Introduction to DNA structure

Hi everybody. Today we're going to be doing an introductory video on DNA the code of life and we're going to be looking at some basic principles in relation to the structure and the function of DNA as well as some revision on things that we already know from previous grades. So what is DNA? Technically DNA is a nucleic acid and essentially it is a molecule that stores all of the necessary information to control cellular activity which is things like specialization of cells and the arrangement of our tissues into organs. DNA is also responsible for controlling the synthesis of proteins which we're going to learn in a lot more detail at a later stage. Now we're going to focus in on particularly deoxyribonucleic acid also known as DNA and the name essentially comes from the structure of the molecule and we can break it up into deoxyribonucleic acid and essentially this is the structure of the molecule in terms of its shape and its backbone. Now if we do a little bit of revision on DNA we need to look at where exactly do we find DNA and DNA is found in almost every cell and in particular the location we want to find the DNA is in the nucleus of a cell. Now generally when we find DNA in a cell we have a nucleus and inside that nucleus we'll have the DNA but it doesn't come in a condensed format as you see here as a chromosome. Instead DNA is generally found in a long spaghetti-like structure we call the chromatin network. Now the DNA strand spends the majority of its time in the form of the chromatin network except when it wants to replicate a cell or to reproduce a new cell in order for reproduction to take place or to repair something the chromatin network will condense and it will form these chromosomes that we see inside of the nucleus. Now a chromosome essentially is a condensed piece of DNA and in humans we have 46 chromosomes. We've inherited 23 from our mother and 23 from our father. Chromosomes essentially are long thin intertwined thread-like structures made up of a strand of DNA that is wound around some proteins and if we were to stretch it out you would see a DNA strand which looks something like a double helix ladder. On these sections of DNA we also find genes and genes essentially are short segments of code that is used to create different proteins. Genes are often called our hereditary information in other words it's the units of hereditary information that we have inherited from our ancestors. Now just a little bit of further recap on chromosomes so that we know where they fit into the bigger picture of DNA. Alongside here I have a single chromosome and I'd like you to know that this chromosome is replicated. In other words generally chromosomes are single stranded however when we want to make another cell they need to double themselves and they need to have two arms in order for this to be successful and so the one alongside here is a replicated chromosome. It's the most common representation of chromosomes and you will notice that the chromosome has two identical arms which we call chromatids and they need to be identical because in order to pass on important DNA information we want both sides of our chromatids to be the same. Then attaching the two chromatids to each other is the centromere and if you look a little bit closer you will see that they have taken one arm of the chromatid and they have zoomed in on it so that you can see that it's actually the DNA molecule that has been spiraled and condensed and pushed into itself in order to form this chromosome. You'll also notice that there's something called the p-arm and the q-arm and essentially this is just the longer and shorter arms of chromosomes. Now let's look at some basic DNA structure when it comes down to the components that make up DNA. So for now we know that the nucleus is where we house the DNA in a cell and generally it is in the form of chromatin network however it will condense into the chromosomes when the cell wants to go through cellular reproduction or we want to repair or replace any missing cells. Now if we were to take that chromosome and stretch it out so that you can see the long thread of DNA that it's made out of you will notice that the thread is wrapped around these histone proteins and histone proteins are essentially globular proteins that give the chromosome a shape and definition as well as protecting the DNA from being broken or lost. Now that we stretch out the DNA strand a little bit further we can see that we have sections of DNA that we would call genes and remember these are our hereditary units and we have many thousands of genes that code for different characteristics. Finally if we get down to the smallest component of DNA we find what we call a nucleotide and the nucleotides are the building blocks of DNA molecules and they are what we also call monomers. So far we've covered these basic DNA terminology points DNA, chromosome, gene, nucleotide and nucleus and the final important terminology that we need to know is the shape that DNA comes in. Now the DNA shape was discovered by Watson and Crick and you will need to know these two scientists' names as a part of your terminology. The defining shape of a DNA strand is what we call a double helix and essentially what it is it's a single strand that spirals around another strand. It's quite difficult to show this in 2D however you can see it a little more clearly in this section of this diagram and essentially it has rungs of a ladder that stretch across it connecting one side of the DNA strand to the other and this particular shape is called a double helix and you will need to know that term when describing its shape. Now that we are aware of the basics around the DNA terminology as well as who discovered it and the overall shape let's look at the smaller building blocks that make up DNA which we call the nucleotides. Nucleotides are what we call the building blocks or the monomers of DNA and it's important to use this word monomer when describing building blocks of any molecule in life sciences. Every nucleotide has three important components that make it up and that is the phosphate group, the sugar and the nitrogen base. Now the phosphate group gives it away. Essentially it is a group of phosphates surrounded by oxygens and it forms a part of what we call the backbone of a DNA molecule. Attached to that backbone is a sugar. Now this sugar is actually very