Mitosis
- Mitosis is the division of a cell's nucleus, and ends in the creation of two daughter cells.
- A cell nucleus contains 49 chromosomes grouped into 23 pairs, one from each parent (Diploid cells means 2 sets of genetics).
- Mitosis takes place in 4 stages, Prophase, Metaphase, Anaphase and Telophase. The phase Cytokinesis works along side Telophase for the final split, but is actually a phase of its own.
- Cells start in Interphase (meaning in between cycles of Mitosis), woking to keep the body and organs functioning.
- During the Interphase, coils of DNA float around loosely in the nucleus of cells.
- When cells become damaged, chemical signals from a protein called Cyclins act like a switch to start the replication (mitosis) process (This signal also works to tell the mitosis process to stop).
- When cells receive the chemical signal from Cyclins, the protein around the nucleus of the cell called Centrioles (Centrosome) duplicate into two.
- The Centrioles (centrosome) regulate the mitosis process in the nucleus.
- The DNA in the nucleus also start to replicate themselves, crating two sets of each pair of chromosome.
The cell then moves into the 4 stages of replication.
Prophase
- This is the longest stage in Mitosis, the DNA (Chromatin) coils up thicker and becomes visible chromosomes. (Normally, Chromosomes in the nucleus is not visible, not even under a telescope. That is until the Prophase starts).
-The duplicated copies of chromosomes stay attached to the original copies.
- The two copies (one left arm and leg, the other the right) make the X-formation of the chromosomes, held together by the centromere. These arms are known as Chromatids.
- The nucleus (the nucleolus makes proteins called rRNA, and sends these out to the cell to make complete Ribosomes) membrane disappears and long microtubule spindles start to form from Centrioles (centrosome) as they move into position at opposite ends of the cell (also known as poles).
- These spindles of microtubules also help to create a structure to the cell.
- These spindles are a essential part of moving chromosomes and the separation on the sister chromatids.
Metaphase
- As metaphase starts, the centromeres of the chromosomes start to line up along the metaphase plate (the middle of the cell) by motor proteins.
- There are two motor proteins on each side of the centromeres, and are known as Centromere Associated Protein E.
- These proteins attach to the microtubule spindles to help stop slack and gain correct positioning.
- Dynein protein works the other end of the microtubular spindle to reduce slack from near the cell membranes.
- The Dynein protein work together at opposite ends of the cell with the microtubular rope, releasing a chemical signal when one becomes too close to the cell wall and promotes the dynein protein the other side to start pulling.
- This push and pull between the Dynein protein ensure the correct positioning of the chromosomes through the middle of the cell.
- The correct positioning of the chromosomes centromere helps to ensure correct chromosome separation.
Anaphase
- This is the phase where the sister chromatids are split up.
- The motor proteins start to pull on the spindle microtubules.
- The forces causes the centromere to split into two, creating two sets of single identical chromosomes.
- The spindles then pull these newly created single chromosomes (daughter chromosomes) to the opposite ends of the cell towards the centrioles (centrosomes).
- At the end of the Anaphase, the spindles degrade and each end of the cell (pole) has a complete set of chromosomes identical to the original DNA at the beginning of the cell cycle,
Telophase
- This is the phase where all the reconstruction begins for the new cell.
- Firstly the chromosome begin to unwind back into chromatin and direct in the cell's metabolic activities.
- A new nucleus wall forms around each new set of chromosomes.
Cytokinesis
- Although this phase works alongside Telophase, it's actually a phase all of its own and is the final step.
- A small creases forms inwards along the equator of the plasma membrane between the two new cells.
- This is called "Cleavage", and the two new cells separated by the nuclei moving away from each other.
- The two new cells contain a complete set of 46 chromosomes and are known as Daughter cells.
- These daughter cells are identical and complete genetic copies to each other and their parent cells.
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Meiosis
- Meiosis happens during fertilization, and is the joining of haploids (singles set unpaired chromosomes) into a diploid cell (complete set of chromosomes).
- Meiosis is the halving of chromosomes, ending with four haploid cells.
- Meiosis only happens in Gametes producing cells (male or female sex cells).
- These germ cells develop into sperm (spermatocyte) or egg cells (oocytes), and are the only cells that undergo Meiosis.
- Meiosis starts following one round of DNA replication ins the female or male sex organs.
-Before Meiosis begins, the cells DNA is replicated and creates chromosomes with two sister chromatids.
- Meiosis involves two nuclear and cell divisions without a interphase, so DNS is not replicated before the second round of division.
- Each division has 4 stages, named Meiosis 1 and 2. Each stage has a Prophase, Metaphase, Anaphase, Telophase and ending with the cytokinesis.
