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 UNIT -6

Information Transfer:The molecular basis of coding and decoding

genetic information is universal Molecular basis of information transfer.
DNA as a genetic material. Hierarchy of DNA structure- from single
stranded to double helix to nucleosomes. Concept of genetic code.
Universality and degeneracy of genetic code. De ne gene in terms of
complementation and recombination

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Information Transfer:The molecular basis of coding and decoding genetic information is universal
Molecular basis of information transfer. point wise explain in details

🧬 Molecular Basis of Information Transfer

✅ 1. Universality of the Genetic Code

• The genetic code is nearly universal across all organisms—from bacteria to humans.
• Each triplet of nucleotides (codon) in mRNA codes for a speci c amino acid.
• Example: AUG codes for Methionine (start codon) in almost all life forms.

✅ 2. DNA: The Genetic Blueprint

• DNA carries hereditary information in its sequence of nitrogen bases: A, T, G, C.

• Genes are speci c segments of DNA that code for proteins.

✅ 3. Replication (Copying the Genetic Information)

• Process by which DNA makes an exact copy of itself before cell division.
• Enzymes like DNA polymerase help in forming the new strands.
• Ensures that genetic information is passed accurately to daughter cells.
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✅ 4. Transcription (DNA → RNA)

• DNA is transcribed into mRNA using the enzyme RNA polymerase.

• Occurs in the nucleus (in eukaryotes).
• The mRNA is a complementary copy of the gene sequence.
• Uracil (U) replaces Thymine (T) in RNA.

✅ 5. RNA Processing (in Eukaryotes)

• The pre-mRNA undergoes processing before leaving the nucleus:

◦ Capping at 5′ end
◦ Poly-A tail at 3′ end
◦ Splicing to remove introns (non-coding regions)

✅ 6. Translation (mRNA → Protein)

• mRNA is read by ribosomes in the cytoplasm.

• Each codon on mRNA is matched by a tRNA carrying the corresponding amino acid.
• The amino acids are joined to form a polypeptide chain (protein).
• Start codon: AUG (Methionine)
• Stop codons: UAA, UAG, UGA
✅ 7. Role of RNA Types

• mRNA (messenger RNA): Carries genetic instructions from DNA.

• tRNA (transfer RNA): Brings amino acids to the ribosome.
• rRNA (ribosomal RNA): Forms part of the ribosome and catalyzes protein synthesis.

✅ 8. Gene Expression Regulation

• Gene expression is regulated at various stages:

◦ Transcriptional level (e.g., operons in bacteria)
◦ Post-transcriptional (e.g., RNA splicing)
◦ Translational and post-translational (protein modi cation)

✅ 9. Mutation and Its Impact

• A mutation is a change in the DNA sequence.

• Can alter the resulting protein’s structure and function.
• Mutations may be silent, harmful, or bene cial.

✅ 10. Central Dogma of Molecular Biology

• Coined by Francis Crick:

DNA → RNA → Protein
◦ This ow of genetic information is fundamental and universal in biology.
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DNA as a genetic material
✅ 1. Full Form:

• DNA = Deoxyribonucleic Acid

✅ 2. Discovery:

• Identi ed by Friedrich Miescher (1869)

• Proved as genetic material by Avery, MacLeod & McCarty (1944) and Hershey-Chase experiment
(1952)

✅ 3. Location:

• Found in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotes.

• Also present in mitochondria and chloroplasts (called extranuclear DNA).

✅ 4. Chemical Composition:

• Made of nucleotides, each consisting of:

◦ A deoxyribose sugar
◦ A phosphate group
◦ A nitrogen base (A, T, G, C)
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✅ 5. Structure (Watson and Crick Model - 1953):

• Double helix structure

• Two strands are antiparallel and complementary
• Base pairing: A–T, G–C (held by hydrogen bonds)

✅ 6. Function as Genetic Material:

• Stores genetic information in the sequence of bases.

• Replicates to pass genetic information to the next generation.
• Controls cell functions by directing protein synthesis via mRNA.

✅ 7. Stability:

• Chemically stable and can be repaired if damaged.

• Its double-stranded nature provides backup for genetic information.

✅ 8. Capable of Variation:

• Mutations in DNA lead to genetic diversity and evolution.

✅ 9. Experimental Proof:

• Avery–MacLeod–McCarty: Showed DNA is responsible for transformation.

