Structural and computational biology
Structural biology studies the 3D shape of biomolecules, because shape decides function. This unit explains why structures are determined, how X-ray crystallography does it in four steps, and how the Protein Data Bank makes structures freely available. It then introduces computational biology and the two kinds of homology (sequence and structural) used to study evolutionary relationships.
Learning outcomes (7)
- Define structural biology.
- Explain that structure determination of biomolecules is important.
- Describe how X-ray crystallography works.
- Outline the online databases where biomolecule structures are available.
- Describe computational biology.
- Define sequence homology.
- Define structural homology.
12.1Structural biology
Biotechnology is an emerging and ever growing branch of biology. It has many subfields, including genetic engineering, synthetic biology, structural biology, computational biology and bioinformatics. This unit deals with structural biology and computational biology.
- Structural biology
- The study of the 3D (three-dimensional) structure and organisation of biological molecules, such as proteins, nucleic acids (DNA and RNA), lipids and carbohydrates, to understand their function, interactions and role in biological processes.
What structural biology emphasises
- Determining the molecular structure.
- Understanding the molecular function.
- Understanding how macromolecular complexes are formed.
Techniques used
- X-ray crystallography.
- Nuclear Magnetic Resonance (NMR) spectroscopy.
- Cryo Electron Microscopy (Cryo-EM).
- Molecular dynamics simulations.
- Bioinformatics and computational modelling.
Aims of structural biology
- Explaining biological mechanisms.
- Developing new treatments.
- Designing novel biomolecules.
- Understanding disease mechanisms.
Applications
The advances in structural biology are applied in drug discovery and development, vaccine design, enzyme engineering, cancer research and the study of neurological disorders.
12.2The importance of structural determination of biomoleculesOften examined
Determining the three-dimensional (3D) structure of biomolecules such as proteins, DNA and RNA is crucial for understanding how they function in our bodies. This information is essential for understanding biological processes, developing life-saving treatments and advancing biotechnology. Continued research in this field will unlock new discoveries, improving human health and quality of life.
Five reasons (a to e)
| Reason | What the structure allows | The book's example |
|---|---|---|
| a. Understanding function and mechanism | The structure of a biomolecule reveals its function. Knowing it helps us understand how biomolecules perform specific tasks. | Enzyme catalysis and protein-protein recognition. |
| b. Designing life-saving treatments | Accurate structural information lets researchers design drugs that bind to a biomolecule and inhibit or activate it, using its shape and chemical properties. | Breakthroughs in treating cancer, HIV and Alzheimer's disease. |
| c. Understanding disease mechanisms | By studying the structure of disease-associated biomolecules, we can identify vulnerabilities to target with treatments. | Understanding the structure of the influenza virus has informed vaccine development. |
| d. Advancing biotechnology | Knowing biomolecular structures enables the design of novel enzymes, biosensors and biomaterials. | Advancements in biofuels, agriculture and biomedical devices. |
| e. Enabling personalised medicine | Understanding individual variations in biomolecular structures lets researchers develop tailored treatments. | Promise in cancer treatment and genetic disorders. |
Structure decides function: the key and lock idea
The book compares a biomolecule with a lock. The structure of the key determines how it fits into the lock. In the same way, the structure of a biomolecule determines how it interacts with other molecules. Only a partner with the matching shape can bind, so a change in shape changes the function.
- Recall the principle: the structure of a biomolecule reveals its function, and only a molecule with a matching shape and chemical properties can bind to its active site (key and lock).
- The 3D structure shows the exact shape and chemical properties of the active site.
- Researchers can therefore design a molecule (the drug) that fits that site and binds to it specifically.
- Once bound, the drug can inhibit the biomolecule (block its function) or activate it, depending on what the treatment needs.
- Because the drug is made for one shape, the therapy is targeted. The book gives the treatment of cancer, HIV and Alzheimer's disease as examples of this approach.
Structure gives shape and chemistry of the site; the drug is designed to fit it; binding inhibits or activates the biomolecule.
12.3X-ray crystallographyOften examined
X-ray crystallography is a powerful tool for determining the 3D structure of biomolecules such as proteins, DNA and RNA. It has revolutionised our understanding of biomolecular structure and function, enabling breakthroughs in medicine, biotechnology and materials science.
