Which are now going to play more significant roles. So in metaphase I, let me draw my cell, so this is the cellular It is metaphase I, metaphase, metaphase I, and it has some similarities I, and you could guess what the next phase is going to be called. But now let's continue with meiosis, and in particular meiosis Just adds more variation as we get into sexual reproduction, so it's a kind of neat They might have different alleles, and then once again, this Sections of the chromosome tend to code for the same genes. Of these homologous pairs crossover, so these In a fairly clean way where homologous sections Thing to happen in meiosis I, and it tends to happen Prophase I in particular is that you have thisĬhromosomal crossover, that is a pretty typical Going into their dense form that has kinda this classic shape that you could see from a microscope, but what was unique or Or starts to disappear, you have the chromosomes Like prophase in mitosis where the nuclear envelope disappears Of time on prophase I because some interesting things happened. Than that, prophase I, but we spent a good bit In the last video, we had just started to get into meiosis, and to be more precise, meiosis I, and to be even more precise It would be less likely that only a single organism type would be the outcome from evolution. Since the resources and environments around the world very so will the adaptions that occur. Then if there are different concentration of A and B in different areas you will find that the groups will migrate to these locations and will share DNA less and over time will drift apart genetically becoming different species without either one needing to die off. When you have a population of organisms that are able to share DNA they have a tenancy to evolve together but is a portion of the population develop a new trait that is not shared because of some limitation like physical separation the genetic drift in these groups will diverge and they will eventually become different species.Īlso if you have an organism that uses a resource A and a portion of the population gets a mutation that allows them to use resource B in addition to resource A there is actually a benefit to both groups if the portion of the population that can use B specializes is using B only since there is less competition for B and this lessens the competition for A as well. There are a number of other factors that are involved in speciation. If the change causes a decrease in ability to survive to pass on its DNA then they will likely die out but if the change is beneficial it will have a greater chance to spread.Įvolution is not that simple. The changes to DNA is random but the selection of what organism survives is not random. It is not correct to say that evolution is random. This is another interesting area that seems incompletely understood. While homologous pairing is not needed during mitosis for proper chromosome segregation, there is some evidence that the homologs tend to stay closer together than you would expect by chance. Keep studying and you may be able to help figure out more about how exactly this works! My understanding is that pairing usually involves recombination, which allows the cell to confirm that two chromosomes have very similar sequences. How the homologs find each other and pair up (known as synapsis) is still being studied and there appear to multiple mechanisms. Occasionally non-homologous chromosomes will pair, but this is almost certain to lead to bad consequences. This means that in healthy cells (for diploid organisms) there will only be one correct pairing. Therefore for two chromosomes to be homologous they must be similar in sequence. First a definition: Homologous means "identical by descent" - so for two chromosomes to be homologous they must have descended from a (potentially much earlier) chromosome that has been duplicated possibly for many generations (the ancestral chromosome might even have existed in a now extinct ancestral species).
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