• Hershey–Chase: Proved DNA, not protein, is the genetic material in viruses.
Hierarchy of DNA structure- from single stranded to double helix to nucleosomes
✅ 1. Nucleotides – The Building Blocks

• DNA is made up of nucleotides, which are the basic structural units.

• Each nucleotide consists of:
◦ Deoxyribose sugar
◦ Phosphate group
◦ Nitrogenous base: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
• Nucleotides link via phosphodiester bonds, forming a single-stranded chain.

✅ 2. Single-Stranded DNA (ssDNA)

• A linear chain of nucleotides bonded together.

• This strand serves as a template during replication and transcription.

✅ 3. Double-Stranded DNA – The Double Helix

• Two complementary strands pair and twist into a double helix.

• Base pairing:
◦ A pairs with T (2 hydrogen bonds)
◦ G pairs with C (3 hydrogen bonds)
• The two strands are antiparallel (5’→3’ and 3’→5’ directions).
• Discovered by Watson & Crick in 1953.
✅ 4. Supercoiling of DNA

• In order to t inside a cell, DNA must be supercoiled.

• Enzymes like topoisomerases help in introducing and relieving these supercoils.
• Supercoiling helps compact DNA and regulate gene expression.

✅ 5. Nucleosomes – DNA Packaging Begins

• In eukaryotes, DNA wraps around histone proteins forming nucleosomes.

• Each nucleosome consists of:
◦ ~146 base pairs of DNA
◦ Wrapped around a histone octamer (2 copies each of H2A, H2B, H3, H4)
◦ Linked by linker DNA, sometimes bound by H1 histone
• Nucleosomes resemble “beads on a string” under an electron microscope.

✅ 6. 30 nm Chromatin Fiber (Solenoid or Zigzag Structure)

• Nucleosomes coil and fold into a 30 nm ber to compact the DNA further.
• Stabilized by histone H1.
• Represents the next level of DNA condensation.
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✅ 7. Looping of Chromatin Fiber

• The 30 nm ber forms looped domains, attached to a protein scaffold.

• Helps in regulating gene expression by organizing active/inactive regions.

✅ 8. Higher-Order Chromatin Condensation

• During cell division, the chromatin bers undergo further condensation.

• Results in the formation of visible metaphase chromosomes.
• Each chromosome contains a single, continuous DNA molecule.
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 Concept of genetic code.

🧬 Concept of Genetic Code

✅ 1. De nition

• The genetic code is a set of rules that determines how the sequence of nucleotide bases (A, T/U, G,
C) in DNA or RNA is translated into amino acids, forming proteins.

✅ 2. Codons

• A codon is a sequence of three nucleotides (triplet) in mRNA.

• Each codon corresponds to one speci c amino acid or a stop signal.

✅ 3. Role in Protein Synthesis

• The genetic code allows cells to read the genetic instructions in DNA (via mRNA) and assemble
amino acids in the correct sequence to make proteins.

🔑 Key Features of the Genetic Code

✅ 1. Triplet Nature

• Each codon is made up of 3 bases (e.g., AUG, GCU).

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✅ 2. Degenerate Code

• Multiple codons can code for the same amino acid.

◦ Example: UCU, UCC, UCA, and UCG all code for serine.
✅ 3. Unambiguous

• Each codon codes for only one amino acid, not multiple.
✅ 4. Start Codon

• AUG is the universal start codon; codes for methionine and signals the start of translation.

✅ 5. Stop Codons

• UAA, UAG, UGA are stop codons; they signal the end of protein synthesis.

✅ 6. Universal Code

• The code is almost the same in all living organisms (from bacteria to humans).

✅ 7. Non-overlapping

• Codons are read one after another with no overlap.

✅ 8. Commaless

• No punctuation between codons; the sequence is read continuously.

 Universality and degeneracy of genetic code
🧬 1. Universality of the Genetic Code

✅ 1.1 De nition

• The genetic code is universal, meaning that the same codons specify the same amino acids in almost all
organisms, from bacteria to humans.
✅ 1.2 Examples

• AUG codes for methionine (start codon) in all known life forms.
• UUU always codes for phenylalanine in prokaryotes, plants, animals, etc.

✅ 1.3 Evidence

• Recombinant DNA from one organism can function in another (e.g., human insulin gene expressed in
bacteria).

