hi there and welcome to learn a over biology for free we're the missus trick if you're new to the channel then I go through a lot of different videos to cover a level biology in this series if you are new then click to subscribe to keep up to date on all the latest videos and if you like this video and find it helpful then click the thumbs up at the bottom for alike this video is going to be on DNA and chromosomes comparing the DNA found in eukaryotic cells to prokaryotic cells and if you follow the AQA course this is the beginning of topic 4 there's quite a lot of questions in this particular video so I would recommend getting self some paper and a pen so you can pause the videos when I suggest have a go at the questions and then go through the word examples with me so we can actually start straight off then with some questions see what you can remember from GCSE so definitions for gene and Allie for chromosomes not so much a definition more a description you might want to do a diagram to demonstrate what a chromosome is so pause at this stage and when you've had a go carry on in the video so first of all then gene and we're going to go through these one at a time a gene is a short section of DNA and that short section codes for a polypeptide and a functional RNA now this is different to the GCSE definition where it was fine to just say it codes for a characteristic but in reality that's far too vague which is why the a-level definition you would have to say is a section of DNA that codes for a polypeptide and a functional RNA now if you haven't already covered the proteins to know what polypeptide chain is I'll link at the top here so you can click and review your protein knowledge so the polypeptide chains that is the primary structure of a protein that will then be folded coiled held together by hydrogen ion accel five bonds to make a functional protein so that is why at GCSE the definition was that a gene is a short section which cares for protein or characteristic it actually just codes for the polypeptide chain and then that gets further processed in the Golgi apparatus so this diagram here is showing us a chromosome made up of tightly coiled DNA and as a small section of that DNA is a gene now over here this is actually linking to a new definition that you'll need to add to your notes and that is the definition locus and locus means locations a locus location and it's the location of where you find a gene so for each species so let's say human species for example all humans have exactly the same genes we have different versions of those gene so which is why we're not all genetically identical but we do all have the same genes so we have a gene for hair color eye color blood group and but you have different versions the gene is always found in exactly the same location on the same chromosome for every person and that location is called the locus now in this example that I've done here it's not about humans it's to do with a particular plant so we've got a pair of chromosomes so homologous pair a term we'll come to shortly and the gene that codes for the flower color is it in exactly the same position on both chromosomes and that would be the locus for flower color they have different versions though and that leads us on to our second definition which is allele so an allele is a different form of the same gene so you might say as a different version and alternative form what we mean by that is individuals have exactly the same genes so a gene to code for a particular polypeptide but you might have a slightly different base sequence or version of that gene and that results in a different protein being created a slightly different protein so in this example again it's from plants we've got the gene for height and we're showing the locus of that gene in this case on these two chromosomes there is the same allele so the same version the gene which will code for a polypeptide which will result in a taller height for the P gene and this is a description of the external surface they could either be a smooth P or a wrinkled surface P so those were our two alleles and the chromosome from parent to has the allele which will code for a smooth surface whereas parent one has the allele which will code for a wrinkled surface so that's what we mean by allele so lastly we have the chromosomes so chromosomes that is how DNA is stored so chromosome is tightly coiled up DNA and in eukaryotic cells chromosomes are located in the nucleus and humans in our somatic cells which means the body cells so not the gametes a sperm of egg in our body cells we have 23 pairs of chromosomes so in total there will be 46 chromosomes in each of our body nuclei and at this point I just want to go through some confusion that can occur with what a chromosome looks like so both of these diagrams are chromosomes the chromosome on the left is what chromosomes look like when the cell isn't undergoing cell division so when the cell isn't going through mitosis isn't going through interphase before that you just have a single thread like or stick like structure and that is our chromosome it still has a centromere band in the middle this here on the right is also a chromosome the difference being this is a chromosome after DNA replication has occurred in interphase of the cell cycle so it's still exactly the same chromosome it's just we now have double the Quan so we've got two copies of that chromosome it's held together in the middle by a centromere and but one of these arms we would call a chromatid and during mitosis or meiosis the centromere will split and the two chromatids are separated back into a chromosome which looks like an individual stick so chromosomes are single sticks when the cells not undergoing division when chromosomes are doubled for cell division this is what they look like but some reason the media have really taken hold of the image on the right and show chromosomes looking as an X structure but that is actually only after a chromosome has replicated so the next thing about chromosomes is this idea of homologous pairs which I mentioned earlier so pairs of matching chromosomes are homologous pairs say for example the two copies of chromosome 1 are homologous pairs so apologist pair is when you have a chromosome which has exactly the same genes as another chromosome they might have different versions there have different alleles but they have identical genes and this occurs from when fertilization happens so when the sperm and the egg Fe's the DNA from those two cells combined and the sperm will contribute one chromosome and the egg contributes the other chromosome and that's how humans end up with 23 pairs yet the sperm and the egg only contain 23 but combine them you have 46 so homologous pair if you were asked to define that in the exam the mark would be this bottom bit here so it's a pair of chromosomes which have exactly the same genes that is what a homologous pair of chromosomes are and just to show you that we've got the 23 pairs of chromosomes that you'd find in a human body cell we call this picture a human karyotype so that is when you have an image of all of the chromosomes organized in their pairs so we can see that they are the same size and the dark colored bands and the light colored bands are in the same position and that's showing us that they