Question

One strand of the given segment of DNA códes for mRNA having the sequence AUC, GCG, UCA
needed for the synthesis of proteins. The strand by which DNA molecule will be responsible for the above mRNA sequence is :

Options are in the attachment. Need a correct ans with explaination for each option.


​

Answer 1

In a typical DNA template, Adenine pairs with Thymine and Gaunine pairs with Cytosine. Since, Thymine is absent in RNA, Uracil is Adenine's complementary pair.

Given mRNA sequence is AUC GCG UCA

Here,

• Adenine will be replaced by Thymine
• Uracil will be replaced by Adenine
• Cytosine will be replaced by Guanine and vice-versa

Likewise,

DNA sequence on one strand would be TAG CGC AGT

The second strand would be complementary - ATC GCG ACT

Option (D) is correct.

Answer 2

☆Information required:

》In general the B form of DNA, they are double stranded consists of nucleotides which are joined by Hydrogen bonds of adjacent strands.

》The pairing of the nucleotides are complement to each other with purine---pyrimidine bonding.

• A--double bond--T
• G--Triple bond--C

》Purine: Adenine,guanine.

Pyrimidines: Thymine, cytosine,uracil.

》During the translation of DNA to mRNA, the thymine is replaces with uracil(by methylation)

☆Solution:

▪︎ let's first apply the rule of elimination in options!

(1) Is totally wrong, bcz uracil is only present in the mRNA strand and, the given are DNA strands.

》Now còde the required mRNA sequence:

• A-- T
• U-- A (as in mRNA 'T' will be còded as 'U')
• C-- G

》next one:

• G-- C
• C-- G
• G-- C

$\star$ now as we know, that during translation, the copying will be first done of one Dna strand and later the mrna strand makes the opposite of complementary strand.

》so, TAG and CGC will be on same single strand of DNA.

• (2) option is eliminated here!

》last one:

• U-- A
• C-- G
• A-- T

》So the required strand should be in sequence:

TAG,CGC,AGT

☆☆(4) is correct! (as in 3 option its TGA which is not sequenced as required, should be as AGT )

--------------