Genetic Variation During Meiosis - Video Tutorials & Practice Problems
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1
concept
Genetic Variation During Meiosis
Video duration:
7m
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Video transcript
In this video, we're going to talk about genetic variation during meiosis. And so recall from our previous lesson videos that meiosis results in 4 haploid cells that are all genetically different from one another, and so meiosis creates genetic variation. And so it turns out that meiosis actually creates genetic diversity or genetic variation via two main events. And so the first main event that creates genetic diversity during meiosis is crossing over. And the second main event that creates genetic diversity during meiosis is independent assortment. Now in this video, we're going to focus mainly on the process of crossing over. But later in our course, in a different video, we'll talk about the process of independent assortment. Now crossing over is the process in which pairs of homologous chromosomes actually exchange their genetic material, essentially swapping segments of DNA. And so you can see that during crossing over, homologous chromosomes are going to cross over and swap or exchange their DNA. Now this process of crossing over is ultimately going to form non identical sister chromatids. And crossing over occurs specifically during prophase 1 of meiosis 1. And so it's important to note that crossing over, this process that creates genetic diversity, only occurs during meiosis 1, specifically prophase 1 of meiosis 1. But crossing over does not occur during mitosis, which recall mitosis creates genetically identical, not genetically diverse cells. Now when your professors are explaining, crossing over and when your textbooks describe crossing over, they're likely going to mention these other terms called synapsis and chiasma. Synapsis is the process where homologous pairs of chromosomes actually align themselves and align their DNA sequences at similar alleles to prompt crossing over. And so synapsis is really just this alignment of the DNA sequences between homologous chromosomes. And chiasma is really just the site of crossing over, the attachment site between 2 homologous chromosomes, allowing these homologous chromosomes to cross over their genetic material, exchange genetic material. But the chiasma is really just the site of crossing over. And so, within this word chiasma, you can see the root chi, which is actually a Greek letter that resembles an x. And so what you'll notice is that the chiasma really does resemble the formation of an x, and it represents the site of crossing over. And so if we take a look at our image down below, notice that we're showing you one replicated chromosome over here that has 2 identical sister chromatids, and we're showing you another replicated chromosome over here that also has 2 identical sister chromatids. And notice that these, replicated chromosomes are very, very similar in size and shape. They carry the same genes, but not necessarily the same versions of the genes or the same alleles. And so that makes these 2 chromosomes here homologous chromosomes. And, again, you can see here that for this particular gene a, they have the same version of the gene, the capital a version of this gene. But notice that for gene b, the blue chromosome has the capital b version of the gene, whereas the red chromosome has the lowercase b version of the gene. And so homologous chromosomes are going to be similar in size, shape, carry the same genes, but not necessarily the same versions of genes. Now what's important to note is that, again, this sister chromatid is exactly identical to this other sister chromatid, and, this is going to be the case before crossing over takes place. And the same goes for this sister chromatid here on this one and this one over here. They are identical to each other before crossing over takes place. And so, again, the process of synapsis is where the homologous pairs of chromosomes are going to align their DNA sequences at similar alleles. And so you can see that synapsis is shown here in the background here, with the yellow background, where it's just showing the alignment here of the chromosomes to prompt crossing over to take place. And then notice over here in the middle image that we have this overlapping, region of the homologous chromosomes called the chiasma. And this represents, again, the site of crossing over, where crossing over is going to take place. And so notice that at the very very end of crossing over that we have a chromosome that is mostly blue but has a little bit of the pink chromosome because there has been an exchange of the genetic material. And then notice over here we have a chromosome that is mostly red, but has a little bit of blue, again, because there has been an exchange of the genetic material in the process of crossing over. And so this process of crossing over takes identical sister chromatids and ends up converting them into non identical sister chromatids. And so notice that this chromatid here is no longer identical to the chromatid that's over here because there has been this exchange of genetic material between these pairs of homologous chromosomes. And so crossing over here, we're showing crossing over at one particular gene, but crossing over can actually occur between 100 or thousands of genes, between homologous chromosomes, and crossing over occurs randomly with every single event of meiosis. And so this is going to create an enormous amount of genetic diversity, swapping little bits and pieces of chromosomes, between homologous pairs of chromosomes. And so again, crossing over is one of the main events that creates genetic diversity during meiosis. And later in our course, we're gonna talk about the second event that creates genetic diversity during meiosis, independent assortment. And so that concludes this video, and I'll see you all in our next one.