- Before Meiosis, a human cell will have 46 chromosomes, 22 pairs of homologous autosomes (Autosomes are chromosomes, homologous are pairs of autosomes that have the same genes but different alleles) and 1 pair of sex chromosomes.
- Humans have either a X (XX = Female) or Y ( XY = Male) sex chromosome.
- These chromosomes are inherited from the parents, one from the father and one from the mother.
Meiosis I
* A pair of Homologous Chromosomes contains chromosomes of similar length, gene position and centromere location. These are made of two chromosomes, one inherited from the father, and one from the mother*
- Known as the reductional division stage, meiosis I separates homologous chromosomes and makes two haploid cells.
- The original diploid cell has 23 pairs of chromosomes (46 chromosomes intotal), each having a sister chromatid joined to the centromere.
- Once cytokinesis is complete, the end result is 2 haploid cells with 23 chromosomes (although each still has a sister chromatid).
- The goal of Meiosis I is to separate homologous pairs of chromosomes and produce haploid cells.
Prophase I
- This is the longest stage in meiosis.
- Like in Mitosis the nuclear membrane breaks down, the chromatin condenses into chromosomes, the centrioles migrate to the correct position and start to form spindles on movement.
- However, what is different is the homologous chromosome line up gene for gene down the tetrads (groups of 2 homologous chromosomes) to allow the gene cross over to occur.
- This gene crossover process is known as Homologous recombination and creates genetic variation.
- Leptotene is the first step in Prophase I, and allows the condensation of chromatin into visible chromosomes found in the nucleus.
- Each chromosome has two sister chromatids, and has a homologue (allows recombination) present in the nucleus.
- This is where Synaptonemal Complex (the formation of the two parallel steps like structures form between the homologous chromosomes) begins.
- The Zygotene phase comes after. This is where Synapsis starts and is the process where chromosome pairs with their homologue, forming pairs. The pairing is specific and exact.
- The chromosome pairs are called ether Bivalent (a group of two chromosome) or Tetrad (A group of four sister chromatids) chromosomes.
- Pachytene stage is where the the genetic recombination occurs, and brings the formation of new Alleles patterns. At the cite of recombination, Chiasmata form,
- Diplotene this is where the synaptonemal complex degrades and the chromosomes start to disassociate, although still bound by the areas of recombination (Chiasmata).
- Lastly, Diakinesis is the further condensing of chromosomes.
Metaphase I
- This is the phase where the 23 pairs of chromosomes line up along the central plate of the cell.
- Chromosome are helped into position by the centroli spindles and attach themselves to the kinetochore and the centromere.
Anaphase I
- In this phase, the spindle fibers shorten and pull the chromosomes apart. This also severs the Chiasmata.
- The split up chromosomes are then pulled towards the poles at opposite sides of the cell, resulting in two 23 chromosome sets at each pole.
- The sister chromatids are still attached at the centromeres.
Telophase 1
- This phase undergoes reduction division sequence and creates two haploid cells.
- Each chromosome has a pair of sister chromatids, although one has undergone recombination so they are no longer identical.
- The spindle fibers disintegrate and the nucleus wall starts to form.
- Cytokinesis begins here and the cell separate by moving away from each other, cleavage separates the two.
- This results in two haploid cells.
*During this period, the cells may go into a rest period called Interkinesis (Interphase II) or start Meiosis II. No DNA replication occurs at this point.*
Meiosis II
After the occurrence of Cytokinesis, the end result is 4 haploid cells containing the information of 23 chromosomes. When two of these cells join in fertilization, the newly formed Diploid Zygote has the correct amount of DNA similar to mitotic cell division.
Prophase II
- Nucleus membrane disappears.
- Chromatin condense down into chromosomes.
- Spindles form from the centrioles as they move into place at opposite sides of the cell wall.
Metaphase II
- Spindle fibers align the 23 chromosomes along the equator of the cell (each contain two chromatids).
- In Metaphase II, the metaphase plate is rotated 90 degrees and is perpendicular to the previous plate.
Anaphase II
- Seperation of the chromatids start here, similar to Mitosis,
- Sister Chromatids are pulled in half from the centromere and moved to opposite poles based at the cell wall.
- This results in 23 chromosomes at each pole of the cell, containing half as much genetic material as at the start of anaphase II.
Telophase II
- The nucleus reforms and the spindle fibers break down.
- The chromosomes uncoil and create chromatin.
- The new nucleus then undergo Cytokinesis and produce four haploid cells though cleavage.
- Each cell has its own unique combination of genes and chromosomes.
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