𧬠Nucleotides, DNA Structure & Packaging
Nucleic Acids β Introduction
- Nucleic acids are polymers of nucleotides that carry genetic information. They are the fundamental molecules of life, responsible for storing and transmitting hereditary instructions from one generation to the next.
- First discovered by Friedrich Miescher (1869) from pus cells (white blood cells); he called it "nuclein". This was the first isolation of what we now know as DNA.
- Two types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). DNA serves as the permanent genetic blueprint, while RNA acts as a temporary working copy used in protein synthesis.
Chemical Composition of Nucleic Acids
Nitrogenous Bases
| Type | Bases | Ring Structure | Mnemonic |
|---|---|---|---|
| Purines | Adenine (A), Guanine (G) | Double ring (9-membered) | Pure As Gold |
| Pyrimidines | Cytosine (C), Thymine (T), Uracil (U) | Single ring (6-membered) | CUT the Py |
- Thymine is found only in DNA.
- Uracil is found only in RNA (replaces thymine).
- 5-Methyluracil = Thymine β thymine is the methylated form of uracil (extra -CHβ group).
Nucleosides and Nucleotides
| Component | Composition | Examples |
|---|---|---|
| Nucleoside | Nitrogenous base + Sugar | Adenosine, Guanosine, Cytidine, Thymidine, Uridine |
| Nucleotide | Nitrogenous base + Sugar + Phosphate group | AMP, GMP, CMP, TMP, UMP |
- Sugar in DNA: 2'-Deoxyribose (lacks -OH at 2' position).
- Sugar in RNA: Ribose (has -OH at 2' position).
- Nucleotides are linked by 3'-5' phosphodiester bonds to form a polynucleotide chain.
Nucleoside Table
| Base | Nucleoside (DNA) | Nucleoside (RNA) |
|---|---|---|
| Adenine | Deoxyadenosine | Adenosine |
| Guanine | Deoxyguanosine | Guanosine |
| Cytosine | Deoxycytidine | Cytidine |
| Thymine | Deoxythymidine | β |
| Uracil | β | Uridine |
TIP
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Nucleic Acids β Introduction
- Nucleic acids are polymers of nucleotides that carry genetic information. They are the fundamental molecules of life, responsible for storing and transmitting hereditary instructions from one generation to the next.
- First discovered by Friedrich Miescher (1869) from pus cells (white blood cells); he called it "nuclein". This was the first isolation of what we now know as DNA.
- Two types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). DNA serves as the permanent genetic blueprint, while RNA acts as a temporary working copy used in protein synthesis.
Chemical Composition of Nucleic Acids
Nitrogenous Bases
| Type | Bases | Ring Structure | Mnemonic |
|---|---|---|---|
| Purines | Adenine (A), Guanine (G) | Double ring (9-membered) | Pure As Gold |
| Pyrimidines | Cytosine (C), Thymine (T), Uracil (U) | Single ring (6-membered) | CUT the Py |
- Thymine is found only in DNA.
- Uracil is found only in RNA (replaces thymine).
- 5-Methyluracil = Thymine β thymine is the methylated form of uracil (extra -CHβ group).
Nucleosides and Nucleotides
| Component | Composition | Examples |
|---|---|---|
| Nucleoside | Nitrogenous base + Sugar | Adenosine, Guanosine, Cytidine, Thymidine, Uridine |
| Nucleotide | Nitrogenous base + Sugar + Phosphate group | AMP, GMP, CMP, TMP, UMP |
- Sugar in DNA: 2'-Deoxyribose (lacks -OH at 2' position).
- Sugar in RNA: Ribose (has -OH at 2' position).
- Nucleotides are linked by 3'-5' phosphodiester bonds to form a polynucleotide chain.
Nucleoside Table
| Base | Nucleoside (DNA) | Nucleoside (RNA) |
|---|---|---|
| Adenine | Deoxyadenosine | Adenosine |
| Guanine | Deoxyguanosine | Guanosine |
| Cytosine | Deoxycytidine | Cytidine |
| Thymine | Deoxythymidine | β |
| Uracil | β | Uridine |
TIP
DNA nucleosides have the prefix "Deoxy-" while RNA nucleosides do not. Thymine is exclusive to DNA; Uracil is exclusive to RNA.
Discovery of DNA as Genetic Material
| Year | Scientist(s) | Contribution |
|---|---|---|
| 1869 | Friedrich Miescher | Discovered "nuclein" (DNA) from pus cells |
| 1928 | Frederick Griffith | Transformation experiment |
| 1944 | Avery, MacLeod, McCarty | Proved DNA is the transforming principle |
| 1950 | Erwin Chargaff | Base pairing ratios |
| 1952 | Hershey and Chase | Confirmed DNA is genetic material (T2 phage + radioactive isotopes) |
| 1953 | Watson and Crick | Double helix model |
Griffith's Transformation Experiment (1928)
- Used Streptococcus pneumoniae β two strains:
- S strain (Smooth) β virulent, has polysaccharide capsule.
