Genetic code Questions in English

(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.

(D) The salient features of the genetic code are as follows:
$1$. The codon is triplet: $3$ nitrogenous bases specify one amino acid.
$2$. It is universal: The same codon specifies the same amino acid in all organisms (with minor exceptions in mitochondria).
$3$. It is non-ambiguous: One codon specifies only one amino acid.
$4$. It is degenerate: Some amino acids are coded by more than one codon.
$5$. It is non-overlapping: The code is read in a continuous manner without overlapping.
$6$. It is commaless: There are no punctuations or gaps between codons.
Therefore,the correct combination is Degenerate,Non-overlapping,and Non-ambiguous.

(N/A) The genetic code is the relationship between the sequence of amino acids in a polypeptide chain and the sequence of nucleotides on $m-RNA$.
During replication and transcription,a nucleic acid is copied into another nucleic acid. In the process of translation,genetic information is transferred from a polymer of nucleotides to a polymer of amino acids.
Changes in nucleic acids (genetic material) lead to changes in the amino acids of proteins. This led to the proposal of the genetic code,which determines the sequence of amino acids during protein synthesis.
The main problem was determining the exact number of nucleotides that code for a single amino acid.
There are only $4$ nitrogenous bases $(A, C, G, U)$ on $m-RNA$ to code for $20$ types of amino acids in organisms.
If the genetic code were a singlet ($1$-letter),it would only provide $4$ codes,which is insufficient to code for $20$ amino acids.
If the genetic code were a doublet ($2$-letter),it would provide $16$ codes,which is also insufficient.
George Gamow $(1954)$,a physicist,suggested that the genetic code is a triplet,meaning each code consists of three nucleotides. This results in $4^3 = 64$ possible codes,which is more than enough to code for $20$ amino acids. Later,Har Gobind Khorana,Holley,and Nirenberg provided experimental evidence for the triplet code.
Marshall Nirenberg's cell-free system for protein synthesis was instrumental in deciphering the code. Severo Ochoa's enzyme (polynucleotide phosphorylase) helped in polymerizing $RNA$ with defined sequences in a template-independent manner.

(N/A) $1$. Universal: The genetic code is universal. The same codon codes for the same amino acid in organisms ranging from bacteria to humans. For example,$UUU$ codes for phenylalanine $(Phe)$ in all organisms,with minor exceptions in mitochondrial codons and some protozoa.
$2$. Specificity: The genetic code is specific. $A$ particular codon always codes for only one specific amino acid.
$3$. Degeneracy: Some amino acids are coded by more than one codon. This property is known as the degeneracy of the genetic code.
$4$. Initiation Codon: The process of translation begins with an initiation codon. $AUG$ codes for methionine and also acts as an initiation codon.
$5$. Non-sense (Termination) Codons: $UAA$,$UGA$,and $UAG$ do not code for any amino acids. They signal the termination of the polypeptide chain synthesis and are therefore called stop or termination codons.
$6$. Colinearity: There is a linear correspondence between the sequence of codons in $mRNA$ and the sequence of amino acids in the polypeptide chain.

(N/A)

Codons	Anticodons
$(1)$ It is a sequence of three nucleotides on $mRNA$ that codes for a specific amino acid.	$(1)$ It is a sequence of three nucleotides on $tRNA$ that is complementary to a specific codon on $mRNA$.
$(2)$ It determines the sequence of amino acids in a polypeptide chain during protein synthesis.	$(2)$ It recognizes and binds to the corresponding codon on $mRNA$ during the process of translation.

(N/A) The genetic code is universal,meaning that a specific codon codes for the same amino acid in all organisms,from bacteria to humans. For example,the codon $UUU$ codes for phenylalanine in almost all living organisms.
However,there are some exceptions to this rule,such as in mitochondrial codons and in some protozoans.

