Mitotic nondisjunction
Mitotic nondisjunction is failure of sister chromatids (identical chromosome copies) to separate, so one daughter cell (of the two formed) gains a copy and the other loses one.
Remember
In mitotic nondisjunction one daughter cell receives an extra copy of the chromosome and the other is short of one copy (monosomy), and the monosomic cell is usually destroyed.
Before a cell divides, each chromosome is copied, giving two identical sister chromatids joined together. In normal mitosis these split at anaphase, when they are pulled apart, and one chromatid goes to each daughter cell.
Nondisjunction is the failure of this separation. Both copies are dragged to the same side, so one daughter cell receives two copies of the chromosome and the other receives none. Each cell also keeps one copy of the partner chromosome inherited from the other parent, so one ends up with three copies (trisomy) and the other with only one (monosomy). A change in chromosome number like this is called aneuploidy.
The two cells can then have very different fates. The cell with the extra chromosome makes more of the products of the genes on it, which can slow or speed its growth. If those genes promote growth, the cell may grow uncontrollably, which may lead to cancer. The cell that has lost a copy of the chromosome (the monosomic cell) is usually destroyed. A cell left with no copy of a chromosome has lost every gene on it, including essential genes (genes needed for the cell's basic functions), so it usually dies.
The consequence depends on when the error happens. If the error happens at the very first division in development, the monosomic cell is usually destroyed, which leaves the trisomic line (the trisomic cell and its descendants), so the whole body is trisomic. If it happens at a later division, the result is a mixture of normal and trisomic cells, called mosaicism (a patch of cells carrying the wrong number of chromosomes). Nondisjunction is most often described in meiosis, but it can occur in mitosis too.
Key facts
- What fails
- the sister chromatids of one chromosome fail to separate at anaphase, so both go to the same daughter cell.
- Result
- one daughter cell has three copies of that chromosome (trisomy) and the other has one (monosomy).
- Extra copy
- the cell makes more of the products of the genes on the extra chromosome, which can slow or speed its growth.
- Lost copy
- the monosomic cell is usually destroyed; a cell left with no copy has lost every gene on that chromosome, including essential genes, so it usually dies.
- Timing
- the monosomic cell is usually destroyed, so an error at the very first division in development makes every cell trisomic, whereas a later error gives mosaicism.
- Where it happens
- nondisjunction can occur in mitosis as well as meiosis, and in a body cell it gives a patch of cells with abnormal chromosome numbers.
Exam traps
- Nondisjunction is not limited to meiosis, because it can also happen in mitosis, where in a body cell it gives a patch of cells with abnormal chromosome numbers, which is mosaicism.
- Sister chromatids separating too early is a timing fault, and nondisjunction is the opposite problem, because chromatids that should separate at anaphase fail to do so.
- Failure of cytokinesis (the splitting of the cytoplasm) after the nuclei have separated does not itself unbalance chromosome numbers, because the chromosomes were already shared evenly between the nuclei.
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