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The causes of genetic mutations

Created by Ross Firestone.

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  • leaf green style avatar for user amk3
    You discussed translocations and inversions but what about deletions(non-disjunction) and duplications(non-disjunction) on chromosomes? How do those work exactly?
    (10 votes)
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    • marcimus pink style avatar for user Rachel Eastman
      Here's what I've understood: mutations occur when there is a change in the sequence of the genetic code. If non-disjunction occurs during cell division, leaving cells with too few or too many copies of a given chromosome, the code has not actually been altered. What has changed is the number of copies of specific parts of its genetic code. Issues related to non-disjunction arise from the over-production or under-production of the protein products coded for by the genes that are present in the wrong number of copies.
      (9 votes)
  • blobby green style avatar for user poulinn91
    This video is not about the causes of mutations as suggested by the title. It really just explains further classifications of mutations introduced in the previous video.
    (9 votes)
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    • piceratops ultimate style avatar for user Thomas Varkey
      No this is the cause of the mutation of the actual protein... While the classification may not seem to be the cause the actual causes of mutations are not well known...As the differ for each case... Take for example the thymine dimer or other major ones... These contribute only minor changes and are not a huge cause of mutations... The causes of mutation by your definition are known as mutagens and are discussed in the next chapter.
      (0 votes)
  • piceratops tree style avatar for user Siyuan Yu
    I'm not quite sure how exactly transition, transversion and especially mispairing has to go...

    So assuming the base pair is A-T, then transition could only mutate it to G-C, so it swaps purine for purine and pyrimidine for pyrimidine, both at the same time; transversion could only mutate it to C-G or T-A, so it swaps the purine for either one of the pyrimidines and then the original matching pyrimidine to the current matching purine, also both at the same time; and mispairing could mutate the pair to A-anything but T/ G-anything but C/ C-anything but G/ T-anything but A, which means it could be switching out either one of the nucleotides or both, as long as the two aren't a proper Watson-Crick pair...?

    Did I get all that right?
    (6 votes)
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  • leafers ultimate style avatar for user aliceliou82
    How does mismatching (mispairing) differ from transition and transversion? Don't transition and transversion basically cause mismatching? Thx!
    (4 votes)
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  • piceratops ultimate style avatar for user Oshien
    In the video it said that inversion refers to two genes on the same chromosome swapping places. I was taught in class that inversion refers to the event when a gene segment breaks away and rejoins at the same spot but in the reversed orientation (so basically the gene is "inverted")

    which definition is correct?
    (4 votes)
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  • piceratops seedling style avatar for user Jessica Lovelace
    Are transposons an example of an inversion (large-scale mutation), or are they two completely different entities?
    (2 votes)
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  • blobby green style avatar for user Alan Warrick
    What causes the substitutions to take place?
    (1 vote)
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    • female robot grace style avatar for user tyersome
      This is a huge subject and there are many different sources of mutation, I'm listing a few and including links to two free online book chapters that will allow you to learn more.


      DNA polymerases make mistakes — this is quite rare and is usually corrected, but sometimes the wrong base remains and is inherited by daughter cells.

      Another major source of spontaneous mutations is instability in the bases that make up DNA. This includes: "tautomerization" where the chemical structure shifts into an isomer that pairs differently and cytosine deamination. This second process creates uracil and is promoted by a common chemical modification (methylation) that is important for regulation of the DNA.

      Mistakes can also be introduced due the effect of UV light, ionizing radiation, or chemicals that modify the DNA — the latter appears to be covered in the next video.

      Further reading:
      https://www.ncbi.nlm.nih.gov/books/NBK21897/
      https://www.ncbi.nlm.nih.gov/books/NBK21578/

      Does that help?
      (2 votes)
  • blobby green style avatar for user 😊
    why there is difference between the number of mutant/ variants at DNA level and at the protein level
    (1 vote)
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  • primosaur sapling style avatar for user Beanie
    How do some DNA fail to copy itself correctly? what is causing its mistake?
    (1 vote)
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    • female robot grace style avatar for user tyersome
      This is a huge subject and there are many different sources of mutation, I'm listing a few and including links to two free online book chapters that will allow you to learn more.


      DNA polymerases make mistakes — this is quite rare and is usually corrected, but sometimes the wrong base remains and is inherited by daughter cells.

      Another major source of spontaneous mutations is instability in the bases that make up DNA. This includes: "tautomerization" where the chemical structure shifts into an isomer that pairs differently and cytosine deamination. This second process creates uracil and is promoted by a common chemical modification (methylation) that is important for regulation of the DNA.

