DNA

A long polymer of deoxyribonucleotides, acting as the genetic material in most organisms, with length defined by the number of base pairs (e.g., Bacteriophage φX174 has 5386 nucleotides, E. coli has 4.6 × 10^6 bp, human haploid DNA has 3.3 × 10^9 bp).

Nucleotide

A molecule with three components: a nitrogenous base (purine or pyrimidine), a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group, linked via N-glycosidic and phosphoester linkages.

Nucleoside

A nitrogenous base linked to the OH of 1’C of a pentose sugar via N-glycosidic linkage (e.g., adenosine, deoxyadenosine, cytidine).

Purines

Nitrogenous bases adenine (A) and guanine (G), present in both DNA and RNA.

Pyrimidines

Nitrogenous bases cytosine (C) in both DNA and RNA, thymine (T) in DNA, and uracil (U) in RNA.

Polynucleotide Chain

A chain of nucleotides linked by 3’-5’ phosphodiester bonds, with a free 5’ phosphate at one end and a free 3’ OH at the other; the backbone is sugar-phosphate, with bases projecting outward.

Double Helix

The structure of DNA proposed by Watson and Crick (1953), with two antiparallel polynucleotide chains (5’→3’ and 3’→5’), base-paired (A-T with two H-bonds, G-C with three H-bonds), coiled right-handed with a 3.4 nm pitch and 10 bp per turn.

Chargaff’s Rule

In double-stranded DNA, the ratio of adenine to thymine and guanine to cytosine is constant and equals one, supporting the base-pairing model.

Central Dogma

The flow of genetic information proposed by Francis Crick: DNA → RNA → Protein, with reverse flow (RNA → DNA) in some viruses.

Nucleosome

A structure in eukaryotes where negatively charged DNA wraps around a positively charged histone octamer (200 bp per nucleosome), forming the ‘beads-on-string’ chromatin structure.

Euchromatin

Loosely packed, transcriptionally active chromatin that stains light in the nucleus.

Heterochromatin

Densely packed, transcriptionally inactive chromatin that stains dark in the nucleus.

Nucleoid

A region in prokaryotes (e.g., E. coli) where negatively charged DNA is held with positively charged proteins in large loops, despite the absence of a defined nucleus.

Transforming Principle

Frederick Griffith’s (1928) observation that a ‘transforming principle’ from heat-killed Streptococcus pneumoniae S strain transformed live R strain into virulent S strain, later identified as DNA.

Hershey-Chase Experiment

Alfred Hershey and Martha Chase (1952) proved DNA is the genetic material using bacteriophages with radioactive phosphorus (labels DNA) and sulfur (labels protein), showing only DNA entered bacteria.

Bacteriophage

A virus that infects bacteria, used in the Hershey-Chase experiment to confirm DNA as the genetic material (e.g., T2 bacteriophage).

RNA World

The hypothesis that RNA was the first genetic material, acting as both genetic material and a catalyst, with DNA evolving later for stability.

Semiconservative Replication

The process where DNA strands separate, each acting as a template for a new complementary strand, resulting in two DNA molecules with one parental and one new strand.

Meselson-Stahl Experiment

Matthew Meselson and Franklin Stahl (1958) proved semiconservative replication in E. coli using heavy nitrogen (15N) and normal nitrogen (14N), showing hybrid and light DNA after one and two generations, respectively.

DNA Polymerase

The main enzyme in DNA replication, catalyzing polymerization of deoxynucleotides in the 5’→3’ direction, requiring a template (e.g., in E. coli, polymerizes 2000 bp/second).

Replication Fork

A small opening in the DNA helix where replication occurs, with continuous synthesis on the 3’→5’ template and discontinuous synthesis (Okazaki fragments) on the 5’→3’ template.

DNA Ligase

An enzyme that joins discontinuously synthesized DNA fragments (Okazaki fragments) during replication.

Transcription

The process of copying genetic information from the 3’→5’ DNA template strand into RNA, guided by complementarity (adenine pairs with uracil).

Transcription Unit

A DNA segment with a promoter (5’-end, binds RNA polymerase), structural gene, and terminator (3’-end, stops transcription).

Promoter

A DNA sequence at the 5’-end of the transcription unit where RNA polymerase binds to initiate transcription.

Terminator

A DNA sequence at the 3’-end of the transcription unit that signals the end of transcription.

Exons

Coding sequences in eukaryotic genes that appear in mature RNA afternearest

Introns

Non-coding sequences in eukaryotic genes, removed during splicing, not present in mature RNA.

mRNA

Messenger RNA, provides the template for protein synthesis by carrying the genetic code from DNA to ribosomes.

tRNA

Transfer RNA, brings amino acids to ribosomes and reads the genetic code via its anticodon during translation.

rRNA

Ribosomal RNA, forms the structural and catalytic core of ribosomes (e.g., 23S rRNA in bacteria acts as a ribozyme).

hnRNA

Heterogeneous nuclear RNA, the primary transcript in eukaryotes containing exons and introns, processed into mature mRNA.

Splicing

The process in eukaryotes where introns are removed from hnRNA and exons are joined to form mature mRNA.

Capping

The addition of a methyl guanosine triphosphate to the 5’-end of hnRNA in eukaryotes to stabilize mRNA.

Tailing

The addition of 200-300 adenylate residues to the 3’-end of hnRNA in eukaryotes, aiding mRNA stability and translation.

Genetic Code

A triplet code of nucleotides in mRNA that specifies amino acids; 61 codons code for 20 amino acids, 3 are stop codons (UAA, UAG, UGA).

Degenerate Code

Multiple codons coding for the same amino acid, e.g., UUU and UUC both code for phenylalanine.

Translation

The process of synthesizing a polypeptide from mRNA, where tRNA matches codons to amino acids at the ribosome.

Anticodon

A triplet of nucleotides on tRNA that base-pairs with the mRNA codon during translation.

Lac Operon

A bacterial operon in E. coli with genes z (beta-galactosidase), y (permease), and a (transacetylase), regulated by lactose as an inducer.

Repressor

A protein (e.g., lac repressor) that binds to the operator in the lac operon, preventing transcription in the absence of lactose.

Inducer

A molecule (e.g., lactose or allolactose) that inactivates the repressor in the lac operon, allowing transcription.

Human Genome Project

A 13-year project (1990-2003) to sequence the 3 billion base pairs of human DNA, identifying 20,000-25,000 genes and enabling advances in medicine and biology.

Expressed Sequence Tags (ESTs)

Sequences of expressed genes (RNA) used in the Human Genome Project to identify coding regions.

Sequence Annotation

Assigning functions to sequenced DNA regions in the Human Genome Project, including coding and non-coding sequences.

BAC

Bacterial artificial chromosome, a vector used in the Human Genome Project to clone DNA fragments for sequencing.

YAC

Yeast artificial chromosome, a vector used in the Human Genome Project to clone large DNA fragments.

DNA Fingerprinting

A technique to identify individuals by analyzing polymorphic repetitive DNA sequences (e.g., VNTRs), used in forensics and paternity testing.

VNTR

Variable Number of Tandem Repeats, a type of minisatellite DNA with varying copy numbers, used in DNA fingerprinting.

Polymorphism

Variations in DNA sequences among individuals, forming the basis of genetic mapping and DNA fingerprinting.