(N/A) There are $20$ types of amino acids found in living organisms. If the genetic code were a singlet (using $1$ nitrogenous base),it would only code for $4$ amino acids. If it were a doublet (using $2$ nitrogenous bases),it would code for $4^2 = 16$ amino acids,which is insufficient to cover all $20$ amino acids. Therefore,as proposed by George Gamow,the genetic code must be a triplet ($4^3 = 64$ combinations),which provides $64$ codons,more than enough to specify the $20$ amino acids.

(N/A) Since some amino acids are coded by more than one codon,a mutation in the first or second position of a codon might change the amino acid,but a mutation in the third position (the wobble position) often does not change the amino acid specified. This redundancy in the genetic code ensures that many point mutations are silent,thereby reducing the overall impact of mutations on protein synthesis.

(N/A) $1.$ Genetic code: The relationship between the sequence of nucleotides on $m-RNA$ and the sequence of amino acids in a polypeptide chain is known as the genetic code.
$2.$ Degenerate codons: $A$ single amino acid can be specified by more than one codon. Such codons are referred to as degenerate codons.

(N/A) George Gamow was a physicist who argued that since there are only $4$ bases and $20$ amino acids to be coded,the code must be a combination of bases. He suggested that in order to code for all $20$ amino acids,the code should be made up of three nucleotides (triplet code).
Marshall Nirenberg was a biochemist who helped crack the genetic code. He developed a cell-free system for protein synthesis and used synthetic $RNA$ (poly-$U$) to demonstrate that the triplet $UUU$ codes for the amino acid phenylalanine.

(N/A) The defect is caused by the substitution of Glutamic acid $(Glu)$ by Valine $(Val)$ at the sixth position of the $\beta$-globin chain of the hemoglobin molecule.
This amino acid substitution in the globin protein occurs due to a single base substitution at the sixth codon of the $\beta$-globin gene,where $GAG$ is replaced by $GUG$.
The mutant hemoglobin molecule undergoes polymerization under low oxygen tension,causing the shape of the $RBC$ to change from a biconcave disc to an elongated,sickle-like structure.
Sickle-shaped red blood cells obstruct capillaries and restrict blood flow to organs,resulting in ischemia,severe pain,tissue necrosis,and often organ damage.

(N/A) The genetic code is degenerate,meaning that some amino acids are encoded by more than one codon. Because of this degeneracy,a single amino acid sequence can correspond to multiple possible nucleotide sequences.
For example,the amino acid Isoleucine (Ile) is coded by three codons: $AUU$,$AUC$,and $AUA$. Therefore,for a dipeptide like Met-Ile,the possible nucleotide sequences include: $(i)$ $AUG-AUU$,$(ii)$ $AUG-AUC$,and $(iii)$ $AUG-AUA$. Conversely,if we deduce the amino acid sequence from any of these three nucleotide sequences,all will code for the same dipeptide,Met-Ile.

(N/A) The statement is correct. Due to the degeneracy of the genetic code,multiple codons can code for the same amino acid. Specifically,mutations occurring at the third base position of a codon (the wobble position) often do not change the amino acid being coded. This type of mutation is known as a 'silent mutation',which results in no change to the protein's primary structure or its function.

(N/A) During replication and transcription,a nucleic acid is copied to form another nucleic acid. Hence,these processes are easy to conceptualize on the basis of complementarity.
The process of translation requires the transfer of genetic information from a polymer of nucleotides to synthesize a polymer of amino acids.
There is no direct complementarity between nucleotides and amino acids,nor could any be drawn theoretically.
However,there was ample evidence to support the notion that changes in nucleic acids (genetic material) were responsible for changes in amino acids in proteins.
This led to the proposition of a genetic code that could direct the sequence of amino acids during protein synthesis.
Historical background and contributions of various scientists:
$1$. George Gamow: In $1954$,a physicist named George Gamow proposed that in order to code for all $20$ amino acids,the code must be made up of three nucleotides (triplet code).
If one base coded for one amino acid,only $4$ amino acids could be coded. If a sequence of two bases coded for one amino acid,the four bases could specify only $16$ $(4 \times 4)$ amino acids,which is inadequate. But if a sequence of three bases coded for one amino acid,the four bases would specify $64$ $(4 \times 4 \times 4)$ amino acids,which is sufficient.
$2$. In the $1960$s,proof regarding the genetic code came from the research of the following scientists:
$(i)$ Har Gobind Khorana: Developed a chemical method for the synthesis of $RNA$ molecules with defined base combinations (homopolymers and copolymers).
$(ii)$ Marshall Nirenberg: Developed a cell-free system for protein synthesis that helped in deciphering the code.
$(iii)$ Severo Ochoa: Showed that the enzyme polynucleotide phosphorylase helped in polymerizing $RNA$ with defined sequences in a template-independent manner (enzymatic $RNA$ synthesis).

