Molecular Basis of Inheritance and Gene Expression: DNA, RNA, and Gene Regulation

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The discovery of DNA as the hereditary material was a collaborative effort involving several key scientists:

Frederick Griffith (1928): Discovered the "transforming principle" in bacteria, suggesting that genetic information could be transferred.
Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944): Demonstrated that DNA is the substance that causes bacterial transformation.
Alfred Hershey and Martha Chase (1952): Used bacteriophages to confirm that DNA, not protein, is the genetic material.
Erwin Chargaff: Established the base pairing rules (A=T, G=C).
Rosalind Franklin and Maurice Wilkins: Used X-ray crystallography to reveal the helical structure of DNA.
James Watson and Francis Crick (1953): Proposed the double helix model of DNA structure.

Nucleotide: The basic unit of DNA, consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, or guanine).
Base Pairing Rule: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
Hydrogen Bonds: A-T pairs form two hydrogen bonds; G-C pairs form three hydrogen bonds.

Double Helix: DNA consists of two antiparallel strands forming a right-handed helix.
Antiparallel: The two strands run in opposite directions: one 5' to 3', the other 3' to 5'.
5’ and 3’ Ends: Refer to the carbon numbers in the deoxyribose sugar; DNA polymerases add nucleotides to the 3’ end.
Leading and Lagging Strands: During replication, the leading strand is synthesized continuously (5’ to 3’), while the lagging strand is synthesized discontinuously in Okazaki fragments.

Prokaryotes: Single origin of replication, circular DNA, replication is faster.
Eukaryotes: Multiple origins of replication, linear chromosomes, more complex regulation.

Nucleosome: DNA wrapped around a core of histone proteins; fundamental unit of chromatin.
Histone Proteins: Positively charged proteins that package and order DNA.
Methylation: Addition of methyl groups to DNA or histones, often represses gene expression.
Acetylation: Addition of acetyl groups to histones, usually promotes gene expression by loosening chromatin structure.

Restriction Enzyme: Enzyme that cuts DNA at specific sequences, used in molecular cloning.
Plasmid: Small, circular DNA molecule in bacteria, used as a vector to transfer genes in genetic engineering.

Codon: A sequence of three nucleotides in mRNA that specifies an amino acid.
Anticodon: A sequence of three nucleotides in tRNA complementary to a codon in mRNA.

Transcription: Synthesis of RNA from a DNA template, occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).
Steps of Transcription:
1. Initiation: RNA polymerase binds to promoter (often includes a TATA box in eukaryotes).
2. Elongation: RNA polymerase synthesizes RNA in the 5’ to 3’ direction, reading the DNA template 3’ to 5’.
3. Termination: RNA polymerase releases the completed RNA transcript.
Translation: Synthesis of a polypeptide using mRNA as a template, occurs in the cytoplasm at ribosomes.
Steps of Translation:
1. Initiation: Ribosome assembles at the start codon (AUG).
2. Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons.
3. Termination: Stop codon is reached, and the polypeptide is released.
Molecules Involved: mRNA, tRNA, rRNA, ribosomes, amino acids, various enzymes.

Pre-mRNA Modification: Includes addition of a 5’ cap, poly-A tail, and splicing to remove introns.
Poly-A Tail: Added to the 3’ end of mRNA, increases stability and aids in export from the nucleus.
Introns and Exons: Introns are non-coding regions removed during splicing; exons are coding regions joined together.

TATA Box: A DNA sequence in the promoter region that helps position RNA polymerase for transcription initiation in eukaryotes.

Point Mutation: Change in a single nucleotide (substitution, insertion, or deletion).
Frameshift Mutation: Insertion or deletion that shifts the reading frame, often resulting in nonfunctional proteins.
Silent, Missense, and Nonsense Mutations: Silent mutations do not change the amino acid; missense mutations change one amino acid; nonsense mutations create a premature stop codon.

Similarities: Both use the genetic code, ribosomes, and tRNAs; transcription and translation are fundamentally similar.
Differences: In prokaryotes, transcription and translation are coupled; in eukaryotes, they are separated by the nuclear envelope and involve mRNA processing.

Codon Chart: Used to translate mRNA codons into amino acids during protein synthesis.
Example: The codon AUG codes for methionine (start codon).

Operon: A cluster of genes under the control of a single promoter and operator.
Repressible Operon: Usually on; can be turned off by a repressor (e.g., trp operon).
Inducible Operon: Usually off; can be turned on by an inducer (e.g., lac operon).
Lac Operon: Inducible; activated in the presence of lactose.
Trp Operon: Repressible; repressed in the presence of tryptophan.

DNA Methylation: Addition of methyl groups to DNA, often silences genes.
Histone Acetylation: Addition of acetyl groups to histones, promotes gene expression by loosening chromatin.
Epigenetics: Heritable changes in gene expression that do not involve changes to the DNA sequence (e.g., methylation, acetylation).

RNA Interference (RNAi): Process by which small RNA molecules inhibit gene expression by degrading mRNA or blocking translation.
miRNA (microRNA): Small RNA molecules that bind to complementary mRNA to inhibit translation.
siRNA (small interfering RNA): Double-stranded RNA molecules that promote mRNA degradation.

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
mRNA Processing	None	5' cap, poly-A tail, splicing
Gene Organization	Operons (polycistronic)	Single genes (monocistronic)
Regulation	Primarily at transcription initiation (operons)	Multiple levels (epigenetic, transcriptional, post-transcriptional, translational, post-translational)

Example of a Protein: Hemoglobin is a protein in red blood cells that transports oxygen throughout the body.
Function of a Repressor Protein: Binds to the operator region of an operon to block transcription.
Splicing: The process by which introns are removed and exons are joined to form mature mRNA.