hi everyone and welcome to miss estrich biology in this video i'm going through all of topic four so if you need that revision boost this one's for you if you just need particular topics so have a look at the time codes on the bottom and skip to the bit that you need most if you are new here then i'm miss estrick a teacher and tutor of over 10 years helping you to get to grips with those most challenging topics in biology improving your study skills technique and most importantly helping you to get the grades that you deserve now this last two years getting ready for your exams must have been so hard because of the pandemic and that is why i've made sure all of my essential revision resources are ready in time for your revision period so if you are struggling with timing stop making revision notes and instead i have created them for you just here so they include all of the key terms for all of the theory in aqa a-level biology as well as the links to the essays or if you struggle more with remembering all of the key information then my active recall workbook is the one for you it has questions covering all of the a level so you can test your knowledge and improve your memory if actually it's time management that's more your difficulty then have a go at my revision timetable maker it comes with a template that you can edit and complete guidelines and how to make a timetable that will be perfect for you and your needs but for now let's get into topic four so we start topic four by comparing the dna in eukaryotic cells and prokaryotic cells and comparisons mean similarities and differences so our two key similarities are the fact that the dna is made up of dna nucleotides for both meaning it contains deoxyribose a phosphate group and a nitrogenous base the nucleotides are also joined together by phosphodiester bonds to make the polymer chain the three key differences are that eukaryotic dna is longer eukaryotic dna is linear so it occurs as straight lines in the chromosomes whereas prokaryotic dna you get circular loops of dna the dna in eukaryotes is associated with histones but for prokaryotic dna it is not associated with proteins those histones so those are your key differences in the dna and this is just showing you here the organization of the dna in a eukaryotic cell so we have our chromosomes within the nucleus that is tightly coiled to make the chromosome but if we unwind that you can then see the nucleosomes which is actually the dna wrapped around the histone proteins and that is how we tightly coil to fit it all in the nucleus now you also need to be able to compare the dna that is found in the mitochondria and chloroplasts of cells to the dna that you find in prokaryotic cells because they actually have quite a few similarities so we can see here our mitochondria and you have just a loop of dna and the same within your chloroplast you just have this circular loop of dna and they have their own dna so that they can transcribe translate their enzymes that they need for photosynthesis and the chloroplast and respiration in the mitochondria so the similarities are they are both short sequences of dna they are both circular and neither of them are wrapped around histone proteins so that then takes us on to looking at what a gene is and a gene is a sequence of dna and it codes for the amino acid sequence for a particular polypeptide and also a functional rna so it codes for an mrna molecule and we can see here one of the extra key terms as well and that is locus and locus is the exact position that one particular gene is found on a chromosome so locus is location so that's the way to remember it locus location of the gene okay so the genetic code you need to know a few features of the genetic code one thing first of all is knowing that a sequence of three bases on dna is called a triplet and those three bases will code for a particular amino acid so the three features are it is a degenerate code it's universal and it's non-overlapping so if we ever think about this idea of degenerates there are 20 amino acids that we said exist and we have four possible dna bases and the way they actually worked out that it was three dna bases that codes for one amino acids was actually mathematically they looked at all the possible code options you could get if you only had one base coding for one amino acid and that would only give you four possible codes g c t or a that's not enough to code for 20 different amino acids so then if you think about could it be two bases if you had only two bases that would give you 16 possible different codes and that's still insufficient so they realize it must be three bases that code for one amino acid because that actually gives you 64 possible different options for what those triplets of bases could be and that is more than enough to code for the 20 amino acids and that is why the genetic code is degenerate and what we literally mean by this definition is there are more than one triple of bases that codes for the same amino acid that would be your key definition and this table here is just showing you an example of that so you can have a look for example at let's look at glycine here gly ggg gga ggc ggu all four of those triplets of bases code for glycine and this is an advantage of the genetic code because if there is a mutation and one of the bases in a triplet is changed you might still have the new triplet coding for the same amino acid and therefore it has no effect on the overall polypeptide chain universal means that the same triplet of bases codes for the same amino acid in all organisms non-overlapping is the fact that each base is only involved in one triplet so if we just draw boxes around this to show you what i mean this base a is only in this one triplets and this c is only in the triplet this g is only in this triplets we don't have this g also making up a second triplet of bases so every codon or triple of bases is read as a discrete unit this is an advantage as if a point mutation occurs it will only affect one codon and therefore one amino acid so it will minimize any potential harm now in your dna you have sections of base sequences which are introns and you have sequences of dna that are exons introns are sequences of dna bases that do not code for polypeptides and you actually have a lot of introns making up your nuclear dna the exons are sequences of dna bases that do code for the amino acids so the exons are the codeine regions