The Search for Genetic Material Questions in English

(B) Frederick Griffith conducted an experiment with $Streptococcus \text{ } pneumoniae$ using two strains: $S$-strain (virulent) and $R$-strain (non-virulent).
He observed that when heat-killed $S$-strain bacteria were mixed with live $R$-strain bacteria and injected into mice, the mice died.
He recovered live $S$-strain bacteria from the dead mice.
This led to the conclusion that some 'transforming principle' was transferred from the heat-killed $S$-strain to the live $R$-strain, which transformed the $R$-strain into the virulent $S$-strain.
Therefore, the main conclusion is that dead bacteria transfer some material to living bacteria, causing them to change their characteristics.

(C) Alfred Hershey and Martha Chase conducted experiments on bacteriophages to prove that $DNA$ is the genetic material.
They used radioactive isotopes to label the components of the bacteriophage.
Proteins contain sulfur but not phosphorus,so they were labeled with radioactive sulfur $(^{35}S)$.
$DNA$ contains phosphorus but not sulfur,so it was labeled with radioactive phosphorus $(^{32}P)$.
Therefore,proteins and $DNA$ are labeled with $^{35}S$ and $^{32}P$ respectively.

(D) In Frederick Griffith's transformation experiment,two strains of Streptococcus pneumoniae were used:
$1$. $S$ strain (Smooth): These bacteria have a mucous (polysaccharide) coat and are virulent,meaning they cause pneumonia.
$2$. $R$ strain (Rough): These bacteria lack the mucous coat and are non-virulent,meaning they do not cause pneumonia.
Therefore,the $R$ strain is characterized as rough and non-virulent.

(B) Frederick Griffith conducted experiments on Streptococcus pneumoniae to identify the transforming principle.
$1$. $R$ strain (Rough) is non-virulent; therefore,mice injected with $R$ strain survive.
$2$. $S$ strain (Smooth) is virulent; therefore,mice injected with $S$ strain die.
$3$. $S$ strain (heat-killed) is non-virulent because the heat kills the bacteria; therefore,mice injected with heat-killed $S$ strain survive.
$4$. $S$ strain (heat-killed) $+ R$ strain (live) results in the transformation of $R$ strain into $S$ strain,making it virulent; therefore,mice injected with this mixture die.
Thus,the mice remain alive in stages $I$ and $III$.

(D) $DNA$ acts as the genetic material in most organisms and is chemically more stable due to the presence of $2'-OH$ group absence and thymine instead of uracil.
$RNA$ performs various active roles such as being a messenger $(mRNA)$,adapter $(tRNA)$,and structural/catalytic molecule $(rRNA)$.

(C) In the original Hershey and Chase experiment,they used radioactive phosphorus $(^{32}P)$ to label $DNA$ and radioactive sulfur $(^{35}S)$ to label proteins.
Since $DNA$ enters the bacterial cell during infection by bacteriophages and proteins do not,they observed radioactive phosphorus inside the bacterial cells,proving $DNA$ is the genetic material.
If the composition were swapped,$DNA$ would contain sulfur and proteins would contain phosphorus.
Upon infection,the $DNA$ (now containing radioactive sulfur) would enter the bacterial cells.
Therefore,radioactive sulfur would be detected inside the bacterial cells,while radioactive phosphorus would remain outside with the protein coat.

(C) The unequivocal proof that $DNA$ is the genetic material came from the experiment conducted by Alfred Hershey and Martha Chase in $1952$ using bacteriophages.
Frederick Griffith conducted the transformation experiments using $Streptococcus \text{ } pneumoniae$ (Pneumococcus).
Avery,Macleod,and McCarty provided the biochemical characterization of the 'Transforming Principle' identified by Griffith.
Wilkins and Franklin produced the $X$-ray diffraction data of $DNA$ structure.

(C) Heavy $\text{DNA}$ (containing $^{15}\text{N}$) can be differentiated from normal $\text{DNA}$ (containing $^{14}\text{N}$) using density gradient centrifugation.
This technique utilizes a $\text{CsCl}$ (Cesium Chloride) density gradient.
This method was famously used by Meselson and Stahl in their experiment to prove the semi-conservative mode of $\text{DNA}$ replication.

(A) Alfred Hershey and Martha Chase conducted experiments in $1952$ using bacteriophages to prove that $DNA$ is the genetic material.
They grew some bacteriophages in a medium containing radioactive phosphorus $(^{32}P)$ to label the $DNA$,as phosphorus is present in $DNA$ but not in proteins.
They grew other bacteriophages in a medium containing radioactive sulfur $(^{35}S)$ to label the proteins,as sulfur is present in amino acids (proteins) but not in $DNA$.
By tracking these isotopes,they demonstrated that only the $DNA$ entered the bacterial cell,confirming it as the genetic material.

