Genetic code Questions in English

(D) The genetic code is a set of rules by which information encoded in genetic material ($DNA$ or $mRNA$ sequences) is translated into proteins by living cells.
In this system, a sequence of three consecutive nucleotides (bases) is called a $codon$.
Each $codon$ specifies a particular $amino acid$ during the process of protein synthesis (translation).
Therefore, a sequence of three consecutive bases specifies an $amino acid$.

(C) Marshall Nirenberg and his colleagues used synthetic $m-RNA$ to decipher the genetic code.
They synthesized a poly-adenine $m-RNA$ (a sequence of $A-A-A-A...$).
Since the resulting polypeptide chain consisted of $11$ lysine amino acids,and the $m-RNA$ had $34$ adenine bases,the ratio of bases to amino acids is $34/11 \approx 3$.
This indicates that a sequence of $3$ adenine bases $(A-A-A)$ codes for one lysine molecule.
Therefore,the genetic code is a triplet.

(B) The 'one gene one enzyme' hypothesis was proposed by George Beadle and Edward Tatum in $1941$.
They conducted experiments on the bread mold $Neurospora crassa$.
They demonstrated that specific genes are responsible for the production of specific enzymes, which in turn catalyze specific biochemical reactions in the metabolic pathway.

(C) The genetic code is based on the combination of $4$ nitrogenous bases $(A, U, G, C)$.
Since the code is a triplet (composed of $3$ nucleotides),the total number of possible combinations is $4^3 = 4 \times 4 \times 4 = 64$.
Therefore,there are $64$ codons in the genetic code.

(B) The genetic code exhibits the property of 'wobble hypothesis' at the third position of the codon. According to this,the third base of the codon (the $3'$ end) shows less specificity and can pair with non-complementary bases in the anticodon of $t-RNA$. Option $A$ is incorrect because $t-RNA$ recognizes amino acids,not $m-RNA$. Option $C$ is incorrect because the genetic code is degenerate,meaning some amino acids are coded by more than one codon. Option $D$ is incorrect because each $t-RNA$ is specific to only one amino acid.

(A) The 'one gene one enzyme' hypothesis was proposed by George Beadle and Edward Tatum in $1941$. They conducted experiments on the bread mold $Neurospora$ $\text{crassa}$ and demonstrated that specific genes are responsible for the production of specific enzymes, thereby establishing a direct link between genes and metabolic pathways.

(A) The genetic code is a triplet code,meaning that a sequence of three nucleotides in $m-RNA$ specifies a single amino acid. This is known as a codon. Since there are $4$ types of nitrogenous bases $(A, U, G, C)$,a triplet code allows for $4^3 = 64$ possible combinations,which is sufficient to code for the $20$ amino acids found in proteins.

(D) The genetic code is based on the sequence of nitrogenous bases in $mRNA$. These bases are Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Uracil $(U)$. Since the genetic code is composed of these four nitrogenous bases,and the options provided list pairs or combinations of these bases,the most appropriate answer is that it involves these bases. However,in the context of standard multiple-choice questions regarding the composition of the genetic code,it is composed of all these nitrogenous bases $(A, G, C, U)$.

(C) codon is a sequence of three nucleotides in $DNA$ or $mRNA$ that specifies a particular amino acid or a stop signal during protein synthesis.
Since there are $4$ types of nitrogenous bases $(A, U, G, C)$,a triplet code ($4^3 = 64$ codons) is required to code for the $20$ amino acids found in living organisms.
Therefore,a codon is composed of three nucleotides.

(D) The 'one gene, one enzyme' hypothesis was proposed by George Beadle and Edward Tatum in $1941$.
They conducted experiments on the bread mold $Neurospora \, crassa$ to demonstrate that genes act by regulating definite chemical events.
They concluded that each gene is responsible for the synthesis of a single specific enzyme, which in turn controls a specific metabolic step.

(A) The process of translation in protein synthesis begins with the initiation codon.
$AUG$ is the universal initiation codon that codes for the amino acid Methionine $(Met)$ in eukaryotes and $N$-formylmethionine $(fMet)$ in prokaryotes.
$UGA$, $UAG$, and $UAA$ are known as stop codons or termination codons, which signal the end of protein synthesis.

