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In Genetics, What Is a Chiasma?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 13 March 2014
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A chiasma is a point of contact between sister chromatids that forms during meiosis, a process of cell division and replication used by a wide variety of organisms. At the chiasma, the chromatids can exchange genetic information, resulting in new combinations of genetic material. When the chromatids separate and become sister chromosomes, they will contain a different mix of genetic material than their parent chromosomes. This allows organisms to evolve genetically, creating new traits and passing them down to descendants.

The chromatids are joined at the centromere, a point roughly in the middle of each chromatid. The formation of a chiasma occurs during meiosis I, the first phase in meiosis, when the paired chromosomes exchange genetic material before splitting into two cells, each containing half the genetic material of the original parent cell. Chiasmata play an important role in the process of replicating genetic material.

By essentially shuffling the genome by exchanging information at the chiasma, organisms are capable of generating theoretically endless combinations of new traits. Some of these combinations do not work out, for a variety of reasons, and they are not passed down. Others prove to be successful, and will start to disseminate through the population as a result. Over time, organisms can undergo major evolutionary shifts as their genomes change and some individuals thrive as a result of their inherited characteristics.

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At times, errors during division can involve the chiasma. If two daughter chromatids fail to separate during meiosis, the product of that particular round of cell division will have an odd number of chromosomes. The exchange of genetic material at this point can also become garbled or confused, resulting in passing down deleterious traits or ending with some cells with an excess of genetic material, while others may be missing pieces of genetic material. In some cases, this can be harmful, as the missing or excess material might be important and could express itself in the form of a genetic defect.

People can see the chiasma with the assistance of microscopy during the process of meiosis, when the chromatids are joined together in a distinctive X shape. This shape is also replicated on charts and graphs depicting meiosis. The term “chiasma,” which comes from the Greek, refers generally to an intersection or crossing. This term is also used in anatomy to refer to bundles of nerves at the point of intersection, like the optic chiasm, where the optic nerves cross each other.

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matthewc23
Post 4

@Emilski - Yes and no about chiasma forming during mitosis. Technically, the sister chromatids are connected in the center, but after reading this article and from what I know, I think chiasma is only used to refer to the joining of centromeres when crossing over happens. Obviously, crossing over is only part of meiosis, hence the term chiasma only being used in that context.

During the talk about chiasma not splitting, I was wondering, do chromosomes ever match up with the wrong chromosome (like chromosome 20 and 21 matching up)? Also, does the chiasma ever form in the wrong place on a set of chromosomes so that they aren't lined up right? What happens in these situations? I would assume the resulting cells are useless.

Emilski
Post 3

@jmc88 - Good explanation. I remember talking about having three sex chromosomes in a class one time. If I remember right, though, having XYY or XXX traits don't matter and will make normal males and females because all of the right genetic material is there. The problem is when you have a combination of both like XXY.

Shouldn't a chiasma also form in mitotic cell division? Right when the chromosomes start to duplicate, aren't they connected in the middle?

jmc88
Post 2

@cardsfan27 - It is only indirectly mentioned in the article, but meiosis is the process organisms use to create sperm and egg cells. Whether the cell will kill itself depends on what chromosomes are present. I can't think of any examples where a cell with only one chromosome ever survives, because all of the genetic material isn't there. There are a lot of cases where organisms will form with three chromosomes, though, because all the genetic material is there plus an extra chromosome.

Probably the one people are most familiar with is Down Syndrome, also known as Trisomy 21, where the 21st chromosome has three sets of information. I don't remember the name of it off the top of my head, but there are also cases where the X and Y sex chromosomes don't split correctly resulting in people or other animals with both male and female traits.

cardsfan27
Post 1

I don't really know anything about biology or how cell division works, but what would happen if the chiasma didn't separate and a new cell ended up with three of the same chromosome? I have heard some cells will kill themselves if there genetics aren't right. Would that happen in a situation like this?

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