Chromosome Mutations

Microevolution is based on the changes at a molecular level that cause species to change over time. These changes may be mutations in DNA, or they could be mistakes that happen during mitosis or meiosis in relation to the chromosomes. If the chromosomes are not split correctly, there may be mutations that affect the entire genetic makeup of the cells.

During mitosis and meiosis, the spindle comes out of the centrioles and attaches to the chromosomes at the centromere during the stage called metaphase. The next stage, anaphase, finds the sister chromatids that are held together by the centromere pulled apart to opposite ends of the cell by the spindle. Eventually, those sister chromatids, which are genetically identical to each other, will end up in different cells.

Sometimes there are mistakes that are made when the sister chromatids get pulled apart (or even before that during crossing over in prophase I of meiosis). It is possible that the chromosomes will not be pulled apart correctly and that could affect the number or amount of genes that are present on the chromosome. Chromosome mutations can cause changes in the gene expression of the species. This may lead to adaptations that could help or hinder a species as they deal with natural selection.

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Duplication can happen when sister chromatids split
Anaphase in an onion root tip. Getty/Ed Reschke

Since sister chromatids are exact copies of each other, if they do not split down the middle, then some genes are duplicated on the chromosome. As the sister chromatids are pulled into different cells, the cell with the duplicated genes will produce more proteins and overexpress the trait. The other gamete that does not have that gene may be fatal.

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During crossing over, deletions can occur

If a mistake is made during meiosis that causes part of a chromosome to break off and become lost, this is called a deletion. If the deletion occurs within a gene that is vital for the survival of an individual, it could cause serious problems and even death for a zygote made from that gamete with the deletion. Other times, the part of the chromosome that is lost does not cause fatality for the offspring. This type of deletion changes the available traits in the gene pool. Sometimes the adaptations are advantageous and will become positively selected for during natural selection. Other times, these deletions actually make the offspring weaker and they will die off before they can reproduce and pass the new gene set down to the next generation.

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Translocation happens when pieces attach to different chromosomes
Chromosome mutation. Getty/Chris Dascher

When a piece of a chromosome breaks off, it is not always lost completely. Sometimes a piece of chromosome will attach onto a different, non-homologous chromosome that has also lost a piece. This type of chromosome mutation is called translocation. Even though the gene is not completely lost, this mutation can cause serious problems by having the genes encoded on the wrong chromosome. Some traits need nearby genes to induce their expression. If they are on the wrong chromosome, then they do not have those helper genes to get them started and they will not be expressed. Also, it is possible the gene was not expressed or inhibited by nearby genes. After translocation, those inhibitors may not be able to stop the expression and the gene will be transcribed and translated. Again, depending on the gene, this could be a positive or negative change for the species.

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Chromosomes reattach incorrectly in inversion
Chromosomes from a human male. Getty/Ed Reschke

Another option for a piece of chromosome that has been broken off is called inversion. During inversion, the piece of the chromosome flips around and becomes reattached to the rest of the chromosome, but upside down. Unless the genes need to be regulated by other genes via direct contact, inversions are not as serious and often keep the chromosome working properly. If there is no effect on the species, the inversion is considered a silent mutation.