Early embryonic development (metaphase in mitosis) over time, from upper left to lower right with tubulin (microtubules) labeled, showing the elongation of the mitotic spindle prior to the first cell division. Courtesy of Ahna Skop, Skop Lab.
In a DNA sequence, one nucleotide is substituted by another nucleotide, such as A substituted by C, resulting a protein sequence with one amino acid different from the original one.