and when we say codon a codon is three bases on mrna that codes for a specific amino acid a start codon is three bases that you find at the start of every gene and that is what initiates translation to occur a stop codon is the final three bases that you have at the end of every gene and those three bases will cause the ribosome to detach during translation and therefore it stops the translation of the polypeptide chain a genome is what we call an organism's complete set of genes in a cell so that is your definition of a genome whereas the proteome is the full range of proteins that a cell is able to produce the genome should never change unless there are mutations whereas the proteome of the cell can constantly change depending on which proteins are needed in a specialized cell because you'll have some genes have been switched off or on and that's what makes it specialized the genome of an organism will really differ between different species so for example a bacteria contain on average six hundred thousand dna base pairs within their genome whereas humans we have three billion dna base pairs so this then starts to take us on to rna before we get into protein synthesis so messenger rna is what mrna stands for and we can see it here on the picture it is short compared to dna because it's only a copy of one gene whereas the dna is the entire genome it's single stranded and it's found in both the cytoplasm and the nucleus so it is made during transcription and that happens in the nucleus but then once it's been modified it leaves the nucleus enters the cytoplasm to attach to a ribosome it's three bases on mrna that are codons so three bases which can code for a particular amino acid trna is transfer rna and this is found in the cytoplasm it has an amino acid binding site which we can see up here at the top and each trna molecule will have a particular or specific amino acid attached to that binding site the trna molecule also has three bases on it at the bottom here and we call those three bases the anticodon and they will be complementary to a particular codon on mrna and when those align they're held in place so that amino acids can start to bond together during translation so trna is involved in translation the second stage of protein synthesis a ribosome will be holding it in place to enable the joining of amino acids it has this clover leaf shape that's what we call this shape and we can see here these lines are representing hydrogen bonds so it's still single stranded it's just folded to create this shape and that shape is held in place by hydrogen bonds so that then takes us onto protein synthesis and it's split into two steps transcription which is where one gene at a time from dna is copied into mrna then we have translation where the mrna will join with a ribosome and corresponding trna molecules will then bring specific amino acids so first of all we have transcription so a complementary mrna copy of one gene on the dna is created in the nucleus mrna is much shorter we've already said and that is because it's only copying one particular gene and therefore is able to leave the nucleus because it is smaller your key steps are all here so if you did have a long answer question describe the process of transcription these would be your key marking points with the key marks put in bold so first of all the dna helix unwinds to expose bases and you have one strand acting as a template and that's our second mark like with dna replication that is caused by dna helicase breaking the hydrogen bonds in the nucleus you then have free floating mrna nucleotides and they will align opposite their complementary dna base pairs on the template strand the enzyme rna polymerase will then join together those rna nucleotides to create the mrna polymer chain once it's copied it then has to be modified and then it leaves the nucleus via a nuclear pore the modification that happens is this here splicing in eukaryotes after transcription we actually call the molecule pre mrna and that is because it still contains the introns which are those non-coding sequences of bases and that's because the dna the gene that was copied there will be introns within it which are the non-coding sequences so the rna that is copied will still contain those introns so the introns need to be removed and we call that splicing they're spliced out so cut out they're spliced out by a protein called a spliceosome and now we have finished mrna that is ready to leave the nucleus that stage doesn't happen in prokaryotes because they don't have introns translation is the next stage in the creation of the polypeptide chain and it involves both mrna and trna if you were asked to describe that whole process again these are your six key marking points and in bold are the key terms you would have to include once a modified mrna has left the nucleus it will then bind to or bind with a ribosome in the cytoplasm the ribosome will attach at the start codon of the mrna molecule the trna molecules with complementary anticodons to the start codon will then align opposite and they're held in place by the ribosome which we can see here in the picture the ribosome holds together two trna molecules at a time the two amino acids that have been delivered by the trna molecule are joined by a peptide bond and that reaction does require energy in the form of atp and an enzyme but you don't need to know the name of it once that happens the trna molecule will be released and the ribosome moves along one codon so the next trna molecule can then align its anticodons to its codons so this continues until the ribosome reaches the stop codon at the end of the mrna molecule and when it does that causes the ribosome to detach and therefore translation ends now the modifications because we now just have a polypeptide chain the modifications will occur in the golgi body for folding to create that secondary tertiary or quaternary structure we then move on to how variation is introduced and gene mutations is one way a change in the base sequence of dna is what a gene mutation is and they randomly occur during dna replication so within the interphase part of the cell cycle these random mutations are more likely to occur if you're exposed to mutagenic agents which can interfere with the dna replication that includes high energy radiation like uv light ionizing radiation like gamma rays and x-rays and also some chemicals which we call carcinogens for example