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'

[gabipavli]

I'm studying A-Level Biology at home by myself as I already have a degree but now want a different one. I dont see a teacher or get to sit in any lessons and these videos are so helpful. I';m glad your resorces are free and so concise.

[edenmarlowe]

thx for going into more detail on why the bases pair with each other and also for the structure of the pentose sugar, its easier to remember now that i actually know whats going on (as far as A-lv goes anyway).

[wgutvnj]

please never stop this, you are amazing and you are helping thousands of people :)

[sunflower.vol.]

Thank you so much sir you made it way easier may more blessings comes into your life.

[pokemonjourneyzippy]

Fantastic video. Just subscribed, really helped my student

[gardisogames]

Thank you very much for explaining it. Now i understand it.👍

[LegendaryHeroponRiki]

do we need to know how the bases bond and know the structure of them?

[carlosjesus]

Hello, first of all thank you for the amazing video and the way is explained, so helpful. I would like to ask, are there any chance to get paper document/notes to print it? I'm studying A level biology and I would like to have some physical note to make comments on it.

[talhazafeer]

Yess ur content is helpful....but maaannnn love ur voice:)

[deets]

hi so i am confused on where does an H come from in formation of H2O. Because the phosphate has O
and the C 3 ' has an OH which doesnt seem to make sense to me as two H and one O is needed

[jcn]

u taught me what 5' to 3' is. my bio teacher couldnt even teach me what it is in 2 months!

[Anna-nwge]

Our body is so complex 😦 even we don't understand it well
Btw I'm the 1st one to comment 🤑

[Muath-Mahmoud]

If I scientist first name is only present then the scientist is most likely British

[passionfruit]

As much as this guy knows his biology this is far too much for A-level. So a lot of time wasted learning what's not in the exam meaning grade 0