The technique uses X-rays that scatter off the electrons in a crystallised biomolecule. This produces a diffraction pattern that reveals the molecule's atomic structure. It has four steps.
- Step 1, crystallisation. The biomolecule is dissolved in a solution, and the solvent is slowly cooled or evaporated so that a crystal lattice forms. The crystal must be of high quality, with uniform orientation and minimal defects.
- Step 2, X-ray diffraction. The crystal is exposed to a beam of X-rays, which scatter off the electrons in the biomolecule. This produces a diffraction pattern of intense spots (reflections). The pattern is captured on a detector such as a CCD camera.
- Step 3, data collection and processing. Specialised software indexes, integrates and scales the diffraction data to give a set of reflection intensities. These represent the amplitude of the scattered X-rays and are used, through mathematical techniques, to reconstruct the 3D electron density map of the biomolecule. The map shows the arrangement of atoms within the molecule.
- Step 4, model building and refinement. Software is used to build a molecular model into the electron density map. The final result is a precise 3D structure of the biomolecule, showing its atomic arrangement, bond lengths and angles.
| Step | Starting material | Result |
|---|---|---|
| 1. Crystallisation | Biomolecule in solution | A high-quality crystal with a regular lattice |
| 2. X-ray diffraction | The crystal and an X-ray beam | A diffraction pattern of spots recorded by a detector (CCD camera) |
| 3. Data collection and processing | Reflection intensities | The 3D electron density map |
| 4. Model building and refinement | The electron density map | The final 3D structure: atoms, bond lengths and angles |
This information is essential for understanding biological mechanisms, designing drugs and engineering biomolecules.
- The book says the crystal must be of high quality, with uniform orientation and minimal defects. The crystal is the sample that scatters the X-rays.
- In step 2 the diffraction pattern is produced by X-rays scattering off the electrons in the regularly arranged molecules. A poor crystal scatters in an irregular way, so the spots (reflections) are not clear.
- In step 3 the electron density map is calculated from the reflection intensities. Unclear reflections give an unreliable map.
- In step 4 the model is built into the electron density map, so a poor map gives a poor 3D structure.
- Each step depends on the one before it, so a poor crystal spoils every later step. This is why the quality of the crystal in step 1 is so important.
A poor crystal gives unclear diffraction spots, an unreliable electron density map and a poor structure model.
12.4Online database for biomolecule structures: the Protein Data Bank (PDB)
- Protein Data Bank (PDB)
- A freely accessible online database (www.wwpdb.org) that stores and provides 3D structural information of biomolecules, including proteins, nucleic acids, lipids and carbohydrates.
- Established in 1971.
- Managed by the Worldwide Protein Data Bank (wwPDB) consortium, which comprises leading research institutions and organisations.
- The database has grown exponentially and now contains over 180,000 biomolecule structures.
- It contains 3D atomic coordinates for biomolecules, structural information, and experimental and functional data.
- It is a cornerstone for biomolecular research, providing essential structural information to advance our understanding of biological processes and develop innovative treatments.
As an example, the PDB entry for insulin is shown in Figure 12.2. Insulin has two polypeptide chains, the A chain of 21 amino acids and the B chain of 30 amino acids. In the book's drawing each amino acid is a circle labelled with its three-letter name and the chains are joined by bridges between cysteine (Cys) residues. The book also shows the 3D ribbon of the molecule with its N and C ends marked.
- The top row is the A chain. Counting its circles from H2N to COOH gives 21 amino acids (Gly at position 1 to Asn at position 21).
- The second and third rows together are the B chain. It starts with Phe at position 1 and ends with Thr at position 30 (HOOC), so it has 30 amino acids.
- The total is 21 + 30 = 51 amino acids.
- The circles joined by highlighted lines are all Cys (cysteine). The lines are A6 to A11 inside the A chain, A7 to B7 and A20 to B19 between the two chains, so there are 3 bridges, each between two Cys residues.
A chain 21, B chain 30, total 51 amino acids; 3 Cys-Cys bridges.
- P is the upper chain, the A chain (21 amino acids). Q is the lower chain, the B chain (30 amino acids).