mustard gas and cigarette smoke a gene mutation can result in either a base being deleted or swapped so substituted for a different base so here are our examples we have our original dna sequence this is showing a substitution it's instead of cytosine that has been swapped or substituted for adenine this one is showing a deletion because that base c has now been deleted and that's actually caused what we call a frame shift everything downstream of the mutation has shifted to the left a base mutation they might have no impact at all because the new codon may still code for the same amino acid and that's because the genetic code is degenerate chromosome mutations can also occur and chromosome mutations are changes in the number of chromosomes and this spontaneously occurs during meiosis and a process called non-disjunction so non-disjunction is when the chromosomes or it could be the chromatids do not equally split during anaphase of either meiosis one or meiosis ii so that's what we can see here non-disjunction occurring because the chromatids didn't separate and instead all of them are being pulled to the same pole of the cell now this can occur in two forms either a change in the whole set of chromosomes which we call polyploidy or changes in the number of individual chromosomes which is aneuploidy so we'll go through polyploidy first which we said is a change in the whole sets of chromosomes so you could end up instead of being diploid having two copies of every chromosome which we have in humans you could have three copies or four copies of every chromosome which would be called triploid or tetraploid now in humans that would be fatal you don't see triploid or tetraploid humans but it's actually quite common in plants so how this would occur then each homologous pair is doubled in replication and that happens in interphase in this example we have non-disjunction in meiosis one for some reason the spindle fibers haven't attached to the chromosome on this side and they have attached to all of the chromosomes on the other side so when the spindle fibers retract it's going to pull all of the chromosomes to one side of the cell and therefore they're all going to be in this cell and there'll be no chromosomes in the next one in meiosis ii that would mean that these two gametes will contain no chromosomes at all so those gametes will not function these gametes though meiosis ii is happening normally and we do have complete separation of all of the chromatids but we now have two copies of every chromosome in the gametes so instead of having a haploid gamete we have a diploid gamete and if a diploid gamete fuses with a haploid gamete that is how we then end up with three copies so we get two chromosomes from this gamete instead of one and we get just one chromosome from the haploid gametes so that is polyploidy changes in the whole sets of chromosomes that could also happen if you have non-disjunction in meiosis ii in this example we can see the chromosomes in meiosis one did separate equally and then we had normal meiosis ii in this example so we have two haploid gametes but for this cell in meiosis ii there was non-disjunction so the spindle fibers didn't form on this side so the chromatids aren't separated equally and instead they're all pulled to this cell so again we end up with a 2n gamete diploid gametes and this gamete has no chromosomes in aneuploidy this is different this is when you have changes in the number of individual chromosomes so sometimes individual homologous pairs of chromosomes fail to separate during meiosis it's still called non-disjunction but instead of it being affecting every single chromosome or chromatid it's just one and this is how down syndrome occurs you have non-disjunction on chromosome 21 so you end up with three copies of that chromosome instead of two so let's see how that might occurred we can see in this one we have non-disjunction occurring at meiosis one because these spindle fibers for just that one chromosome or that one homologous pair of chromosomes is attaching and it pulls them both to this cell and this cell does not get a copy of that red chromosome if meiosis ii occurs normally so no non-disjunction all of the chromatids are separated equally however because of the non-disjunction of myosis one this gamete is haploid it has one copy of all the chromosomes except for the red so we describe that as haploid plus one extra chromosome so n plus one these two are still haploid but they're missing a chromosome so we describe it as n for haploid minus one now if an m plus one so a haploid with an additional chromosome is to fuse with a haploid chromosome that is how you can get which means three copies so tri trisomy is three three copies of one particular chromosome and that is how down syndrome occurs three copies of chromosome 21. now you could also have non-disjunction occurring in meiosis ii so we can see there normal cell division occurred in the first round of meiosis but now we have non-dysjunction in meiosis ii because the chromatids are not separated equally for the red chromosome they're all pulled to this one gamete so that would be n plus one and this one is missing the red chromosome so it's n minus one now another way that variation can be introduced is in meiosis and meiosis creates gametes and it creates four genetically different haploid gametes by two nuclear divisions so meiosis is how variation can be introduced as well and that's through two mechanisms independent segregation of homologous chromosomes and crossing over between homologous chromosomes and both of these occur within the first round of division in meiosis independent segregation is when the homologous pairs of chromosomes line up opposite each other at the equator to form by valence it is random which side of the equator the paternal and maternal chromosomes from each homologous pair align so we can see on this side by chance two purples two reds but equally it could have been a purple and a red red and a purple in meiosis those homologous pairs of chromosomes are separated in meiosis one so one of each homologous pair ends up in the daughter cells eventually this creates a large number of possible combinations of chromosomes in the daughter cells produced and you can actually calculate this by doing 2 to the power of n 2 because it is homologous pairs so your pairs of chromosomes and n you would substitute in for how many homologous pairs of chromosomes