Genetic code Questions in English

(B) The genetic code is a triplet,meaning $1$ codon consists of $3$ bases.
When a base at position $901$ is deleted,it causes a frameshift mutation from that point onwards.
The total number of bases remaining after the deletion is $998$.
The number of bases from the point of deletion $(901)$ to the end of the $RNA$ is $999 - 901 + 1 = 99$ bases.
Since each codon consists of $3$ bases,the number of codons affected by this frameshift is $99 / 3 = 33$ codons.
Therefore,$33$ codons will be altered.

(C) The correct answer is $C$.
Polypeptide synthesis is initiated by specific start codons.
The primary start codon is $AUG$,which codes for the amino acid methionine.
In some rare cases,$GUG$ (which codes for valine) can also act as a start codon.
The other options ($UAA$,$UAG$,and $UGA$) are stop codons (also known as termination codons) that signal the end of protein synthesis.

(B) The correct answer is $B$. The genetic code is non-ambiguous. $A$ non-ambiguous code means that one codon specifies only one particular amino acid. There are $64$ codons in total. Out of these,$3$ are stop codons (nonsense codons) which do not code for any amino acid,and the remaining $61$ code for $20$ amino acids. Since one codon always codes for only one specific amino acid,the code is not ambiguous.

(B) The genetic code is non-ambiguous. $A$ non-ambiguous code means that there is no ambiguity regarding a particular codon. One codon specifies only one amino acid and not any other. There are $64$ codons in total. Out of these $64$,$3$ are stop codons (nonsense codons),which do not code for any amino acid,while the remaining $61$ code for one of the $20$ amino acids. Each codon is specific to a single amino acid,thus the code is not ambiguous.

(D) The genetic code was deciphered in the $1960$s by scientists including $Crick$, $Ochoa$, $Nirenberg$, $Mathaei$, and $Khorana$.
Specifically, Marshall $Nirenberg$ and $Heinrich$ $Mathaei$ performed the cell-free protein synthesis experiment using synthetic $RNA$ (poly-$U$), which provided the first evidence that a sequence of three nucleotides (a triplet) codes for a specific amino acid (phenylalanine).
This discovery unequivocally established the triplet nature of the genetic code.

(D) The correct answer is $D$. The genetic code is the relationship between the sequence of amino acids in a polypeptide and the nucleotide sequence of $DNA$ or $mRNA$.
Genetic code is continuous and does not possess pauses (commas) between the triplets.
Therefore,the statement that a codon in $mRNA$ is read in a non-contiguous fashion is false.
If a nucleotide is deleted or added,the entire reading frame shifts,which is known as a frameshift mutation.

(D) $AUG$ codes for methionine and is the initiation or start codon which starts the synthesis of the polypeptide chain.
$UAA$ (ochre),$UAG$ (amber),and $UGA$ (opal) are stop codons that do not specify any amino acid; therefore,they are called termination codons.
$UUA, UUG, CUU, CUC, CUA,$ and $CUG$ code for leucine.
$GCU, GCC, GCA,$ and $GCG$ code for alanine.
Thus,the pair $UAG, UGA$ is correctly matched as stop codons.

(C) Sickle cell anaemia is a point mutation caused by the substitution of a single nucleotide in the gene for the $\beta$-globin chain of haemoglobin.
In the normal haemoglobin gene,the codon $GAG$ codes for glutamic acid at the sixth position of the $\beta$-globin chain.
Due to a mutation,$GAG$ is replaced by $GUG$,which codes for valine.
Therefore,$GAG$ is the codon for glutamic acid,and $GUG$ is the codon for valine.
Both $GAA$ and $GAG$ code for glutamic acid,but in the context of the sickle cell anaemia mutation,$GAG$ is the specific codon that undergoes substitution to $GUG$.

(D) The genetic code has several key properties:
$1$. It is unambiguous: One codon codes for only one specific amino acid.
$2$. It is nearly universal: The same codon codes for the same amino acid in almost all organisms.
$3$. It is degenerate: Some amino acids are coded by more than one codon.
$4$. It is read in a contiguous fashion: The $mRNA$ is read in a continuous sequence of three nucleotides without any punctuation or gaps.
Therefore,the statement that the codon is read in a non-contiguous fashion is incorrect.

(D) The genetic code is a set of rules by which information encoded in genetic material is translated into proteins.
$AUG$ is the start codon and codes for Methionine $(Met)$,not Phenylalanine $(Phe)$.
$UUU$ codes for Phenylalanine $(Phe)$,not Leucine $(Leu)$.
$UUA$ codes for Leucine $(Leu)$,not Serine $(Ser)$.
$GUU$ codes for Valine $(Val)$.
Therefore,the correct pair is $GUU - Val$.