- R is a bridge that joins two Cys residues inside the same chain (A6 to A11).
- S marks the bridges that join the two chains (A7 to B7 and A20 to B19).
- T is the left end of the upper chain, the free amino end, written H2N in Figure 12.2.
P A chain, Q B chain, R bridge within a chain, S bridges between chains, T amino (H2N) end.
12.5Computational biology
- Computational biology
- An interdisciplinary field that combines computer science, mathematics and biology to analyse and model biological systems. It uses computational tools and methods to understand complex biological processes, interpret large datasets and make predictions.
Fields related to computational biology
Computer science, mathematics and biology are the three fields combined. Figure 12.3 shows the key areas of computational biology.
Applications
- Personalised treatments based on individual genetic profiles.
- Understanding gene regulation and function.
- Modelling protein 3D structures.
- Rapid analysis of large data.
- Understanding complex biological systems and disease mechanisms.
- Phylogenetic and synthetic biology.
Tools and methods
- Algorithms.
- Machine learning.
- Database management.
- Programming languages.
By integrating computer science and biology, computational biology continues to advance our understanding of life and improve human health.
12.6Computational evolutionary biology
Computational evolutionary biology uses computational methods and algorithms to study evolution, particularly by analysing large data sets of genomics, transcriptomics and other related biological information.
- Homology
- The similarity of the structure, physiology or development of different species of organisms based upon their descent from a common evolutionary ancestor.
Two kinds of homology are discussed in the next sections: sequence homology and structural homology.
12.7Sequence homology
- Sequence homology
- The study of similarities and relationships between biological sequences, such as DNA, RNA or protein sequences. It is the presence of shared sequences or structural features between two or more biological molecules, indicating an ancestral origin.
This concept is fundamental to understanding evolutionary relationships, functional conservation and genetic diversity. Sequence homology provides a powerful tool for understanding the intricate relationships between biological sequences, shedding light on the complex history of life on Earth.
Reading a sequence comparison (alignment)
Figure 12.4 in the book shows the sequence of the protein BauA in 25 samples, one below the other. Parts of the sequence that are the same in most samples are conserved. BauA is a protective antigen found in mice, and that is why its sequence is conserved in most samples. A small version of the same idea is drawn below, with the conserved positions shaded.
- Compare the sequences position by position. A line joins two identical letters and a cross marks a difference.
- Positions 3 (T and S), 6 (I and L) and 11 (Q and E) differ, so there are 3 differences.
- The other positions are identical, so 12 - 3 = 9 positions are the same out of 12.
- Identical positions are shared sequence. A high number of shared positions suggests that the two molecules come from a common ancestor, which is what sequence homology means.
9 of 12 positions identical, 3 different: the molecules are probably related by descent.
12.8Structural homology
- Structural homology
- Similarity in 3D structures among biological molecules, such as proteins, RNA or DNA. It reveals shared architectural features, indicating evolutionary relationships or functional conservation. It is the presence of similar 3D structures or substructures between two or more biological molecules, suggesting a common ancestral origin or functional analogy.
Structural homology provides valuable insights into the shared architectural features of biological molecules, illuminating the complex relationships between structure, function and evolution.
Figure 12.5 in the book shows the structural homology of the myoglobin molecule in different mammals: human, horse, sperm whale, sea turtle and tuna are placed on a branching tree, and the 3D shape of myoglobin is similar in all of them.
Sequence homology and structural homology compared
| Point | Sequence homology | Structural homology |
|---|---|---|
| What is compared | The order of nucleotides or amino acids in DNA, RNA or protein sequences | The 3D structures of molecules such as proteins, RNA or DNA |
| What is shared | Shared sequences between two or more molecules | Similar 3D structures or substructures |
| What it indicates | Ancestral origin, functional conservation and genetic diversity | A common ancestral origin or functional relationship, shared architecture |
| Example in the book | BauA protein sequence in 25 samples (Figure 12.4) | Myoglobin in different mammals (Figure 12.5) |
- Ask what is being compared: an order of letters, or a 3D shape.
- (a) The order of amino acids is a sequence, so this is sequence homology (Figure 12.4).