(N/A) The salient features of the genetic code are as follows:
$(i)$ The codon is a triplet. $61$ codons code for amino acids and $3$ codons do not code for any amino acids; hence,they function as stop codons.
$(ii)$ One codon codes for only one amino acid; hence,it is unambiguous and specific.
$(iii)$ Some amino acids are coded by more than one codon; hence,the code is degenerate.
$(iv)$ The codon is read in $mRNA$ in a contiguous fashion. There are no punctuations.
$(v)$ The code is nearly universal: for example,from bacteria to humans,$UUU$ codes for Phenylalanine $(Phe)$. Some exceptions to this rule have been found in mitochondrial codons and in some protozoans.
$(vi)$ $AUG$ has dual functions. It codes for Methionine $(Met)$ and also acts as an initiator codon.

(N/A) Mutation is defined as a sudden inheritable change in the genetic material.
It is categorized into the following types:
$(i)$ Point mutation: This involves a change in a single base pair where one base is replaced by another. $A$ classical example is the change of a single base pair in the gene for the $\beta$-globin chain,which results in the substitution of the amino acid glutamate with valine. This leads to a condition known as sickle cell anemia.
$(ii)$ Frameshift mutation: This is a change in the reading frame of the genetic code due to the insertion or deletion of base pairs.
$(a)$ Insertion: If one or more nucleotides are added to the $DNA$ segment,it is called insertion. If three or a multiple of three bases are added,the reading frame remains unchanged,but a protein with additional amino acids is formed.
$(b)$ Deletion: If one or more nucleotides are removed from the $DNA$ segment,it is called deletion. Similarly,if three or a multiple of three bases are removed,one or more amino acids are deleted from the polypeptide chain.

(N/A) Unambiguous code means that one codon codes for only one amino acid; for example,$AUG$ codes only for methionine.
Genetic code is universal,meaning a particular codon codes for the same amino acid in all organisms.
It is degenerate because some amino acids are coded by more than one codon; for example,$UUU$ and $UUC$ both code for phenylalanine.

(A-3, B-4, C-1, D-5, E-2) The genetic code is a set of rules by which information encoded in genetic material is translated into proteins.
- $(a)$ $UUU$ codes for Phenylalanine $(3)$.
- $(b)$ $GGG$ codes for Glycine $(4)$.
- $(c)$ $UCU$ codes for Serine $(1)$.
- $(d)$ $CCC$ codes for Proline $(5)$.
- $(e)$ $AUG$ codes for Methionine $(2)$,which also acts as the start codon.
Therefore,the correct matching is $(a-3, b-4, c-1, d-5, e-2)$.

(A) The sequence of amino acids in a protein is determined by the sequence of nucleotides in the $DNA$ molecule.
During the process of protein synthesis,$DNA$ is transcribed into $mRNA$,which is then translated into a polypeptide chain by ribosomes.
Any mutation or change in the nucleotide sequence of the $DNA$ (such as point mutations,insertions,or deletions) directly alters the codon sequence in the $mRNA$.
This change in the $mRNA$ sequence results in the incorporation of a different amino acid or a change in the total number of amino acids in the resulting protein chain.