2
Problem
Problem
Which of the following processes occurs when homologous chromosomes cross over in meiosis I?
a) Two sister chromatids get tangled, resulting in one re-sequencing its DNA.
b) Two sister chromatids exchange identical pieces of DNA.
c) Maternal alleles are "corrected" to be like paternal alleles and vice versa.
d) Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.
A
Two sister chromatids get tangled, resulting in one re-sequencing its DNA.
B
Two sister chromatids exchange identical pieces of DNA.
C
Maternal alleles are "corrected" to be like paternal alleles and vice versa.
D
Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.
3
Problem
Problem
Crossing over involves each of the following EXCEPT:
a) The transfer of DNA between two non-sister chromatids.
b) The transfer of DNA between two sister chromatids.
c) The formation of a synaptonemal complex.
d) The alignment of homologous chromosomes.
e) All of the above are involved in crossing over.
A
The transfer of DNA between two non-sister chromatids.
B
The transfer of DNA between two sister chromatids.
C
The formation of a synaptonemal complex.
D
The alignment of homologous chromosomes.
E
All of the above are involved in crossing over.
4
concept
Independent Assortment
Video duration:
8m
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Video transcript
So now that we've covered crossing over in our last lesson video, the first main event that creates genetic diversity during meiosis. In this video, we're going to talk about the second main event that creates genetic diversity during meiosis, and that is independent assortment. And so independent assortment occurs specifically during metaphase 1 of meiosis 1. And of course during metaphase, we know that the chromosomes are going to align themselves in the middle of the cell. And specifically during metaphase 1 of meiosis 1, the chromosomes are going to align themselves in homologous pairs in 2 rows on the metaphase plate. And so independent assortment refers to the ability of these pairs of homologous chromosomes to independently and randomly align themselves on the metaphase plate during metaphase 1 of meiosis 1. And so when these pairs of homologous chromosomes independently and randomly align themselves during metaphase 1, this results in an enormous amount of possible genetic combinations during meiosis. And again, independent assortment along with crossing over is part of the reason why meiosis results in 4 haploid cells that are all genetically different from one another. And so independent assortment helps to create more possible genetic combinations, making it very, very difficult to get 2 cells that are genetically identical during the process of meiosis. And so it's actually possible to calculate the number of combinations due to independent assortment by using the equation 2 raised to the power of n, where n here is the exponent, and n represents the haploid number of chromosomes in a cell. And so we'll be able to get some practice applying this equation here to calculate the number of combinations due to independent assortment when we get to our image down below. And so if we take a look at our image down below, notice that we're showing you 2 possibilities, possibility number 1 and possibility number 2 for independent assortment as it occurs during meiosis. And so notice that possibility number 1 is over here generating combination 1 and combination 2, and possibility number 2 is over here generating combination 3 and combination 4. And so, notice that this first row right here represents metaphase 1 of meiosis 1. And, of course, during metaphase, we know that the chromosomes are going to align themselves in the middle of the cell. But during metaphase 1 of meiosis 1, homologous chromosomes are going to pair up and they're going to align themselves in 2 rows on the metaphase plate. And so notice here in our representation, we're using a total of 4 chromosomes, and these are replicated chromosome. And you can see here that, one possibility, for these chromosomes to align themselves is that all of the maternal chromosomes inherited from the mother line up on one side, and all of the paternal chromosomes inherited from the father line up on the other side. So that's one possibility if these homologous chromosomes are independently and randomly aligning during metaphase 1. But if they're independently and randomly aligning on the metaphase plate here, it's also possible for not for all of the mother's chromosomes to not be on one side. And so notice here on this image that they're not lined up on one side, and, the father's chromosomes are also not lined up on one side. So this is another possibility for how these chromosomes can independently and randomly align. And so notice that each of these possibilities here is going to result in a different genetic combination. And so, over here in this possibility, notice that both of the mother's homologous chromosomes are going to go to the left to create this cell here that has 2, of the mother's chromosomes in one cell. And these 2 from the father would both separate to the right to create a cell that has both of the father's chromosomes. But if they aligned with this possibility here, notice that one of the mothers is going to go to the left and one of the fathers is gonna go to the left. And so you end up getting a cell that has, one of the mothers and one of the fathers chromosomes. And the same goes for this other cell over here. 1 of the mother fathers goes to the right and one of the mothers goes to the right as well, and you get this combination right here. And so these combinations are different. All of these combinations are