- (b) A 3D shape is a structure, so this is structural homology (Figure 12.5).
- (c) The order of bases in an RNA molecule is a sequence, so it is sequence homology. If it is similar in all organisms, it shows that they have evolved from a common ancestor.
(a) sequence, (b) structural, (c) sequence homology.
Key terms
- Structural biology
- The study of the 3D structure and organisation of biological molecules to understand their function, interactions and role in biological processes.
- 3D structure
- The three-dimensional shape and organisation of a biomolecule, which determines how it functions and interacts.
- Biomolecules
- Biological molecules such as proteins, nucleic acids (DNA and RNA), lipids and carbohydrates.
- Nuclear Magnetic Resonance (NMR) spectroscopy
- A technique used in structural biology to study molecular structure.
- Cryo Electron Microscopy (Cryo-EM)
- A technique used in structural biology to study molecular structure.
- Molecular dynamics simulations
- Computer simulations used in structural biology.
- Macromolecular complex
- A large assembly of biomolecules; structural biology studies how such complexes form.
- Drug discovery and development
- An application of structural biology in which drugs are designed from the structure of a target biomolecule.
- Vaccine design
- An application of structural biology, for example the use of the structure of the influenza virus.
- Enzyme engineering
- An application of structural biology in which novel enzymes are designed from known structures.
- Targeted therapy
- A treatment designed from accurate structural information to bind and inhibit or activate a specific biomolecule.
- Personalised medicine
- Treatment tailored to individual variations in biomolecular structures.
- X-ray crystallography
- A technique that uses X-rays scattered by the electrons of a crystallised biomolecule to produce a diffraction pattern that reveals its atomic structure.
- Crystallisation
- Step 1 of X-ray crystallography: dissolving the biomolecule and slowly cooling or evaporating the solvent to form a high-quality crystal lattice.
- Crystal lattice
- The regular arrangement of molecules in a crystal.
- Diffraction pattern
- The pattern of intense spots (reflections) produced when X-rays scatter off the electrons of a crystal; recorded by a detector such as a CCD camera.
- Reflection intensities
- Values obtained after the diffraction data are indexed, integrated and scaled; they represent the amplitude of the scattered X-rays.
- 3D electron density map
- The map of the biomolecule reconstructed mathematically from the reflection intensities; it shows the arrangement of atoms.
- Model building and refinement
- Step 4 of X-ray crystallography: building a molecular model into the electron density map to give the final 3D structure with atomic arrangement, bond lengths and angles.
- Protein Data Bank (PDB)
- A freely accessible online database that stores and provides 3D structural information of biomolecules.
- Worldwide Protein Data Bank (wwPDB)
- The consortium of leading research institutions and organisations that manages the PDB.
- Atomic coordinates
- The positions of atoms in 3D space that the PDB stores for biomolecules.
- Computational biology
- An interdisciplinary field that combines computer science, mathematics and biology to analyse and model biological systems.
- Algorithms
- Computational tools used in computational biology.
- Machine learning
- A computational method used in computational biology.
- Computational evolutionary biology
- The use of computational methods and algorithms to study evolution by analysing large data sets of genomics, transcriptomics and other biological information.
- Homology
- The similarity of the structure, physiology or development of different species based upon descent from a common evolutionary ancestor.
- Sequence homology
- The study of similarities and relationships between biological sequences (DNA, RNA or protein), indicating shared ancestral origin.
- Conserved sequence
- A sequence that stays the same in most samples, as the BauA sequence in 25 samples.
- Structural homology
- Similarity in 3D structures among biological molecules, indicating evolutionary relationships or functional conservation.
Multiple choice questionsp. 242–243
- Asize
- Bnumber of amino acids
- C3D structureAnswer
- Dtypes of amino acids
Protein function depends on the three-dimensional shape the chain folds into. That shape creates the active site or binding surface that fits another molecule (the key and lock idea in section 12.2).
The number and types of amino acids set the sequence, but it is the folded 3D shape that decides how the protein works.
- Aspread of diseases
- Bmechanism of diseasesAnswer
- Ccause of diseases
- Dsymptoms of diseases
Section 12.2 (c) explains that studying the structure of disease-associated biomolecules shows how a disease works and where it can be treated. That is the mechanism.