(C) Frederick Griffith's experiment $(1928)$ demonstrated bacterial transformation.
Statement $I$ is correct: $S$-strain (Smooth) bacteria possess a polysaccharide coat (mucus),which makes them appear smooth.
Statement $II$ is correct: $S$-strain is virulent (pathogenic) and causes pneumonia,whereas $R$-strain (Rough) lacks the coat and is non-virulent.
Statement $III$ is incorrect: The transforming principle is associated with the genetic material of the $S$-strain,not the $R$-strain.
Statement $IV$ is correct: Transformation can occur in vitro (in a test tube) when heat-killed $S$-strain is mixed with live $R$-strain.
Therefore,statements $I, II,$ and $IV$ are correct.

(C) In the Meselson and Stahl experiment,$E. coli$ was grown in a medium containing $^{15}N$ (heavy isotope) for many generations,resulting in $DNA$ that was heavy $(^{15}N-^{15}N)$.
When these bacteria were transferred to a medium containing $^{14}N$ (light isotope),the first generation (after $20$ min) showed only hybrid $DNA$ $(^{15}N-^{14}N)$.
After the second generation (after $40$ min),the $DNA$ consisted of $50\%$ hybrid $(^{15}N-^{14}N)$ and $50\%$ light $(^{14}N-^{14}N)$ $DNA$.
Therefore,the test tube would show two bands: one light band at the top and one hybrid band at the bottom. This corresponds to option $C$.

(C) Hershey and Chase conducted experiments using bacteriophages to prove that $DNA$ is the genetic material.
They grew some viruses on a medium containing radioactive phosphorus $(^{32}P)$ to label the $DNA$,as $DNA$ contains phosphorus but not sulfur.
They grew other viruses on a medium containing radioactive sulfur $(^{35}S)$ to label the proteins,as proteins contain sulfur (in amino acids like methionine and cysteine) but not phosphorus.
Therefore,the correct radioactive elements used were phosphorus and sulfur.

(B) The relationship between genes and $\text{DNA}$ is best understood through mutation studies.
Mutation studies,such as those conducted by Beadle and Tatum,demonstrated that specific changes in $\text{DNA}$ (mutations) lead to specific changes in the phenotype (e.g.,loss of a specific enzyme function).
This provided the foundation for the 'one gene-one enzyme' hypothesis,establishing that a gene is a segment of $\text{DNA}$ that codes for a specific functional product.

(C) Nuclein was the term coined by Friedrich Miescher for the substance he isolated from the nuclei of white blood cells found in pus. It is chemically a mixture of nucleic acids and proteins,not lipids. Therefore,the statement that it is a mixture of lipids and nucleic acids is incorrect.

(B) In the Hershey and Chase experiment,they used radioactive phosphorus $(^{32}P)$ to label $DNA$ and radioactive sulfur $(^{35}S)$ to label proteins. The statement in option $B$ is incorrect because it incorrectly assigns the isotopes: phosphorus is $^{32}P$ and sulfur is $^{35}S$,whereas the option states $^{35}P$ and $^{32}S$.

(B) The central dogma of molecular biology states that genetic information flows from $DNA \rightarrow RNA \rightarrow \text{Protein}$.
However, in retroviruses, the flow of information is reversed, where $RNA$ is converted into $DNA$ by the enzyme reverse transcriptase.
This process is known as reverse transcription.
This concept was independently discovered and proposed by Howard Temin and David Baltimore in $1970$.

(A) In $1869$, Friedrich Miescher first identified $DNA$. He isolated a substance from the nuclei of pus cells (leukocytes) and named it 'nuclein'. Later, due to its acidic nature, it was renamed 'nucleic acid'.

(C) The transforming principle was first identified by Frederick Griffith in $1928$,but he could not determine its biochemical nature.
Oswald Avery,Colin MacLeod,and Maclyn McCarty conducted experiments in $1944$ to determine the biochemical nature of the transforming principle.
They purified biochemicals (proteins,$DNA$,$RNA$,etc.) from the heat-killed $S$ strain bacteria and observed that only $DNA$ caused the transformation of $R$ strain bacteria into $S$ strain bacteria.
They also discovered that protein-digesting enzymes (proteases) and $RNA$-digesting enzymes (RNases) did not affect transformation,whereas $DNA$-digesting enzymes (DNases) inhibited it.
Thus,they concluded that $DNA$ is the hereditary material.

(C) Nucleic acid was first discovered by $Friedrich \ Miescher$ in $1869$.
He isolated a substance from the nuclei of pus cells (which are primarily $white \ blood \ cells$ or leukocytes) and named it 'nuclein'.
Later, it was identified as nucleic acid.

