RNA is unstable:
- 2’-OH group present at every nucleotide (ribose sugar) in RNA is a reactive group and makes RNA liable and easily degradable.
- RNA is also now known as catalyst, hence reactive.
- RNA is unstable and mutates faster. Consequently the viruses having RNA genome and having shorter life span mutate and evolve faster.
DNA is more stable:
- Stability as one of the properties of genetic material was very evident in Griffith’s ‘transforming principle’ itself that heat, which killed the bacteria at least did not destroy some of the properties of genetic material.
- Two strands being complementary if separated by heating come together, when appropriate conditions are provided.
- Presence of Thymine in place of uracil confers additional stability to DNA
- DNA is chemically less reactive and structurally more stable when compared to RNA.
- Therefore among the two nucleic acids the DNA is a better genetic material.
Better genetic material (DNA or RNA)
- Presence of thymine at the place of uracil confers more stability to DNA.
- Both DNA and RNA are able to mutate.
- In fact RNA being unstable mutate at a faster rate.
- RNA can directly code for the synthesis of proteins, hence easily express.
- DNA however depends on RNA for protein synthesis.
- The protein synthesis machinery has evolved around RNA.
- Both RNA and DNA can functions as genetic material, but DNA being more stable is preferred for storage of genetic information.
- For the transmission of genetic information RNA is better.
RNA WORLD:
- RNA is the first genetic material.
- Essential life processes evolved around RNA.
- RNA used to act as a genetic material as well as catalyst.
- But RNA being catalyst was reactive and hence unstable.
- Hence DNA has evolved from RNA with chemical modifications that make it more stable.
- DNA being double stranded and having complementary strand further resists changes by evolving a process of repair.
REPLICATION: THE PROCESS:
- Watson and Crick proposed a scheme for replication of DNA.
- The Original statement that “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material (Watson and Crick, 1953)
- The scheme suggested that the two strands would separate and act as template for the synthesis of new complementary strands.
- New DNA molecule must have one parental strand and one new strand.
- This scheme of replication is called Semiconservative type of replication.
Experimental Proof of semiconservative nature of replication:
- It is now proved experimentally that replication is semiconservative type.
- It was first shown in Escherichia coli and subsequently in higher organism.
- Mathew Messelson and Franklin Stahl performed the following experiment in 1958.
STEPS OF THE EXPERIMENTS:
- They grew E.coli in 15NH4Cl medium for many generations. (15N is heavy nitrogen not radioactive element)
- The result was that 15N was incorporated into newly synthesized DNA and other nitrogen containing compound as well.
- This heavy DNA molecule could be distinguished from normal DNA by centrifugation in a cesium chloride (CsCl) density gradient.
- Then they transferred the E.coli into a medium with normal 14NH4Cl and let them grow.(E.coli divides in 20 minutes)
- They took samples at definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices.
- Various samples were separated independently on CsCl gradients to measure the densities of DNA.
- The DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium had a hybrid or intermediate density.
- DNA extracted from the culture after another generation (after 40 min.) was composed of equal amount of this hybrid DNA and of ‘light ‘DNA.
Experiment by Taylor and colleagues:
- Used radioactive thymidine to detect distribution of newly synthesized DNA in the chromosomes.
- They performed the experiment on Vicia faba (faba beans) in 1958.
- They proved the semiconservative nature of DNA replication in eukaryotes.
Replication Machinery and Enzymes:
- In all living cells such as E.coli replication requires a set of enzymes.
- E.coli completes the replication of its DNA in within 38 min.
- The average rate of polymerization has to be approx. 2000 bp per sec.
- The polymerization process must be accurate; any mistake during replication would result into mutation.
- Deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP) serve dual purposes:
- Provide energy for polymerization.
- Acts as substrates for polymerization.
- The replication process occurs within a small opening of the DNA helix called replication fork.
- The region where, replication fork formed is called origin of replication.
- The replication fork is formed by an enzyme called helicase.
- Two separated strand is called template strands.
- Main enzyme is DNA-dependent DNA polymerase, since it uses a DNA template to catalyze the polymerization of deoxyribonucleotides.
- DNA polymerase catalyses polymerization only in one direction i.e. 5’→3’.
- On one strand (template with 3’→5’ polarity) the replication is continuous hence called leading strand.
- In another strand (template with 5’→3’ polarity) the polymerization takes place in the form of short fragment called Okazaki fragment.
- The short fragments are joined by DNA ligase, hence called lagging strand.
- In eukaryotes replication takes place in S-phase of cell cycle.
- A failure of cytokinesis after replication results into polyploidy.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]