Option C is tempting, but structure shows how the molecule goes wrong, not the wider causes such as infection. The spread and the symptoms of a disease are not explained by molecular structure.
- Aactive site of enzyme and substrate
- Bhormones and their receptors
- Cantigens recognized by antibodies
- Dall of aboveAnswer
In each pair, one molecule fits the other by shape: the substrate fits the active site of an enzyme, a hormone fits its receptor, and an antigen fits the binding site of an antibody. So all three pairs are correct.
Option A alone is only one example of the model, so it is not the full answer.
- Abacterial infections
- Bviral infectionsAnswer
- Cmetabolic diseases
- Dsyndromes
Targeted therapy uses a drug designed to bind one specific biomolecule. The book's examples are HIV, a viral infection, in section 12.2 (b), and the influenza virus, whose structure guided vaccine development, in section 12.2 (c).
A syndrome is a group of symptoms rather than one molecular target, so option D is not the best answer.
- Adifferent name
- Bdifferent effect on each person
- Cspecific cell surface markerAnswer
- Ddifferent set of symptoms
Personalized and targeted treatment works by binding a specific feature of the cancer cells. A specific cell surface marker is such a feature, so each cancer needs a medicine chosen for its own marker.
Option D describes the results of the disease, not the target a drug can bind. Option A is only a name, and option B describes the outcome for each person rather than the reason the treatment differs.
- A3D electron maps are formed on solids
- BX-ray diffraction is done on crystalsAnswer
- Csolutions on evaporation form solids
- Dgases on slow cooling can form liquids
X-ray diffraction needs a crystal. The regular repeat of molecules in a crystal produces the sharp diffraction spots (section 12.3, steps 1 and 2). Liquids and gases do not form such a regular lattice, so no clear pattern is obtained.
Options A, C and D describe solids or changes of state. They do not explain why the method cannot be used on liquids or gases.
- Aby using VPN
- Bafter paying require fee
- Cfreely via internetAnswer
- Dif you get reference letter from concerned department
The Protein Data Bank is a freely accessible online database (www.wwpdb.org), as section 12.4 states. No VPN, payment or letter is needed to use it.
- Ause ribosomes for protein
- Bhave same size of ribosomes
- Chave evolved from common ancestorAnswer
- Dhave same types of ribosomes
Sequence homology is a shared sequence that indicates a common ancestral origin (section 12.7). The rRNA sequence is shared by all organisms, so all organisms have evolved from a common ancestor.
Options A and D describe what ribosomes do or what type they are, and option B is about size. None of these is shown by sequence homology.
- Ait gives their meat red colour
- Bsimilar shapes are related with similar functionsAnswer
- Cmyoglobin can replace haemoglobin function when required
- Ddifferent mammals eat each other so they can use it in their bodies
Structural homology means that similar 3D structures can indicate a common ancestry or functional analogy (section 12.8). Similar shapes are linked with similar functions, so option B is the best answer.
Options A, C and D are not supported by the book's text.
Note. The book's caption to Fig. 12.5 says "different mammals", but the figure includes tuna, which is a fish. The correct wording is "different vertebrates".
- Aall molecules
- Ball proteins
- Call enzymes
- Dall biomoleculesAnswer
The PDB stores 3D structures of biomolecules, including proteins, nucleic acids, lipids and carbohydrates (section 12.4). Option D covers all of them.
Option A is too broad because the database is about biomolecules, not every molecule. Options B and C cover only part of what is stored.
Short answer questionsp. 243
Three points link structure with function:
- Shape decides fit. The 3D shape of a biomolecule determines which molecules it can bind to, as in the lock and key model: the active site of an enzyme fits its substrate.
- Chemical properties decide interactions. The arrangement of charged, polar and non-polar groups on the surface decides how the molecule interacts with other molecules, for example in protein-protein recognition.
- Change in structure changes function. Because the shape controls binding, altering the structure alters or stops the molecule's task.
Structural biology gives three reasons for hope:
- Targeted drugs: knowing the shape and chemical properties of a cancer-related biomolecule lets researchers design drugs that bind it specifically and inhibit or activate it.
