Unit 1: Introduction to biophysics - Subjective Questions

Biomolecules are the organic molecules that form the structural and functional basis of all living organisms. They participate in the maintenance, growth, and reproduction of cells.

Major categories of biomolecules:

• Carbohydrates: Composed of carbon, hydrogen, and oxygen (general formula ). Examples: glucose, starch, cellulose.
• Proteins: Polymers of amino acids linked by peptide bonds. Examples: enzymes, hemoglobin, collagen.
• Lipids: Hydrophobic molecules such as fats, phospholipids, and steroids. Examples: triglycerides, cholesterol.
• Nucleic acids: Polymers of nucleotides carrying genetic information. Examples: DNA and RNA.

Significance:

• They provide energy (carbohydrates, lipids).
• They act as structural components (proteins, lipids).
• They store and transmit genetic information (nucleic acids).
• They catalyze reactions (enzymes).

Chemical bonds hold biomolecules together and determine their structure and function. They are broadly divided into strong (covalent) and weak (non-covalent) interactions.

1. Covalent bonds:

• Formed by sharing of electron pairs between atoms.
• Strong, with bond energies of .
• Example: peptide bonds, phosphodiester bonds.

2. Ionic (electrostatic) bonds:

• Attraction between oppositely charged groups.
• Example: salt bridges between and in proteins.

3. Hydrogen bonds:

• Attraction between a hydrogen atom bound to an electronegative atom and another electronegative atom.
• Bond energy .
• Example: base pairing in DNA, secondary structure of proteins.

4. Van der Waals forces:

• Weak, short-range attractions from transient dipoles.
• Important in molecular packing and recognition.

5. Hydrophobic interactions:

• Tendency of nonpolar groups to cluster away from water.
• Drive protein folding and membrane formation.

Biological relevance: These bonds collectively stabilize macromolecular structures, mediate molecular recognition, and allow reversible interactions crucial for life processes.

DNA can adopt several conformations depending on hydration, base sequence, and ionic conditions.

Key points:

• B-DNA is the most common physiological form described by Watson and Crick.
• A-DNA forms under dehydrating conditions and is seen in DNA-RNA hybrids.
• Z-DNA has a zig-zag backbone and may play a role in gene regulation.

The Watson-Crick model (1953) describes DNA as a double helix.

Structural features:

• Two polynucleotide strands wound around a common axis.
• The strands are antiparallel ( and ).
• The sugar-phosphate backbone lies on the outside; bases point inward.
• The helix is right-handed with ~10 base pairs per turn and a rise of per base pair (B-form).

Complementary base pairing:

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Significance of complementary pairing:

• Explains Chargaff's rules (, ).
• Provides the basis for semiconservative replication — each strand acts as a template.
• Enables accurate transmission of genetic information.
• Underlies techniques like PCR and hybridization.

Transcription is the process by which the genetic information in a DNA template is copied into a complementary RNA molecule by the enzyme RNA polymerase.

Stages of transcription:

1. Initiation:

• RNA polymerase binds to the promoter region of the gene.
• The DNA double helix unwinds locally, forming a transcription bubble.

2. Elongation:

• RNA polymerase moves along the template strand in the direction.
• RNA is synthesized in the direction.
• Ribonucleotides are added following base-pairing rules (A-U, G-C); note that uracil (U) replaces thymine.

3. Termination:

• RNA synthesis stops at a terminator sequence.
• The newly formed RNA and RNA polymerase are released.

Post-transcriptional processing (in eukaryotes):

• 5' capping
• 3' polyadenylation (poly-A tail)
• Splicing to remove introns

Result: A functional messenger RNA (mRNA) that carries the genetic code to ribosomes for translation.

Both DNA and RNA are nucleic acids made of nucleotide monomers, but they differ in several key aspects.

Key structural note: The presence of the reactive -hydroxyl group in RNA makes it more susceptible to hydrolysis and allows it to adopt complex folded structures. The absence of thymine (replaced by uracil) is another defining difference.

Conformational changes in DNA refer to the structural transitions the DNA molecule undergoes in response to environmental and sequence-dependent factors, switching between forms such as A, B, and Z-DNA.

