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Course: MCAT > Unit 5
Lesson 4: DNA- DNA questions
- Central dogma of molecular biology
- Central dogma - revisited
- Early experiments on the genetic code
- Eukaryotic gene transcription: Going from DNA to mRNA
- DNA
- Molecular structure of DNA
- Antiparallel structure of DNA strands
- Telomeres and single copy DNA vs repetitive DNA
- Leading and lagging strands in DNA replication
- Transcription and mRNA processing
- Speed and precision of DNA replication
- Translation (mRNA to protein)
- Differences in translation between prokaryotes and eukaryotes
- DNA repair 1
- DNA repair 2
- Semi conservative replication
- Protein modifications
- Jacob Monod lac operon
- DNA structure and function
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Telomeres and single copy DNA vs repetitive DNA
Telomeres, the protective caps on chromosome ends, prevent deterioration and chromosome sticking. They contain no genes, acting as a buffer zone during replication. Telomerase can lengthen telomeres, allowing cells to replicate more. The video also discusses single copy and repetitive DNA, with telomeres being highly repetitive DNA.
Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Efrat Bruck.
Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Efrat Bruck.
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Please try to make the writing more clear. Thanks!(55 votes)- At5:18, she says the telomerase has a repeating sequence of GGTTAG, that is wrong. It has a repeating sequence of TTAGGG(5 votes)
TTAGGG is the repeating sequence in telomeres, but in the actual lengthening process telomerase (the enzyme responsible for lengthening telomeres) repeatedly adds GGTTAG. Since the element is repeated many times, the repeating sequence just depends on where you "start" in the sequence e.g. put two GGTTAG's together and you have GG then TTAGGG then TTAG.(33 votes)
Hi! It's impossible to read the text on this video - it is very blurry. Is it possible to re-upload this video with better resolution? I love the MCAT videos - they are AWESOME. Thank you!(13 votes)
Why are repetitive DNA so prone to mutations?(4 votes)
Repeats are prone to something called slipped-strand mispairing - basically they cause DNA to get misaligned easier during DNA synthesis. If not fixed, these errors cause deletions/frameshift mutations in the DNA. Interestingly, some bacteria make use of this mechanism so that they achieve constant genetic/phenotypic variability in their surface structures as to evade the immune system :)(17 votes)
- Great video. General question though in an experiment how would you go about manipulating a telomere binding sequence to represent another one. For example, TTAGGG to ATACGG. This was a topic that came up in my molecular biology exam.(6 votes)
Telomeres are an example of highly repetitive DNA. I think that was the connection.
It was to help show that because telomeres are a buffering area they are not involved in intense transcription and translation, like single copy DNA is.(3 votes)
Is the amount of telomerase present in cells the only (or main) factor that prevents certain cells in the body (like neurons) from replicating frequently? If so, is it possible to allow more replication of these cells by increasing the amount of telomerase (and thus lengthening the telomeres) int these cells?
(Just as a side note, how would the amount of telomerase present affect the ability of cells to retain their telomeres? -- since telomerase is an enzyme, and thus inexhaustible, the amount present would only affect the rate of telomere regeneration and not the presence of telomeres, right? )(4 votes)
Enzymes aren't "exhaustible". If you study any type of molecular bio in the future, you'll see that proteins have half lives (which can range from a few seconds to a few years). So, a protein will be made and used until degraded, and more can be made when needed by signaling to the nucleus, which leads to translation of more of it. So what you really mean to ask is if certain cells stop expressing (transcribing and translating genes encoding telomerase) telomerase after some time. And the answer is yes: basically all of your cells except eggs and sperms.
When your young, you have active telomeres, but once you grow up, telomere activity drastically decreases everywhere...except in your gametes. And yes, this leads to signs of aging, and potentially cancer, much later in life. Generally, your body detects if telomeres are getting shorter than the acceptable threshold, so they will do apoptosis. The ones that get away become cancerous.
The reason why certain specialized cells don't divide is usually linked to their complex structure, not so much to telomere length. The reason why brain cells don't divide too often is complex. Firstly, your brain can't risk having too many brain cells since they wouldn't fit. Secondly, they don't just grow and split like skin cells. Brain cells develope in embryos in a really complex path where they travel to their destination and start extending axons and what not. So it's a little hard to make new ones at an older age. But you do have a point, because some evolutionary scientists believe that our brains evolved to not continually undergo cell division in order to decrease the chance of cancer occurrence in our most vital organ. But it's still a hypothesis.(4 votes)
Is the loss of telomeres the cause of aging?(2 votes)- Aging is mostly attributed to oxidative stress, but mutations do pile up throughout our lifetime, leading to genetic-based complications such as cancer when we're old. That's why cancer usually shows up later in life (more than about 6 mutations have to accumulate in a cell line for that cell to become cancerous). So telomere degradation, which leads to deletion mutations eventually, leads to such complications at older ages. I believe telomerase activity also decreases with age leading to more telomere degradation...though I may be wrong. There's definitely a link between telomere length and life expectancy, but it's not yet quite known if telomere degredation is a cause or a consequence of aging.
