what is the difference between dna and chromosomes a chromosome is in fact a dna molecule and in prokaryotes such as bacteria dna is basically the same thing as a chromosome however in eukaryotic cells the dna inside our nuclei is a really big jumble on the right is a reconstruction of the chromosomes in a yeast cell and you can see how jumbled up and intertwined the chromosomes are if you took the dna in a human cell and stretched it out like a string that string would stretch for three meters considering that a typical cell is only 10 microns wide the dna has been compressed by a factor of one is to three hundred thousand so you have this big jumble of dna in in the nucleus but when the cell divides you want to be able to organize this mess and reliably segregate the dna so that each daughter cell gets an equal amount of dna and that's why um the dna in eukaryotic cells doesn't exist in isolation it is in fact in complex with a set of proteins called histones and the dna is wrapped around the histones and this complex of dna and histone proteins organizes itself into more complicated structures which we will learn about later on in the course together this complex is called chromatin and it is essential for many many biological processes including cell division and gene expression and therefore in eukaryotic cells the difference between dna and chromosome is that chromosome is dna plus other proteins that organize the dna in the space inside the nucleus what do the terms haploid and diploid mean many organisms such as bacteria but also life stages in in organisms such as the red blood red bread mold neurospora as well as our gametes human gametes have one of each type of chromosome in the cell for example you may have a long chromosome and a short chromosome and therefore this cell has two types of chromosomes and the cell is said to have a haploid number of two humans and human gametes have 23 chromosomes and therefore humans are said to have a haploid number of 23. but other life stages in sexually reproducing organisms such as humans and flowering plants namely for humans and flowering plants it's the somatic cells the cells of our body that have not one but two of each type of chromosome and i'm going to show the other homolog of each type of chromosome in yellow they are called homologues because the two chromosomes of each type they are similar to each other but not identical one of the homologs comes from the father and so is the paternal derived chromosome and the other homolog comes from the mother and is the maternal derived chromosome so this diploid cell then has a total of four chromosomes so the haploid number is two and the diploid number is four how does chromosome number change during the life cycle of a sexually reproducing organism such an organism may start life out as a single cell zygote and the zygote will be deployed so it will have two n chromosomes where n is the haploid number of the organism now the zygote is going to go undergo mitosis repeated cell division until you get an adult individual and we'll talk about it in a bit but during mitosis um cell division the dna is copied and therefore the number of chromosomes on the amount of dna does not change in the cell and therefore this adult is also diploid and has two n chromosomes then this adult is going to undergo meiosis and one of the most important function of meiosis is to half the number of chromosomes so you may get sperm you may get eggs but these gametes are going to be haploid and therefore they will only have n chromosomes and then these gametes are going to fertilize undergo fertilization and each gamete brings with it in n chromosomes and therefore in the zygote you end up with two n chromosomes and evidently for each pair of homologous chromosomes in the zygote one of the pair came from the egg and the other came from the sperm and that's why this is the paternal and the other one is the maternal homolog of a pair of homologous chromosomes and therefore in the life cycle of a sexually reproducing animal there are two key steps that change chromosome number during fertilization by definition since fertilization involves the fusing of the sperm and the egg there is a doubling of chromosome number from n to 2n and consequently there is a necessary need for a halving of chromosome number from 2 and 2n therefore fertilization and meiosis serve opposing roles so that fertilization doubles the number of chromosomes and there is a necessity for meiosis to halve the number of chromosomes and i would like you to imagine what would happen if there was no meiosis to halve the number of chromosomes when you produce gametes yet you had fertilization that doubles the number of chromosomes dividing cells undergo um the cell cycle and there are four phases of the cell cycle the s phase and the s stands for synthesis this is when the dna is replicated in the cell in preparation for cell division there is a necessary need to replicate the dna since if the cell divides without this replication the daughter cells will have less than their full complement of dna and would not be able to survive then there's the m phase where the m stands for mitosis or meiosis and then there are two gaps g1 say gap and g2 which also stands for gap and the cell cycle proceeds in the clockwise