DNA Structure | A-level Biology | OCR, AQA, Edexcel

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The key points covered of this video include:

1. The DNA Double Helix
2. Complementary Base Pairing
3. Antiparallel Strands

The DNA Double Helix

The structure of DNA was first discovered by the scientists, Watson and Crick. However, their discovery was based on pioneering research by Rosalind Franklin. Watson and Crick worked out that DNA was composed of two very long polynucleotide chains twisted into a double helix. The sides of the DNA double helix are made from pentose sugar and phosphate groups forming a sugar-phosphate backbone. The two strands of DNA are held together by hydrogen bonds between the bases forming the "rungs" of the DNA double helix. The double helix of DNA is also antiparallel - the strands of DNA run in opposite directions to each other.

Complementary Base Pairing

In DNA, the two polynucleotide strands are held together by hydrogen bonds between two bases. The base pairing in DNA is very specific: The correct bases pair up due to: The number of hydrogen bonds each pair of bases can form, The size of the bases - a larger purine must always pair with a smaller pyrimidine. Because the structure and bonding of the bases makes the pairing specific, we say the bases are complementary to each other. Due to base pairing, there will always be the same percentage of complementary bases in a DNA molecule. This is known as Chargaff's Rule after the scientist who discovered it.

Antiparallel Strands

The two polynucleotide strands in DNA run in opposite directions to each other - they are antiparallel. One of the strands runs 5' to 3' while the other strand runs 3' to 5'. The directions are named according to the position number of the carbon atoms in the pentose sugar of each nucleotide. In a mononucleotide, carbon five is bonded to the phosphate group and carbon three has a hydroxyl group. However, in a polynucleotide, carbon three is instead bonded to the phosphate group of the next nucleotide, creating a 5' to 3' strand. In DNA, the strands run in opposite directions, creating one 5' to 3' strand and one 3' to 5' strand.

Summary

DNA is composed of two polynucleotide chains twisted into a double helix
DNA has a sugar-phosphate backbone and bases joined together by hydrogen bonds
Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three hydrogen bonds
The two DNA strands are antiparallel, with one running 5' to 3' and the other running 3' to 5'

Видео DNA Structure | A-level Biology | OCR, AQA, Edexcel канала SnapRevise