DNA structure and replication 1. From the experiments covered in class, answer t

Question

DNA structure and replication 1. From the experiments covered in class, answer the following questions a. How were scientists able to determine that DNA, and not some other molecule serves as the genetic material in bacteria and bacteriophage? How was it determined that the structure of DNA is a double helix with the two strands held together by hydrogen bonds formed between complementary nitrogenous bases? How do we know that G pairs with C and that A pairs with T as complementary base pairs are formed? b. c. 2. When Avery and his colleagues had obtained what was concluded to be the transforming factor from the IIIS virulent cells, they treated the fraction with protease, RNase, and DNase, followed in each case by the assay for retention or loss of transforming ability. What were the purpose and results of these experiments? What conclusions were drawn? 3. Phage are radiolabelled with radioactive sulfur and then allowed to infect bacteria. a. Will the phage progeny from lysis of those bacteria be radloactive? What famous pair of scientists conducted an experiment similar to this? What was the conclusion of their experiment? b. c. if the GC content of a DNA molecule is 22%, what are the percentages for each of the individual nucleotides? 4, 5, why is the melting temperature of DNA (%) related to base composition? 6. Each nucleotide pair of a DNA double helix weighs about 1 x 1041 g. The human body contains approximately 0.5 g of DNA. How many nucleotide pairs of DNA are in the human body? One nucleotide strand of a DNA molecule has the base sequence shown below: 5'-ATTGCTACGG-3 7. Give the base sequence and label the 5' and 3' ends of the complementary DNA nucleotide strand. 8. Define and indicate the significance of the following for DNA replication. a. Okazaki fragments b. DNA ligase c. Primer RNA characteristics. Predict what enzyme or function is being affected by each mutation, and explain your prediction 9. Several temperature-sensitive mutant strains of E. coli display the following

Explanation / Answer

1 .a.

There are two landmark studies on bacteria nd bacteriophage that support the fact that DNA is the genetic material.

i)Transformation Experiment :-

The experiment was initially done by Griffith. what he did-

It was seen that the RII cells got transformed into SIII. Phenomena known as transformation.

Avery,Macleod and McCarthy repeated the experiment in vitro to identify the cause of transformation.

they reported that th e virulence of Pneumococcus depends on a polysaccharide capsule which is present in virulent strain SIII and absent in avirulent of non-virulent strain RII.

The strains of non capsulated type RII were treated with an extract of DNA from capsulated strain SII. A few cells of SII type could be isolated from the mixture.

RII(non virulent)+DNA extract from SIII(extract is non virulent but living SIII are virulent)--->many non virulent RII+ few virulent SIII

When the extract was treated with DNAse that destroys the DNA this transfoming abilty was lost.But treatment with protease or RNAse did no affect the transforming ability.

This experiment on bacteria indicated that DNA is the genetic material.

ii)Hershey -Chase experiment:

E.coli cells were infeected with 32PDNa labeled phage,after allowing them to grow for a certain time they were agitated in a blender which sheared off the phage coats.the pahge coat and the infected cells were then separeted by centrifugation.most of the radioactivity was found in the cells.

the same esperiment was performed using 35S labelled cells(labells the phage coat protein) most of the radioactivity was found outside the cells and in the suspension mostly after centrifugation.Since phage reproduction (both DNA synthesis and new coat protein synthesis) occur intside the infected cells and since only the phage DNA enters the host cell,the DNA and not the protien must carry the genetic information.

b)Rosalind Franklins Xray crystallographic studies showed that DNA was a double helix.Further Watson Crick's study showed that the helix are joined by hydrogen bonds. Chargaff's experiment depicted the pairing of the nitrogenous bases.

c)A and T and Gand C pairing specificity is known from the discovery of Chargaff.His rules regarding this pairing is known as Chargaff's rules.

2.Avery and collegues treated the extract with

protease:protease degrades protiens .so if the tranformation abilty is within the protien component then treating the extract with protease would cause loss of transformation ability--that was not lost actually--thus trotein is not the genetic material.

RNAse degrades RNA.so digestion by this enzyme would cause loss of transformation abilty if RNA was the genetic material.But cells were transfromed.---thus RNA was not the genetic matrial.

DNAse degrades DNA .treating the eaxtract with this enzyme caused the loss of transformtion ability. So it was proved that DNA was the gentic material.

  

3)

a)35S radiolabelles the coat proteins of the phage virus. As we kow during infecting a bacteria as the DNA only enters the cell which is not labelled thereby the daughter phages produced will not have any radioactivity.

b)A.D. Hershey and M.Chase did a similar experiment.

c) From their experiment they concluded that DNA but not protein carreis the genetic informations or simply DNA is the genetic material.

4) according to question GC content is 22%.

we know G pairs with C only.

so both G and C will have same percentage.

since,G+C=22%

so G=C=22/2=11%

Again,rest is AT=100%-22%=78%

similar to G and C

A=T=78/2=39%

ans:G and C ,A and T are 11% ,39% respectively.

5)melting of DNA is separation of teh 2 DNA strands that are held togather by the hydrogen bonds between the nitrogenous base pairs.Now we know that there are 2 hydrogen bonds between A and T while 3 between C and G.So breaking 3 bonds will need more heat than breaking 2 bonds.Thus more AT will lower the melting temperature while more GC will increase it and vice versa.

6) 1x10^-21g weighs 1 double helix

so,0.5g weighs 0.5/1x10^-21 = 0.5x10^21 double helix (ans.)

7)We know tha A pairs with T and G pairs with C and the DNA strands are antiparallal.So following that the sequence 5'-ATTGCTACGG-3' has the complemntery sequence of 3'-TAACGATGCC-5'.

I am really sorry for not being able to answer all the questions as we also get a limited time to answer. This is olso why I could not elaborately explain many things. Please be considerate.

Thank you and hope this is helpful.

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