✅ 1.4 Exceptions (Rare)

• Some mitochondria and certain protozoa show slight variations:

◦ In human mitochondria, UGA codes for tryptophan instead of being a stop codon.
✅ 1.5 Importance

• Universality supports the idea of a common evolutionary origin of life.

• Enables genetic engineering and biotechnology applications across species.
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🧬 2. Degeneracy of the Genetic Code

✅ 2.1 De nition

• The genetic code is degenerate, meaning more than one codon can specify the same amino acid.
✅ 2.2 Examples

• Leucine: coded by 6 codons (UUA, UUG, CUU, CUC, CUA, CUG)

• Arginine and Serine: also coded by 6 codons
• Methionine and Tryptophan: only 1 codon each (AUG and UGG)
✅ 2.3 Wobble Hypothesis

• Proposed by Francis Crick

• Explains that the third base in a codon is often less critical, allowing tRNA exibility in pairing.
◦ Example: GGU, GGC, GGA, GGG all code for glycine.
✅ 2.4 Importance of Degeneracy

• Provides protection against mutations:

◦ Silent mutations (change in base but not amino acid) do not affect protein function.
◦ Ensures functional stability despite genetic changes.
✅ 2.5 Evolutionary Advantage

• Degeneracy increases genetic robustness, allowing organisms to tolerate point mutations without lethal
effects.
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 De ne gene in terms of complementation and recombination

✅ 1. Gene (General De nition)

• A gene is the basic unit of heredity that codes for a functional product, usually a protein or RNA.

✅ 2. Gene as a Unit of Complementation

🔹 What is Complementation?

• Complementation occurs when two different mutations in a diploid organism restore the normal
phenotype because the mutations affect different genes.
🔹 Gene De nition:

• A gene is de ned as a unit of function in complementation analysis.

• If two mutations do not complement each other (no functional product is formed), they are in the same
gene.
🔹 Example:

• If two mutants (each with a different defective gene) produce a normal phenotype when combined, the
mutations are in different genes.
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✅ 3. Gene as a Unit of Recombination

🔹 What is Recombination?

• Recombination is the exchange of genetic material between two DNA molecules or chromosomes.
🔹 Gene De nition:

• A gene is also a unit of recombination, meaning it is the smallest segment of DNA within which no
recombination can be detected.
🔹 Cistron Concept (Seymour Benzer’s work):

• Cistron: Functional unit within DNA; equivalent to a gene in terms of coding a complete polypeptide.
• Recombination can occur within a gene, dividing it into smaller recombination units.
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How DNA acts as a Genetic Meterial [ Grif th experiment ]

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🧬 Grif th’s Experiment – Discovery of the "Transforming Principle"

✅ Background

• Frederick Grif th was studying Streptococcus pneumoniae, a bacterium that causes pneumonia.
• He aimed to understand how bacteria could change their form and virulence.

✅ Types of Bacteria Used

1. S strain (Smooth)
◦ Virulent (disease-causing)
◦ Has a polysaccharide capsule that protects it from the host immune system.
◦ Kills mice
2. R strain (Rough)
◦ Non-virulent (harmless)
◦ Lacks capsule, so it's destroyed by the host immune system.
◦ Mice survive
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✅ Conclusion by Grif th

• The harmless R strain was transformed into the deadly S strain.

• Some "transforming principle" from the dead S strain converted R strain into a virulent form.
• He did not identify what the transforming material was, but it carried genetic information.
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🔬 Follow-up Experiment – Avery, MacLeod, and McCarty (1944)

• Isolated components from heat-killed S strain (DNA, RNA, and proteins).

• Treated each with enzymes that destroy one type:
◦ Protease → destroyed proteins
◦ RNase → destroyed RNA
◦ DNase → destroyed DNA
• Only when DNA was destroyed, transformation did not occur.
✅ Conclusion:

• DNA is the "transforming principle" and thus the genetic material responsible for inheritance.

🧠 Why Grif th’s Experiment is Important

1. First experimental evidence that a chemical substance (later proven as DNA) can transfer genetic traits.
2. Led to the understanding that DNA carries hereditary information, not proteins.
3. Opened the path for molecular genetics and the discovery of the DNA double helix.
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 What is DNA Replication ? What is transcription ? What is translation ?

