No machine is perfect, and so too are our cells. Here we explain how mutations often arise when the cell makes mistakes as a transcriber, spell-checker, and pair of magnets.
A genetic mutation is simply a change in a person’s genome (genetic material present in a cell). Some genetic changes are protective, some are harmful, and most end up not having much of an impact. How do these changes occur in the first place?
Mutations will often arise at a few specific times:
- While the cell is replicating its DNA in order to divide
- During the repair of damage to the DNA molecule
- During reproduction in the process of creating sperm and egg cells
Mutations during DNA replication
The molecular machinery that copies a cell’s DNA is not always perfectly accurate. You can think of DNA replication like a person transcribing text into a new document. Every time a cell divides, it needs to copy over all of its genetic material over to the replicated cell. The cell will use the old copy as a template. Sometimes, an error gets introduced when the wrong DNA base or section of DNA sequence gets incorporated into the new strand. The cell has ways to correct the errors, but some errors will remain in the genetic code.
A person’s gamete cells (egg or sperm) have gone through many rounds of cell division — from the time you started off as only a single cell to the time your body produces sperm or eggs. With each cell division, there is a possibility to introduce new copy errors during DNA replication, which could then be incorporated into the genetic makeup of any resulting sperm or egg cells and can be passed down to future generations. These new mutations that can occur in any embryo are called de novo mutations. Orchid is the only embryo screening offering the option of screening for specific de novo changes associated with severe disease.
Mutations during DNA repair
DNA can be damaged by a number of things from the environment, such as chemicals that interact with DNA — like those found in cigarette smoke — and ionizing radiation — like ultraviolet rays. Individual DNA bases can be structurally damaged, or one or both DNA strands can be physically broken.
The cell will attempt to repair any damaged DNA through various methods, with some more error-prone than others. This is like a spell-checker — sometimes misspellings or typos are identified and fixed, but other times they are not. In addition, if the repair is done by a more error-prone method, it’s possible that extra bases are added or some bases get deleted at the site of damage. These mutations can be harmless, but they can also be harmful changes that increase risk for diseases like cancer.
Since DNA repair mechanisms are imperfect, it’s best to limit exposure to environmental sources of DNA damage, like smoking or repeated sunburns. Research has also suggested that some chemicals that induce mutations, such as those in first- and second-hand smoke, can cause DNA breaks and mutations in sperm.
Mutations during reproduction
The interesting thing about sperm and eggs is that they only have one copy of each chromosome. The body does this so that a fertilized egg gets half of its genetic material from the egg and the other half from the sperm: 23 + 23 to give a total of 46 chromosomes.
The special process of creating sperm and eggs so that they only have one copy of each chromosome is called “meiosis.” During meiosis, bundles of matching chromosomes are physically pulled apart so that one of each chromosome ends up in each new cell. This is kind of like a pair of magnets that remain stuck together.
One of the more commonly seen mistakes that can occur is when a set of chromosomes does not separate properly before the cell divides. When the chromosomes fail to separate, this can result in sperm or egg cells with an extra or missing copy of a chromosome. This is one of the most common ways genetic conditions like Down syndrome can happen. All of Orchid's embryo screening includes screening for chromosomal differences.