(A) George Gamow was a physicist who proposed that if there are $4$ bases and $20$ amino acids,the code must be made up of three nucleotides.
He suggested that the genetic code is a triplet,meaning $4^3 = 64$ codons,which is sufficient to code for $20$ amino acids.
This was a significant theoretical contribution to understanding the nature of the genetic code.

(D) The genetic code is based on $4$ nitrogenous bases: Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Uracil $(U)$.
Since the genetic code is a triplet,each codon consists of $3$ nitrogenous bases.
The total number of possible codons is calculated by the formula $4^n$,where $n$ is the number of bases per codon.
Here,$n = 3$,so the total number of codons = $4^3 = 4 \times 4 \times 4 = 64$.

(C) Har Gobind Khorana developed a chemical method for the synthesis of $RNA$ molecules with defined combinations of bases (homopolymers and copolymers). This was a crucial step in deciphering the genetic code,as it allowed researchers to determine which amino acids were coded by specific nucleotide sequences.

(C) Marshall Nirenberg's cell-free system for protein synthesis was instrumental in deciphering the genetic code. By using synthetic $RNA$ templates in a cell-free environment,he was able to determine which amino acids were coded by specific nucleotide triplets (codons). While Har Gobind Khorana also made significant contributions by synthesizing $RNA$ molecules with defined combinations of bases,the development of the cell-free system is primarily attributed to Marshall Nirenberg.

(A) The cell-free system,developed by Marshall Nirenberg and Heinrich Matthaei,was instrumental in deciphering the genetic code. By using synthetic mRNA in a cell-free environment,they were able to determine which specific codons correspond to which amino acids. This breakthrough allowed scientists to crack the genetic code,identifying that a sequence of three nucleotides (a codon) codes for a specific amino acid.

(B) Severo Ochoa was a Spanish-American biochemist who made significant contributions to the understanding of genetic code.
He discovered the enzyme $Polynucleotide$ $phosphorylase$,which is also known as the $Ochoa$ $enzyme$.
This enzyme helps in the polymerization of $RNA$ in a template-independent manner.
It allowed scientists to synthesize $RNA$ molecules with defined sequences of bases in a test tube,which was a crucial step in deciphering the genetic code.

(D) codon is a sequence of three nucleotides found on $mRNA$ (messenger $RNA$) that codes for a specific amino acid.
Therefore,codons are present in $RNA$ molecules.

(A) The genetic code consists of $64$ codons in total.
Out of these $64$ codons,$61$ codons code for amino acids.
The remaining $3$ codons ($UAA$,$UAG$,and $UGA$) are stop codons (also known as termination codons) and do not code for any amino acid.
Therefore,the correct answer is $61$.

(C) Nonsense codons,also known as stop codons or termination codons,are sequences in mRNA that signal the end of protein synthesis.
There are $3$ such codons in the genetic code: $UAA$,$UAG$,and $UGA$.
These codons do not code for any amino acid and are recognized by release factors rather than tRNA molecules.
Therefore,the correct answer is $3$.

(C) degenerate codon refers to a codon that codes for the same amino acid as another codon. Most amino acids are specified by more than one codon.
$AUG$ is the start codon,which codes for methionine. It is unique because it is the only codon that codes for methionine (along with $UGG$ for tryptophan),making it a non-degenerate codon.
$UUU$ codes for phenylalanine (along with $UUC$).
$AGU$ codes for serine (along with $AGC$,$UCA$,$UCG$,$UCC$,$UCU$).
$GUU$ codes for valine (along with $GUC$,$GUA$,$GUG$).
Therefore,$AUG$ is the correct answer.

(B) The initiation codon is the specific sequence of three nucleotides in 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 $(Met)$.
$AUG$ also acts as a start signal for the ribosome to begin the translation process.

(A) The genetic code is degenerate,meaning that some amino acids are encoded by more than one codon.
For example,leucine and arginine are each specified by $6$ different codons.
This property helps in reducing the impact of mutations,as a change in the third base of a codon often results in the same amino acid being incorporated into the polypeptide chain.