(B) Stop codons,also known as termination codons or nonsense codons,are specific nucleotide triplets in mRNA that signal the end of protein synthesis.
There are three stop codons in the genetic code: $UAA$ (ochre),$UAG$ (amber),and $UGA$ (opal).
These codons do not code for any amino acid and are recognized by release factors rather than tRNA molecules,which leads to the termination of translation.

(C) The genetic code is degenerate,meaning some amino acids are specified by more than one codon.
Proline is an amino acid that is specified by $4$ different codons in the genetic code.
These codons are $CCU$,$CCC$,$CCA$,and $CCG$.
Therefore,the correct answer is $4$.

(D) The salient features of the genetic code include:
$1$. The codon is triplet: $61$ codons code for amino acids and $3$ codons do not code for any amino acids (stop codons).
$2$. One codon codes for only one amino acid,hence it is unambiguous and specific.
$3$. Some amino acids are coded by more than one codon,so the code is degenerate.
$4$. The codon is read in $mRNA$ in a contiguous fashion. There are no punctuations.
$5$. The code is nearly universal: For example,from bacteria to human,$UUU$ would code for Phenylalanine.
$6$. $AUG$ has dual functions: It codes for Methionine $(met)$ and it also acts as an initiator codon.

(C) The genetic code for the amino acid leucine (Leu) consists of six codons.
These codons are $UUA, UUG, CUU, CUC, CUA,$ and $CUG$.
Among the given options,option $C$ contains the correct codons for leucine.
Note: $AUU$ codes for isoleucine,$UCC$ codes for serine,$UAC$ codes for tyrosine,$UGA$ is a stop codon,and $UCA$ codes for serine.

(D) In the genetic code,there are three stop codons (also known as termination codons) that signal the end of protein synthesis. These are $UAA$,$UAG$,and $UGA$.
$AUG$ is the start codon,which codes for methionine.
$CUA$ codes for leucine.
$UGG$ codes for tryptophan.
Therefore,$UAG$ is the correct stop codon.

(C) 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.
Each codon consists of a sequence of three nucleotides.
$UUU$ is a specific codon in the genetic code.
According to the standard genetic code table,the codon $UUU$ codes for the amino acid Phenylalanine.
Therefore,the correct option is $C$.

(B) The genetic code determines the translation of mRNA codons into amino acids:
$1$. $AUG$ is the start codon and codes for Methionine $(A-2)$.
$2$. $UAA$ is one of the three stop codons $(UAA, UAG, UGA)$ and does not code for any amino acid $(B-4)$.
$3$. $UUU$ codes for Phenylalanine $(C-1)$.
$4$. $UGG$ codes for Tryptophan $(D-3)$.
Therefore,the correct matching is $(A-2), (B-4), (C-1), (D-3)$.

(A) The genetic code is a set of rules by which information encoded in genetic material is translated into proteins.
$GUU$ is a codon consisting of three nucleotides: Guanine $(G)$,Uracil $(U)$,and Uracil $(U)$.
According to the standard genetic code table,the codon $GUU$ specifies the amino acid Valine $(Val)$.
Therefore,the correct option is $A$.

(B) $AUG$ is a dual-function codon in the genetic code.
$1$. It codes for the amino acid methionine $(Met)$.
$2$. It acts as the initiator codon,signaling the start of protein synthesis (translation) on the $mRNA$ strand.

(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 $(U)$.
$A$ codon consists of a triplet of these bases.
Since all four nitrogenous bases $(A, G, C, U)$ are involved in forming the genetic code,the correct answer is 'All of the above'.

(D) The genetic code consists of $64$ codons,which are triplet sequences of nucleotides $(4^3 = 64)$.
Out of these $64$ codons,$61$ codons code for $20$ amino acids,while $3$ codons $(UAA, UAG, UGA)$ are stop codons (nonsense codons) that do not code for any amino acid.
Therefore,the statement that the genetic code is triplet is correct.

(D) Har Gobind Khorana developed a chemical method to synthesize $RNA$ molecules with defined combinations of bases (homopolymers and copolymers).
By using these synthetic $RNA$ molecules in a cell-free protein-synthesizing system,he was able to decipher the genetic code.
Specifically,he demonstrated that the triplet codons $UUU$ (which codes for Phenylalanine) and $AUG$ (which codes for Methionine) were among the first to be deciphered using his experimental approach.

(C) The genetic code is described as $degenerate$ because some amino acids are encoded by more than one codon. For example,there are $64$ possible codons,but only $20$ amino acids. This redundancy allows multiple codons to specify the same amino acid,which provides a buffer against mutations.