(D) In the genetic code,there are $64$ codons in total. Out of these,$61$ codons code for amino acids,while $3$ codons do not code for any amino acid and act as stop signals (termination codons).
These $3$ stop codons are $UAA$,$UAG$,and $UGA$.
Therefore,all the given options are correct.

(B) For an $m-RNA$ to be completely translated,it must contain a start codon $(AUG)$ to initiate translation and a stop codon ($UAA, UAG,$ or $UGA$) to terminate it,with the sequence between them being in-frame.
$1$. Option $(a)$: $5'-AUG\ UGA\ UUA\ AAG\ AAA-3'$. Here,$UGA$ is a stop codon immediately following the start codon $AUG$. This will result in a very short peptide or no translation,as the stop codon is encountered too early.
$2$. Option $(b)$: $5'-AUG\ AUA\ UUG\ CCC\ UGA-3'$. This sequence starts with $AUG$ (start codon) and ends with $UGA$ (stop codon). The codons in between $(AUA, UUG, CCC)$ are sense codons. Thus,it can be translated.
$3$. Option $(c)$: $5'-AGU\ UCC\ AGA\ CUC\ UAA-3'$. This sequence lacks a start codon $(AUG)$. Therefore,translation cannot be initiated.
$4$. Option $(d)$: $5'-AUG\ UAC\ AGU\ AAC\ UAG-3'$. This sequence starts with $AUG$ (start codon) and ends with $UAG$ (stop codon). The codons in between $(UAC, AGU, AAC)$ are sense codons. Thus,it can be translated.
Therefore,sequences $(b)$ and $(d)$ can be completely translated.

(A) Marshall Nirenberg and Heinrich Matthaei were the first to decipher the genetic code in $1961$.
They used a cell-free system to synthesize a polypeptide chain using a synthetic poly-$U$ $RNA$ sequence,which resulted in the formation of polyphenylalanine.
This experiment proved that the codon $UUU$ codes for the amino acid phenylalanine,marking the beginning of the deciphering of the genetic code.

(A) The process of protein synthesis (translation) begins with an initiation codon.
In almost all organisms,$AUG$ acts as the initiation codon.
It codes for the amino acid methionine $(Met)$.
$UAG$,$UGA$,and $UAA$ are stop codons (termination codons) that signal the end of protein synthesis.

(D) The genetic code was deciphered through the work of several scientists. Marshall $W$. Nirenberg and his colleagues used cell-free systems to synthesize trinucleotides (synthetic $RNA$) to determine which amino acids were coded by specific codons. This breakthrough allowed for the identification of the genetic code. Therefore,the correct answer is $M.W. Nirenberg$.

(A) nonsense codon,also known as a stop codon or termination codon,is a nucleotide triplet within $mRNA$ that signals the termination of translation. The three stop codons are $UAA$,$UAG$,and $UGA$. These codons do not code for any amino acid. Instead,they are recognized by release factors,which cause the ribosome to release the completed polypeptide chain from the $tRNA$ and dissociate from the $mRNA$ molecule. Therefore,the correct function is to release the polypeptide chain.

(D) In the genetic code,$AUG$ acts as the start codon and codes for methionine.
$UAA$,$UAG$,and $UGA$ are the three stop codons (nonsense codons) that do not code for any amino acid.
$UAC$ is the codon for the amino acid tyrosine.
$AUC$ codes for isoleucine,$UUA$ codes for leucine,and $UGU$ codes for cysteine.
Therefore,the correct match is $UAC$ - Tyrosine.

(B) The given $m-RNA$ sequence is $AUGGCAGUGCCA$,which has a total of $12$ nucleotides.
In an overlapping genetic code,each nucleotide is part of multiple consecutive codons.
For a triplet code (where each codon consists of $3$ nucleotides),the number of codons in an overlapping system is equal to the total number of nucleotides minus the length of the codon plus one.
Number of codons = $N - n + 1$,where $N$ is the total number of nucleotides $(12)$ and $n$ is the number of nucleotides per codon $(3)$.
Calculation: $12 - 3 + 1 = 10$.
Therefore,there are $10$ possible overlapping codons in this sequence.

(C) The genetic code consists of $64$ codons in total.
Out of these $64$ codons,$61$ codons code for amino acids.
The remaining $3$ codons $(UAA, UAG, UGA)$ are stop codons (also known as termination codons) and do not code for any amino acid.
Since there are $20$ types of amino acids that need to be encoded,$61$ codons are used to specify these $20$ amino acids,as some amino acids are coded by more than one codon (degeneracy of the genetic code).

