Hi and welcome back to Free Science Lessons. This is the second part of a two-part video on meiosis. You should be able to describe the stages of meiosis and how meiosis can lead to genetic variation. In the last video we looked at crossing over and how that produces genetic variation in meiosis.
If you haven't watched that video then you need to watch it now. Remember that in meiosis we start with a diploid cell. in other words with chromosomes in pairs, and we produce four haploid gametes.
These gametes have individual chromosomes, not pairs of chromosomes. Now a key idea you need to understand is that meiosis actually involves two rounds of nuclear division. In meiosis I, homologous chromosomes are separated from each other, and in meiosis II, sister chromatids are separated from each other.
Let's look at the stages of meiosis. Before meiosis starts, the cell will have been through interphase. During interphase, the cell copies the chromosomes in the organelles. Remember though that the chromosomes are not visible as distinct structures during interphase. Okay, first the cell enters meiosis I, and the first stage of this is prophase I.
During prophase I, the chromosomes condense and become visible. Homologous chromosomes link together, forming Chia's motor. Remember that when the homologous chromosomes are paired like this, we call this a bivalent.
At this point, crossing over can take place, exchanging alleles between the homologous chromosomes. During prophase I, the nuclear membrane also breaks down, and the centrioles move to opposite poles of the cell. Spindle fibres also start to assemble into the spindle apparatus. In metaphase I, the pairs of homologous chromosomes are now lined up on the equator of the spindle apparatus. Okay, now we have anaphase I.
During anaphase I, the spindle fibres shorten and the homologous chromosomes move towards opposite poles. For this to happen, the chi is martyr between homologous chromosomes break. Okay, finally in meiosis I, we've got telophase I. In telophase I, the chromosomes have now reached the poles of the cell. At this point, the nuclear membranes reform and the chromosomes uncoil back to their chromatin state.
At this point, the cell undergoes cytokinesis, dividing into two cells. Now, these cells are haploid because they no longer contain pairs of homologous chromosomes. OK, now the cells enter meiosis II. In prophase II, the chromosomes condense and become visible again.
Again, the nuclear membrane breaks down and spindle fibres begin to develop. In metaphase II, the chromosomes are lined upon the equator of the spindle apparatus. In anaphase II, the centromere of each chromosome divides, and the spindle fibres shorten. The chromatids are now pulled towards opposite poles of the cell. OK, now in telophase II, the chromatids have reached the poles of the cell, and we now call them chromosomes.
Just like before, the nuclear membranes reform and the chromosomes uncoil back to their chromatin state. And finally each cell undergoes cytokinesis to produce two haploid cells. So as you can see meiosis starts with one diploid cell and produces four haploid cells. Because the chromosome number halves, scientists say that meiosis is reduction division.
Now each gamete made by meiosis is genetically different to the others. And we've already seen that crossing over is a major source of genetic variation in meiosis. But meiosis increases genetic variation in another way as well.
I'm showing you here the homologous chromosome pairs lined up on the spindle during metaphase 1. And to keep things simple, I'm not showing crossing over. The key idea you need to understand is that when homologous chromosome pairs line up on the spindle, we cannot predict whether the paternal or maternal chromosome will end up in which gamete. Scientists call this independent assortment. Looking at cell A, the homologous chromosome pairs have lined up so that the two paternal chromosomes are on the left and the two maternal chromosomes are on the right.
However, looking at cell B, in this case, the homologous chromosome pairs have lined up so that one paternal and one maternal chromosome are now on both the left and right. So, when the homologous chromosomes separate, we can produce four genetically different cells like this, and each cell may have different alleles depending on whether it contains a paternal or maternal chromosome. Now I'm showing this for a cell which only has two chromosome pairs, a long pair and a short pair.
But human cells actually have 23 chromosome pairs. Now the number of genetically different gametes produced by independent assortment is 2 to the power of n, where n is the number of homologous chromosome pairs. 2 to the power of 23 gives us over 8 million genetically different gametes.
And remember that's only considering independent assortment of chromosomes. When we factor in genetic variation due to crossing over, the number of possible gametes becomes really enormous. So meiosis has two ways to produce genetic variation in the gametes.
These are crossing over and independent assortment of chromosomes. There is one final factor to consider. Most organisms produce a vast number of genetically different gametes. And during fertilization, male and female gametes fuse randomly with each other. In other words, we cannot predict which male gamete will fuse with which female gamete.
And this random fusion of gametes introduces a whole extra level of genetic variation in the offspring. OK, so hopefully now you can describe the stages of meiosis, and how meiosis can lead to genetic variation.