Chapter 2.2 Flashcards
DNA is a stable molecule because:
- The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
- Hydrogen bonds like the organic base pairs forming bridges (rungs) between the phosphodiester uprights. As there are three hydrogen bonds between cystine and guanine, the higher proportion of C-G pairings, the staler the DNA molecule
- There are other interactive forces between the base pairs that hold the molecule together (e.g. base stacking)
What is the function of DNA??
It is a hereditary material responsible for passing genetic information from cell to cell and generation to generation.
- How many base pairs are in DNA?
2. What does this mean?
- There are 3.2 billion base pairs in the DNA
- meaning there is almost an infinite variety of sequences of bases. Therefore, there is there is great genetic diversity.
The DNA is adapted to carry out its function in the following ways?
- It is a very stable structure and normally passes from generation to generation without change. It rarely mutates.
- Its two separate strands are joined only with hydrogen bonds which allows them to separate during DNA replication and protein synthesis.
- It is an extremely large molecule and therefore carries immense amount of genetic information.
- The deoxyribose-phosphate backbone and the bases being within the helical cylinder protects the bases from being corrupted by outside chemical and physical forces.
- Base pairing leads to DNA being able to replicate and to transfer information as mRNA.
What is the pneumonia experience?
- A harmful form that causes pneumonia is known as the S-strain.
- Mice were separately injected with living bacteria of the safe form and dead bacteria from the harmful form. All the mice remained healthy
- Then, the group of mice were injected with both types together. These mice then developed pneumonia
There are 3 possible reasons why this happened:
-Experimental error, for example, the harmful forms in the mixture were not killed - The living safe form had mutated into the harmful form. This is possible but extremely unlikely, especially given that the experiment was repeated many times were not all killed.
Step 6: Pneumonia is caused by a toxin. The harmful form of the bacterium has the information on how to make the toxin but, being dead, cannot do so. The safe form has the means of making the toxin but lacks the information on how to do so. The information on how to make the toxin may have been transferred from the harmful form to the safe form, which then produces it.
Step 6 was then investigated by: - The living harmful bacteria that were found in the mice with pneumonia, were collected
- Various substances were isolated from these bacteria and purified
- Each substance was added to suspensions of living safe bacteria to see whether it would transform them into the harmful form.
- The only substance that produced this transformation was purified DNA
- When an enzyme that breaks down DNA was added, the ability to carry out the transformation ceased.
How does DNA replication go:
- Starts at the origin identified by the DNA sequence. DNA has multiple origins.
- At the origin, helicase (unzipping enzyme) breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the DNA unwind to form two single strands.
- The original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides that have been activated bind specifically to their complementary exposed base on each original template strand – A with T and C with G.
- Condensation reactions join the nucleotides of the new strands together by a phosphodiester bond– catalysed by the enzyme DNA polymerase. Meanwhile the remaining unpaired bases continue to attract their complementary nucleotides. Hydrogen bonds form between the bases on the original and new strands.
- The leading strand is in a 3’ to 5’direction, the DNA polymerase is moving in the complementary 5’ to 3’ direction in order to make the complementary 5’ to 3’ strand. (Think of the leading strand as something easy = counting: 1, 2, 3, 4, 5…, therefore it is in the 3’ to 5’ direction, the DNA polymerase is then complementary to this thus 5’ to 3’). The DNA can replicate continuously when the strand is in 3’ to 5’ direction because the active site of DNA is complementary to the 3’ end of the newly forming DNA strand, so the enzyme can only add nucleotides to the new strand at the 3’ end.
The two strands however are antiparallel. This means, the lagging strand runs in the opposite direction to the leading strand, therefore the lagging strands run in 5’ to 3’. This means to replicate the 5’ to 3’ strand, the DNA polymerase has to move in the opposite direction as it did in the 3’ to 5’ strand. The new strand it forms is in the 3’ to 5’ direction. It replicates this strand discontinuously and the short Okazaki fragments being formed are later linked together by an enzyme called ligase. - Each new strand of DNA contains one strand of contains one original DNA strand and one new DNA strand. This process is termed semi-conservative model.
What does the semi-conservative model show?
The model that shows that that a new DNA molecule is made up of both 1 strand of original DNA and one strand of new DNA
What were the two scientist that found this?
The model that shows that that a new DNA molecule is made up of both 1 strand of original DNA and one strand of new DNA. This is theory was founded by two scientists Stahl and Melson, by doing this one experiment:
What 3 facts were the experiment based on?
- All the bases in DNA contain nitrogen
- Nitrogen as two forms: N14 and N15. N14 is lighter and N15 is heavier and the isotope.
- Bacteria will incorporate nitrogen from their growing medium, into any new DNA they make.
What is the semi-conservative model?
Step 1: They grew some bacteria in N14 medium which was used as the control, the bacteria grew for some generations.
Step 2: They grew some bacteria some bacteria in a N15 and let it divide for generations by binary fission.
Step 3: They then transferred the bacteria that was grew in the N15 medium over into the N14 medium and let the a=bacteria multiply. They noticed threat the new bacteria that were made in the N14 medium contain N14 and N15 both in their DNA. This therefore shows that DNA must be both made out of both one stand of parental DNA and one strand out of new DNA.
Step 4: They proved this by putting the DNA in a centrifuge, and spun at very high speed- check. The bacteria that was just grew in the N15 medium sunk to the bottom, as N15 bacteria is heavy. They then the DNA that grew in the N14 medium in the centrifuge and spun at a high speed and because N14 is light it floated near the top of the test tube. They then put the DNA from the bacteria in the centrifuge that was grew in both the N14 and N15 medium and was spun at high speeds. This floated to the middle of the test tube. This shows that because it floated to the middle of the test tube 1 strand of DNA must be parental DNA and one strand of DNA must be new DNA therefore the semi-
What are the differences between DNA and RNA
RNA: single stranded ribose sugar Has A,U,C and G The ribose sugar has two OH hydroxyl groups the deoxyribose one OH group hydroxyl group made in the nucleus Function in the cytoplasm/ribosome DNA double stranded deoxyribose sugar Has one OH groups hydroxyl group Has organic bases, A,T,C,G Only in the nucleus
