IGCSE Biology Paper-1: Specimen Questions with Answers 208 - 208 of 208

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Question 208

Question

MCQ▾

Given diagram is showing segment of double stranded DNA.

The Phosphodiester Bonds

What does P, Q, R and S represent?

What Does P, Q, R and S Represent?
PQRS
AATGC
BAUGC
CTAGC
DUACG

Choices

Choice (4)

a.

B

b.

A

c.

D

d.

C

Answer

d.

Explanation

  • To explain base equivalence and other properties of DNA, Watson, and Crick (1953) , based on X-ray diffraction studies, proposed double helical structure of DNA. Such structure has 2 right-handed helical polynucleotide chains (strands) around a central axis resembling a spiral staircase.
  • The two strands of helical are anti parallel, means 5 ‘3.’
  • Phosphodiester bonds (Sugar-phosphate groups) are oriented in opposite direction in 2-strands, i.e.. , the 5′ end of one strand is opposite to the 3′ end of the other strand.
  • The bases are held like steps (rungs) of spiral staircase by hydrogen bonds.
  • The phosphodiester bonds (with sugar) form rails or the sides of ladder.
  • These bonds connect 2-nucleotides. Between A & T there are 2 hydrogen bonds and in between C & G there are 3 hydrogen bonds .
  • The base pair has more stability due to triple bond, as compared to the base pair.
The Phosphodiester Bonds

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