Factors influencing DNA conformation:

• Hydration/humidity: High humidity favors B-DNA; low humidity favors A-DNA.
• Salt concentration: High ionic strength (e.g., high , ) can promote Z-DNA.
• Base sequence: Alternating purine-pyrimidine sequences (e.g., ) favor Z-DNA.
• Supercoiling: Negative supercoiling stabilizes underwound structures like Z-DNA.
• Methylation: Cytosine methylation can favor Z-form.
• Protein binding: Proteins can bend, unwind, or stabilize particular conformations.

Biological importance:

• Conformational flexibility allows DNA to interact with regulatory proteins.
• Local conformational changes are important in replication, transcription, and recombination.
• Bending and looping enable long-range gene regulation.

RNA molecules perform diverse structural, catalytic, and informational roles in the cell.

Major types of RNA:

• Messenger RNA (mRNA):

  • Carries genetic information from DNA to ribosomes.
  • Serves as the template for protein synthesis.

• Transfer RNA (tRNA):

  • Adaptor molecule (~76-90 nucleotides) with a cloverleaf secondary structure.
  • Carries specific amino acids to the ribosome; contains an anticodon.

• Ribosomal RNA (rRNA):

  • Structural and catalytic component of ribosomes.
  • Catalyzes peptide bond formation (ribozyme activity).

• Small nuclear RNA (snRNA):

  • Involved in splicing of pre-mRNA (part of the spliceosome).

• MicroRNA (miRNA) and small interfering RNA (siRNA):

  • Regulate gene expression by silencing target mRNAs.

• Long non-coding RNA (lncRNA):

  • Regulatory roles in chromatin remodeling and transcription.

Significance: RNA acts not only as a genetic messenger but also as a catalyst and regulator, supporting the RNA world hypothesis.

Unlike DNA, single-stranded RNA folds into complex secondary and tertiary structures that are essential for its function.

Basis of RNA folding:

• Intramolecular base pairing: Complementary regions within the same strand pair to form stems and loops.
• -OH group: The reactive hydroxyl provides additional hydrogen-bonding and catalytic potential.
• Base stacking: Hydrophobic and van der Waals interactions between stacked bases stabilize helical regions.

Common secondary structural motifs:

• Hairpin/stem-loop
• Bulge loops
• Internal loops
• Pseudoknots

Tertiary interactions:

• Coaxial stacking of helices.
• Metal ion (e.g., ) coordination, which neutralizes phosphate charge and stabilizes folding.
• Non-canonical base pairs and tertiary contacts.

Thermodynamics:

• Folding is driven by a favorable free energy change () from base pairing and stacking.
• The folded state represents a minimum in the free energy landscape.

Functional consequence: Folding allows RNA to act as an enzyme (ribozyme), form functional cores of ribosomes and tRNA, and regulate gene expression.

The stability of the DNA double helix arises from a combination of hydrogen bonding and base stacking interactions.

1. Hydrogen bonding:

• Occurs between complementary bases: A=T (2 bonds) and G≡C (3 bonds).
• Provides specificity of base pairing rather than being the dominant source of stability.
• The total contribution depends on GC content; higher GC gives more hydrogen bonds and higher melting temperature .

2. Base stacking:

• Adjacent base pairs stack on top of one another due to van der Waals and hydrophobic interactions between the planar aromatic rings.
• This is the major stabilizing force of the double helix.

Thermodynamic view:
The overall free energy of duplex formation can be expressed as:

• The enthalpy () contribution comes largely from stacking and hydrogen bonds.
• The entropy term () reflects loss of conformational freedom and release of ordered water.

Melting temperature relationship:

• increases with GC content and ionic strength.
• A common approximation: for short oligonucleotides.

Conclusion: While hydrogen bonds confer specificity, base stacking provides the bulk of the thermodynamic stability of the double helix.

A peptide bond is a covalent amide linkage formed between the carboxyl group () of one amino acid and the amino group () of another, with the elimination of a water molecule (condensation reaction).

