The sperm and ovum fuse to form a single cell called zygote and this zygote has the potential to form a complete organism with multi trillion cells.
The sperm and ovum consist of how many chromosomes? 23 chromosomes. 23 chromosomes from sperm and 23 chromosomes from ovum and when they combine they form a zygote that consists of 46 chromosomes. From zygote, with each cell division 1 to 2, 2 to 4, 4 to 8 and so on many different cells are formed until a complete organism is formed. And interestingly each cell of a human being consists of 46 chromosomes. So from zygote onward the chromosome number remains same that is 46. So from zygote the cell division that leads to formation of same number of chromosome that is called mitosis. The cell division that leads to formation of sperm and ovum is called meiosis and in meiosis number of chromosomes is decreased.
The cell from which male and female gametes they are formed that cell consists of 46 chromosomes but these gametes consists of 23 chromosomes i.e. 1/2 number of chromosomes. But from zygote onwards whatever there is cell division that leads to formation of cells that consist of same number of chromosome and this type of cell division is called mitosis.
Two types of cell divisions are there
1-Mitosis that occurs in somatic cell (all of the bodily cells)
2-Meiosis it occurs only in gametes (the sex cells)
Mitosis consists of two parts
The main part is division of nucleus. So let’s first discuss about that. Do you know what is the length of DNA? The DNA of single cell is about 1.8 meter long. And do you know what is the size of an average human cell? That is 40 to 50 micrometer and such a long DNA is packed inside not only that's so much small cell but a part of that cell, it is packed inside nucleus of that very small cell. How DNA is packed inside the nucleus of a cell? Let's discuss about that. DNA consists of nucleotide bases. The code of our all the proteins of our body that is present inside the DNA and it is in the form of nucleotide sequence.
The double-stranded DNA is coiled around each other to form a double helical structure. Then this double helical DNA it interacts with histone proteins. Histones are positively charged proteins because they have abundant amount of basic amino acids. And DNA is negatively charged because of phosphate groups. The negatively charged DNA interacts with positively charged histone proteins. Eight histone proteins form an octamer and DNA is woven around this histone octamer. And this histone complex with DNA around it is called nucleosome. This nucleosome complex is then further coiled around itself to form the structure that is called chromatin.
Then this chromatin is further bound around each other to form a three-dimensional geometry called chromatid. So what is chromatid? Chromatid consists of chromatin that is further coiled around each other and there are certain proteins that hold up this structure and these proteins are called scaffolding proteins. And it forms this beautiful three-dimensional structure called chromatid. And this is centromere.
We will discuss later that what is chromosomes? So here is a cell and in the nucleus of cell DNA is present in the form of chromatin and that is before the division starts.i.e. mitosis start. When mitosis start it is further packed up in the form of chromatid. At first it was in the form of chromatin, loosely arranged but then when cell division starts it is packed up in the form of chromatids.
That is just like when you have to carry some luggage from one place to another you have to further pack it up you cannot take it away in loosely arranged fashion. You have to pack it up in briefcase or something. That's why it is further packed up so that it becomes easier to divide it. So this first phase of mitosis in which DNA is further condensed or packed up this is called Prophase. P for packed P for Prophase. Nuclear membrane starts to disappear so that division of DNA becomes much easier.
We have 46 double helical DNA molecules arranged in the form of chromatids in each cell. But here only 4 are shown representing 46 chromatids . Now let's suppose if we divide these chromatids that half goes to one cell and the other half goes to other cell now each cell will have half number of chromatids or half number of nuclear material or half number of DNA and that's not how mitosis should occur. How we can solve this problem?
Actually before prophase of mitosis starts there is an other phase in which cell prepares itself for mitosis that is called interphase.
And in interphase these chromatids replicate and a copy of this chromatid is formed in front of it so that it can be easily divided. Before mitosis start in the interphase exact copy of this DNA is formed in front of it by the process of DNA replication. And this DNA copies again but they're coiled, interacts with histones and coiled to form chromatin. And this copy of chromatin in prophase again it is condensed to form two sister chromatids, two chromatids that are exactly same. you don't exactly resemble your sister but for the DNA, for the chromatids, their sister resembles each other. Things are different at molecular level. And two sister chromatids are attached with each other at centromere and this whole structure is called chromosome.
So in prophase there are two sister chromatids not just one and these are paired together to form a chromosome and this is prophase. Remember, DNA is not replicated in prophase. DNA is replicated even before prophase starts in interphase, specifically S phase, synthesis phase of interphase.
In next phase chromosomes are arranged on midline and they form a plate like structure and this is called mitotic metaphase plate or metaphase plate. So this phase in which chromosomes are arranged on midline it is called metaphase and M for midline M for metaphase.
Then there are special organelles called centrosomes and from centrosomes special fibers originate and they attach to chromosome at centromere, and not exactly at the centromere but it is a part of centromere to get hooked with a special protien called kinetochore. Why it is called kinetochore? By kineto means to move and core means chromatids. This kinetochore helps to move the chromatid away from each other so it is called kinetochore. And these fibers which attach to centrosome add kinetochore they are called kinetochore fibers. Then there are the fibers that are not attached with Kinetochore and they are called non-kinetochore fibers or polar fibers and both of these are spindle fibers.
Then kinetochore fibers shorten in length. And when they shorten they pull chromatids away from each other with the help of hook or kinetochore. And non kinetochore fibers or polar fiber elongate due to lengthening of non-kinetochore fibers, cell also elongate. And this phase is called anaphase. And in this phase chromatids move away from each other. A for away and A for Anaphase.
Chromatids ultimately reach their final destination with a splendid hard work of the spindle fibers. These sisters are now separated and this happens in telophase. Chromatids are now far away they can only see each other with telescopes. Of course, they do not have some molecular telescopes inside the cell but that is just a mnemonic, don't tell anyone that they are carrying the telescope inside the cell. The nuclear membrane also reappears. And that is about the telophase, the last phase of nuclear division of mitosis.
Now what is cytokinesis? It is division of cytoplasm, organelles and plasma membrane. So in cytokinesis there is formation of special contractile ring in the cytoplasmic side of cell. It consists of actin and myosin proteins. And it leads to formation of cleavage furrow and it will ultimately lead to division of cytoplasm, organelles and plasma membrane into two and as a result two daughter cells from one cell are formed and each daughter cell has equal number of DNA.
Of course, they have chromatids but before next mitosis start in the interphase they will form another chromatid in front of it to form a complete chromosome. yes with each cycle of mitosis the number of organelles are also reduced to half then what happens in the subsequent divisions to organelles. Will they be vanished out?
The organelle will not vanish. Actually in interphase not only DNA is replicated but organelles are also copied they are also replicated so that we have a copy usually more than one copies of organelles, each organelle so that with the cell division the organelle is are distributed equally to daughter cells. In interphase many different things occur there like DNA is replicated, organelles are replicated and cell prepares itself for cell division for mitosis.
What do you think spindle fibers are formed in which phase? Metaphase?
No, actually spindle fibers, they start to form in prophase but they attach with kinetochore and they, they complete, they undergo the complete synthesis in metaphase. The spindle fibers start to form in prophase.