Chapter 9, Problem 18QP

The following is a portion of a protein: met-trp-tyr-arg-gly-pro-thr-Various mutant forms of this protein have been recovered. Using the normal and mutant sequences, determine the DNA and mRNA sequences that code for this portion of the protein, and explain each of the mutations.

1. a. met-trp-
2. b. met-cys-ile-val-val-leu-gln-
3. c. met-trp-tyr-arg-ser-pro-thr-
4. d. met-trp-tyr-arg-gly-ala-val-ile-ser-pro-thr-

Summary Introduction

To determine: The DNA and mRNA sequences that code for this normal protein “met-trp-tyr-arg-gly-pro-thr” and explain the mutation in “met-trp”.

Introduction: The proteins are coded from triplet codons. There are 64 triplet codons, which code for 20 amino acids in which initiation and stop codons are also included. The initiation codons allow the mRNA to transcribe the DNA template while stop codons restrict the translation.

Explanation of Solution

The possible triplet codons for the normal protein “met-trp-tyr-arg-gly-pro-thr” are as follows:

Met	Trp	Tyr	Arg	Gly	Pro	Thr
AUG	UGG	UAU	AGA	GGU	CCU	ACU
		UAC	AGG	GGC	CCC	ACC
				GGA	CCA	ACA
				GGG	CCG	ACG

These are the possible triplet codons for “met-trp-tyr-arg-gly-pro-thr” protein. Any mutation in the above sequences may lead to mutation.

The first mutation occurred and formed “met-trp” protein. Possibly, the mutation occurred at ‘tyr’, which resulted in the termination. The termination is caused by a stop codon. The possible triplet codons after mutation in “met-trp” are :

Met	Trp	Stop codons
AUG	UGG	UAA
		UAG

In the original sequence, tyrosine is present. However in mutated sequences,  stop codons are formed, which shorten the polypeptide to Trp.

Summary Introduction

To determine: The DNA and mRNA sequences that code for this normal protein “met-trp-tyr-arg-gly-pro-thr” and explain the mutation in “met-cys-ile-val-val-leu-gln”.

Introduction: The proteins are coded from triplet codons. There are 64 triplet codons which code for 20 amino acids in which initiation and stop codons are also included. The initiation codon allows the mRNA to transcribe the DNA template; whereas, stop codons restrict the translation.

Explanation of Solution

The possible triplet codons for the normal protein “met-trp-tyr-arg-gly-pro-thr” are as follows:

Met	Trp	Tyr	Arg	Gly	Pro	Thr
AUG	UGG	UAU	AGA	GGU	CCU	ACU
		UAC	AGG	GGC	CCC	ACC
				GGA	CCA	ACA
				GGG	CCG	ACG

These are the possible triplet codons for “met-trp-tyr-arg-gly-pro-thr” protein. Any mutation in the above sequences may lead to mutation.

The second mutation occurred and formed “met-cys-ile-val-val-leu-gln” protein. Possibly the point mutation occurred which caused changes in the reading frame. The triplet codons of the mutated protein “met-cys-ile-val-val-leu-gln” are :

Met	Cys	Ile	Val	Val	Leu	Gln
AUG	UGU	AUA	GUG	GUC	CUA	ACU
			GUG			ACC
						ACA
						ACG

The highlighted codons are mutated. The single place deletion or insertion may arise this type of situation where the complete sequence is changed.

Summary Introduction

To determine: The DNA and mRNA sequences that code for this normal protein “met-trp-tyr-arg-gly-pro-thr” and explain the mutation in “met-trp-tyr-arg-ser-pro-thr”.

Introduction: The proteins are coded from triplet codons. There are 64 triplet codons which code for 20 amino acids in which initiation and stop codons are also included. The initiation codon allows the mRNA to transcribe the DNA template; whereas, the stop codons restrict the translation.

Explanation of Solution

The possible triplet codons for the normal protein “met-trp-tyr-arg-gly-pro-thr” are as follows:

Met	Trp	Tyr	Arg	Gly	Pro	Thr
AUG	UGG	UAU	AGA	GGU	CCU	ACU
		UAC	AGG	GGC	CCC	ACC
				GGA	CCA	ACA
				GGG	CCG	ACG

These are the possible triplet codons for “met-trp-tyr-arg-gly-pro-thr” protein. Any mutation in the above sequences may lead to mutation.

The third mutation occurred and formed “met-trp-tyr-arg-ser-pro-thr” protein. Possibly the frameshift mutation occurred, which caused changes in the reading frame. The triplet codons of the mutated protein “met-trp-tyr-arg-ser-pro-thr” are :

Met	Trp	Tyr	Arg	Ser	Pro	Thr
AUG	UGG	UAU	AGA	AGU	CCU	ACU
		UAC	AGG		CCC	ACC
					CCA	ACA
					CCG	ACG

Here, the only codon for glycine is completely changed. The single frame is inserted and deleted from the same place.

Summary Introduction

To determine: The DNA and mRNA sequences that code for this normal protein “met-trp-tyr-arg-gly-pro-thr” and explain the mutation in “met-trp-tyr-arg-gly-ala-val-ile-ser-pro-thr-”.

Introduction: The proteins are coded from triplet codons. There are 64 triplet codons, which code for 20 amino acids in which initiation and stop codons are also included. The initiation codon allows the mRNA to transcribe the DNA template; whereas, the stop codons restrict the translation.

Explanation of Solution

The possible triplet codons for the normal protein “met-trp-tyr-arg-gly-pro-thr” are as follows:

Met	Trp	Tyr	Arg	Gly	Pro	Thr
AUG	UGG	UAU	AGA	GGU	CCU	ACU
		UAC	AGG	GGC	CCC	ACC
				GGA	CCA	ACA
				GGG	CCG	ACG

These are the possible triplet codons for “met-trp-tyr-arg-gly-pro-thr” protein. Any mutation in the above sequences may lead to mutation.

The second mutation occurred and formed “met-trp-tyr-arg-gly-ala-val-ile-ser-pro-thr-” this protein. This is a type of mutation where the reading frame is shifted because this protein is relatively larger than the normal protein.

Hence, frame insertion is a possible way due to which the protein is completely changed.