direction going from synthesis or doubling of the dna copying of the dna to g2 or gap phase two to mitosis or meiosis and then the first cap g1 and then returning back to the s phase and a further division is that everything other than mitosis is called interphase now the role of meiosis is to half the number of chromosomes however given the constraints of the cell cycle this happens in a roundabout manner in two stages meiosis one and meiosis ii in meiosis one the cell is still in the cell cycle and it undergoes dna replication in the preceding interface in the interface that precedes the meiosis and therefore for a diploid organism if you started out with two n chromosomes where n is the haploid number of chromosomes you will end up at the start of meiosis a start of m phase with four n chromosomes and then in meiosis one the um the cell divides in two and what that means is that the daughter cells at the end of meiosis one have two n chromosomes once again and we can look at the number of cells so if you started with one cell you will end up with two cells at the end of meiosis one the cell proceeds straight to meiosis ii and there is no dna replication during meiosis 2 there is cell division and therefore the number of chromosomes goes down or halves from 2n to n giving you a haploid gamete and you started out with two cells and you divided each cylinder two therefore you end up with four gametes so every myocyte generates four gametes let's take a cell with a haploid number of one and therefore it the cell has one type of chromosome and it's a diploid cell so therefore there are two homologues of this chromosome perhaps this is from the mother and the other one is from the father now during interface during the cell cycle in the interface in the s phase the dna is going to be copied and therefore we're going to end up with two doubling the number of chromosomes and therefore this cell had two n chromosomes and now we have four n chromosomes and n is is equal to one during prophase one these chromosomes are going to condense into chromatids and the two copies resulting from dna replication are called sister chromatids so these two are sister chromatids and perhaps the one on the left came from this copy of the homologous chromosome the other one came from this copy of the homologous chromosome this dot in the middle is the centromere and this is where the kinetochore is going to form so that the chromosomes can be pulled by the spindle apparatus to the opposite poles of the cell during anaphase and similar to the paternal chromatids the maternal chromatids would also be condensed and the two copies would be called sister chromatids now this pair or each pair of sister chromatids is called a diad and there are two diads here one from the [Music] maternal chromosomes and the other from the paternal chromosomes now the two diets are not floating floating around freely in fact in prophase one the two diodes are attached to each other through a complex of proteins called the synaptonemal complex and the importance of the synaptonemal complex or attaching the two diats to each other is to make sure that during metaphase one each diad aligns on the opposite side of the metaphase plate so that eventually each daughter cell of meiosis one and therefore each gamete in um in at the end of meiosis ii receives one of each type of chromosome the chromat the chromatids coming from the paternal and the maternal chromosomes that is the chromatids that are not the result of dna replication are referred to as non-sister chromatids this whole unit of four chromatids is called a bivalent and finally the four chromatids together are referred to as a tetrad with all this terminology out of the way let's see what happens in metaphase one and keep in mind that the number of chromosomes hasn't changed in prophase it's still 4n and so it will be in metaphase one what happens in metaphase one is that each diad aligns on one side of the metaphase plate so that at the end of meiosis one one pair of sister chromatids are in one daughter cell and the other pair of sister chromatids are in the other daughter cell and since cytokinesis has taken place the number of chromosomes has reduced from 4n to 2n and i will not go into all the details of meiosis 2. but the bottom line for meiosis 2 is that the sister chromatids get separated and therefore two gametes get the paternal homolog of the chromosome while two gametes get the maternal homolog of the chromosome and since there's cytokinesis or cell division during meiosis ii number of chromosomes halves so that you end up with gametes having half the number of chromosomes n chromosomes which corresponds to the haploid number for the organism the final point that i would like to underline here is that meiosis underlies mendel's law of equal segregation there is segregation happening or separation of the chromosomes into the gametes since half the gametes get the paternal chromosome and half the gametes get the maternal chromosome furthermore it's clear that after every meiosis or every myocyte gives two gametes that contain the paternal chromosome and two gametes that contain the maternal chromosome and therefore the segregation is equal so that there are equal numbers of camets that carry the paternal or the maternal chromosome