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Post by Miss DiFederico on Apr 6, 2012 8:21:03 GMT -5
Feel free to post any questions you have about course material, assignments, test, etc.
Please try to answer each other's questions.
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Post by melanie on Apr 14, 2012 15:30:31 GMT -5
Hello, I have a few questions 1.) I dont understand how the conservative model for DNA replication proposed by Meselson and Stahl was supposed to work.( how did they predict a 3:1 ratio of light:heavy after the second replication) ?? The conservative model says that the two DNA strands stay together during replication.
So after one round of replication, you have a)a set of double stranded DNA (2 strands) that are the originals b) and the newly replicated double stranded DNA (2 strands) which are both new. This would give one heavy band from the original DNA and one light band from the new DNA (1:1).
During the second round of replication, a) the double stranded DNA made of original DNA will replicated giving 2 sets of double stranded DNA --> one made of two original strands, and one made of two new strands. This gives one heavy band and one light band b) the double stranded DNA made of new DNA will replicate. Both new sets of double stranded DNA will now be made of new DNA. This will give two light bands.
Overall, after two rounds of replication, you get a 3:1 ratio new:original sets of double stranded DNA.2.) I am a little confused by DNA replication and I was wondering if anyone could explain to me what the difference is between DNA polymerase III and the RNA primer ?? Do these enzymes both serve the same purpose and just work with each other The RNA primer acts as a signal to tell the DNA poly III where to begin to add the new nucleotides. So the RNA primer will attach first, then the DNA poly III will recognize the primer and know to begin adding complimentary bases at the 3' end of the primer.
Does this help?\ yess thanks miss !!! I understand now
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Post by alexbardoel on Apr 14, 2012 16:25:52 GMT -5
Hi Miss! I was just reviewing my notes and wondered if we had to be familiar with the structure of the four complimentary bases?
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Post by Miss DiFederico on Apr 14, 2012 16:43:30 GMT -5
Hello, Yes and no. You need to know that the purines (adenine and guanine) are a DOUBLE ring structure and that the pyrimidines (thymine and cytosine) are SINGLE rings. You do NOT need to know how to draw, label or recognize a diagram of any of the bases. Absolutely nothing on the test will involve any diagrams. Hope this helps
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Post by Rebecca B on Apr 14, 2012 20:45:11 GMT -5
Hello i have a question How does the fact that DNA replicates semiconservatively decreases the possibility of errors made during DNA replication? Describe another mechanism that minimizes DNA replication error By replicating semiconseratively, the DNA strands are separated. Once this happens, new nucleotides can be added one at a time. If the wrong base is added, the cell has mechanisms to recognize and correct this error. Also, if one nucleotide is incorrect, this is a small scale error. If DNA replicated conservatively and the entire double stranded molecule was replicated incorrectly, this of course would be a much more sever error.
Another mechanism that reduces error is what we talked about in class on Friday (not on test). Many amino acids have multiple codons that code for that same amino acid. This means if the wrong base if added during replication, this will not necessarily change the amino acid and thus there is what we call a silent mutation. And I was wondering if all of the questions were going to be from the class notes or also from the homework? Most of the stuff will be from class. I liked some of homework questions as well. I mentioned in class that I liked p216 # 6 and 7 (we took these up in class). Also, I said to add p. 223 #7 to your list of homework questions because I think it's a good question...if you know what I mean. Thanks!
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Post by melanie on Apr 14, 2012 20:49:05 GMT -5
I have another question ... will we have to know the years/dates and order of who discovered what first that each scientists made a contribution to genetics (like the summary on pg 215 of our txt ... i dont know if you have a copy of the textbook)
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Post by Miss DiFederico on Apr 14, 2012 21:33:54 GMT -5
No years, dates or need to know the order in which the discoveries were made.
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Post by melanie on Apr 14, 2012 21:35:56 GMT -5
No years, dates or need to know the order in which the discoveries were made. THANK YOU !
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seema
New Member
Posts: 15
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Post by seema on Apr 14, 2012 21:45:01 GMT -5
Hey Miss! I am a bit confused about the Meselson and Stahl experiment; their results in the three test tubes were (a) heavy DNA (b) layer of intermediate DNA and (c) layer of light DNA and layer of intermediate DNA I don't really understand what they mean by heavy/light/intermediate and what those three represented in their experiment or how they proved that DNA replicates semi-conservatively by it?