Structural/biophysical characteristics:

• Partial double-bond character: Due to resonance, the C-N bond has ~40% double-bond character, making it rigid and planar.
• Planarity: The six atoms of the peptide unit (, C, O, N, H, ) lie in a plane.
• Trans configuration: The trans form is favored to minimize steric clashes.
• No free rotation: Rotation is restricted around the C-N bond but allowed around the (phi) and (psi) angles.

Significance:

• The rigidity and defined geometry of peptide bonds determine the backbone conformation of proteins.
• The allowed and values are represented in the Ramachandran plot.

The central dogma of molecular biology, proposed by Francis Crick (1958), describes the directional flow of genetic information within a biological system.

Main flow:

Processes:

• Replication: DNA is copied to produce identical DNA molecules.
• Transcription: Information in DNA is transcribed into mRNA.
• Translation: mRNA is decoded by ribosomes to synthesize proteins.

Special cases (exceptions):

• Reverse transcription: RNA → DNA (in retroviruses, using reverse transcriptase).
• RNA replication: RNA → RNA (in some RNA viruses).

Significance:

• Explains how genetic information is stored, transmitted, and expressed.
• Forms the foundation for molecular biology, genetics, and biotechnology.
• Highlights the central role of RNA as an intermediary between DNA and proteins.

Amino acids are the building blocks of proteins. Each amino acid consists of a central alpha carbon () bonded to four groups.

General structure:

• Amino group ()
• Carboxyl group ()
• Hydrogen atom ()
• Side chain / R group (variable)

At physiological pH, amino acids exist as zwitterions ( and ).

Classification based on R group:

• Nonpolar (hydrophobic): e.g., alanine, valine, leucine.
• Polar uncharged: e.g., serine, threonine, glutamine.
• Acidic (negatively charged): e.g., aspartate, glutamate.
• Basic (positively charged): e.g., lysine, arginine, histidine.

How properties determine protein structure:

• Hydrophobic residues cluster in the protein interior, driving folding.
• Charged/polar residues face the aqueous exterior.
• Cysteine forms disulfide bonds stabilizing tertiary structure.
• Proline introduces kinks; glycine provides flexibility.

Conclusion: The chemical nature of side chains governs folding, stability, and function of the resulting protein.

Hydrophilic and hydrophobic interactions describe how molecules behave in relation to water and are central to the organization of biological structures.

Hydrophilic interactions:

• "Water-loving" — favorable interaction with water molecules.
• Involve polar and charged groups (e.g., , , ).
• Stabilized by hydrogen bonds and ion-dipole interactions.

Hydrophobic interactions:

• "Water-fearing" — nonpolar groups avoid contact with water.
• Nonpolar molecules aggregate to minimize disruption of water structure.
• Driven largely by an increase in entropy of surrounding water.

Comparison:

Role in biomolecular organization:

• Protein folding: Hydrophobic core, hydrophilic surface.
• Membrane formation: Phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward.
• Molecular recognition and self-assembly of macromolecular complexes.

The melting temperature () of DNA is the temperature at which 50% of the double-stranded DNA has denatured (separated) into single strands.

Denaturation:

• Heat disrupts hydrogen bonds and base stacking, separating the two strands.
• Monitored by an increase in UV absorbance at 260 nm — the hyperchromic effect.

Factors affecting :

• GC content: More G-C pairs (3 H-bonds) raise . A useful approximation:
  
• Ionic strength: Higher salt concentration shields phosphate repulsion, stabilizing the duplex and raising .
• pH: Extreme pH disrupts hydrogen bonds and lowers stability.
• Length of DNA: Longer molecules generally have higher .
• Denaturants: Urea and formamide lower .

Significance:

• Used in PCR primer design, hybridization, and characterization of DNA sequences.
• Reflects the stability of the double helix.

A nucleotide is the monomeric unit of nucleic acids (DNA and RNA), consisting of three components.

Components of a nucleotide:

• Nitrogenous base: A purine (adenine, guanine) or pyrimidine (cytosine, thymine/uracil).
• Pentose sugar: Deoxyribose (DNA) or ribose (RNA).
• Phosphate group(s): One or more phosphate groups attached to the carbon.

A nucleoside is the base + sugar (without phosphate); adding phosphate makes it a nucleotide.

