 remind yourself of what a chromosome is it's nothing more than one long string of DNA a chromosome is one molecule of DNA I call it like I mean it's it's a long it's a string of DNA and technically this is going to be important to us at some point the DNA actually winds around special proteins called histone proteins so the molecule of DNA actually like loops around these histone proteins it can loop around these proteins and they're like organizing structures like they help the DNA not get super tangled in the nucleus imagine I mean there's 46 molecules in almost all of your cells 46 molecules that make up six feet if you unwound all the DNA you could have a six foot tall that's the size of my kids a six foot tall piece of DNA in one of your cells and you have a hundred trillion cells like this is sort of a mind-boggling concept of how much DNA is actually found in your nucleus and organized into chromosomes thank you histone proteins for helping us do this while we're talking about the histone proteins we can wind the DNA can wind loosely oh my gosh is that how you spell loosely or tightly and if it is loosely wound it actually can end up looking like a tangle the DNA plus the histone protein is called chromatin and so you can have loosely wound chromatin or tightly wound chromatin so we must look at yarn I when I have spare time I like to knit I don't have a lot of spare time so I don't do it very often do you agree that this like when you buy a log of yarn you could unwind this whole log of yarn and it would be one long string this is tightly wound DNA tightly wound chromatin and I don't know I think of it as if I wound up my DNA I think of it as looking something like this and I call it a chromosome if ha this is actually representing a chromosome say chromosome number one from an egg parent and chromosome number two I mean chromosome number one same chromosome right it's the exact same thing it's just a different color from sperm parent do you remember what those were called the chromosomes that are the same I didn't draw that very well that's supposed to be exactly the same but ones from sperm parent ones from egg parent do you remember what those were called those are called homologous chromosomes I think this is probably maybe review I can't remember do you agree that we could unwind these tightly wound tight tight tight oh loosely wound this is loosely wound tangled mess look do you agree that I just drew exactly the same oh I'm not gonna draw them I'm gonna draw them so that and of course I do the whatever do you see what I'm doing loose tight tight tight now my children who I just said were now six feet tall when they were not six feet tall and they were cute and little they took this and they unwound it all and they turned it this and if I mean I bet now hmm this could be an interest I wonder what I'd have to give them to have them take this and wind it back up could they do it yes would they do it highly unlikely I when I see the tangle I call the tangle chromatin and I call the little logs chromosomes but inside the tangle are chromosomes and that's the truth like it's the same thing it just matters how tightly wound around the histone proteins the DNA is okay I do want you to label I drew little center bubbles on my chromosomes and those are important structures that help organize the chromosomes when we get ready to divide them and that's called the centromere homologous chromosomes everybody has a centromere in fact you can count centromeres to figure out how many chromosomes you have and then remember that we can actually visualize all these chromosomes in a karyotype this is what your chromosomes look like these are human chromosomes and you can see that there are 23 pairs of chromosomes for a total of 46 chromosomes this these chromosomes are in log form I do have a picture here of the chromosomes in not log form hold on a minute I have to make that go away you can't see it yet but there it is for real that right there is a microscope image of DNA in tangled chromatin form do you see the resemblance do you see why I was like dude and then hold on hold your horses can you imagine trying to divide the DNA if it was in that like this format are you kidding me can you imagine like trying to make sure make a copy of all the DNA and then try to make sure that you get it equally divided no honey that's like not ever gonna work for you you you'll be much better off if you instead go with this strategy put it into a log it's why it matters that's why we're paying attention to what our chromosomes are named okay I think I've told you everything that you need to know and I think we're ready to look at the cell cycle because the DNA changes its form it goes back and forth between loose chromatin form and tight chromatin form between chromatin the tangle and chromosomes the logs let's see if we can figure out when and why it would go back and forth like that