Molecular Basis of Inheritance
Complete High-Yield Keywords for NEET UG & Boards
Nucleic Acid Fundamentals
DNA (Deoxyribonucleic Acid)Long polymer of deoxyribonucleotides that acts as the primary genetic material in most organisms; stores hereditary information.
RNA (Ribonucleic Acid)Polymer of ribonucleotides that functions as messenger, adapter, structural component, and catalyst; serves as genetic material only in some viruses.
NucleotideBasic monomeric unit consisting of three components: nitrogenous base, pentose sugar (deoxyribose in DNA, ribose in RNA), and phosphate group.
NucleosideCompound formed when a nitrogenous base links to the 1'C OH of pentose sugar via N-glycosidic linkage (without phosphate group).
PurinesDouble-ring nitrogenous bases—Adenine (A) and Guanine (G)—found in both DNA and RNA.
PyrimidinesSingle-ring nitrogenous bases: Cytosine (C) in both DNA/RNA, Thymine (T) in DNA only, and Uracil (U) in RNA only.
Phosphodiester LinkageCovalent bond formed between 3'OH of one sugar and 5'OH of next sugar via phosphate group; forms backbone of polynucleotide chains.
5'-end / 3'-endTerminal ends of polynucleotide chain—5' end has free phosphate group attached to 5'C of sugar; 3' end has free OH group attached to 3'C of sugar.
DNA Structure & Packaging
Double HelixRight-handed coiled structure of DNA with two anti-parallel polynucleotide strands held together by hydrogen bonds between complementary bases.
Base PairingSpecific hydrogen bonding between complementary bases—Adenine with Thymine (2 H-bonds) and Guanine with Cytosine (3 H-bonds).
Complementary StrandsTwo DNA strands where sequence of bases in one strand determines the sequence in the other strand due to specific base pairing rules.
Anti-parallelOrientation where one DNA strand runs 5'→3' while the complementary strand runs 3'→5'.
HistonesPositively charged basic proteins rich in lysine and arginine residues that package DNA in eukaryotes.
Histone OctamerUnit of eight histone molecules (two copies each of H2A, H2B, H3, H4) around which DNA wraps.
NucleosomeFundamental repeating unit of chromatin where ~200 bp of DNA is wrapped around a histone octamer; appears as "beads-on-string" under electron microscope.
ChromatinComplex of DNA and proteins (histones + non-histone chromosomal proteins) forming thread-like structures in the nucleus.
EuchromatinLoosely packed region of chromatin that stains light and is transcriptionally active.
HeterochromatinDensely packed region of chromatin that stains dark and is transcriptionally inactive.
Genetic Material Discovery
NucleinOriginal name given by Friedrich Miescher (1869) to the acidic substance isolated from nucleus (later identified as DNA).
Transforming PrincipleSubstance transferred from heat-killed S strain to live R strain bacteria in Griffith's experiment (1928); later identified as DNA by Avery, MacLeod & McCarty (1944).
DNA Replication
Semiconservative ReplicationMode of DNA replication where each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Replication ForkSmall opening in the DNA helix where active replication occurs; formed due to unwinding of double helix.
Origin of ReplicationSpecific DNA sequence where replication initiates; required for propagation of DNA fragments in recombinant DNA technology.
DNA-dependent DNA PolymeraseMain enzyme catalyzing DNA synthesis using a DNA template; polymerizes only in 5'→3' direction.
DNA LigaseEnzyme that joins discontinuously synthesized DNA fragments (Okazaki fragments) on the lagging strand.
Transcription
TranscriptionProcess of copying genetic information from one strand of DNA into RNA using DNA-dependent RNA polymerase.
Template StrandDNA strand with 3'→5' polarity that serves as template for RNA synthesis; also called antisense strand.
Coding StrandDNA strand with 5'→3' polarity having same sequence as RNA (except thymine instead of uracil); also called sense strand.
PromoterDNA sequence located upstream (toward 5' end) of structural gene that provides binding site for RNA polymerase to initiate transcription.
TerminatorDNA sequence located downstream (toward 3' end) of structural gene that defines the end of transcription process.
Structural GeneSegment of DNA that codes for RNA or protein product.
Monocistronic GeneGene coding for a single polypeptide; characteristic of most eukaryotic genes.
Polycistronic GeneGene coding for multiple polypeptides under control of a single promoter; characteristic of prokaryotic operons.
ExonsExpressed/coding sequences in eukaryotic genes that appear in mature processed RNA.
IntronsIntervening/non-coding sequences in eukaryotic genes that are removed during RNA processing.
hnRNAHeterogeneous Nuclear RNA: primary transcript in eukaryotes containing both exons and introns; precursor to mRNA.
SplicingProcess of removing introns and joining exons in a defined order to produce mature functional RNA.
Capping & TailingPost-transcriptional modifications: Capping adds methyl guanosine triphosphate to 5' end; Tailing adds adenylate residues to 3' end.
RNA Types & Translation
mRNA (Messenger RNA)RNA that carries genetic information from DNA to ribosomes; provides template for translation.
tRNA (Transfer RNA)Adapter molecule with anticodon loop and amino acid acceptor end; brings specific amino acids to ribosome during translation.
rRNA (Ribosomal RNA)Structural and catalytic component of ribosomes; 23S rRNA in bacteria acts as ribozyme.
Genetic CodeTriplet nucleotide code in mRNA that specifies sequence of amino acids during protein synthesis.
CodonSequence of three nucleotides in mRNA that codes for a specific amino acid or stop signal.
Degenerate CodeProperty where multiple codons code for the same amino acid.
Start Codon (AUG)Initiator codon that codes for Methionine and signals beginning of translation.
Stop CodonsUAA, UAG, UGA: Terminator codons that do not code for any amino acid; signal end of translation.
TranslationProcess of polymerization of amino acids to form polypeptide chain based on mRNA sequence; occurs on ribosomes.
Charging of tRNAActivation of amino acid in presence of ATP and its attachment to cognate tRNA; first step in translation.
Untranslated Regions (UTRs)Sequences at 5' and 3' ends of mRNA not translated but essential for translation efficiency.
Gene Regulation & Genomics
OperonFunctional unit of DNA in prokaryotes containing polycistronic structural genes under control of a single promoter.
Lac OperonPrototype inducible operon in E. coli containing genes for lactose metabolism regulated by repressor protein.
InducerMolecule (lactose/allolactose) that inactivates repressor protein, allowing transcription.
Human Genome Project (HGP)13-year mega project (1990-2003) aimed at sequencing all 3.3 billion base pairs of human DNA.
SNPsSingle Nucleotide Polymorphisms: locations where single-base DNA differences occur among individuals.
DNA FingerprintingTechnique to identify individuals based on polymorphisms in repetitive DNA sequences; used in forensics.
VNTRVariable Number of Tandem Repeats: mini-satellite DNA where a short sequence is repeated; copy number shows high polymorphism.
Conceptual Frameworks
Central DogmaFundamental principle stating flow of genetic information: DNA → RNA → Protein.
RNA World HypothesisTheory proposing that RNA was the first genetic material and catalyst; DNA evolved later for stability.
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