(D) In protein synthesis,the process of translation begins with an initiation codon.
For both prokaryotes and eukaryotes,the codon $AUG$ serves as the initiation codon.
$AUG$ codes for the amino acid Methionine.
Therefore,the correct option is $D$.

(B) The genetic code is a triplet code,meaning each codon consists of $3$ nucleotides. There are $4$ types of nitrogenous bases $(A, U, G, C)$. The total number of possible codons is calculated as $4^3 = 4 \times 4 \times 4 = 64$. Out of these $64$ codons,$61$ codons code for amino acids,while $3$ codons $(UAA, UAG, UGA)$ are stop codons (nonsense codons) that do not code for any amino acid. Therefore,the number of codons that code for amino acids is $61$.

(B) In the process of protein synthesis,translation is terminated when the ribosome encounters a stop codon on the mRNA molecule.
There are three stop codons: $UAA$,$UAG$,and $UGA$.
These codons do not code for any amino acid and are recognized by release factors rather than tRNA molecules.
Among the given options,$UAA$ is a stop codon.

(D) In the process of translation,the termination of the polypeptide chain is signaled by stop codons (also known as nonsense codons).
These codons do not code for any amino acid.
The three stop codons in the genetic code are $UAA$,$UAG$,and $UGA$.
Therefore,the correct option is $D$.

(C) In molecular biology,a $Gene$ is defined as the functional unit of inheritance.
According to the $One$ $Gene-One$ $Enzyme$ hypothesis (later modified to $One$ $Gene-One$ $Polypeptide$ hypothesis),a single $Gene$ codes for a single polypeptide chain.
$A$ $Cistron$ is defined as a segment of $DNA$ coding for a polypeptide.
Therefore,in eukaryotes,since a $Gene$ typically codes for one polypeptide,it is functionally equivalent to one $Cistron$ (monocistronic). Thus,the terms are often used synonymously.

(C) In the genetic code,$AUG$ is the universal start codon,which codes for the amino acid Methionine.
Stop codons,also known as termination codons,are $UAA$,$UAG$,and $UGA$.
Among the given options,$AUG$ is correctly identified as the start codon.
$UGU$ codes for Cysteine,$UAC$ codes for Tyrosine,and $UUU$ codes for Phenylalanine.

(C) The genetic code is described as 'degenerate' because more than one codon can code for the same amino acid.
This phenomenon is primarily due to the 'wobble hypothesis' proposed by Francis Crick.
According to this hypothesis,the base pairing between the $3'$ end of the codon and the $5'$ end of the anticodon is less strict.
Therefore,the third base (the $3'$ position) of the codon is the most flexible and is responsible for the degeneracy of the genetic code.

(C) The codon $UAU$ codes for the amino acid Tyrosine.
$UAA$ is a stop codon (nonsense codon) that does not code for any amino acid and signals the termination of protein synthesis.
Since the mutation occurs at the $25^{th}$ position,the translation process will stop after the $24^{th}$ amino acid is incorporated.
Therefore,a polypeptide chain consisting of only $24$ amino acids will be formed.

(A) The genetic code consists of $64$ total codons.
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.
Therefore,$61$ codons are used to code for the $20$ amino acids.

(A) gene is a segment of $DNA$ that codes for a specific polypeptide or protein.
When a mutation occurs at the gene level,the sequence of nucleotides in the $DNA$ changes.
This altered $DNA$ sequence leads to the transcription of an altered $mRNA$ sequence.
Consequently,the translation process results in the synthesis of a protein with a different amino acid sequence,which alters the protein's structure and function.
Since proteins are responsible for expressing traits (phenotypes),a change in protein structure leads to a change in the organism's traits.

(B) The sequence of amino acids in a polypeptide chain is determined by the sequence of nucleotides in the $m-RNA$ (messenger $RNA$).
During the process of translation,the $m-RNA$ acts as a template,and the codons present on it are read by the $t-RNA$ to bring the specific amino acids in the correct order.
Therefore,$m-RNA$ carries the genetic information from the $DNA$ to the ribosome,which dictates the primary structure of the protein.

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

(B) The genetic code is degenerate and specific.
$1$. $GUU$ codes for Valine,while $GCU$ codes for Alanine. Thus,option $A$ is incorrect.
$2$. $UAA$ and $UGA$ are two of the three stop (termination) codons,which signal the end of protein synthesis. Thus,option $B$ is correct.
$3$. $AUG$ is the initiator codon (Methionine),but $ACG$ codes for Threonine. Thus,option $C$ is incorrect.
$4$. $UUA$ codes for Leucine,but $UCA$ codes for Serine. Thus,option $D$ is incorrect.