      Mistakes can also be introduced due the effect of UV light, ionizing radiation, or chemicals that modify the DNA — the latter appears to be covered in the next video.

      Further reading:
      https://www.ncbi.nlm.nih.gov/books/NBK21897/
      https://www.ncbi.nlm.nih.gov/books/NBK21578/

      Does that help?
      (1 vote)
  • leafers seed style avatar for user Chelsea Newman
    Are point mutations the same thing as base pair mutations?
    (1 vote)
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Video transcript

Voiceover: So, today we're going to talk about the causes of genetic mutations, but first let's just do a quick review of the idea that mutations are mistakes in a cell's DNA, and there are two main types of mutations that we see when we look at a cell's DNA, and the first is called point mutations, and that's when one DNA base is switched out for another, which usually results in a change to one codon in the RNA sequence. Frame-shift mutations are when the reading frame of the RNA is altered, and while the actual nucleotides in the RNA sequence haven't changed that much, the reading frame of the RNA strand has shifted, meaning that many different RNA codons will change as a result, and we're going to take a look into what causes these point and frame-shift mutations. So, point mutations are caused by base substitution, which is when one DNA base is substituted for another, and there are a couple of different types of base substitution. A transition is when you have a substitution of adenine for guanine or vice versa, which is a swap between two purines, or a substitution of cytosine for thymine or also vice versa, which is a swap between two pyrimidines. A transversion is when either adenine or guanine is swapped for either cytosine or thymine, and in this type of base substitution, you have either a purine being replaced with a pyrimidine or a pyrimidine being replaced with a purine. Now, the last kind of mutation that can lead to a point mutation is a mispairing, which some people call mismatching, and that's when a DNA strand has a non-Watson-Crick base pairing. Normally, A pairs with T and G pairs with C, but when you have a mispairing, that's when A and C pair up or when G and T pair up, and it's much more common for mispairings to occur between a purine and pyrimidine, as opposed to between two purines, like A and G pairing up, or two pyrimidines like C and T pairing up. Next, we're going to talk about frame-shift mutations. So, let's say that we have this DNA strand here, with three repeating CTC units and an extra C on the end. This would then be transcribed into an RNA strand with repeating GAG units and an extra G on the end, and our three codons would be the three GAG units, which would then each translate to a glutamate amino acid. Now, one way you can cause a frame-shift mutation is through an insertion, and that's when an extra DNA base finds its way into our sequence. So, here we have this extra cytosine base, that I've underlined, falling into our sequence, and this additional C base would lead to an extra G being thrown into our RNA sequence, which would then shift the codon reading frame of our RNA strands during translation. So now, instead of three GAG codons, we have just one GAG codon and two GGA codons, with two extra bases on the end. This would then code for one glutamate residue and two glycine residues, instead of three glutamates. The other way that you can cause a frame-shift mutation is through a base deletion. So, in a deletion, we drop off one of our bases from our original sequence. So, here I've dropped that first thymine base, and this would also result in a shift of the RNA reading frame. Now, instead of having three GAG codons, we have a GGG codon and two AGG codons, which would lead to a protein with a glycine and two arginine amino acids. So, overall, insertions and deletions can both lead to frame-shift mutations. Now, we can also talk about large-scale mutations, which instead of being at the level of individual nucleotides, are usually seen at the chromosomal level and can affect many genes, instead of just a few base pairs. So, first we'll talk about translocation, which is when a gene from one chromosome is swapped for another gene on a different chromosome. Now, it's important to see that translocation refers to gene swapping between nonhomologous chromosomes, which means that if this blue chromosome were chromosome 10, then the green one could be any chromosome aside from chromosome 10, and this is what sets translocation apart from the process of crossing over that occurs during meiosis between homologous chromosomes. The next large-scale mutation we'll talk about is chromosomal inversion, and that's when two genes on the same chromosome switch places. So, here our green and blue genes are being swapped and end up on different parts of the chromosome after the mutation. Now, since both of these mutations don't always affect the individual nucleotides coding for a gene, it's important to see that many of these types of mutations affect how a gene's expression is regulated, in addition to changing what the genes actually code for. Remember that the position of a gene on a chromosome partly determines how it's regulated, and this could be due to histone configuration, promoter regions, or any other regulatory process. So, what did we learn? Well, first we learned that small-scale mutations affect the DNA at the nucleotide level, and of these small-scale mutations, we have point mutations, which can be caused by transitions, transversions, and mispairings, and we also have frame-shift mutations, which can be caused by insertions or deletions. Next, we talked about large-scale mutations, which affect the DNA at the chromosomal level, and the two large-scale mutations we talked about were translocation and inversion.