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Post by Miss DiFederico on Apr 14, 2012 22:12:16 GMT -5
Original DNA- heavy nitrogen therefore heavy band New DNA (all strands which are made by replicating the parent DNA)- light nitrogen therefore light band Mixed weight band- here you have both old and new DNA together in the same double stranded DNA molecule (ie. semiconservative had one strand of new DNA and one strand of old DNA)
Parent DNA: both strands are original --> heavy
One round of replication a) conservative: this model says DNA strands stay together. So you would have one double stranded DNA molecule where both strands are made of original DNA (heavy) and one where both strands are made of new DNA (light) This will give you one heavy and one light band in the test tube. Note: this is NOT seen therefore conservative is not the correct model
b) semiconservative: this model says the DNA strands separate. For each original strand, a new strand is built. So after one round of replication you will have two DNA molecules each composed of one original strand and one new strand (mixed weight because both new and old DNA are present in the same molecule of DNA) This will give you an mixed/intermediate weight band in the test tube. Note: this is seen
c) dispersive: this model says pieces of original DNA are present in each of the newly replicated DNA molecules and the gaps are filled in with new DNA (mixed weight because both new and old DNA are present in the same molecule of DNA) Note: this is seen
Because the mixed weight band after ONE round of replication supports both the semiconservative and the dispersive model, a second round of replication must be done.
After a second round of replication: a) conservative (* this model was already disproved after the first round of replication, however, if you were to do a second round, this is what you would expect to see if the conservative model was correct): you were left with a double stranded DNA mole cue made of only old DNA and one made of only new DNA after the first round of replication. If you were to do a second round of replication, you would have one molecule where both strands are made of original DNA (because in this model the strands do not separate) and three molecules made of only new DNA. You would see a light band and a heavy band.
b) semiconservative: You were left with two molecules with one strand of old and one strand of new DNA after the first round of replication. Each of these strands will separate and a new strand will be build with each. So you get two DNA molecules that contain one old strand and a newly built strand. You also have two DNA molecules composed of only new DNA. This will give a mixed band and a light band. Note: this banding pattern is seen --> proves DNA replicates semiconservatively
c) dispersive: all DNA molecules will have both old and new DNA because pieces of the molecules from the first round of replication (that contain both new and old DNA) will break off and the gaps will be filled in with new DNA. This will give a mixed weight band. --> not seen therefore not the dispersive model
Does this help?
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Post by Lisa Kim on Apr 15, 2012 9:10:32 GMT -5
Hi Miss! I have a question Is DNA ligase only present in lagging strands? It says that it joins Okazaki fragments in lagging strands but in the diagram, I saw that there is DNA ligase in the leading strand too. So i was wondering if DNA ligase have any other function than joining Okazaki fragments.. Thank you!
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Post by Miss DiFederico on Apr 15, 2012 10:04:26 GMT -5
DNA ligase is able to join any two fragments of DNA. Mainly we think of the Okazaki fragments in the lagging strand. However, recall that DNA poly I removes the RNA primer from both the leading strand (only 1 primer) and the lagging strand (multiple primers) and fills in the gap where the primer was with DNA nucleotides. This small area must then be joined to the rest of the replicated DNA molecule. DNA ligase is used to do this. Because RNA primers are present in both leading and lagging strands, DNA ligase is used in both strands. Note that in the lagging strand with the Okazaki fragment, DNA ligase will of course be used more often.
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Post by sarahgraham on Apr 15, 2012 11:13:10 GMT -5
Hey Miss, I was just wondering if we have to know Frederick Griffith's experiment in detail and how he used the virulent and nonvirulent bacteria on the mice. I'm still not sure how this experiment worked to show transfermation. How can the cells that were destroyed by heat convert the unencapsuled cells to the virulent form?
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Post by Miss DiFederico on Apr 15, 2012 12:10:44 GMT -5
We did not discuss this experiment in depth in class, so that means that it will not be on the test. You only need to know what was discussed on the slides. That is true for any other information not covered in class. If it was not on my slides, I promise it will not be on my test.
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Post by Brittany on Apr 15, 2012 13:04:25 GMT -5
Hi Miss! I was just wondering what the format of the test is going to be. Thanks!
25 multiple choice 25 marks worth of short answer - 4 questions application, 4 questions of communication
You can answer in point from
NO diagrams
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