(A) Marshall Nirenberg and Heinrich Matthaei performed cell-free protein synthesis experiments. By using synthetic $RNA$ sequences (poly-$U$),they determined that a sequence of three nucleotides (a triplet) codes for a specific amino acid. This work was crucial in deciphering the genetic code.

(A) The genetic code has several key properties:
$1$. It is universal: The same codon codes for the same amino acid in almost all organisms.
$2$. It is degenerate: Some amino acids are coded by more than one codon.
$3$. It is unambiguous: One codon codes for only one specific amino acid.
$4$. It is non-overlapping and comma-less: The codons in $m-RNA$ are read in a contiguous fashion without any gaps or commas.
Therefore,the statement that codons in $m-RNA$ are not contiguous is false.

(D) The initiation codon is the specific sequence of three nucleotides in mRNA that signals the start of protein synthesis.
In almost all organisms,$AUG$ acts as the initiation codon.
It codes for the amino acid methionine $(Met)$.
$UGA$,$UAA$,and $UAG$ are stop codons (termination codons) that signal the end of protein synthesis.

(C) The genetic code is read in groups of $3$ bases,known as codons.
Since the deletion occurs at position $901$,all codons from this point onwards will be affected.
The total number of bases is $999$.
The deletion occurs at position $901$.
The number of bases remaining after the deletion point is $999 - 901 + 1 = 99$ bases.
Since each codon consists of $3$ bases,the number of codons affected is $99 / 3 = 33$ codons.
Therefore,$33$ codons will be altered due to the frame-shift mutation.

(D) The genetic code is read in triplets (codons). $A$ change in the reading frame (frameshift mutation) occurs when the number of nucleotides inserted or deleted is not a multiple of $3$.
If $3$ nucleotides (or a multiple of $3$) are inserted or deleted,the reading frame remains unchanged,although one or more amino acids may be added or removed.
In the given $mRNA$ sequence $5'-AAC-AGC-GGU-GCU-AUU-3'$,deleting $GGU$ (which consists of $3$ nucleotides) from the $7^{th}, 8^{th}$,and $9^{th}$ positions removes exactly one codon.
This results in the loss of one amino acid,but the subsequent codons remain in the same reading frame.
Therefore,option $D$ is the correct condition.

(C) The genetic code is considered nearly universal,meaning that the same codons specify the same amino acids in almost all organisms,from bacteria to humans.
Because of this universality,a human gene (such as the one for insulin) can be inserted into a bacterial cell (like $E. coli$) using recombinant $DNA$ technology.
The bacterial machinery recognizes the human codons and translates them into the correct human protein (insulin),as the genetic language is shared across species.

(C) Protein synthesis is terminated when the ribosome encounters a stop codon on the $mRNA$ strand.
These stop codons are also known as nonsense codons because they do not code for any amino acid.
The three stop codons are $UAA$ (Ochre),$UAG$ (Amber),and $UGA$ (Opal).
Therefore,the correct group is $UAG, UGA, UAA$.

(A) The term 'degenerate' in the context of the genetic code usually refers to the property where multiple codons code for the same amino acid. However,in many textbooks,the term 'degenerate' is incorrectly used to refer to 'stop codons' or 'nonsense codons'. Based on the provided options,the question asks for the set of stop codons.
Stop codons (also known as nonsense or termination codons) do not code for any amino acids and signal the end of protein synthesis.
The three stop codons are $UAA$ (ochre),$UAG$ (amber),and $UGA$ (opal).

$(A)$ The genetic code specifies the relationship between codons and amino acids:
$1$. $UUU$ codes for Phenylalanine $(A-III)$.
$2$. $GGG$ codes for Glycine $(B-IV)$.
$3$. $UCU$ codes for Serine $(C-I)$.
$4$. $CCC$ codes for Proline $(D-V)$.
$5$. $AUG$ codes for Methionine $(E-II)$.
Thus, the correct matching is $A-III, B-IV, C-I, D-V, E-II$.

(A) The synthesis of a polypeptide chain terminates when a stop codon (nonsense codon) of $mRNA$ reaches the $A-$site of the ribosome.
There are three stop codons: $UAA, UAG$,and $UGA$.
These codons do not code for any amino acid and are not recognized by any $tRNA$ molecule.
Because no $tRNA$ can bind to these codons,no further aminoacyl-$tRNA$ reaches the $A-$site.
Consequently,the polypeptide chain is released from the $P-$site $tRNA$ in the presence of release factors,terminating the translation process.