Joining of nucleotides — phosphodiester bond:

• Nucleotides are linked by phosphodiester bonds between the -OH group of one sugar and the -phosphate of the next.
• This creates a sugar-phosphate backbone with directionality ().

Result:

• A polynucleotide chain with a free -phosphate at one end and a free -OH at the other.
• The sequence of bases encodes genetic information.

DNA supercoiling refers to the over- or under-winding of the DNA double helix, causing the molecule to coil upon itself. It arises because DNA in cells is topologically constrained (e.g., circular bacterial DNA or looped eukaryotic DNA).

Types of supercoiling:

• Positive supercoiling: Overwinding (extra twists in the same direction as the helix).
• Negative supercoiling: Underwinding (twists opposite to helix direction); most cellular DNA is negatively supercoiled.

Topological relationship:

where = linking number, = twist, and = writhe.

Enzymes controlling supercoiling:

• Topoisomerase I: Relaxes supercoils by cutting one strand.
• Topoisomerase II (gyrase): Introduces negative supercoils using ATP.

Biological importance:

• Compaction of large DNA into the cell.
• Facilitates strand separation during replication and transcription.
• Regulates gene expression and stabilizes alternative conformations like Z-DNA.

RNA world hypothesis:
The hypothesis proposes that early life relied on RNA as both the genetic material and the catalyst, before DNA and proteins evolved.

Roles of RNA proposed in the RNA world:

• Information storage (like DNA today).
• Catalysis of biochemical reactions (like enzymes today).
• Self-replication capabilities.

Roles of RNA in modern cells:

• mRNA: Carries genetic message.
• tRNA: Adaptor in translation.
• rRNA: Catalyzes peptide bond formation.
• Regulatory RNAs: miRNA, siRNA, lncRNA control gene expression.

Catalytic ability of RNA — ribozymes:

• Ribozymes are RNA molecules with enzymatic activity.
• Examples: the ribosome (peptidyl transferase activity of rRNA), self-splicing introns, and RNase P.
• Catalysis is possible because RNA can fold into complex 3D shapes and use the reactive -OH group and coordinated metal ions.

Significance:

• The discovery of ribozymes (Cech and Altman, Nobel Prize 1989) provided strong evidence for the RNA world.
• RNA bridges the gap between information storage and catalysis, supporting its ancestral role.

In the B-form DNA double helix, the two sugar-phosphate backbones are unequally spaced, creating two grooves of different sizes.

Major groove:

• Wider (~22 Å) and deeper region.
• Exposes more base-specific chemical information (hydrogen bond donors/acceptors, methyl groups).

Minor groove:

• Narrower (~12 Å) region.
• Provides less sequence-specific information.

Origin of grooves:

• The grooves arise because the glycosidic bonds of a base pair are not diametrically opposite each other, but form angles that create unequal spacing.

Biological significance:

• DNA-binding proteins (e.g., transcription factors) primarily read the sequence through the major groove, since it exposes distinctive patterns for each base pair.
• Some proteins and drugs (e.g., certain antibiotics) bind the minor groove.
• Groove recognition is fundamental to gene regulation, replication, and repair.

Proteins are organized into four hierarchical levels of structure, each stabilized by specific interactions.

1. Primary structure:

• The linear sequence of amino acids joined by peptide bonds (covalent).
• Determines all higher levels of structure.

2. Secondary structure:

• Local folding into regular patterns such as the -helix and -pleated sheet.
• Stabilized primarily by hydrogen bonds between backbone and groups.

3. Tertiary structure:

• The overall 3D folding of a single polypeptide chain.
• Stabilized by:
  • Hydrophobic interactions (major driving force)
  • Hydrogen bonds
  • Ionic bonds (salt bridges)
  • Disulfide bonds (covalent, between cysteines)
  • Van der Waals forces

4. Quaternary structure:

• Assembly of multiple polypeptide subunits into a functional complex (e.g., hemoglobin).
• Stabilized by the same non-covalent interactions and sometimes disulfide bonds.

Significance: The correct folding through these levels is essential for protein function; misfolding can cause diseases (e.g., Alzheimer's).