That's too much to explain in a single post allowed here.
MEIOSIS and MITOSIS
Meiosis and mitosis are the two types of cell division. Technically, they are forms of nuclear division, but they are usually coupled with cytokinesis -- division of the cytoplasm. Meiosis produces gametes (sperm and eggs) whereas mitosis produces somatic (body) cells.
Mitosis involves a SINGLE ROUND of division and produces TWO daughter cells that are DIPLOID and are GENETICIALLY IDENTICAL to each other and to the parent cell. Crossing over does NOT occur.
Meiosis involves TWO ROUNDS of division and produces FOUR daughter cells that are HAPLOID and are GENETICALLY DIFFERENT from each other and from the parent cell. Crossing over DOES occur.
Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase.
1. Prophase:
(1) Chromatin condenses into visible structures called chromosomes: each chromosome consists of two genetically identical sister chromatids joined together at their centromeres
(2) Microtubules project from the cell's 2 centrosomes, the beginning of the formation of the mitotic spindle. Later in prophase, microtubules called spindle fibers (or kinetochore microtubules) attach to the centomeres of sister chromatids at disks of proteins called kinetochores
(3) The cell's two centrosomes migrate away from each other towards opposite poles of the cell, possibly being pushed apart by their interacting microtubules
(4) The nucleolus (or nucleoli) disappear.
(5) The nuclear envelope completely disassembles
2. Metaphase
Kinetochore microtubules from opposite poles push and pull on the sister chromatids' centromeres, leading to the chromosomes being lined up at the equator of the cell (at the metaphase plate).
3. Anaphase
(1) The centromeres of the sister chromatids of each chromosome separate and the two sister chromatids are freed, each becoming a chromosome. The sister chromatids (each a chromosome now) are pulled towards opposite poles of the cell by the kinetochore microtubules.
(2) The cell elongates. Microtubules from the centrosomes that were attached not to kinetochores of chromosomes, but rather to microtubules from the other centrosome at the opposite pole, are called polar microtubules: the polar microtubules push against one another causing the cell to elongate.
4. Telophase:
The steps of telophase are essentially the reverse of those from prophase.
(1) Chromosomes decondense back into chromatin.
(2) The mitotic spindle is disassembled.
(3) The cell now has only 1 centrosome.
(4) The nucleolus (or nucleoli) reappear.
(5) The nuclear envelope is reassembled.
CYTOKINESIS
Starting in mid-to-late anaphase, the cell itself (not just the nucleus) begins to divide. It is cytokinesis that actually forms 2 cells from 1.
In animal cells, a contractile ring of proteins forms just under the plasma membrane, encircling the position of the earlier metaphase plate. The ring contracts and creates a depression in the plasma membrane called a cleavage furrow. Continued contraction of the ring of proteins leads to continued deepening of the cleavage furrow until the cell is pinched in two.
Meiosis consists of two rounds of division, called meiosis I and meiosis II. Each round contains the same 4 above stages, so meiosis consists of:
prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, and telophase II.
An important process called crossing over occurs during prophase I. Homologous chromosomes, each consisting of two genetically identically identical sister chromatids, pair up along their lengths -- a process called synapsis -- to form what are called either bivalents (they consist of 2 chromosomes, bi = 2) or tetrads (they consist of 4 chromatids, tetra = 4). Then corresponding segments of non-sister chromatids are exchanged. This increases genetic variation in the population.
DNA and its role as hereditary information
The DNA of each somatic (body) cell contains the complete genetic information of an entire human. However, a different collection of genes are turned or turned off in each different type of cell (neuron, white blood cell, kidney cell, etc.).
REPLICATION: DNA is replicated before cell division (in what is known as the S phase of interphase) to make sure that each daughter cell has a complete copy of all the parent cell's DNA.
TRANSCRIPTION: In the trascription of a gene, one section of one DNA strand is copied by an enzyme called RNA polymerase into a complementary RNA molecule (rRNA, tRNA, mRNA, microRNA, etc.). mRNA is messenger RNA and carries the sequence information for a protein from the nucleus (where DNA is stored) to the ribosomes in the cytoplasm.
TRANSLATION: The three main types of RNA cooperate to make a protein. The rRNA (ribosomal RNA) is part of the ribosome, and the tRNAs (transfer RNAs) bring the appropriate amino acids to the ribosome, as specified by the mRNA codons (a codon is a series of three consecutive mRNA bases).
Thus, the base sequences in DNA specify the order in which amino acids are strung together to form all of the particular proteins of all cell types of a human.
DNA transmits this information from one generation to the next. Meiosis produces eggs (in women) and sperm (in males). Both have only 23 chromosomes. When a sperm fertilizes an egg, the resulting cell -- called a zygote -- again has a full 46 chromosomes: all the information needed to produce an adult human.
For cell organelles, try running through this tour of a eukaryotic cell (note: it shows a plant cell, with cell walls and chloroplasts, which human cells don't have).
http://bcs.whfreeman.com/thelifewire/content/chp04/0402001.html
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