(D) In $1928$,Frederick Griffith performed an experiment with $Streptococcus$ $pneumoniae$ bacteria.
He observed that when non-virulent rough $(R)$ strain bacteria were mixed with heat-killed virulent smooth $(S)$ strain bacteria and injected into mice,the mice died.
He recovered living $S$-type bacteria from the dead mice.
This phenomenon,where the genetic material from the heat-killed $S$-strain was taken up by the $R$-strain,thereby changing its phenotype to $S$-type,is known as transformation.

(D) The Hershey and Chase experiment involved three main steps to demonstrate that $DNA$ is the genetic material:
$1$. $Infection$: Bacteriophages were allowed to attach to $E. coli$ bacteria and inject their genetic material.
$2$. $Blending$: The culture was agitated in a blender to separate the viral protein coats (ghosts) from the bacterial cells.
$3$. $Centrifugation$: The mixture was centrifuged to separate the heavier bacterial cells (pellet) from the lighter viral protein coats (supernatant).
Therefore,the correct sequence is $Infection - Blending - Centrifugation$.

(C) The Hershey-Chase experiment $(1952)$ was designed to determine whether $DNA$ or protein is the genetic material.
$1$. They grew some bacteriophages on a medium containing radioactive phosphorus $(^{32}P)$ to label their $DNA$,and others on a medium containing radioactive sulfur $(^{35}S)$ to label their protein coats.
$2$. When the phages labelled with $^{35}S$ (protein) infected bacteria,the radioactivity remained in the supernatant after centrifugation,indicating that the protein did not enter the cell.
$3$. When the phages labelled with $^{32}P$ $(DNA)$ infected bacteria,the radioactivity was detected inside the bacterial cells,proving that $DNA$ is the genetic material.
$4$. Option $C$ correctly describes the result for the $^{35}S$ experiment: the radioactive protein coat remains outside the cell in the supernatant.

(D) The unequivocal proof that $DNA$ is the genetic material came from the experiments of Alfred Hershey and Martha Chase in $1952$.
They worked with bacteriophages,which are viruses that infect bacteria.
By using radioactive isotopes ($^{32}P$ to label $DNA$ and $^{35}S$ to label proteins),they demonstrated that only the $DNA$ entered the bacterial cell,while the protein coat remained outside.
This confirmed that $DNA$ is the hereditary material that carries genetic information.

(C) The correct answer is $C$.
In $1928$,Frederick Griffith performed a series of experiments using the bacterium $Streptococcus$ $pneumoniae$ (the bacterium responsible for pneumonia).
He observed that a non-virulent strain of bacteria could be transformed into a virulent strain by a 'transforming principle' present in heat-killed virulent bacteria.
This experiment provided the first evidence that genetic material could be transferred between organisms.

(B) The correct option is $R$ only.
In the Griffith experiment:
$P$: $R$ strain is non-virulent (rough),so mice injected with it survive. The statement says they die,which is incorrect.
$Q$: $S$ strain (Heat-killed) is non-virulent because the heat kills the bacteria,so mice injected with it survive. The statement says they die,which is incorrect.
$R$: $R$ strain is non-virulent,so mice injected with it survive. This statement is correct.

(B) In $1869$,the Swiss physiological chemist $Friedrich \ Miescher$ isolated a substance from the nuclei of pus cells (leukocytes) obtained from discarded surgical bandages.
He named this substance 'Nuclein' because it was found specifically in the nucleus.
Later,this substance was identified as $DNA$ (Deoxyribonucleic acid).
Therefore,the correct answer is $Friedrich \ Miescher$.

(A) The correct answer is $A$.
Oswald Avery, Colin MacLeod, and Maclyn McCarty worked to determine the biochemical nature of the 'transforming principle' in Griffith's experiment.
They purified biochemicals (proteins, $DNA$, $RNA$) from the heat-killed '$S$' cells to see which ones could transform live '$R$' cells into '$S$' cells.
They discovered that digestion with $proteases$ and $RNAases$ did not affect transformation, meaning neither protein nor $RNA$ was the genetic material.
However, digestion with $DNAase$ inhibited the transformation, proving that $DNA$ is the substance that causes the transformation of '$R$' strain into '$S$' strain.
Therefore, the experiment that conclusively proved $DNA$ is the genetic material involved the addition of $DNAase$ to the mixture, which prevented the transformation.

(A) genetic material of $RNA$ viruses.
In $RNA$ viruses,genetic $RNA$ refers to the $RNA$ that encodes the virus's genetic information,allowing it to replicate and produce new viral particles.
This $RNA$ acts as the virus's genetic blueprint,serving the same function as $DNA$ does in other organisms.