(C) The correct answer is $C$.
$AUG$ is the initiation codon and codes for methionine. It does not perform three functions.
Genetic code is triplet (three nitrogenous bases code for one amino acid).
Genetic code is universal,meaning a specific codon codes for the same amino acid in all organisms from bacteria to humans.
The code is read on $mRNA$ in a contiguous fashion without any punctuation.

(D) The genetic code consists of $64$ codons,out of which $61$ code for amino acids and $3$ are stop codons (termination codons).
$UAA$,$UAG$,and $UGA$ are the three stop codons that do not code for any amino acid.
$UGG$ is a codon that codes for the amino acid Tryptophan.
Therefore,$UGG$ is the odd one out as it is a sense codon,while the others are stop codons.

(D) To determine the amino acid sequence,we use the standard genetic code table:
$1$. $AUG$ codes for Methionine $(Met)$.
$2$. $UUU$ codes for Phenylalanine $(Phe)$.
$3$. $CUU$ codes for Leucine $(Leu)$.
$4$. $AAC$ codes for Asparagine $(Asn)$.
$5$. $GCA$ codes for Alanine $(Ala)$.
$6$. $CAC$ codes for Histidine $(His)$.
Therefore,the sequence of amino acids is $Met-Phe-Leu-Asn-Ala-His$.

(B) The genetic code is degenerate,meaning most amino acids are specified by more than one codon. However,there are two exceptions: $Methionine$ (encoded by $AUG$) and $Tryptophan$ (encoded by $UGG$). These two amino acids are specified by only a single codon each. Among the given options,$Methionine$ is the correct answer.

(D) The genetic code is nearly universal,meaning that a specific codon codes for the same amino acid in almost all organisms,from bacteria to humans. However,there are exceptions to this universality. Mitochondrial codons often deviate from the standard genetic code (checkerboard table) used by nuclear $DNA$. Therefore,mitochondrial codons are the correct answer as they do not follow the standard universal genetic code.

(C) The genetic code is read in groups of three bases,known as codons.
If one or two bases are added or deleted,it causes a frameshift mutation,which alters the entire reading frame of the mRNA sequence downstream of the mutation.
However,if three bases (or a multiple of three) are added or deleted,the reading frame remains intact because the addition or deletion of a full codon does not shift the grouping of the subsequent nucleotides.
Therefore,the addition or deletion of $3$ bases (or multiples of $3$) does not change the reading frame.

(D) The genetic code is composed of triplets of nucleotides,known as codons.
Since there are $4$ types of nitrogenous bases $(A, U, G, C)$ in $RNA$,and each codon consists of $3$ bases,the total number of possible combinations is $4^3 = 4 \times 4 \times 4 = 64$.
Out of these $64$ codons,$61$ code for amino acids,and $3$ are stop codons (nonsense codons) that do not code for any amino acid.
Therefore,the total number of possible codons is $64$.

(C) The process of translation involves the synthesis of a polypeptide chain from an $m-RNA$ template.
$m-RNA$ carries the genetic information in the form of a sequence of codons,which are triplets of nucleotides.
Each codon specifies a particular amino acid.
Therefore,the sequence of amino acids in the resulting polypeptide chain is directly determined by the sequence of codons present on the $m-RNA$ molecule.

(A) The genetic code is read in the $5' \rightarrow 3'$ direction on mRNA. The anticodon on tRNA is complementary to the codon on mRNA.
According to the wobble hypothesis and the standard base pairing rules,the anticodon must be able to base pair with a valid codon.
However,the question asks which anticodon is 'not possible'. In standard biological contexts,all listed anticodons $(UAC, AAA, UUU, AUU)$ are chemically possible as sequences.
If this question refers to the absence of a corresponding stop codon or a specific biological constraint,it is often noted that anticodons for stop codons $(UAA, UAG, UGA)$ are not typically present in the cell.
The codon $UAA$ would require an anticodon $UUA$. The codon $UAG$ would require an anticodon $CUA$. The codon $UGA$ would require an anticodon $UCA$.
Since none of these are listed,and assuming the question implies a standard codon-anticodon pairing,all options are technically possible. However,in many competitive exam contexts,if one option is considered 'not possible',it is often due to a specific codon-anticodon interaction rule. Given the standard set,all are valid sequences for tRNA.

