🔹Many damaged, dead, and worn out cells can be replacedby growth and division of other similar cells.
🔹The frequency with which cell division occurs varies with different types of tissue.
This is normally carefully
regulated to allow effective maintenance and repair ofbody tissues.
🔹At the end of their natural lifespan, ageing cells are programmed to ‘self destruct’ and their components are removed by phagocytosis; a process known as apoptosis.
🔹Cells with nuclei have 46 chromosomes and divide by mitosis, a process that results in two new genetically identical daughter cells.
🔹 The only exception to this is the formation of gametes (sex cells), i.e. ova and spermatozoa,which takes place by meiosis.
🔹The period between two cell divisions is known as the cell cycle, which has two phases that can be seen on light
microscopy: mitosis (M phase) and interphase.
Prophase
🔹Prophase possesses over portion of mitosis. The atomic layer separates to shape various little vesicles and the nucleolus breaks down. A design known as the centrosome copies itself to shape two little girl centrosomes that relocate to furthest edges of the cell. The centrosomes sort out the creation of microtubules that structure the shaft filaments that comprise the mitotic axle. The chromosomes consolidate into minimal designs. Each reproduced chromosome would now be able to be believed to comprise of two indistinguishable chromatids (or sister chromatids) held together by a design known as the centromere.
🔹Prometaphase
The chromosomes, driven by their centromeres, move to the tropical plane in the mid-line of the cell - at right-points to the pivot shaped by the centrosomes. This locale of the mitotic shaft is known as the metaphase plate. The shaft filaments tie to a construction related with the centromere of every chromosome called a kinetochore. Singular axle filaments tie to a kinetochore structure on each side of the centromere. The chromosomes keep on gathering.
🔹Metaphase
The chromosomes adjust themselves along the metaphase plate of the axle mechanical assembly.
🔹Anaphase
The briefest phase of mitosis. The centromeres partition, and the sister chromatids of every chromosome are pulled separated - or 'detach' - and move to the far edges of the cell, pulled by axle strands joined to the kinetochore locales. The isolated sister chromatids are currently alluded to as girl chromosomes. (It is the arrangement and division in metaphase and anaphase that is significant in guaranteeing that every girl cell gets a duplicate of each chromosome.)
🔹Telophase
The last phase of mitosis, and an inversion of a considerable lot of the cycles saw during prophase. The atomic film changes around the chromosomes gathered at one or the other shaft of the cell, the chromosomes uncoil and get diffuse, and the axle filaments vanish.
🔹Cytokinesis
The last cell division to shape two new cells. In plants a cell plate structures along the line of the metaphase plate; in creatures there is a tightening of the cytoplasm. The cell at that point enters interphase - the stretch between mitotic divisions.
🔹Meiosis
Meiosis is the type of eukaryotic cell division that produces haploid sex cells or gametes (which contain a solitary duplicate of every chromosome) from diploid cells (which contain two duplicates of every chromosome). The interaction appears as one DNA replication followed by two progressive atomic and cell divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is gone before by an interaction of DNA replication that changes over every chromosome into two sister chromatids.
🔹Meiosis I
Meiosis I isolates the sets of homologous chromosomes.
In Meiosis I an uncommon cell division decreases the cell from diploid to haploid.
🔹Prophase I
The homologous chromosomes pair and trade DNA to shape recombinant chromosomes. Prophase I is partitioned into five stages:
🔹Leptotene: chromosomes begin to consolidate.
🔹Zygotene: homologous chromosomes become firmly related (synapsis) to frame sets of chromosomes (bivalents) comprising of four chromatids (quadruplicates).
🔹Pachytene: getting over between sets of homologous chromosomes to shape chiasmata (sing. chiasma).
🔹Diplotene: homologous chromosomes begin to isolate however stay joined by chiasmata.
🔹Diakinesis: homologous chromosomes proceed to isolate, and chiasmata move to the closures of the chromosomes.
🔹Prometaphase I
Shaft mechanical assembly framed, and chromosomes connected to axle filaments by kinetochores.
🔹Metaphase I
Homologous sets of chromosomes (bivalents) masterminded as a twofold line along the metaphase plate. The plan of the matched chromosomes as for the shafts of the axle device is arbitrary along the metaphase plate. (This is a wellspring of hereditary variety through arbitrary arrangement, as the fatherly and maternal chromosomes in a homologous pair are comparable however not indistinguishable. The quantity of potential plans is 2n, where n is the quantity of chromosomes in a haploid set. Individuals have 23 distinct chromosomes, so the quantity of potential mixes is 223, which is more than 8 million.)
🔹Anaphase I
The homologous chromosomes in each bivalent are isolated and move to the contrary posts of the cell
🔹Telophase I
The chromosomes become diffuse and the atomic film changes.
🔹Cytokinesis
The last cell division to frame two new cells, trailed by Meiosis II. Meiosis I is a decrease division: the first diploid cell had two duplicates of every chromosome; the recently framed haploid cells have one duplicate of every chromosome.
🔹Meiosis II
Meiosis II isolates every chromosome into two chromatids.
The occasions of Meiosis II are similar to those of a mitotic division, albeit the quantity of chromosomes included has been split.
🔻Meiosis creates hereditary variety through:
the trading of hereditary material between homologous chromosomes during Meiosis I
the arbitrary arrangement of maternal and fatherly chromosomes in Meiosis I
the irregular arrangement of the sister chromatids at Meiosis II