🧬 1. What is DNA Replication?

✅ De nition:
DNA replication is the process of making an exact copy of the DNA molecule before cell division.
✅ Key Features:
• Semi-conservative: Each new DNA molecule has one old strand and one new strand.
• Occurs in the nucleus (in eukaryotes) during S-phase of the cell cycle.
• Enzymes involved:
◦ Helicase: Unwinds the DNA double helix.
◦ DNA polymerase: Adds new nucleotides.
◦ Primase: Synthesizes RNA primer.
◦ Ligase: Seals gaps between DNA fragments.
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🧬 2. What is Transcription?
✅ De nition: Transcription is the process by which DNA is copied into mRNA (messenger RNA).
✅ Key Features:
• Template: One strand of DNA acts as a template.
• Enzyme: RNA polymerase builds the RNA strand.
• Base pairing: A–U (instead of A–T), G–C.
• Occurs in the nucleus (in eukaryotes).
• mRNA carries the genetic code from DNA to ribosomes for protein synthesis.

🧬 3. What is Translation?
✅ De nition: Translation is the process by which the genetic code in mRNA is read by ribosomes
to synthesize a protein (polypeptide).
✅ Key Features:
• Occurs in the cytoplasm at the ribosome.
• mRNA provides the codon sequence.
• tRNA brings the correct amino acids using its anticodon.
• rRNA is a structural part of ribosomes.
• Begins at the start codon (AUG) and ends at a stop codon (UAA, UAG, UGA).
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 The concept of central dogma .
🧬 The Concept of Central Dogma of Molecular Biology
The Central Dogma is a fundamental concept in molecular biology that describes the ow of genetic
information within a biological system.

✅ De nition

• The Central Dogma explains how genetic information in DNA is transcribed into RNA and then
translated into proteins, which perform cellular functions.

🔁 Flow of Information
DNA → RNA → Protein

✅ 1. DNA Replication

• DNA copies itself to pass genetic information to daughter cells.

• Ensures genetic continuity.

✅ 2. Transcription

• Information from DNA is transcribed into messenger RNA (mRNA).

• Occurs in the nucleus (in eukaryotes).
• Enzyme involved: RNA polymerase.
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✅ 3. Translation

• mRNA is translated into a protein at the ribosome.

• tRNA brings amino acids based on codons in the mRNA.
• Proteins are made, which carry out structural and functional roles in the cell.

✅ Exceptions (Reverse Flow)

Although the central dogma is mostly unidirectional, some exceptions exist:
• Reverse Transcription:
◦ RNA → DNA
◦ Seen in retroviruses (e.g., HIV), using reverse transcriptase enzyme.

🧠 Importance of Central Dogma

• Explains how genes control cell function by coding for proteins.

• Foundation for genetic engineering, molecular biology, and biotechnology.
• Essential to understanding gene expression and regulation.
 describe the different models proposed for DNA replication ( conservative , semi
conservative , dispersive )

🧬 Models of DNA Replication

Before the actual mechanism of DNA replication was con rmed, scientists proposed three models to explain
how DNA could be copied:

✅ 1. Conservative Model

• Explanation:
In this model, the entire original (parental) DNA molecule remains intact, and a completely new double-
stranded DNA molecule is synthesized.
• Result after replication:
◦ One DNA molecule is entirely old,
◦ The other is entirely new.
• Visual:
Old + Old → new + new
• Status:
Rejected by experimental evidence.
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✅ 2. Semi-Conservative Model (✔ Correct Model)

• Explanation:
Each daughter DNA molecule consists of one original (parental) strand and one newly synthesized
strand.
• Result after replication:
◦ Both DNA molecules have one old + one new strand.
• Experimental proof:
Meselson and Stahl experiment (1958) con rmed this model using isotope labeling (¹⁵N and ¹⁴N) in E.
coli.
• Status:
✔ Accepted as the correct mechanism of DNA replication.

✅ 3. Dispersive Model

• Explanation:
The original DNA is broken into fragments, and new DNA is synthesized in short segments.
Both strands of the daughter DNA have intermixed segments of old and new DNA.
• Result after replication:
◦ Each strand is a mosaic of old and new DNA pieces.
• Status:
Rejected — inconsistent with experimental results (e.g., Meselson-Stahl experiment).
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