Molecular structure of DNA and RNA - Structure of the DNA double helix

16 important questions on Molecular structure of DNA and RNA - Structure of the DNA double helix

How is the double-stranded structure of DNA stabilized?

By base pairs (bp), pairs of bases in opposite strands that are hydrogen bonded to each other.

When you have gone 360 degrees around the axis of the helix, how many base pairs have you count and what distance have you passed?

10 bp, which is a lineair distance of 3,4 nm.

What does the AT/GC rule indicate about purines and pyrimidines?

Purines always bond with pyrimidines, so no purine-purine binding.
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How many hydrogen bonds occur between G en C and between A and T?

- Between G and C: 3 hydrogen bonds;
- Between A and T: 2 hydrogen bonds.

Which sequence is more stable, one with a high or low proportion of G and C?

High, because of the 3 hydrogen bonds between G and C.

What rule indicates that the sequence of a strand is known when the sequence of the opposite strand is known?

The AT/GC rule.

One strand is running in the 5' to 3' direction, and the other in the 3' to 5' direction. How is this opposite orientation called?

An antiparallel arrangement.

What is base stacking?

The bases have a flattened structure. The flat sides of the bases are facing each other, so they are stacked in top of each other in the DNA structure.

What does it mean when the DNA double helix is in a right-handed direction?

If the double helix is laid on your desk, then it spirals in a clockwise direction away from you (left-handed: counterclockwise).

What are the major and minor grooves of the DNA double helix?

Two grooves in the backbone, places where atoms of the bases are in contact with the water in the surrounding cellular fluid.

There are different types of structures of the DNA double helix. What are two of them?

B DNA and Z DNA.

Which type of DNA structure is the predominant form of DNA in living cells?

B DNA.

What are 3 differences between B DNA and Z DNA?

- B DNA is right-handed, and Z DNA is left-handed;
- The number of bp per 360 degrees turn are 10 in B DNA and 12 in Z DNA;
- In B DNA, the bases are more perpendicular to the central axis, and in Z DNA, the bases are more tilted relative to the central axis.

Which 3 factors favor the formation of a Z DNA conformation (out of a B DNA conformation)?

- High ionic strength: a sequence of bases that alternates between purines and pyrimidines (CGCGCGCG);
- Lower ionic strength: the methylation of cytosine;
- Negative supercoiling.

What is the biological significance of Z DNA?

There is a lot unknown about this topic, but accumulating evidence suggests a possible role for Z DNA in the process of transcription.

A double-stranded molecule of B DNA contains 340 nucleotides. How many complete turns occur in this double helix?

- A double helix with 340 nucleotides, so 340/2 = 170 nucleotides in a row;
- B DNA, so 10.0 bp per complete turn, so 170/10 = 17 complete turns in this double helix.

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