that species has so for humans that would be 2 to the power of 23. we have 23 pairs of chromosomes which means we can make over 8 million different possible gametes just from independent segregation now crossing over also occurs sometimes it's actually quite rare but it can occur and again it occurs when the homologous pairs of chromosomes line up at the equator and form a bivalent which is what we call it when you have both of them next to each other and you have chromatids from each of those chromosomes cross over and they can get twisted around each other that puts tension on the chromatids causing part of the chromatid to break and swap and in doing that we create new combinations of alleles which is represented by the letters here so originally this chromosome only had the dominant allele but now we've got a dominant and recessive one that's pulled apart we've got new combinations of alleles on that chromosome now comparing meiosis and mitosis meiosis is two nuclear divisions whereas mitosis is only one that's why meiosis results in haploid cells whereas mitosis is diploid cells meiosis introduces genetic variation through crossing over and independent segregation mitosis creates genetically identical cells now you could be asked to identify meiosis in an unfamiliar life cycle and what you need to do here is look for where you have cells that were diploid or 2n dividing to then create cells that are haploid because that's what happens in meiosis you go from 2n to n it won't always be gametes because not all organisms have life cycles like humans where it is the creation of the gametes that is meiosis so for example we can see here we have the zygote which is 2n and then it makes something called zeus force which are n so that would be the meiosis stage so that's what you're looking for 2n moving to n so again that there would be meiosis genetic diversity this is the number of different alleles of genes in a population and this is what enables natural selection natural selection can only occur if there is genetic diversity natural selection is the process that leads to evolution and our definition of evolution is the change in allele frequency over many generations natural selection is really important to the survival of the whole species because it results in the species becoming better adapted to their environment and these adaptations might be anatomical physiological or behavioral so the key marking points that you'd need to describe for this process are first of all you would have new alleles for a gene being created by random mutations if those new alleles increase the chances of the individual surviving in that particular environment then they're more likely to survive and therefore more likely to reproduce and pass on that new advantageous allele to the next generation over many generations of that occurring that new allele will now become more common in the gene pool or in other words we've increased the allele frequency now the types of selection that occur are directional selection and stabilizing directional selection is when the advantageous allele is coding for an extreme trait so this links to your antibiotic resistance example and if we think about the traits being low resistance medium high resistance the extreme traits would be either very low resistance or very high resistance and in the case of antibiotic resistance when there was a change in the environment which is introduction of antibiotics the bacteria which had the alleles for high resistance antibiotics were more likely to survive and pass on that allele for antibiotic resistance and that's why we saw this shift and antibiotic resistance allele became far more common in these species stabilizing selection is when whatever is the middling trait remains the selective advantage and that would be the case if there's no change in the environment and this is exemplified by human birth weights so being a middling birth weight increases your likelihood of surviving because if you're very very small then you might be very premature and might have under underdeveloped lungs difficulties regulating your temperature are more vulnerable to infection if you're very very heavy then it's going to be a more complicated birth and that could result in difficulties so that would mean that the modal trait remains a selective advantage and we can see that the range of traits or alleles decreases over many many generations the range and the standard deviation decrease next then it's thinking about species and taxonomy so a species is when two organisms are able to produce fertile offspring and a species must reproduce and pass on advantageous alleles in order for the whole species to be able to survive and this is where courtship behavior comes in this behavior is essential for successful mating meaning mating and creating fertile offspring in order for species to survive so courtship behavior is a sequence of actions which is unique to every species so it is genetically coded for this behavior and it is how animals are able to identify members of their own species to make sure they are reproducing with members of their own species to make sure that they can create fertile offspring the behavior the sequence of actions is normally carried out by the male and then the female picks whether they are worthy of mating with now this sequence could include dance moves creation of sounds release of pheromones display of feathers fighting whatever it is is always a unique sequence to that species the female then observes the ritual and decides if they look like they have a good enough set of alleles based on their fitness their performance to mate with so the reason this is important is we said to ensure successful reproduction and the way it does that is first of all because the behavior is unique to every species it allows them to recognize members of their own species and also the opposite sex that is because to make fertile offspring you'd need sperm and egg to fuse it also synchronizes mating behavior and what this means is it makes sure that the male and the female are mating when they are sexually mature so the female is releasing eggs and the male is able to produce and release sperm it can actually also help the survival of the offspring once it's born in some animals and that's because this behavior this courtship ritual can help form a really strong bond which we call a pair bond between the parents so they're more