(D) The genetic code has several key characteristics:
$1$. It is universal,meaning a specific codon codes for the same amino acid in all organisms.
$2$. It is specific (or unambiguous),meaning one codon codes for only one specific amino acid.
$3$. It is degenerate,meaning some amino acids are coded by more than one codon.
$4$. It is non-overlapping and comma-less.
Therefore,the genetic code is $NOT$ ambiguous; it is unambiguous.

(C) The initiation codon is the specific sequence of three nucleotides in messenger $RNA$ $(mRNA)$ that signals the start of protein synthesis during translation.
In almost all organisms,the initiation codon is $AUG$,which codes for the amino acid methionine $(Met)$.
$UAG$ is a stop codon (amber codon).
$AUC$ codes for isoleucine.
$CCU$ codes for proline.

(B) $1$. The given $m-RNA$ sequence is $5'-AUG\ CUA\ UAC\ CUC\ CUU\ UAU\ CUG\ UGA-3'$.
$2$. The translation process begins at the start codon $AUG$,which codes for Methionine.
$3$. The codons are read in triplets: $AUG$ (Met),$CUA$ (Leu),$UAC$ (Tyr),$CUC$ (Leu),$CUU$ (Leu),$UAU$ (Tyr),$CUG$ (Leu).
$4$. The sequence $UGA$ is a stop codon (nonsense codon) and does not code for any amino acid.
$5$. Counting the codons before the stop codon: $AUG, CUA, UAC, CUC, CUU, UAU, CUG$ equals $7$ codons.
$6$. Therefore,the polypeptide chain will consist of $7$ amino acids.

(D) The genetic code consists of $64$ codons.
$AUG$ acts as the start codon and codes for methionine.
$UAG$,$UAA$,and $UGA$ are the three stop (termination) codons that do not code for any amino acid.
Option $D$ is correct because $UAG$ and $UGA$ are both stop codons.
Option $A$ is incorrect because $ACG$ codes for threonine,not methionine.
Option $B$ is incorrect because $UUA$ codes for leucine,but $UCA$ codes for serine.
Option $C$ is incorrect because $GUU$ codes for valine,while $GCU$ codes for alanine.

(A) The codon $UAA$ is a stop codon (also known as a nonsense codon).
When the $25^{th}$ codon mutates to $UAA$,the translation process will terminate prematurely at the $24^{th}$ amino acid position.
Since the ribosome stops adding amino acids after the $24^{th}$ one,a truncated polypeptide chain consisting of only $24$ amino acids will be synthesized.
Therefore,the correct answer is that a polypeptide of $24$ amino acids will be formed.

(D) The genetic code was deciphered through the collaborative efforts of several scientists. Marshall Nirenberg and Heinrich Matthaei performed the first successful experiment in $1961$ to crack the genetic code using synthetic poly-uracil $RNA$. Har Gobind Khorana later developed methods to synthesize $RNA$ molecules with defined combinations of bases,which helped in deciphering the complete genetic code. Severo Ochoa discovered the enzyme polynucleotide phosphorylase,which was instrumental in polymerizing $RNA$ in a template-independent manner. However,the primary credit for deciphering the code is given to Nirenberg and Matthaei for the initial breakthrough.

(C) In $1961$,Marshall $Nirenberg$ and Heinrich $Matthaei$ performed a landmark experiment to crack the genetic code.
They used a synthetic poly-uracil $(poly-U)$ $RNA$ sequence in a cell-free system.
They observed that this $poly-U$ sequence directed the synthesis of a polypeptide chain consisting entirely of the amino acid phenylalanine.
Since the codon $UUU$ was the only repeating unit in the $poly-U$ sequence,they concluded that $UUU$ codes for phenylalanine.
Thus,$UUU$ was the first codon to be deciphered.

(C) The $RNA$ sequence is $UGU\ GUG\ UGU\ GUG$.
This sequence is read in triplets (codons) as $UGU, GUG, UGU, GUG$.
The resulting polypeptide chain is Cysteine-Valine-Cysteine-Valine.
Therefore, the codon $UGU$ codes for Cysteine, and the codon $GUG$ codes for Valine.
Thus, the correct answer is $UGU \text{ and } GUG$.

