Molecular Biology Glossary

A highly regulated form of programmed cell death characterized by cell shrinkage, chromatin condensation, DNA fragmentation, and membrane blebbing. Apoptosis is essential for development, tissue homeostasis, and immune function, and its dysregulation contributes to cancer and autoimmune diseases.

The complex of DNA and histone proteins that makes up chromosomes in eukaryotic cells. Chromatin exists in two forms: euchromatin (loosely packed, transcriptionally active) and heterochromatin (tightly packed, generally transcriptionally silent).

A sequence of three nucleotides in mRNA that specifies a particular amino acid during translation or signals the termination of the polypeptide chain (stop codon). The genetic code consists of 64 codons mapping to 20 amino acids and 3 stop signals.

A double-stranded helical molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. Its two strands are held together by hydrogen bonds between complementary base pairs (A-T, G-C).

A protein (or in some cases RNA) that catalyzes a specific biochemical reaction by lowering its activation energy. Enzymes are highly specific for their substrates and are essential for virtually all metabolic processes in living organisms.

A segment of a gene that is represented in the mature mRNA after splicing. Exons contain the coding sequences that are translated into protein, as well as the 5' and 3' untranslated regions of the mRNA.

A segment of DNA that contains the instructions for building one or more functional molecules, typically proteins or functional RNA. Genes include coding sequences (exons), non-coding sequences (introns), and regulatory regions.

An enzyme that unwinds the DNA double helix by breaking hydrogen bonds between complementary base pairs, creating a replication fork. DNA helicase is essential for DNA replication, repair, and transcription.

Small, positively charged proteins around which DNA wraps to form nucleosomes, the basic units of chromatin structure. Histones undergo post-translational modifications (acetylation, methylation, phosphorylation) that regulate chromatin accessibility and gene expression.

A non-coding segment of a gene that is transcribed into pre-mRNA but is removed by the spliceosome during RNA processing before translation. Introns may contain regulatory elements and enable alternative splicing.

An enzyme that catalyzes the joining of two DNA fragments by forming a phosphodiester bond between the 3'-hydroxyl end of one fragment and the 5'-phosphate end of another. DNA ligase is critical for sealing Okazaki fragments during replication and for molecular cloning.

A permanent alteration in the nucleotide sequence of DNA. Mutations can be spontaneous (arising from replication errors or DNA damage) or induced (by mutagens such as chemicals, radiation, or viruses) and may be beneficial, neutral, or deleterious to the organism.

The basic building block of nucleic acids, consisting of a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and one or more phosphate groups.

A functional unit of gene regulation in prokaryotes consisting of a cluster of genes under the control of a single promoter and operator. The genes in an operon are co-transcribed into a single polycistronic mRNA and typically encode proteins involved in a common metabolic pathway.

A small, circular, extrachromosomal DNA molecule that replicates independently within a bacterial cell. Plasmids often carry genes for antibiotic resistance and are widely used as vectors in recombinant DNA technology for cloning and gene expression.

A regulatory DNA sequence located upstream of a gene that serves as the binding site for RNA polymerase and transcription factors to initiate transcription. Promoter strength and associated regulatory elements control the rate and timing of gene expression.

A large molecular complex composed of ribosomal RNA and proteins that serves as the site of protein synthesis (translation). Ribosomes read mRNA codons and facilitate the assembly of amino acids into polypeptide chains.

A single-stranded nucleic acid molecule involved in various roles including coding (mRNA), decoding (tRNA), regulation (miRNA, siRNA), and catalysis (ribozymes). It uses ribose sugar and uracil instead of thymine.

The process by which an extracellular signal (such as a hormone or growth factor) is transmitted through a series of molecular events inside a cell, ultimately leading to a cellular response such as changes in gene expression, metabolism, or cell division.

A large ribonucleoprotein complex composed of small nuclear RNAs (snRNAs) and associated proteins that catalyzes the removal of introns from pre-mRNA in eukaryotic cells through a two-step transesterification mechanism.

A reverse transcriptase enzyme that extends telomeric DNA sequences at chromosome ends using an internal RNA template. It is active in stem cells, germ cells, and most cancer cells, preventing telomere shortening and enabling indefinite cell division.

Repetitive nucleotide sequences (TTAGGG in vertebrates) at the ends of chromosomes that protect against degradation, end-to-end fusion, and loss of genetic information during DNA replication. Telomeres shorten with each cell division in somatic cells.

The process by which a cell takes up exogenous DNA from its environment. In molecular biology, transformation refers specifically to the introduction of recombinant plasmid DNA into competent bacterial cells, often achieved through heat shock or electroporation.

A DNA molecule used as a vehicle to carry and deliver foreign genetic material into a host cell. Common vectors include plasmids, bacteriophages, cosmids, bacterial artificial chromosomes (BACs), and viral vectors, each suited for different insert sizes and applications.

An analytical technique used to detect specific proteins in a sample. Proteins are separated by gel electrophoresis, transferred to a membrane, and detected using antibodies specific to the target protein. Named by analogy with Edwin Southern's DNA blotting technique.