here are different. And what you see here in this row, this row right here, is representing metaphase 2 of meiosis 2. And, of course, during metaphase 2 of meiosis 2, the chromosomes are aligning in 1 single file line, like they do in mitosis as well. And so you can see here the alignment of the chromosomes in one single file line. And so, this is gonna result in different genetic combinations. So notice here we have this, combination number 1 is, one possibility, And then you have combination number 2 over here is another possibility. And then, over here, if independent assortment occurred in this way, you'll get combination 3 and combination 4, which are different than combinations 12. And all of these represent different genetic combinations. And they all resulted from independent assortment, how these chromosomes can independently and randomly align themselves during metaphase 1 of meiosis 1. And so, ultimately, what we're saying here is that independent assortment, this event that occurs during metaphase 1 here, the random alignment of these chrom homologous pairs of chromosome, it's gonna create a lot of possible genetic combinations. And this is just showing 4 combinations here using just 4 total chromosomes. But we know that in human cells there are actually 46 chromosomes. And with 46 chromosomes, that would create a lot more possibilities. And so, you can again calculate the number of possible combinations by using this formula here, 2 raised to the power of n, where the variable n again represents the haploid number of chromosomes. And so notice, here in this example that we're showing you, we would say that 2 to the n if we were using this equation, the n here, the haploid number of chromosomes in our cells is 2 chromosomes because the end result of meiosis we know creates haploid cells and the haploid number here in this example is 2. So we would take 2 and the n here would be a 2. And so 2 raised to the 2, 2 squared essentially is 2 times 2, which is 4. And so the number of possible genetic combinations when there are, the haploid number is 2 is 4 genetic possible combination. And that's what we're showing you here, the 4 genetic possible combinations, that result if you were to only have 4, you know, total chromosomes in the diploid cell and 2 total chromosomes in the haploid cell. And so this here concludes our brief introduction to independent assortment, how it occurs during metaphase 1, and really how it just consists of homologous chromosomes independently and randomly aligning to create an enormous amount of possible genetic combinations during meiosis. And we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
5
example
Genetic Variation During Meiosis Example 1
Video duration:
2m
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Video transcript
Alright. So here we have an example problem that says for a species with a haploid number of 23 chromosomes, which would be humans, Humans have a haploid number of 23 chromosomes. How many combinations of maternal and paternal chromosomes are possible for the gametes based on the independent assortment of chromosomes during meiosis? And we've got these 4 potential answer options down below. So really what this problem is trying to ask us is how many possible genetic combinations are there if the haploid number of chromosomes is 23? And so what we need to recall from our last lesson video is that there is an equation to calculate the number of possible genetic combinations due to independent assortment. And so that equation is right here. The number of possible genetic combinations due to independent assortment is equal to 2 raised to the power of n, where n is equal to the haploid number of chromosomes. And so for this equation, all we need to do is take 2 and raise it to the power of n, again, the haploid number of chromosomes, and we're told that the haploid number of chromosomes is 23. So 2 raised to the 23. And when you type this into your calculator, 2 raised to the power of 23, you'll get an answer of 8,000,388,008 possible genetic combinations. And this is a lot of possible genetic combinations when it comes to just independent assortment on its own, and this is every single time meiosis occurs. And so the likelihood that these 4, gamete cells that result are going to have the same genetic combination is really unlikely if there are 8,000,388,608 possible genetic combinations. And so of course, when we look at the answer options, option d says about 8,000,000. And of course, that is going to be the closest one to the correct answer here. So option d here is going to be the correct answer to this example. And that concludes this example. So I'll see you all in our next video.
6
Problem
Problem
How many genetically unique gametes can be created in an organism with 4 chromosomes?
a) 256.
b) 23.
c) 16.
d) 1 million.
e) 4.
A
256.
B
23.
C
16.
D
1 million.
E
4.
7
Problem
Problem
During which of the following processes does independent assortment of chromosomes occur?
a) In meiosis I only.
b) In meiosis II only.
c) In mitosis and meiosis I.
d) In mitosis and meiosis II.
e) In meiosis I and meiosis II.
A
In meiosis I only.
B
In meiosis II only.
C
In mitosis and meiosis I.
D
In mitosis and meiosis II.
E
In meiosis I and meiosis II.
8
Problem
Problem
Independent assortment of chromosomes is a result of which of the following processes?
a) The random way each pair of homologous chromosomes lines up at the metaphase plate.
b) The random combinations of eggs and sperm during fertilization.
c) The random distribution of the sister chromatids into the two daughter cells.
d) The diverse combination of alleles that may be found within any given chromosome.