- R strain (Rough) β non-virulent, lacks capsule.
| Treatment | Result |
|---|---|
| Live S strain β Mouse | Dies |
| Live R strain β Mouse | Lives |
| Heat-killed S β Mouse | Lives |
| Heat-killed S + Live R β Mouse | Dies (Transformation) |
- Conclusion: A "transforming principle" from heat-killed S converted R strain into S. Later identified as DNA.
IMPORTANT
Griffith did NOT identify DNA as the transforming principle β only that transformation occurs. That proof came from Avery et al. in 1944.
Avery, MacLeod & McCarty Experiment (1944)
- Purified the transforming principle from heat-killed S strain; tested with enzymes:
- Protease (degrades protein) β Transformation still occurred.
- RNase (degrades RNA) β Transformation still occurred.
- DNase (degrades DNA) β Transformation did NOT occur.
- Conclusion: DNA is the genetic material (transforming principle).
Hershey-Chase Experiment (1952)
- Used T2 bacteriophage infecting E. coli.
- Labeled DNA with Β³Β²P (radioactive phosphorus); protein with Β³β΅S (radioactive sulfur).
- After infection + blending + centrifugation:
- Β³Β²P (DNA) found inside bacterial cells (pellet).
- Β³β΅S (protein) found outside in supernatant.
- Conclusion: DNA (not protein) enters the host and directs phage reproduction.
TIP
Why Β³Β²P and Β³β΅S? DNA contains phosphorus but no sulfur. Protein contains sulfur but little phosphorus. This allowed selective tracking.
Image Generation Prompt
A step-by-step diagram of the Hershey-Chase experiment: (1) T2 phage with P-32 labeled DNA and S-35 labeled protein coat, (2) phage infecting E. coli bacteria, (3) blending to separate phage ghosts from bacteria, (4) centrifugation showing P-32 in pellet (inside bacteria) and S-35 in supernatant (phage coats). Label each step clearly.
Chargaff's Rules (1950)
- A = T and G = C (complementary base pairing).
- A + G = T + C (total purines = total pyrimidines).
- A+T / G+C ratio varies between species (species-specific) but is constant within a species.
- A/T = 1 and G/C = 1.
Watson and Crick Model of DNA (1953)
James Watson and Francis Crick proposed the double helix model. Nobel Prize: 1962 (shared with Wilkins). Based on:
- X-ray crystallography data of Rosalind Franklin and Maurice Wilkins.
- Chargaff's base composition rules.
Key Features of B-DNA
- Right-handed double helix β two antiparallel polynucleotide strands.
- Antiparallel β one strand 5'β3'; the other 3'β5'.
- Sugar-phosphate backbone on outside; bases project inward.
- Base pairing (complementary):
- A = T β 2 hydrogen bonds
- G β‘ C β 3 hydrogen bonds
- Diameter = 20 Γ (2 nm)
- Pitch (one complete turn) = 34 Γ (3.4 nm)
- Rise per base pair = 3.4 Γ (0.34 nm)
- Base pairs per turn = 10 bp
- Major groove (wider) and minor groove (narrower).
TIP
Quick memory trick for dimensions: 2-3.4-0.34-10 β 2 nm diameter, 3.4 nm pitch, 0.34 nm per bp, 10 bp per turn.
Image Generation Prompt
A detailed diagram of the Watson-Crick DNA double helix showing: antiparallel strands (5'β3' and 3'β5'), A=T (2 H-bonds) and Gβ‘C (3 H-bonds), major/minor grooves, and dimensions (3.4 nm pitch, 2 nm diameter, 0.34 nm per bp, 10 bp per turn). Clean scientific illustration.
Types of DNA
| Feature | A-DNA | B-DNA | Z-DNA |
|---|---|---|---|
| Helix direction | Right-handed | Right-handed | Left-handed |
| Diameter | 23 Γ | 20 Γ | 18 Γ |
| Base pairs per turn | 11 | 10 | 12 |
| Rise per bp | 2.56 Γ | 3.4 Γ | 3.7 Γ |
| Conditions | Dehydrated DNA | Physiological | High salt concentration |
IMPORTANT
B-DNA is the most common form in living cells. Z-DNA is the only left-handed form β a frequent exam question.
Palindromic Sequences
- DNA sequences that read the same on both strands in the 5'β3' direction.
- Example: EcoRI recognition site:
5'βGAATTCβ3' 3'βCTTAAGβ5' - Are recognition sites for restriction endonucleases (molecular scissors used in genetic engineering).
DNA Packaging
Problem
- Human DNA: ~2 meters long, must fit inside a nucleus of ~5 ΞΌm diameter (~10,000-fold compaction).
Nucleosome Model (Roger Kornberg, 1974)
- Nobel Prize in 2006 for studies on eukaryotic transcription.
- Nucleosome = fundamental unit of chromatin.