important when we actually name DNA. DNA is called DNA or deoxyribose because that refers to the name of the sugar. The sugar is a ribose sugar however it has been deoxygenated so it is a deoxyribose. These sugars are also known as pentose sugars and for the simple reason that there is five carbons that form the structure of the sugar. Lastly but most importantly are the nitrogenous bases. These nitrogenous bases are the foundation of our genetic code. In other words these bases are the DNA code that will code for every protein that is needed in order to grow an organism and also to maintain it. Now there are four nitrogenous bases that we need to be aware of and that is adenine, thymine, cytosine and guanine. What's important to note about the nitrogenous bases is that they are complementary to one another and they will only ever pair with their complementary base. In other words A which is adenine always joins to T which is thymine and G always joins to C. Guanine always joins to cytosine. We call these complementary base pairs. So now that we know that nucleotides are consisting of a phosphate group, a sugar and a nitrogenous base and we refer to these monomers as the building of DNA we now need to see how do we put many of these nucleotides together in order to form one strand of DNA. We've looked at nucleotides which are the monomers of DNA but now we need to take those monomers and we need to form them into a functional piece of DNA and so what we have here in the diagram is a piece of DNA which we've flattened out so it's not in its double helix shape and you will notice a couple of things that we've already discussed and so let's just look at them now side by side. You have the basic building blocks of a DNA strand which is a nucleotide which if I draw a box around it this is one nucleotide and this nucleotide contains its phosphate group which is down here the letter P. It has its sugar and it has its nitrogenous base attached to it. In this instance it's the letter G which means that that is guanine. On the opposite side you will notice that there is the exact repeat except this time you have cytosine instead of guanine and you will know that because we've learnt about base pairs being complementary. G always joins to C and A always joins to T. Now in order to hold all of the nucleotides together we need some kind of foundation and that is where we find the sugar phosphate backbone. Essentially what that is is all of the phosphates and sugars that run along down each side of the double strand that we find in the double helix and we call this the sugar phosphate backbone. Now we also know that we need to keep our complementary nucleotides attached to one another. In other words we need to attach C to G and A to T and the only way to do that is to use what we would like to call weak hydrogen bonds and you will see these bonds is represented as lines connecting the two nitrogenous bases to one another and you will notice that we refer to them as weak hydrogen bonds. Now we call them weak hydrogen bonds because simply in relation to the other bonds that surround the DNA molecule these hydrogen bonds are weaker and they are breakable and that's really important for some processes that we are going to do later on in particular DNA replication and protein synthesis and the reason why we want them to be weak is because eventually we want to be able to pull the DNA molecule apart so we can get at the information inside of the DNA molecule. The next thing you will notice about our DNA structure is that our thymine and adenine are different in their structures and you will notice that thymine is a single ring whereas adenine is a double ring. Likewise guanine is a double ringed structure whereas cytosine is a single ring and these two groupings have terminology names that we need to know and the two ring structures so the ones that we see in adenine and guanine are known as purines. The single ring structures are known as pyrimidines and they are thymine and cytosine. Now that we have a good understanding of the basic structure of DNA and its nucleotides we now need to take these structures and transform them into their function which is ultimately providing the DNA sequence or the DNA code that we would use to create proteins and that we would use to regulate cellular activities. What's very interesting about the DNA code is that those four letters A, T, G and C are found in every organism on earth. In other words every organism has the same four letters that their DNA is written on. The only difference between organisms is the order in which those letters are found. For example the order in which the letters are found such as A, T, T, G, C, A will produce a very different protein in a different organism that perhaps has the code T, A, A, G, A, C. Both of these codes will code for something in that organism, a characteristic perhaps, and even though they don't code for the same protein they are written in the same four letters. It's important to know that these code sequences of letters are millions of letters long and it would be impossible for us to write out all of the letters and so generally when we deal with DNA we only deal with sections of DNA at a time. We now need to look at the basic functions of DNA and the first function is DNA carries our hereditary information. Now this hereditary information is stored in the form of genes and like I mentioned earlier we have thousands of genes that code for different characteristics and that can be things like the genes that code for your eye colour, the genes that code for your blood group, the genes that code for the growth of your muscle, the production of saliva. These are all genes. The second function of DNA is that it provides the code for an organism's growth and development by coding for protein synthesis and essentially what that means is the DNA provides the necessary recipe or blueprint to make the proteins that are required for the growth and development of that organism. Last but not least the function of DNA is linked to the ability to pass DNA on to the next generation and DNA needs to be able to replicate itself in order to be passed on through cellular division which ensures that genetic code is passed on. Essentially what that means is we want every organism that is reproduced to have some part of the genetic code from its parents therefore ensuring the survivability of that species.