(A) In the process of translation,the termination of protein synthesis occurs when a stop codon reaches the $A$-site of the ribosome.
There are three stop codons: $UAA$,$UAG$,and $UGA$.
These codons are recognized by release factors,which facilitate the release of the polypeptide chain from the ribosome.
$AUG$ is the start codon,which codes for the amino acid methionine and does not act as a stop codon or a signal for release factors.

(A) The codon $UAU$ codes for the amino acid Tyrosine.
$UAA$ is a stop codon (nonsense codon) that does not code for any amino acid.
When the $25$th codon mutates from $UAU$ to $UAA$,the translation process will terminate prematurely at the $24$th amino acid.
Therefore,the ribosome will release a polypeptide chain consisting of only $24$ amino acids instead of the original $50$.

(B) The genetic code determines the amino acid sequence during protein synthesis. Based on the standard genetic code table:
- $UUU$ codes for Phenylalanine $(Phe)$. So, $p-3$.
- $AAA$ codes for Lysine $(Lys)$. So, $q-4$.
- $CCC$ codes for Proline $(Pro)$. So, $r-1$.
- $GGG$ codes for Glycine $(Gly)$. So, $s-2$.
Therefore, the correct matching is $p-3, q-4, r-1, s-2$.

(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 observed that specific mutations in genes resulted in the loss of specific enzymes, leading to the conclusion that each gene is responsible for the synthesis of a single enzyme.

(B) $1$. For $Met$ (Methionine),the start codon is $AUG$. The corresponding anticodon is $UAC$.
$2$. For the codon $GGA$,the amino acid is $Gly$ (Glycine). The corresponding anticodon is $CCU$.
$3$. For $Leu$ (Leucine),the codon $UUA$ has the anticodon $AAU$.
$4$. For the anticodon $ACA$,the corresponding codon is $UGU$,which codes for $Cys$ (Cysteine).
Thus,the correct sequence is $a-AUG, b-UAC, c-Gly, d-CCU, e-UUA, f-AAU, g-UGU, h-Cys$.

(D) Genetic code is defined as the sequence of nucleotides in a polynucleotide chain that determines the sequence of amino acids in a polypeptide chain.
Because the genetic code is universal,a specific codon codes for the same amino acid in all living organisms,ranging from bacteria to humans.
Therefore,when a human gene is inserted into a bacterial cell,the bacteria can read the genetic instructions and synthesize the corresponding human protein.

(C) The codons for the amino acid glycine are $GGU, GGC, GGA,$ and $GGG$.
In the genetic code,a codon is a sequence of three nucleotides (triplet) that specifies a particular amino acid.
Glycine is a non-essential amino acid,and its codons all start with the nucleotides $GG$ followed by any of the four bases $(U, C, A, G)$.

(A) The genetic code is triplet in nature. Three nitrogenous bases together form one codon that is specific for a particular amino acid.
The amino acid proline is encoded by four codons: $CCU, CCC, CCA,$ and $CCG$.
Among the given options,$CCC, CCG,$ and $CCU$ are the codons for proline.

(A) Alanine is an amino acid that is encoded by four specific codon triplets: $GCU, GCC, GCA,$ and $GCG.$
Among the given options,the sequence $GCU, GCC, GCA$ correctly represents codons for Alanine.

(C) The amino acid Glycine is encoded by four codons in the genetic code: $GGU, GGC, GGA,$ and $GGG$.
Comparing this with the given options,option $C$ contains three of these codons $(GGU, GGA, GGC)$,making it the correct choice.