Aneuploidy and Nondisjunction

in deployed organisms there's a 2end number of chromosomes in every single somatic cell so for example in humans every single somatic cell has 46 individual chromosomes or 23 pairs of homologous chromosomes under normal conditions now a cotype is basically a pictorial description of all the chromosomes found within that particular organism within that particular individual now in humans every normal human cotype will show 23 pairs of homologous chromosomes or 46 individual chromosomes now what exactly does a cotype in a human actually look like let's take a look at the following picture that describes the human karot type under normal conditions so we have chromosome pair number one chromosome pair number two chromosome pair number three all the way to chromosome pair number 22 and all of these chromosome pairs one through 22 are known as autosomal homologous chromosome pairs the final chromosome pair the 23rd one is called The Sex homologous chromosome pair now in males in normal males we have one x sex chromosome and one yex chromosome and a normal female Naes we have one X and the other one is also an X so notice that because each one of these pairs consist of two individual chromosomes that means on the normal conditions we have two multiplied by 23 or 46 individual chromosomes within the human cotype so in every single somatic cell of our body under normal conditions these are the chromosomes that we going to find in the nucleus of those somatic cells so now that we know what a cotype actually looks like in a normal healthy human individual let's now discuss chromosomal abnormalities so one of the common type of chromosomal abnormalities is anupy so in some individuals within the somatic cells of some individuals we can either have an extra copy of a chromosome or we can have one less chromosome than we normally have so we can either have 47 chromosomes or 45 chromosomes and in either case these conditions are known as anupy so once again as we saw just a moment ago when we discussed the human karot type we saw that every single one of these chromosomes came with a pair and this is is known as disomic so a um disomic conditions and so each one of these pairs describes a daic condition because we have only two per pair sometimes however we can either have a trimic individual or we can have a monosomic individual and what that means is one of these pairs actually has an extra copy of a chromosome so three in this particular case or we can have one less so we can have a monosomic condition so that is what we mean by anupy anupy is a type of chromosomal abnormality in which we either have an extra copy of one of the chromosomes or we have one less than we should normally have now the next question is why exactly does an employ actually take place how does it arise well there are two types of cell cycle processes we have mitosis and we have meiosis and both of these processes can actually lead to anupy and the specific process that leads to anupy is known as nondisjunction so the most common reason for anupy is non-disjunction of chromosomes that takes place during anaphase of mitosis or during anaphase of meiosis so let's begin by focusing on nondisjunction taking place in mitosis now normally what happens in mitosis if once again we look at this karot type so mitosis is the process by which a somatic cell in our body chooses to divide and that somatic cell produces two identical daughter cells that have the same exact genetic information so what happens during mitosis in uh during interphase what happens is every single one of these chromosomes is replicated so this chromosome is replicated this chromosome is replicated this one is replicated this one is replicated and so forth and let's say if this one is replicated what we produce is a pair of identical cyto chromatids remember Cy chromatids are two chromatids two chromosomes that are exactly the same they have the same exact genetic information now in this particular picture instead of drawing out all these 46 pairs of cister chromatids we're only going to look at two to basically save space so this is our chromosome uh this is our somatic cell and inside the somatic cell let's say we have chromosome one and chromosome 2 and we replicate them so these are the identical cyto chromatids so these these two are identical and these two are identical now normally what happens under normal conditions is these mitotic spindle apparatuses form they extend these fibers and these fibers attach themselves onto these sections on each one of these cyto chromatids and so during metaphase of mitosis we have these extensions and these connections that form and normally these two will move to that side these other cister chromatids will move to the other side and in humans we have 46 chromosomes moving this way 46 chromosomes moving the other way and so during anaphase we should have in this particular picture we should see two chromosomes moving this way and two chromosomes moving the other way but if nondisjunction take takes place what that means is one of the these fibers actually fails to form a proper connection with one of the cyto chromatids and so let's say that this connection formed this connection formed and this connection formed this connection did not actually form and so now what happens during anaphase when these fibers begin to pull these cyto chromatins apart these are pulled apart correctly so these begin moving to opposite poles but then this one doesn't move apart correctly in fact this pair of identical cyto chromatids moves to the other side to one side and this one fails to move to this side and so at the end when we produce our two daughter cells these will no longer be identical because they will not carry the same amount of genetic information in this particular case we're going to have a daughter cell a somatic cell that has one extra chromosome than it should have so we have a trimic condition and this one will lack that particular chromosome and so it will have a monosomic condition now one important Point must be made about mitosis if mitosis so let's suppose I'm a normal individual and what that means is inside every somatic cell of my of of my body I have 23 pairs or 46 individual