do contain exactly the same genes so we've got all of these homologous pairs numbered except for the very final the 23rd pair is a numbered we've got here x and y and that's because the 23rd pair are the sex chromosomes so they will determine biologically whether you are male or female so in this human karyotype the chromosomes are x and y so biologically this individual would be a male because males have a Y chromosome so how is all of this DNA stored then because there's a lot of DNA that has to fit into every single cell so he said in eukaryotic cells the DNA is stored in the nucleus the chromosomes are linear in shape so you either have just a single stick like structure or again we see this double structure that X like shape this would be just before mitosis or meiosis was about to occur the way all that DNA fits in though is a chromosome is made up of tightly tightly coiled DNA and to help make sure that DNA doesn't get tangled if you imagine trying to pack away your Christmas lights on your Christmas tree often they just get shoved into the box next year you try and take them out on they're all tangled up so what helps DNA not get tangled is it's wrapped around a structure and those structures are called histone proteins so the DNA gets wrapped around these histone proteins so it is tightly coiled it should make it less likely that the DNA gets tangled up and this image here where it says nucleosomes that is what we call the complex where you do have DNA wrapped around a histone protein so that's what these two paragraphs us here are describing so the histone is the name of the protein DNA is wrapping around that histone 2 create nucleosomes so that's how the DNA is stored in a eukaryotic cell in comparison in prokaryotic cells the DNA is still as a chromosome but it is much shorter so you have much less DNA and instead of it being a linear stick like structure it is circular so those are two key differences the DNA is much shorter and it is circular instead of linear in structure secondly because it's much shorter it's not wrapped around histone proteins so it's not protein bound or it's not associated with proteins the DNA does not wrap around the histones instead it just super coils to fit into the cell and there's no nucleus in a prokaryotic cell so it's not within the nucleus it's just free within the cytoplasm so the last thing here is two organelles which are in eukaryotic cells the mitochondria and the chloroplasts and these two organelles contain their own DNA that's because both of these organelles have essential reactions so RESP aerobic respiration in mitochondria and faith' synthesis in the chloroplast and that DNA is to code for enzymes which are essential for those reactions what you need to know on the spec though is how the DNA in these two organelles is similar to the DNA in prokaryotic DNA or prokaryotic cells so the similarities are the DNA is much shorter much like in prokaryotic cells it's circular like prokaryotic cells and it's not histone bound either so this DNA that you find inside of the mitochondria and chloroplasts doesn't resemble typical eukaryotic cell DNA in fact it's very very similar to prokaryotic DNA so at this stage we're going to go through some questions to check your understanding of that theory but also to introduce the math skills so you can start to see some of the basic math style questions that you could be asked so pause the video I'd say spend maybe about five minutes on these questions and when you're ready press play to go through the answers with me so question one explain how the considerable length of DNA molecule is compacted into the chromosome so this is for the eukaryotic cells so the double helix gets tightly tightly coiled around the histone proteins to make that nuclear zone complex and then it that is what forms of chromosome question T is quite similar and what's the function of a histone protein so those proteins associate with the DNA to assist in that tightly coiling so the histone protein is there for the DNA to wrap around so it can tightly coil fit into the nucleus as part of the chromosome then get to the Matis questions so we're told that in this example the DNA in one human muscle cell is two point three meters in length if all the DNA were distributed equally between the chromosomes calculate the mean length of DNA in each one so we know the length is 2 4 3 meters you have 46 chromosomes in a muscle cell so 2.3 divided by 46 is naught point naught 5 meters Part B we're asked to calculate in millimeters the length of DNA in a human brain cell so the first part of this question was actually testing do you know that in every body cell you have exactly the same DNA so if you have 2 point 3 meters of DNA in a muscle cell you're going to have exactly the same DNA in a brain cell so it also be 2 point 3 meters of DNA in a brain cell so all we had to do for this question was work that out and then convert the meters into millimeters so that would be times a thousand so 2.3 times a thousand is 2,300 millimeter I'm just going to link up here a video on microscopes because in that video there is a section there about how to convert different units so if you weren't clear on the conversion part click the video just have a recap on that there's a timestamp on that video as well to let you know exactly at what point in the video that is occurring so the last question the human genome contains approximately 3 billion base pairs which resides in the 23 pairs of chromosomes within the nucleus of all our cells if all the base pairs were equally distributed between the chromosomes calculate how many base pairs each chromosome would have and you have to give your answer in standard form so first of all 3 billion is 3 times 10 to the 9 if we have 23 pairs of chromosomes that means we have 46 in total we're told to assume that it is equally distributed between the chromosomes so 3 times 10 to the 9 divided by 46 and that comes to six point five times ten to the seven for our standard form so that's it for our introductions topic for DNA and chromosomes just as a recap then genes are sections of DNA which code for polypeptides and functional RNA alleles are alternative forms of a gene humans have 23 pairs of chromosomes and we call those pairs homologous and homologous pairs means that it's two chromosomes with exactly the same genes in eukaryotic cells the DNA is stored in chromosomes and the DNA tightly coils around histone proteins and it's linear in shape prokaryotic DNA is quilt and stored as chromosomes still but it's not going to be associated with proteins there's no histones they're much shorter and they are circular in shape lastly there is DNA found inside of the two organelles chloroplasts and mitochondria and that DNA is very similar to prokaryotic DNA because it's also short and secular so that's it for the DNA and chromosomes if you want to try some practice questions to test your knowledge on this then go along to miss Esther ich komm I'll put a link for exactly where you can find those questions in the description box for this video so you can find those and if you haven't subscribed already click the subscription logo just here to keep up to date with all the latest videos and if you've enjoyed today's video found helpful then please click the thumbs up at the bottom