA replication), Reading the Code (protein synthesis) and Controlling the Code (gene regulation).
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© OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.
OCR acknowledges the use of the following content:
Learner Resource 2: Molecular diagram: Antiparallel nature of the DNA double helix © Visionlearning, Inc • Learner Resource 5: Watson and Crick with their DNA model: A. Barrington Brown/Science Photo Library