ual chromosomes now if inside one of my somatic cells mitosis takes place and nondisjunction takes place then what that means is I will only have this anlo condition Within These two daughter cells that are formed as a result of the non-disjunction in mitosis all the other somatic cells of my body will still be normal and that's exactly why nondisjunction Tak place in mitosis is not as dangerous as nondisjunction taking place in meiosis because in meioses as we'll see in just a moment what ends up happening is all the somatic cells of that individual will have an abnormal number of chromosomes they will have anupy while in this case only these somatic cells produced via this non-disjunction mitosis will have that anupy condition so although this still can be dangerous because it can lead to abnormal cells it can form cancer cells as long as those abnormal cells are actually destroyed either by our immune system or by the process of uh uh programmed cell death if that happens it is not as dangerous as in the case of meiosis so let's move on to nondisjunction taking place in meiosis now because meiosis actually consists of meiosis one and meiosis 2 that means there are two different places two different times where nondisjunction can actually take place so let's begin by assuming that nondisjunction only takes place during uh during meiosis one so once again we're dealing with uh only well if we take a look at our uh normal human kot type technically speaking we should be showing all these individual chromosomes within the cell but to save time I'm only going to focus on the 23d chromosome pair our sex chromosome pair so we're not going to consider the autosomal chromosomes in this particular case so before meiosis actually takes place and before this male individual can produce sperm cells those chromosomes must replicate themselves during the process of interface and so the X chromosome is replicated to produce this identical X chromosome and the Y chromosome is also replicated to produce this identical cyto chromatid so normally what should happen during normal conditions is during metaphase one of meiosis these pairs basically line up at the equator of our cell so in humans we're going to have 23 pairs of these chromosomes line up at the mdle and during during uh and then during anaphase if these connections are formed correctly these 23 pairs of chromosomes are basic basically moved to opposite poles now if nondisjunction takes place what that means is once again we fail to form this fiber attachment and so instead of this attaching here and this attaching here let's say what happens is this attaches here and also attaches there and so now what happens is we have nondisjunction takes place and both of these pairs of chromosomes basically move to one side of the pole and so when we have when we produce those two cells one of the cells will lack the sex chromosome and the other one will have an extra pair of sex chromosomes so this is where nonis Junction took place in meiosis one in anaphase one of meiosis now let's suppose we have metaphase 2 take place and metaphase 2 takes place normally so all these chromosome pairs line up at the equator as shown and these fibers form correctly and we have the separation of these chromosomes to that side and these chromosomes to the other side and so in this particular case we form these two identical sperm cells and in this particular case we form these two sperm cells that don't have those sex chromosomes so all of these sperm cells are abnormal because in this case in case one and two we have one more than we should remember each sperm cell should have only one sex chromosome in this case we have two in this case we have none so what will happen next well remember the entire purpose of forming the sperm cells by the male individual was to basically take the sperm cell and combine it with an X cell so let's suppose we take either one of these two sperm cells our aneuploid sperm cell and we combine it with a ma with a female normal X cell remember X cells always or normal X cells always have One X chromosome now if these two gametes actually combine we what we're going to form a zygote that has the anupy condition and what that means is we're going to have two of these X chromosomes one of these Y chromosomes and what happens is we know under normal conditions we should have 1 x one y or 1 x1x but because of this rearrangement we're going to have an extra copy of that X chromosome and because we have an extra copy instead of having 46 chromosomes we're going to have 47 chromosomes in this Z zote and when the zygote divides to form the many different cells of that individual every single somatic cell of that individual will have this anoyed condition and that's why nondisjunction in meiosis is much more dangerous than nondisjunction is in in mitosis because if it takes place in meiosis every somatic cell of the body will end up having this anoy condition but it might mitosis it's only those two daughter cells that are formed by that process that will have that proc that will have that condition so as long as our immune system can protect our body from those abnormal cells we should have no problem so if a normal X cell combines with either sperm cell one or sperm cell two the zygote will have an extra X chromosome copy so we'll have XX y now if the normal X cell combines with either cell sperm cell three or four we're going to have a zygote in which we're going to lack the Y chromosome we're going to have only one X chromosome and this condition is known as XO where o means we don't have that second sex chromosome so this is nonis Junction taking place in meiosis one more specifically in anaphase one of meiosis now we can also have non this Junction taking place in meiosis 2 more specifically in anaphase 2 of meiosis so

Channel: Andrey K

Views: 127,825

Keywords: karyotype, human karyotype, aneuploidy, nondisjunction, nondisjunction in mitosis, nondisjunction in meiosis, introduction to aneuploidy, non-disjunction of chromosomes, chromosomal abnormalities, nondisjunction in meiosis I, nondisjunction in meiosis II, nondisjunction in humans, genetic abnormalities

Id: BxmaH5LiIkg

Length: 16min 6sec (966 seconds)

Published: Thu Jan 15 2015