(C) In protein synthesis,the process of translation is terminated when the ribosome encounters a stop codon on the mRNA strand.
These stop codons do not code for any amino acid and instead signal the release of the polypeptide chain.
The three stop codons (also known as termination codons or nonsense codons) are $UAA$,$UAG$,and $UGA$.
Therefore,the correct option is $C$.

(B) In the genetic code,there are $64$ codons in total,out of which $61$ code for amino acids and $3$ are stop codons (also known as termination codons).
These $3$ stop codons are $UAA$ (Ochre),$UAG$ (Amber),and $UGA$ (Opal).
They do not code for any amino acid and signal the termination of the polypeptide chain synthesis during translation.
Therefore,the correct set of stop codons is $UGA, UAG, UAA$.

(D) The genetic code is degenerate,meaning that some amino acids are encoded by more than one codon.
This phenomenon is primarily due to the 'wobble hypothesis' proposed by Francis Crick.
According to this hypothesis,the pairing between the third base of the codon (at the $3'$ end) and the first base of the anticodon (at the $5'$ end) is less strict and allows for non-canonical base pairing.
Therefore,the third base of the codon is the most flexible,leading to the degeneracy of the genetic code.

(A) The genetic code is said to be degenerate because some amino acids are coded by more than one codon.
Since there are $64$ possible codons and only $20$ amino acids,multiple codons often specify the same amino acid.
This property prevents the effect of mutations at the third position of the codon (wobble hypothesis).
Therefore,this characteristic is known as the degeneracy of the genetic code.

(B) The genetic code has several key properties:
$1$. It is degenerate,meaning some amino acids are coded by more than one codon,but a single codon does not code for more than one amino acid. Thus,statement $(a)$ is incorrect.
$2$. The first two bases of a codon are more specific in determining the amino acid,while the third base is less specific. Thus,statement $(b)$ is correct.
$3$. The third base of the codon often exhibits the 'wobble' phenomenon,allowing for non-canonical base pairing. Thus,statement $(c)$ is correct.
$4$. The genetic code is universal,meaning it is the same in almost all organisms. Thus,statement $(d)$ is correct.
Therefore,statements $(b), (c),$ and $(d)$ are correct.

(B) The genetic code is a set of rules by which information encoded in genetic material is translated into proteins by living cells.
$1$. $UUU$ codes for Phenylalanine,not Leucine.
$2$. $AAA$ codes for Lysine. This is a correct match.
$3$. $AUG$ codes for Methionine (it also acts as a start codon),not Cysteine.
$4$. $CCC$ codes for Proline,not Alanine.
Therefore,the correct pair is $AAA$ - Lysine.

(C) In the process of protein synthesis (translation),the termination of the polypeptide chain is signaled by specific codons known as stop codons or termination codons.
These codons do not code for any amino acid.
The three stop codons are $UAG$ (Amber),$UGA$ (Opal),and $UAA$ (Ochre).
Therefore,the set of codons that can terminate the process is $UAG, UGA, UAA$.

(B) To determine the number of bases per codon,we use the formula $n^x \geq N$,where $n$ is the number of available bases,$x$ is the number of bases per codon,and $N$ is the number of amino acids to be coded.
Given $n = 12$ and $N = 96$.
If $x = 1$,$12^1 = 12 < 96$.
If $x = 2$,$12^2 = 144$.
Since $144 \geq 96$,the minimum number of bases required for each codon is $2$.

(A) The genetic code consists of $64$ codons. Out of these,$61$ codons code for amino acids,while $3$ codons $(UAA, UAG, UGA)$ act as stop codons (termination codons) and do not code for any amino acid. Therefore,statement $ii$ is incorrect because not all $64$ codons code for amino acids. Statements $i, iii, iv,$ and $v$ are correct: the code is a triplet,it is unambiguous (one codon codes for only one amino acid),it is universal (the same codon codes for the same amino acid in all organisms),and $AUG$ has dual functions (it codes for methionine and acts as an initiator codon).

(A) Let $n$ be the number of nitrogen bases per codon and $b$ be the number of types of nitrogen bases available.
Given $b = 6$ and the number of amino acids to be coded is $40$.
The number of possible codons is given by $b^n$.
We need to find the minimum integer $n$ such that $b^n \geq 40$.
For $n = 1$: $6^1 = 6 < 40$.
For $n = 2$: $6^2 = 36 < 40$.
For $n = 3$: $6^3 = 216 \geq 40$.
Therefore,the minimum number of nitrogen bases required to form a codon is $3$.

