hello everybody my name is Iman welcome back to my YouTube channel today we're continuing chapter 16. last time we covered the first objective which is DNA is the genetic material today we're moving into our second objective which states a chromosome consists of a DNA molecule packed together with proteins now this right here is a structure called a chromosome DNA here in this chromosome is compacted all right it's condensed into this structure but DNA is not always compacted into condensed chromosomes as seen in this illustration in fact DNA only takes on this appearance as the cell prepares to undergo division now at this point this is the most compacted form that the DNA can can take when cells are not preparing for cell division the 46 chromosomes are mixed together in a form called chromatin this is still highly compacted but it's less so compact than the condensed chromosome structure all right and what we want to do today is we're going to explore both the chromatin structure and the chromosome structure now besides the nucleus there is DNA inside the mitochondria and of course in Plants sometimes in the chloroplast now these organelles they perform important functions that provide this cell with energy and genes for these processes are located within the organelle's DNA mitochondria mitochondrial and chloroplast DNA are are both circular in nature so how is DNA compacted from essentially two meters of double helix into the nucleus of a tiny cell well DNA goes through several levels of compaction first the double helix is coiled around specialized proteins called histenes to form nucleosomes all right so here's our double helix DNA all right its first form of compaction is well we can have this DNA all right wrap around these proteins called histenes to form this nucleosome unit all right then these nucleosomes they're coiled again all right around each other so here we form these nucleosome units these are parts of DNA that are coiled around histine proteins and then all these units can be further coiled around each other closely together all right to form chromatin all right these nucleosomes are coiled again to form chromatin now as mentioned cells that are not dividing will stop at this step but if the cell is preparing for division then there's another level of compaction that happens to form the condensed chromosome structure so these and these chromatin structures will coil even further to form chromosome now DNA compaction it involves a group of proteins that can be divided into two categories histine proteins and non-histine proteins histenes are specialized structural proteins that are going to be able to interact with DNA very very closely this interaction is due to histines and DNA having opposite charge to each other histides tend to carry a positive charge because they contain many positively charged amino acids like Arginine and lysine and these positively charged amino acids and histenes can then bind to you guessed it the negatively charged phosphate groups in the sugar phosphate backbone of DNA now there are also non-histine proteins that include some structural proteins that are involved in the spatial organization of DNA inside the nucleus as well as as a few regulatory proteins that are involved in gene expression now these regulatory proteins determine which proteins and enzymes are produced from DNA nevertheless important to know all right that DNA can be tightly coiled around a group of histine proteins to form a structure called nucleosomes all right these nucleosomes contain eight histine proteins and about 146 base pairs of DNA nucleosomes and DNA resemble beads on a string all right one looked at with a high powered microscope now DNA can then be tightly cooled around a group of histine proteins even further all right nucleosomes can then be assembled further I'm sorry to form that next level of compaction called chromatin chromatin involves histine and non-histine proteins all right and they contain about the same amount of DNA and proteins and then they coil even further to form this chromatin state and chromatin in non-dividing cells chromatin is how DNA is compacted in the nucleus and and you can actually um see chromatin versus chromosome with high powered micro microscopes as well and that's how you can distinguish all right if a cells are prepared to die that chromatin structure will further compact in this form in chromatin form right your DNA is not your your cell is not dividing when your cell is ready to divide that chromatin will form chromosomes all right so that's the difference that we want to remember all right DNA coils around histine uh histenes that forms a nucleosome unit then your nucleosomes can can further compact to form chromatin and then that chromatin can then further condense on itself to form chromosomes so that's the three level three levels of compaction here now what we want to also discuss is um two different kinds of Chromatin here all right so this step right here before we condense further to chromosome we have this chromatin State and there are two kinds of Chromatin here heterochrom chromatin and eurochromatin all right when we have heterochromatin this is dense transcriptionally silent DNA that's going to appear dark under a light microscope all right as opposed to you chromatin Which is less dense transcriptionally active DNA that appears light under a light microscope now if we have the condensed form of Chromatin AKA we have chromosomes because your cell is prepare to die to divide not die um there are two parts of a chromosome that chromosome that are important to to understand and Define here all right that is telomeres telomeres are the ends of chromosomes all right they contain High GC guanine cytos in content to prevent unraveling of the DNA now during replication telomeres are slightly shortened although this can be partially reversed by enzymes called telomerase all right another important part of our chromosome chromosome is this centromere all right these are located in the middle of chromosomes and they hold sister chromatids together all right until they are separated during anaphase in mitosis they also contain a high GC content to maintain a strong bond between chromatids all right remember G and C they pair um through three hydrogen bonding as opposed to a and t which has two hydrogen bonds that keep them together so it makes sense that in places where it's important that things really stay together that you'll see a high GC content all right it helps maintain a strong bond because of the three hydrogen bonds that keep G and C together all right now with all this information we want to move into a topic that's very important to discuss all right and that is DNA replication all right we're just going to briefly go over the DNA replication process we'll even repeat this in the next chapter when we talk about transcription and translation further but here's the premise of DNA replication all right DNA replication is the process of creating two identical DNA molecules from a single original DNA molecule now DNA replication is semi-conservative meaning that the two resulting DNA molecules each contain a strand from the original DNA molecule DNA replication is also an all or non-process meaning that once it starts it will proceed to completion now the process of DNA replication begins at a site called the origin origin of replication all right it begins at a site called the origin of replication where an initiator protein will bind all right so your initiator protein binds all right to the double-stranded DNA at the origin of replication and it slightly unwinds the DNA all right it is important to note