If you ever look up Dolly the sheep, she was the first cloned animal. They took the body cell of another sheep, extracted its DNA, and inserted it into an empty egg. This egg became Dolly. Interestingly, once Dolly was born, she only lived to half the life span of an average sheep. One reason they think this might be is due to the fact that the DNA used to clone Dolly came from a middle-aged sheep whose telomeres had already been degraded a bit. If this does in fact end up being the reason for her short age, then telomeres would be linked to length of life! Isn't that so cool?! Genetics is honestly so cool.(4 votes)
When you refer to single copy and highly repetitive DNA, is this the same thing as 'euchromatin' and 'heterochromatin' ?(1 vote)
Euchromatin refers to DNA that is actively being transcribed and translated. Heterochromatin refers to inactive DNA that is coiled around histones.(5 votes)
Can someone explain the mechanism by which the telomeres get shorter and shorter with replication?(1 vote)
It's essentially due to the fact that a short segment at the end of DNA cannot be copied: if you recall, lagging strands have to keep laying down new primers as the next stretch of DNA is exposed and replicated. The last primer is removed at the end of replication and no further nucleotides can be added, so that's the area that's not replicated. I've linked below a Khan article that discusses this and another site that has some nice diagrams and a good analogy in the middle of the article. Hope that helps!
https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/telomeres-telomerase
https://learn.genetics.utah.edu/content/basics/telomeres/(2 votes)
- is single copy DNA similar between people whereas the repeated DNA varies between people? and that is why forensics use repeated DNA?? thanks(1 vote)
yes, the difference in the umber of repetitive sequence between individuals varies so it is used as a marker to distinguish between people. I recommend to search for VNTR,STR and other different types(1 vote)
5:18
Video transcript
- [Voiceover] Here is
a pair of chromosomes as they would appear during mitosis, and the ends of chromosomes
are capped with an area known as telomeres, and telomeres are mainly found
in eukaryotic chromosomes because usually prokaryotes
just have one circular chromosome, so it doesn't have any ends. And what do telomeres do? Well, they protect chromosomes, or protect the ends of
chromosomes from deterioration. Why would the ends of
chromosomes deteriorate? So, the enzymes that replicate chromosomes are not able to get to the very,
very end of the chromosome. So there's gonna be, they're
gonna get to, let's say this area. So there's gonna be a small spot over here that's not replicated, and since the telomeres
don't have any genes in them, it's not really harmful, it doesn't really matter. So, what would happen if
there were no telomeres? Well, let's take a look
at the other chromosome. If there were no telomeres, and let's say, the chromosome was only
replicated 'til about here, there would be this area with
useful genes that wouldn't be replicated, and that would
be pretty problematic. So basically, telomeres
act as a buffer zone, because they do not contain
any important genes. Another thing that telomeres do is they prevent chromosomes from
sticking to each other. If chromosomes stuck to each other, then a lot of the genes would be scrambled and genes wouldn't be where
they're supposed to be and that would be pretty problematic. And here's actually a
picture of human chromosomes where the telomeres are
highlighted in this florescent, so you can see the telomeres over here. So you can see how at both
ends of each chromatid, there are telomeres, and so what happens is that with each time the chromosomes replicate, the telomeres get a little bit shorter
and shorter and shorter, so there's an enzyme known as telomerase and telomerase is able
to lengthen telomeres and bring them back to
their original length. So there are some cells
that replicate a lot and they have a lot of telomerase. This cell can keep on
replicating and replicating, but then there are other
cells that do not have a lot of telomerase, and when telomeres are
basically non-existent anymore, because the chromosomes
replicated many, many times, let's just get rid of the telomeres, so the chromosomes will actually
not be able to replicate, and so the cell will not divide again, and it will kind of die. Now that we're talking about telomeres, I want to bring up a topic
that's tangentially related, and that is single copy
DNA and repetitive DNA. So, single copy DNA is when
you have a DNA sequence, I'm just gonna make one up, let's say, A T C C, that basically
does not repeat itself, so it might be flanked
by other DNA sequences, as opposed to repetitive
DNA, which is when you have a DNA sequence that
keeps repeating itself, so you might have it A T C
C and then again, A T C C, A T C C, et cetera. So what's the difference
between single copy DNA and repetitive DNA? So here we have a spectrum. On the left we have single copy DNA, in the middle we have DNA
that's somewhat repetitive, and on the right we have
highly repetitive DNA. So, single copy DNA holds
most of the organism's, there should be an apostrophe there, important genetic information, so basically most of the
important genes are going to be single copy, so since the important genes are single copy DNA, single copy DNA is
transcribed and translated and it has a low mutation
rate, which is a good thing because of course, we don't
want there to be mutations in the important genes. Repetitive DNA, or DNA
that's somewhat repetitive is found, well at least
in mammals and in insects, near the centromeres. If you recall, the
centromeres are the center of the chromatid, or when
you have chromatids that are duplicated, the chromatids are
attached by the centromere, by the, that middle
part in the chromosome, and they may contain genes
that are transcribed and translated, but then there
might also be parts of the repetitive DNA that don't contain genes, and those parts are not
transcribed and translated, and repetitive DNA has a
higher mutation rate than single copy DNA. Now let's take a look at DNA
that's highly repetitive. So, it contains no genes, and
because it contains no genes, it is not transcribed and not translated, and highly repetitive DNA
has an even higher rate of mutation than DNA
that's somewhat repetitive. So there's lots of highly
repetitive DNA that we're not exactly sure what its purpose is. Scientists are currently
trying to figure out what the purpose of this
highly repetitive DNA is, but there are some sections
of highly repetitive DNA that we do know what their purpose is. For example, telomeres. Telomeres are sections
of highly repetitive DNA and as I've explained
before, their purpose is to basically act as a buffer
zone for the important part of the chromosome, and in fact, the DNA sequence that's
repeated in telomeres is this right over here, G G T T A G, and in human chromosomes,
the telomeres are made up of approximately 2,000 repeats
of this DNA sequence, G G T T A G.