- Disease mechanism: the structure shows how the molecule works in the cancer cell, so the vulnerable point to attack can be found.
- Personalized treatment: individual variation in biomolecular structure allows tailored treatments, which the book says show promise in cancer treatment.
- Search the Protein Data Bank (www.wwpdb.org) for a stored 3D structure of pepsin. The PDB holds more than 180,000 biomolecule structures with their atomic coordinates.
- If no structure is stored, determine it by X-ray crystallography: crystallize pure pepsin, expose the crystal to X-rays, record the diffraction pattern, reconstruct the electron density map and build the atomic model.
- Examine the 3D structure, including the arrangement of atoms, bond lengths and angles, to relate its shape to how the enzyme works.
Computational biology combines three fields of science:
| Field | Role in computational biology |
|---|---|
| Computer science | algorithms, machine learning, database management and programming languages |
| Mathematics | mathematical methods to model biological systems and analyse data |
| Biology | the biological systems, processes and sequences that are modelled and analysed |
| Term | Definition |
|---|---|
| Structural biology | the study of the 3D structure and organization of biological molecules, such as proteins, nucleic acids, lipids and carbohydrates, to understand their function and interactions |
| X-ray crystallography | a technique in which X-rays scattered by the electrons of a crystallized biomolecule form a diffraction pattern, from which a 3D electron density map and the atomic structure are obtained |
| Computational biology | an interdisciplinary field that combines computer science, mathematics and biology to analyse and model biological systems, interpret large datasets and make predictions |
| PDB (Protein Data Bank) | a freely accessible online database (www.wwpdb.org) that stores 3D structural information of biomolecules |
| Sequence homology | the presence of shared sequences of DNA, RNA or protein between two or more biological molecules, indicating a common ancestral origin |
| Structural homology | the presence of similar 3D structures or substructures between two or more biological molecules, suggesting a common ancestral origin or functional analogy |
Extensive answer questionsp. 243
Biomolecules such as proteins, DNA and RNA carry out their functions through their three-dimensional shape. Determining that structure is necessary for five reasons, which the book gives in section 12.2 (a) to (e).
Understanding function and mechanism
The structure reveals the function. Like a key fitting a lock, the shape decides how a molecule interacts with other molecules. This explains enzyme catalysis and protein-protein recognition.
Designing life-saving treatments
Knowing the shape and chemical properties of a biomolecule allows drugs that specifically bind to it and inhibit or activate it. This has led to treatments for cancer, HIV and Alzheimer's.
Understanding disease mechanisms
Studying the structure of disease-associated biomolecules shows where the disease can be attacked. Knowing the structure of the influenza virus informed vaccine development.
Advancing biotechnology
Knowing biomolecular structures enables the design of novel enzymes, biosensors and biomaterials. This has advanced biofuels, agriculture and biomedical devices.
Enabling personalized medicine
Understanding individual variation in biomolecular structure allows tailored treatments. This approach shows promise in cancer treatment and genetic disorders.
X-ray crystallography determines the three-dimensional structure of a biomolecule in four steps. Each step produces the input for the next one, as Fig. 12.1 shows.
Steps
- Crystallization. The biomolecule is dissolved in a solution, which is then cooled slowly or left to evaporate so that the solvent leaves a crystal lattice. The crystal must be of high quality, with uniform orientation and few defects, because the regular repeat is what makes a clear pattern.
- X-ray diffraction. A beam of X-rays is directed at the crystal. The X-rays scatter off the electrons of the biomolecule and produce a diffraction pattern of intense spots (reflections), which is captured on a detector such as a CCD camera.
- Data collection and processing. Specialized software indexes, integrates and scales the diffraction data to give a set of reflection intensities. These represent the amplitudes of the scattered X-rays. Mathematical techniques use them to reconstruct a 3D electron density map, which shows how the atoms are arranged in the molecule.
- Model building and refinement. A molecular model is built into the electron density map with software and refined. The final result is a precise 3D structure that gives the atomic arrangement, bond lengths and angles of the biomolecule.