that the origin of replication between prokaryotes and eukaryotes differ prokaryotes have a circular chromosome that has one origin of replication DNA replication starts at that one position in circular chromosomes and it works around the entire chromosome eukaryotes on the other hand consist of multiple linear chromosomes with multiple origins of replication in a single chromosome having so many origins of replication in your in eukaryotes it helps speed up the process of DNA replication all right like we said DNA replication first begins when an initiated protein binds to the origin of replication when this happens the initiator protein will unwind the DNA all right however apart from this basic function it doesn't do much else all right the first significant enzyme however is DNA helicase DNA helicase binds all right DNA helicase is will bind to the unwind DNA all right it's going to bind to the double stranded DNA and it's going to unwind it to form a replication form creating two single stranded DNA templates all right one DNA helicase separates the DNA into two strands it creates a little bit of a strain in the DNA molecule obviously this strain or or tension arises because of the double helical structure of the DNA so when the DNA unwinds sometimes it creates over winds ahead of the replication form and it could become Tangled therefore we have an enzyme that's very important that comes in to help with that strain and tension and it's called topoisomerase or in other words DNA gyrase all right it's going to cut through the phosphate backbone on one or both strands of DNA ahead of the replication fork and it's going to help untangle the DNA and and relieve any built up strain okay so so far we had our first step initiator protein binds to origin of replication Second Step you have DNA helicase that binds to the unwind DNA it continues to unwind it and for the tension that's created ahead we have DNA gyrase or topoisomerase that promotes strand Separation by removing super coils in front of the DNA helicase all right single strand DNA binding proteins will also attach to stabilize the single strands of DNA that we form from unwinding the double helix all right this is going to allow the separated strands to serve as templates all right so those are the first two steps of DNA replication The Next Step all right the next step involves an enzyme called DNA primase DNA primase binds to the leading strand of the DNA it lays short it lays down a short strand of RNA complementary to the single stranded DNA template this short fragment of RNA is called the RNA primer and the purpose of creating the RNA primer is so that DNA polymerase which is an enzyme that synthesizes new fragments of DNA can function all right now because DNA polymerase needs an existing nucleotide with a three Prime oh group to which the next nucleotide can be added DNA primase helps initiate the process of elongating the new nucleotide sequence also it's very important to note that DNA primase is making RNA and not DNA and in this way RNA Primal will eventually have to be removed all right so in this third step RNA primase binds to the leading strand all right it binds to the leading strand all right it binds to that 3 Prime to five Prime strand all right um and it synthesizes a short RNA primer that is complementary to the DNA template all right so we have this RNA primer that is added to the leading strand of DNA all right and it helps start this process now once the RNA primer is present DNA polymerase can Now function all right DNA polymerase can Now function there are several types of DNA polymerase enzymes in both prokaryotes and eukaryotes it's not important to understand each of their specific functions right now but here what we see is that DNA polymerase 3 comes in all right and it uses the RNA primer to initiate DNA synthesis by adding deoxyribonucleotides to its three prime end so the leading strand requires only one RNA primer became because DNA synthesis is continuous in the five Prime to three prime Direction all right so we had in step three that DNA primase attach and insert an RNA primer now in this fourth step DNA polymerase 3 can begin to synthesize a new Strand and this New Strand is five Prime two three prime all right because we're using the leading strand of dna3 prime to five Prime to create a complementary strand so of course that complementary strand has to be anti-parallel and so that New Strand is five Prime to three prime all right very important to know now in terms in terms of that all right DNA is antiparallel let me repeat and DNA polymerase adds nucleotides in the five Prime to three prime Direction which is the opposite strand traveling all right from the leading strand now DNA polymerase can only work in the five Prime to five three prime Direction so what about that opposite strand all right this is what we've been talking about this leading strand of DNA what about this other strand right here all right it's five Prime to three prime so we're gonna have to be synthesizing a complementary a strand that's three prime to five Prime what happens to this lagging strand all right well the lagging strand it's synthesized differently and so we can move into our fifth step for that in terms of the lagging strand it's gonna involve multiple RNA primers all right it's going to involve multiple RNA primers as DNA helicase unwinds the DNA DNA primase is going to create an RNA primer near the replication fork all right DNA polymerase can create a new strand of DNA using the the that primer in five Prime to three prime however as helicase continues to unwind the strand of DNA DNA primase has to create a new RNA primer once again all right DNA polymerase 3 once again creates a new strand of DNA in the five Prime to three prime until it reaches the fragment of DNA it created before in this way the lagging strand is created in fragments known as the okazaki fragments all right so an RNA primer is made for the lagging strand and DNA polymerase then extends the Strat but it happens in short fragments all right so for so here in the sixth strip for the lagging strand DNA synthesis is really discontinuous and it requires multiple RNA primers DNA is synthesized at the three prime end of each primer generating these okazaki fragments that grow until they meet adjacent fragments the RNA primer is then removed and replaced with DNA by DNA polymerase one all right so for the leading strand when we create a complementary stand it happens all out once using DNA polymerase all right as for the lagging strand here which is five Prime to three prime well you're going to have to create little segments one at a time and you can have need a lot of RNA primers to create those fragments that are then all connected together where DNA polymerase 1 does that by removing the primer at the very end now the last step here is that DNA ligase links together adjacent okazaki fragments here after then the DNA ligase polymerase 1 polymerase 3 DNA primase DNA helicase and DNA gyrase all work simultaneously in the vicinity of this replication fork to create two new dnas all right each new DNA has a old Strand and a new strand from the original DNA all right so for that last piece of information DNA ligase all right links together adjacent okazaki fragments and so you can see how all these enzymes work together in the vicinity of that replication fork to generate now two new DNA strands all right with that we end our chapter all right so we finish chapter 16. let me know if you have any questions comments concerns down below other than that I will see you in the next chapter good luck happy studying and have a beautiful beautiful day