(D) The initiator codon is the specific sequence of three nucleotides in mRNA that signals the start of protein synthesis. In almost all organisms,the codon $AUG$ serves as the initiator codon. It codes for the amino acid methionine $(Met)$ in eukaryotes and $N$-formylmethionine $(fMet)$ in prokaryotes.

(D) The 'one gene-one enzyme' hypothesis was proposed by George Beadle and Edward Tatum in $1941$.
They conducted experiments on the bread mold $Neurospora$ $\text{crassa}$.
They demonstrated that each gene is responsible for the production of a specific enzyme, which in turn controls a specific metabolic step in the organism.
Therefore, the correct option is $D$.

(C) Beadle and Tatum proposed the '$One$ $Gene$ - $One$ $Enzyme$' hypothesis based on their experiments with the bread mold $Neurospora$ $crassa$.
They observed that specific mutations in the organism led to the loss of specific enzymatic activities, which prevented the synthesis of essential amino acids.
This led to the conclusion that genes carry the genetic information necessary to synthesize specific proteins (enzymes), which catalyze biochemical reactions in the cell.
Therefore, the correct conclusion from their work is that genes contain the information to make proteins.

(B) Aminoglycoside antibiotics such as streptomycin,kanamycin,and neomycin bind to the $30S$ small subunit of prokaryotic ribosomes.
This binding interferes with the decoding process,leading to the misreading of the genetic code on the $mRNA$.
As a result,incorrect amino acids are incorporated into the polypeptide chain,leading to the production of non-functional or defective proteins.
Therefore,the correct option is $B$.

(D) The production of human proteins in bacteria is possible because the genetic code is universal.
This means that the same codons specify the same amino acids in almost all living organisms,from bacteria to humans.
Therefore,when a human gene is inserted into a bacterial plasmid,the bacterial machinery can correctly transcribe and translate the human gene into the corresponding human protein.

(A) $m-RNA$ (messenger $RNA$) is the type of $RNA$ that carries the genetic information from $DNA$ to the ribosomes,where it serves as a template for protein synthesis. The sequence of nucleotides in $m-RNA$ determines the sequence of amino acids in the polypeptide chain.

(C) In the genetic code,there are $64$ codons in total.
Out of these $64$ codons,$3$ are stop codons $(UAA, UAG, UGA)$ which do not code for any amino acid.
Therefore,there are $61$ codons that code for amino acids.
Since each $t-RNA$ molecule has an anticodon that pairs with a specific codon on $mRNA$,there are $61$ types of $t-RNA$ molecules corresponding to the $61$ sense codons.

(D) The correct answer is $D$.
$m-RNA$ (messenger $RNA$) is synthesized in the nucleus during transcription.
$r-RNA$ (ribosomal $RNA$) is a structural component of ribosomes.
$t-RNA$ (transfer $RNA$) typically consists of about $73-93$ nucleotides,so $75$ is a valid average length.
There are $64$ codons in the genetic code,but only $61$ code for amino acids (the other $3$ are stop codons). Therefore,there are $61$ types of $t-RNA$ molecules,not $63$.

(B) Statement $P$ is incorrect because an $m-RNA$ molecule can have a variable number of nucleotides depending on the length of the polypeptide chain it encodes. It is not fixed at $75$.
Statement $Q$ is incorrect because there are $64$ codons in the genetic code,out of which $61$ code for amino acids and $3$ are stop codons. Furthermore,these codons are present on the $m-RNA$ molecule,not in the cytoplasm as free entities. Therefore,both statements are incorrect.

(A) The primary structure of a protein refers to the linear sequence of amino acids in a polypeptide chain.
This sequence is determined by the genetic information stored in the $DNA$.
Specifically,a $Gene$ (a segment of $DNA$) contains the instructions for the specific order of amino acids,which is transcribed into $mRNA$ and then translated into the protein sequence.
Therefore,the primary structure is controlled by the $Gene$.

(B) The $DNA$ molecule contains the genetic code,which acts as a blueprint for the synthesis of proteins and other essential chemicals in an organism. This code is determined by the sequence of nitrogenous bases $(A, T, G, C)$ within the $DNA$ strand. These instructions are passed from parents to offspring,ensuring that the necessary biochemical processes are maintained according to the inherited genetic information.