BOL

Mutation refers to the actual change in sequence: D614G is an aspartic acid-to-glycine substitution at position 614 of the spike glycoprotein. Genomes that differ in sequence are often called variants. This term is somewhat less precise because 2 variants can differ by 1 mutation or many. Strictly speaking, a variant is a strain when it has a demonstrably different phenotype (eg, a difference in antigenicity, transmissibility, or virulence).

The terms "mutation," "variant," and "strain" are often used in biology and genetics to describe different aspects of genetic diversity and change in organisms. While they are related, they have distinct meanings and implications:

Mutation:

Definition: A mutation is a change in the DNA sequence of an organism's genome. It can involve the substitution, deletion, insertion, or rearrangement of nucleotides in the DNA.
Scope: Mutations can occur at the level of a single nucleotide (point mutation), affect a larger segment of DNA (e.g., insertion or deletion of multiple nucleotides), or involve more extensive genetic rearrangements.
Consequences: Mutations can have various consequences, ranging from no observable effect to causing diseases, altering the function of genes, or providing genetic variation for evolution.
Variant:

Definition: A variant is a specific version of a gene or DNA sequence that differs from the reference or wild-type sequence. Variants can result from mutations or natural genetic variations within a population.
Scope: Variants can encompass single nucleotide changes (single nucleotide polymorphisms or SNPs), insertions, deletions, or larger structural variations.
Consequences: Variants may or may not have functional significance. Some variants are associated with specific traits, diseases, or drug responses, while others may be benign and not affect an organism's phenotype.
Strain:

Definition: A strain typically refers to a subgroup or genetically distinct population within a species. It can also refer to a specific lineage of microorganisms (e.g., bacteria or viruses) with unique characteristics.
Scope: Strains are often used in microbiology to describe different isolates of microorganisms that may have genetic differences, leading to variations in characteristics such as virulence, drug resistance, or metabolic pathways.
Consequences: Strains may exhibit differences in phenotype due to their genetic diversity. For example, bacterial strains may have varying antibiotic susceptibility profiles or different antigenic properties.
In summary, mutations are the fundamental genetic changes that can occur in DNA, variants refer to specific genetic differences or versions of a gene or DNA sequence, and strains are distinct populations within a species or lineages of microorganisms. These terms are used in different contexts and convey different levels of genetic diversity and variation.