hello and welcome to learn a level biology for free with miss estrich in this video i'm going to be covering dna the structure of it and how that links to its function if you are new here click subscribe so you don't miss any of the videos so dna first of all what does it stand for deoxyribonucleic acid and it codes for amino acid sequences which are the primary structure in proteins and that primary structure is what determines the final 3d shape of a protein and therefore the function of a protein and if you haven't already seen in my video on proteins for a level biology i'll link it just here so you can go and check that out as well so this is why dna is essential and every single cell has its own copy except for red blood cells and it needs to be protected because this is your hard copy that is the code for all the proteins in a cell so we'll be looking at the structure and how that structure links to this function now dna does count as one of the polymers in the biological molecules unit and we'll be looking at its dna double helix structure as well so first of all a dna nucleotide we'll be looking at the monomer which makes up a dna polymer so this dna nucleotide you'll be familiar with this from gcse it contains a phosphate group a pentose sugar and a nitrogen containing base now one of the key differences from gcse is you would have to say the name of that pentose sugar so for dna you would have to state it's deoxyribose and then you need to know the four names of the nitrogenous bases not just the letters representing them so for example g is guanine c is cytosine a is adenine and t is thymine so that is your dna nucleotide and the next thing is knowing how that dna nucleotide will then go through a condensation reaction to create the polymer and the polymer is known as a polynucleotide so over here on the right i have four dna nucleotides and in order to join them together a bond forms between the phosphate group and the deoxyribose and this is via condensation reactions now these are actually covered these condensation reactions in one of my earlier videos in carbohydrates so i'll link that just here so you can see the full details on condensation reactions but essentially it's when a water molecule is removed to form a bond and join two molecules together and the bond that is formed is called a phosphodiester bond and that's what i've shown here in this bold blue the molecules are joined together are the nucleotides now a phosphodiester bond is a really strong covalent bond and that is important for dna because that then means these nucleotides are held together by strong bonds and the bases are what form the genetic code because three of these bases will code for one amino acid so the final thing just to point out here is sometimes you might see the phrase a sugar phosphate backbone and that's referring to this bit here that i've just highlighted so it looks a bit like a spine you've got these sections in a straight line and it's really really strong because of those covalent bonds so that is the strong backbone holding the molecule together dna though doesn't just exist as one polymer in the nucleus of our cell we have two polymer chains that's double stranded and those two double strands are held together and they twist and coil up to make a double helix structure so i'm going to go through how it's possible to create that structure so the first thing is we have complementary base pairs and again you should be familiar with this from gcse so we can see that cytosine and guanine are complementary base pairs and adenine and thymine are complementary base pairs so those two bases and these two bases are able to align opposite and then hydrogen bonds can form between these bases and that's what holds together the structure and we can see just a little bit of extra detail here guanine and cytosine can actually form three hydrogen bonds because of the molecular structure of cytosine and guanine adenine thymine can form two hydrogen bonds and this idea of complementary base pairing is really important when it comes to dna replication because that is what ensures that identical copies of dna are created and i'll link my dna replication video just up here so the final thing is knowing then these structures that we've looked at in dna in particular the polymer how does that relate to its function of being a genetic information carrier but also very very stable strong molecule so the first thing links back to the covalent bonds those phosphodiester bonds and that is one of the key things that makes it a stable structure so those phosphodiester bonds create this sugar phosphate backbone and you then get a double helix forming and because of that double helix that then means the strong covalent bonds are on the outside of the helix and the weaker hydrogen bonds are in the middle of the helix but they're protected by the stronger bonds on the outside it's a double stranded molecule so when it does come to dna replication you have those two strands and they can act as a template to create identical copies of new dna the hydrogen bonds which are forming between the bases to hold the two strands together are weak and that is an advantage for dna replication because you want to be able to break those bonds with very little energy so that you can then replicate dna over and over to create new cells it's also a very very large molecule but because dna forms a double helix it can then further coil and coil and coil and it does twist around proteins in eukaryotic cells called histones and this enables it to condense and coil so tightly that you can fit lots and lots of information into that molecule in a relatively small space final advantage is the complementary base pairing and this allows identical copies to be made in dna replication so that is it for dna the structure the function and how those linked together i hope you found it helpful if you have give it a thumbs up [Music] you