- Structure: ~146 bp of DNA wrapped 1.65 turns around a histone octamer.
- Histone octamer = 2 copies each of H2A, H2B, H3, H4.
- Linker DNA = ~54 bp connecting adjacent nucleosomes.
- Histone H1 = linker histone; seals the nucleosome (not part of the octamer).
- "Beads on a string" appearance (10 nm fiber) under electron microscope.
Levels of DNA Packaging
| Level | Structure | Diameter |
|---|---|---|
| 1 | DNA double helix | 2 nm |
| 2 | Nucleosome ("beads on string") | 10 nm |
| 3 | Solenoid (30 nm fiber) | 30 nm |
| 4 | Chromatin loops (anchored to nuclear scaffold) | 300 nm |
| 5 | Chromatid | 700 nm |
| 6 | Metaphase chromosome | 1400 nm |
TIP
Remember the diameter progression: 2 β 10 β 30 β 300 β 700 β 1400 nm.
Euchromatin vs Heterochromatin
| Feature | Euchromatin | Heterochromatin |
|---|---|---|
| Packaging | Loosely packed | Tightly packed |
| Transcription | Active | Inactive |
| Staining | Lightly stained | Darkly stained |
| Replication | Early S phase | Late S phase |
Key Points to Remember
- Miescher (1869) discovered nuclein from pus cells; nucleotides = base + sugar + phosphate
- Thymine only in DNA; Uracil only in RNA; 5-methyluracil = Thymine
- Griffith (1928) β transformation; Avery (1944) β DNA is transforming principle; Hershey-Chase (1952) β DNA enters host (Β³Β²P in pellet)
- Chargaff: A=T, G=C; purines = pyrimidines
- B-DNA: right-handed, 10 bp/turn, 3.4 nm pitch, 2 nm diameter; A=T (2H bonds), Gβ‘C (3H bonds)
- Z-DNA = only left-handed form
- Nucleosome model (Kornberg, 1974): ~146 bp around H2A/H2B/H3/H4 octamer; H1 = linker histone
- Packaging: 2 nm β 10 nm (beads) β 30 nm (solenoid) β 1400 nm (chromosome)
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Nucleic acids | Polymers of nucleotides; two types: DNA and RNA |
| Discovered by | Friedrich Miescher (1869) from pus cells; called it "nuclein" |
| Purines | Adenine (A), Guanine (G) β double ring; mnemonic: Pure As Gold |
| Pyrimidines | Cytosine (C), Thymine (T), Uracil (U) β single ring; mnemonic: CUT the Py |
| Thymine vs Uracil | Thymine only in DNA; Uracil only in RNA; 5-Methyluracil = Thymine |
| Nucleoside vs Nucleotide | Nucleoside = Base + Sugar Nucleotide = Base + Sugar + Phosphate |
| DNA sugar vs RNA sugar | DNA = 2'-Deoxyribose (no -OH at 2') RNA = Ribose (has -OH at 2') |
| Polynucleotide linkage | 3'-5' phosphodiester bonds |
| Griffith (1928) | Transformation in S. pneumoniae; heat-killed S + live R β mouse dies; did NOT identify DNA |
| Avery, MacLeod, McCarty (1944) | Proved DNA is transforming principle; DNase stopped transformation |
| Hershey-Chase (1952) | T2 phage; Β³Β²P (DNA) in pellet; Β³β΅S (protein) in supernatant; DNA = genetic material |
| Chargaff's Rules (1950) | A = T, G = C; purines = pyrimidines; A+T/G+C ratio is species-specific |
| Watson-Crick Model (1953) | Double helix; Nobel 1962 (with Wilkins); based on X-ray data of Rosalind Franklin |
| B-DNA dimensions | Diameter = 2 nm, Pitch = 3.4 nm, Rise/bp = 0.34 nm, 10 bp/turn |
| H-bonds in base pairing | A=T β 2 H-bonds; Gβ‘C β 3 H-bonds |
| B-DNA features | Right-handed; antiparallel (5'β3' and 3'β5'); major and minor grooves |
| Z-DNA | Only left-handed form; 18 A diameter; 12 bp/turn; high salt |
| A-DNA | Right-handed; 23 A diameter; 11 bp/turn; dehydrated |
| Palindromic sequences | Read same on both strands 5'β3'; restriction enzyme recognition sites |
| Human DNA length | ~2 meters; fits in ~5 ΞΌm nucleus |
| Nucleosome model | Roger Kornberg (1974); Nobel 2006 |
| Nucleosome structure | ~146 bp wrapped 1.65 turns around histone octamer (2x each: H2A, H2B, H3, H4) |
| Linker DNA and H1 | Linker = ~54 bp; H1 = linker histone (seals nucleosome) |
| Packaging progression | 2 β 10 β 30 β 300 β 700 β 1400 nm |
| Euchromatin | Loosely packed, active transcription, lightly stained, early S phase |
| Heterochromatin | Tightly packed, inactive, darkly stained, late S phase |
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