A
The random way each pair of homologous chromosomes lines up at the metaphase plate.
B
The random combinations of eggs and sperm during fertilization.
C
The random distribution of the sister chromatids into the two daughter cells.
D
The diverse combination of alleles that may be found within any given chromosome.
9
concept
Nondisjunction
Video duration:
5m
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Video transcript
In this video, we're going to introduce nondisjunction. And so nondisjunction is an error that can occur during either meiosis 1 or meiosis 2, when chromosomes fail to separate from each other, which can result in aneuploid cells. Now aneuploid cells are cells that are going to contain either too many or too few chromosomes. And, again, aneuploid cells can be the result of a nondisjunction. Now nondisjunctions resulting in aneuploid cells can lead to genetic disorders. For example, trisomy 21 or Down syndrome, or it could even lead to cell death. And so let's take a look at our image down below which is showing you nondisjunction during anaphase 1 of meiosis 1 or, anaphase 2 of meiosis 2. And so notice, here at the top we're showing you a cell undergoing meiosis 1, and here at the bottom we're showing you a cell undergoing meiosis 2. And again, nondisjunction can occur either during meiosis 1 or meiosis 2 when these chromosomes are going to fail to separate properly. And so here what you can see is during metaphase 1 of meiosis 1, the homologous chromosomes are going to, randomly and, independently align themselves in 2 rows on the metaphase plate. And so what you'll see here is in this image, we're showing you the nondisjunction of a small blue chromosome. And so this small blue chromosome, if anaphase were to occur properly, it should shift over to this side of the cell. However, if a nondisjunction occurs of the small blue chromosome, notice that the small blue chromosome is actually going to go to the same side as its homologous chromosome pair. And so this side over here is going to be missing the homologous chromosome, whereas this side over here is going to have an additional homologous chromosome. And so what you'll notice is that this cell here, which would represent the cell here at the top, is going to have too many chromosomes. And so if you have too many chromosomes, that is going to be a type of aneuploid cell. And so notice that this cell over here only has one replicated chromosome, so it has 2 few chromosomes. And so you can see something similar occurring with meiosis 2 down below. And again in meiosis 2, the chromosomes all line up in 1 single file row. And this time, you can see that there's nondisjunction of the large red sister chromatids. And so, usually during, meiosis 2, this sister chromatid would go, to the bottom and the other sister chromatid would go to the top. But if there is nondisjunction, then notice that this bottom sister chromatid here is not going to separate. It's going to fail to separate. And so, notice that both sister chromatids would end up over in the same cell, and this cell would be lacking a sister chromatid. And so again, you would end up getting a cell that has too many chromosomes and a cell that has too few chromosomes. And so, again, if a cell has either too many or too few chromosomes, they're referred to as aneuploid cells. And again, this nondisjunction can lead to genetic disorders such as, again, trisomy 21 or Down syndrome. And trisomy 21, as you can see here, tri is a root that means 3, and, trisomy 21 is referring to having 3 copies of chromosome 21. And so if we take a look at this karyotype that we have over here on the right, notice that there are 2 pairs of every single chromosome except for this pair right here. And this pair, notice that there are actually 3 copies of this chromosome, chromosome 21, and that leads to, the genetic disorder trisomy 21 which is Down syndrome. And so we're showing you a baby here, cute baby with Down syndrome. And so, what you'll notice here is that, we have finished our lesson here introducing nondisjunction and how nondisjunction is an error that can occur during meiosis, where chromosomes or sister chromatids failed to separate resulting in aneuploid cells with either too many or too few chromosomes. And so we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
10
Problem
Problem
Non-disjunction means that chromosomes:
a) Do not cross over during synapsis.
b) Do not replicate during interphase.
c) Have serious mutations.
d) Do not separate correctly from one another during anaphase.
A
Do not cross over during synapsis.
B
Do not replicate during interphase.
C
Have serious mutations.
D
Do not separate correctly from one another during anaphase.
11
Problem
Problem
During meiosis, if nondisjunction occurs during anaphase II (shown below in cell #3) what are the possible gametes that can be produced from cell #3?
a) A only.
b) B only.
c) C only.
d) A & C.
e) B & C.
f) A & B.
g) All three.
A
A only.
B
B only.
C
C only.
D
A & C.
E
B & C
F
A & B
G
All three.
12
Problem
Problem
________ cells have an extra or missing chromosomes after meiosis due to nondisjunction during Meiosis I.