likely to stay together and if they are together for some animals like penguins it increases their likelihood of survival because you need one penguin to look after the chick one penguin to go and find food it also enables that strong and healthy mates are selected for and therefore the advantageous alleles are being passed on to the next generations to ensure the survival of the entire species so this could be used as a way for us to identify how closely related different species are as well so for example we've been shown three ducks here and the sequence of their courtship behavior and we can see that ducks one and two must be more closely related because their sequence of behaviors in the courtship ritual are more similar than duck one and three or ducks two and three and because this behavior is genetically coded for because the sequence of behaviors is more similar their dna based sequence is likely to be more similar too phylogenetic classification is another way to look at how closely related different species are and also how recent their shared common ancestors were phylogenetic classification is arranging groups according to evolutionary origins and relationships so humans and chimpanzees we can see are most closely related to each other because they branch in the tree most recently compared to the others so that means they evolved from a shared common ancestor more recently than they did compared to any of the other species so that means they have had less time to accumulate different mutations in the human and chimpanzee populations compared to let's say the human and the horse population because humans and horses their recent common ancestor is back here and there's no time scale on this but this is normally going back like this would probably be at the very start here this could be looking at maybe 13 20 million years ago and so on so it is millions of years of accumulating mutations in these separate species now you can also classify and group using a hierarchy and a hierarchy is when you have smaller groups arranged within larger groups and there's no overlap between those groups so this is one particular hierarchy you need to know off by heart domain kingdom phylum class order family genus and species and we can see here that within one genus you can have multiple different species so that is our smaller groups within larger groups but there's no overlap in those species they're still distinct groups the binomial system is a universal way of identifying organisms and it's using two names that's what binomial means the first name is the genus and the second name is the species so for humans our genus is homo and our species is sapiens so our binomial name is homo sapiens and we can see an example here of common names which would be unique to every language versus the binomial system which is universally used and that gives you more information on how closely related they are because calling them both robins is misleading but they are closely related but actually we can see they're not the same species and they're not even the same genus so the final thing is looking at biodiversity and there's different ways that you can classify and measure biodiversity it could be looking at the range of different habitats that exist it could be the variety of genes amongst all the individuals of a population of one species or it could be looking at the number of different species and individuals within each species in a community species diversity as well if we go into that further that takes into account species richness and species richness just means the number of different species in a community biodiversity can be used to describe a range of habitats you could be looking at the biodiversity of a very small local habitat like a forest or it could be talking about the entire earth having a low biodiversity isn't actually a cause for concern for example in the arctic or deserts you would expect that to be low but if you have a decrease in biodiversity that is a cause of concern because that could be caused by human activity causing destruction and farming techniques is an example of that farming techniques can reduce biodiversity but it is needed to provide food for humans so we have to come up with some kind of compromise between the two you could get application questions where you have to suggest different farming techniques that could reduce the biodiversity so i've got some of those listed here and if you want more detail on that then go to my actual biodiversity video to hear all of the examples and how it reduces the biodiversity and what the compromise would be the way that we measure biodiversity is using the index of diversity and that describes the relationship between the number of species in a community which is the species richness and the number of individuals within each species so this is the formula to calculate it and they would give you that in the exam but they won't give you this that capital n is the total number of organisms of all species lowercase n is the population size of one species one is the lowest value you can get and the larger the value the greater that index of diversity the greater the biodiversity so here's an example if you want to have a go pause the video now and write it down and then have a look at the answer if not let's go through so the first bit the formula we're going to look at is the lowercase n times n minus one so that would be six as our lower case the population size of species a and n minus one that'd be five six times five would be thirty so we then do that for all of the others and we need the sum of that so the sum of that column is 180 capital m we said was the total number of all individuals and in this example it's 25. so 25 times 24 divided by 180 is 3.3 genetic diversity within or between species can also be measured by comparing different factors so you can compare observable characteristics but that can actually be quite inaccurate because members of different species that aren't even closely related might look similar because they live in similar environments so more accurate ways to compare how closely related species are is through comparing the dna base sequence the mrna-based sequence or the amino acid sequence for proteins and the more similar those sequences the more closely related they must be so that is it for topic four i hope you found it helpful and if you have please give this video a thumbs up