Both sequence homology and structural homology show a similarity inherited from a common ancestor, but they compare different things at different levels of organization.
| Point | Sequence homology | Structural homology |
|---|---|---|
| Molecules compared | DNA, RNA and protein sequences | 3D structures of proteins, RNA and DNA |
| What is compared | the order of nucleotides or amino acids, position by position | the overall three-dimensional shape and the arrangement of its parts |
| Meaning of similarity | a shared sequence indicates a common ancestral origin | a similar shape suggests a common ancestral origin or functional analogy |
| Example from the book | the BauA sequence is conserved in most of the 25 samples (Fig. 12.4) | myoglobin in human, horse, sperm whale, sea turtle and tuna has a similar 3D structure (Fig. 12.5) |
Key difference
Sequence homology looks at the order of the letters in a sequence, while structural homology looks at the folded shape. Two molecules can have different sequences but a similar 3D structure, as panel (b) shows.
FBISE 2026 questions
Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.
- AFins of fish and wings of birds
- BHuman arm and whale flipperAnswer
- CWings of birds and insects
- DEye of human and eye of octopus
Homologous structures are similar because the organisms descended from a common ancestor. The human arm and the whale flipper are built on the same bone plan inherited from a common ancestor, even though they are used differently.
The other pairs have a similar function but a different origin: fins of fish and wings of birds, wings of birds and insects, and the eyes of a human and an octopus all do a similar job in unrelated structures. Similar function without a common ancestral structure is not homology.
a. Sequence homology (1.5 marks)
Sequence homology is the presence of shared sequences between two or more DNA, RNA or protein molecules, which indicates a common ancestral origin. It helps to understand evolutionary relationships, functional conservation and genetic diversity.
Example: the sequence of the protein BauA in 25 samples is the same in most positions (conserved). Another example is the rRNA sequence, which shows homology in all organisms.
b. Structural homology (1.5 marks)
Structural homology is the similarity of the 3D structures of biological molecules such as proteins, RNA or DNA. It shows shared architectural features and suggests a common ancestral origin or functional relationship.
Example: the 3D shape of the myoglobin molecule is similar in human, horse, sperm whale, sea turtle and tuna.
| Point | Sequence homology | Structural homology |
|---|---|---|
| Compared | Order of nucleotides or amino acids | 3D shape of the molecule |
| Example | BauA protein in 25 samples | Myoglobin in different animals |
- ACompletely different organs
- BUnrelated species features
- CSimilar structure due to common ancestryAnswer
- DSimilar function but different structure
Homology is the similarity of structure, physiology or development of different species based on their descent from a common evolutionary ancestor. Structural homology is therefore a similar structure due to common ancestry.
Option D (similar function but different structure) describes analogous structures, which are the opposite case. Options A and B describe features that are different or unrelated, so they do not show homology.
Homologous structures have a common ancestral origin; analogous structures only have a similar function. Any three points of difference earn the marks.
| Point | Homologous structures | Analogous structures |
|---|---|---|
| Origin | Inherited from a common ancestor | Have different evolutionary origins, no common ancestral structure |
| Structure | Similar basic structure | Different basic structure |
| Function | May be used for different functions | Perform a similar function |
| Example | Human arm and whale flipper | Wings of birds and wings of insects |
Exam focus
- Differentiate between sequence homology and structural homology, with an example of each. (short)Set B Q.2 (xiii) asked both with examples (1.5+1.5) and book long question 3; MCQ 8 and 9 ask the rRNA and myoglobin examples.Go to answer
- Homology means similarity due to a common ancestor; homologous against analogous structures. (mcq)Asked three times in 2026: Set B MCQ 17, Set D MCQ 1 and Set D Q.2 (xiii) (differentiate homologous and analogous).Go to answer
- Describe the four steps of X-ray crystallography. (long)No paper question in 2026, but it is book long question 2 and the only technique the unit describes in detail.Go to answer
- Why is it necessary to determine the structure of biomolecules? (five reasons, key and lock) (long)No paper question in 2026; book long question 1 and MCQ 1 and 2.Go to answer
- Define PDB and read the insulin structure (A chain 21, B chain 30, three Cys bridges). (short)No paper question in 2026; book short question 5 and MCQ 7 and 10.Go to answer
- Define computational biology and name the fields related to it. (short)No paper question in 2026; book short question 4.Go to answer