GCSE Biology - What is DNA? (Structure and Function of DNA) #65

today's video is all about dna so we're going to run through the structure of dna with a focus on nucleotides and complementary base pairing and at the end we're going to briefly go through how a gene can code for a protein the first thing to understand about dna is that it's made from two strands that are wrapped around each other in this twisting shape that we call a double helix to understand the rest of the structure though let's take just one of these strands and then show it as a 2d diagram as though it's been

untwisted and laid out flat so we're now looking at this section here of the original strand now hopefully you can see that dna is actually a polymer because it's made up of lots of these tiny little units called monomers with dna we call each monomer a nucleotide and if we look at the nucleotide in more detail we can see each one is actually made up of three different parts at the top we've got a phosphate which is connected to a sugar and then on the side we have a base importantly every nucleotide has exactly the

same phosphate and sugar but when it comes to bases there are four different types namely a t c or g which stand for adenine thymine cytosine and guanine and this means that there are effectively four different nucleotides in dna one type for each of the four different bases if you take a look at these two nucleotides here we can see how they're combined together to form a polymer basically the phosphate of one nucleotide bonds to the sugar of the next nucleotide and this process then keeps repeating for thousands of nucleotides so that the sugars and

phosphates form one long chain which we call a sugar phosphate backbone and if we look back at our full dna molecule over on the left the sugar phosphate backbone is this outside part so it's effectively forming a protective outer casing around those bases in the middle if we look back at our single long chain though you can see that all the bases stick out to the side and these are what hold the two strands in the double helix together if we line up a second strand of dna facing the opposite way you can see how

the bases could pair up and hold the two strands together importantly though only complementary bases can pair to each other so a always has to pair with t and c always has to pair with g we call this concept complementary base pairing and it allows us to figure out what the complementary sequence of a strand will be for example if we have a strand of dna that reads a g t g c t t a c then we can use this sequence to work out what the sequence of bases on the complementary strand must be

because we know that a always pairs to t and g always pairs to c so we know that the first base on our complementary strand must be a t because that's complementary to the a then a second base must be a c because that's complementary to g then the third must be in a then c then g and so on as a last point whenever you hear the term genetic code it's the sequence of bases that they're talking about and a gene is just a particular sequence of bases that are codes for a particular protein

to do this each group of three bases is called a triplet and codes for a specific amino acid for example agt would code for one amino acid while gct might go to a different one and tac would go to a third to understand how this helps us code for a protein let's take a longer sequence of bases and work through the steps for how it would form a protein first our cells would read this dna based sequence as a series of triple codes which remember are three bases each then it will take the amino acids

that each triplet codes for and combine them all in that same order then lastly this long chain of amino acids that we formed will fold up all by itself and form a protein now the important thing about proteins is that because each type is made from a different sequence of amino acids each type will have a unique shape which allows to carry out a particular function so within each of our cells we have loads of different proteins that carry out loads of different things the main uses of proteins though are in enzymes which act as

biological catalysts to speed up the rate of chemical reactions hormones which carry messages around the body and structural proteins which add strength to our cells and tissues anyways that's everything for this video so hope that made some sense and helped and we'll see you again soon