EFB325 Cell Physiology

Focus on the M phase

• M phase includes mitosis (nuclear division) and cytokinesis (cell division)

Mitosis and meiosis

Mitosis is the process of division in eukaryotes, which yields daughter cells that are genetically identical to the original cell. Remember that most of the cells in the plant or animal body are diploid, which means that they have two similar versions (or copies) of each chromosome=homologous chromosomes. Humans have 23 pairs of chromosomes, which includes the sex chromosomes, where the X and Y chromosomes are quite different from each other. All the other pairs will have chromosomes that are very similar (but not identical) to one another.

Mitosis

During mitosis, the replicated chromosomes condense, are aligned between the two poles of the cell, then sister chromatids are split and each chromatid is pulled to the opposite poles

Prophase

• the chromosomes condense, revealing in the microscope sister chromatids attached at the centromere
• the nucleolus dissipates
• the centrosomes form outside of the nucleus-in animals each centrosome has a pair of centrioles
• the cytoskeleton in the cytoplasm disassembles and the microtubules reassemble, starting at the centrosomes, to form the mitotic spindles
• the centrosomes begin to migrate to opposite poles of the cell

Prometaphase

• nuclear envelope (membranes) and the nuclear lamina (made up of intermediate filaments) break down
• the spindle microtubules extend to chromosomes and bind to the centromere in a junction called the kinetochore
• centrosomes are now at opposite poles

Metaphase

• three type of microtubules exist in the mitotic spindle=kinetochore MTs, interpolar MTs, and aster MTs
• these three function to align the chromosomes across the metaphase plate, perpendicular to the spindle, tugging them equally to each pole

Anaphase

• the two sister chromatids separate and are pulled by the kinetochore MTs toward the poles (anaphase A), then the interpolar MTs push the poles apart and the aster MTs pull the poles toward the cell cortex (anaphase B)
• this pulling and pushing is accomplished by motor proteins associated with the microtubules: dynein-type motor proteins would pull the kinetochore toward the spindle pole while the kinetochore MTs get shorter; dynein-type motor proteins would also pull the spindle poles toward the cell cortex as the aster MTs get shorter; kinesin-type motor proteins would slide the interpolar MTs opposite each other as the polar MTs get longer
• each pole has a daughter chromosome from each replicated chromosome

Telophase

• the spindle dissociates, the nuclear envelope and nuclear lamina begin to reform, the nucleolus reforms, the chromosomes begin to uncoil

Cytokinesis

in animals

• a cleavage furrow (puckering of the plasma membrane) forms perpendicular to the orientation of the mitotic spindles
• a belt of actin and myosin microfilaments contracts (similar to muscle contraction) to cleave the cytoplasm in half (usually in half, sometimes the two daughter cells are different sizes)

in plants

• a new cell wall is deposited between the two poles centered at the phragmoplast
• vesicles from the Golgi carry polysaccharides and cell wall proteins to the phragmoplast in the process of building the new cell wall

Meiosis-from diploid to haploid

In the process of sexual reproduction, haploid gametes are produced by special cells in plants and animals=germ cells. Gametes contain only one chromosome representative of each chromosome pair. Each chromosome is, by chance, one originally from the mother or from the father. When two gametes fuse together during fertilization, then the diploid state is restored, since each chromosome is a member of a pair of nearly identical chromosomes.

Meiosis involves two steps of nuclear and cell division=meiosis I and meiosis II

• as with mitosis, the cell has replicated its chromosomes in the S phase, so each chromosome has a pair of sister chromatids

Meiosis I

Prophase I

• similar events as mitotic prophase, such as: condensing of the chromosomes, appearance of centrosomes, which migrate to poles, and disassembly of the nuclear envelope
• the DIFFERENCE from mitotic prophase is: homologous chromosomes (each with 2 sister chromatids) align next to one another to form bivalents=four sister chromatids, 2 from each homologous chromosome
• homologous chromosomes are held together by the synaptonemal complex
• when the homologous chromosomes are aligned together, crossing over (recombination) may occur between non-sister chromatids=exchange of portions of each chromatid

Metaphase I

• the homologous chromosomes are attached to spindle microtubules at the kinetochores, with the 2 kinetochores from a single centromere on a chromosome facing toward one of poles (which one is random)
• the bivalents align along the metaphase plate

Anaphase I

• the chromosomes separate and are pulled to the poles by the spindle microtubules
• NOTE the difference from mitosis: homologous chromosomes are separated from one another, each chromosome, with two sister chromatids, goes to one of the poles

Telophase I and cytokinesis

• the nuclear envelope reforms and the cells divide, essentially as in mitosis except that the daughter cells are now haploid

Meiosis II

Prophase II

• essentially the same as mitotic prophase, EXCEPT there are half as many chromosomes (there is only one chromosome from each of the pairs)

Metaphase II

• the chromosomes align at the metaphase plate and spindle microtubules attach to the kinetochores of each sister chromatid, facing opposite poles

Anaphase II

• the sister chromatids are separated and pulled to the poles

Telophase II and cytokinesis

• the nuclear envelope reforms and the cells divide
• each cell has a daughter chromosome from only one of the chromosomes in each pair (the cells are haploid)

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