Cell Cycles: Interphase, Mitosis, Cytokinesis

• Cell Cycle: nuclear division, cytokinesis
• Parental cell: genetic copies of parental cell
• 3 process: checks/ regulators for each step to ensure timely progression, replication process to synthesis DNA into two copies, interwoven “cables” and “motors” of mitotic cytoskeletons.
• Chromosomes: nuclear units of genetic information; DNA molecules combined with proteins.
• In eukaryotes, heredity information of nucleus is distributed among several linear molecules.
• Eukaryotes= diploid (2n); 23 pairs of chromosomes, 46 chromosomes
• Ploidy: number of chromosomes sets
• Sister Chromatids: replication of DNA; identical molecules; precise division.
• Chromosome Segregation: equal distribution of daughter chromosomes to each of the two daughter cells
• Interphase: longest phase of cell cycle
• External factors that influence cells: signal molecules, hormones, growth factors, death factors; bind to surface of cell and trigger a reaction. Stimulate or inhibit phosphate groups from being added to CDK complexs; slow/ speed up cell division.
• G1 phase: growth stage, cell makes RNAs, proteins, other cellular molecules

• S phase: cell duplicates chromosomal proteins, DNA,
• G2 phase: continues to synthesis proteins/ RNA
• G1 phase, time in this stage can vary in length in species. Once DNA replication begins 10-12 hrs through S phase, 4-6 G2, 1-4 mitosis.
• G1 phase, stop dividing, cycle through G0 phase once mature, cell cycle arrest
• Prophase: replicated chromosomes condense, compact/ rod-like; packs long units into small enough units to divide. (nucleolus= disappears, stop RNA synthesis) centrosomes: divided into two parts

Interphase

The time interval between nuclear divisions. During this phase, a cell increases in mass, roughly doubles the cytoplasmic components and duplicates its chromosomes

During interphase cells grow, make structural proteins that repair damaged parts, transport nutrients to where they are needed, eliminate wastes, and prepare themselves for mitosis by building proteins. Proteins also function in the construction of enzymes that aid chemical reactions, most important of these reactions are those that control the synthesis of DNA and the replication of the genetic information in the chromosomes

During interphase genetic material is called chromatin

Chromatin – all the DNA molecules and associated proteins in the nucleus, when referred to as chromatin, the chromosomes are long, thin strands dispersed throughout the nucleus in a tangles, fibrous mass

• each chromosome duplicates itself during interphase and the original and duplicate are attached to each other by a centromere
• when attached to each other, original and duplicate are referred to as (‘identical twin’) sister chromatids
• since sister chromatids contain identical genetic information, the pair, attached at the centromere, is considered to be one chromosome

Prophase

• the FIRST phase of MITOSIS
• the chromosomes become visible under a microscope as the shorten and thicken
• in animal cells, a small body of cytoplasm separates and its parts move to opposite poles of the cell as the chromosomes become visible
• centrioles provide attachment for spindle fibres which serve as guide wires for the attachment and movement of the chromosomes during cell division
• centrioles + spindle fibres = spindle apparatus
• most plant cells do not have centrioles, but spindle fibres still form and serve a similar purpose
• centromere helps anchor the chromosomes to the spindle fibres
• when viewed under a microscope the nuclear membrane appears to fade; in effect it is dissolving allowing the separation of chromosomes and cell organelles

Metaphase

• the SECOND phase of MITOSIS
• chromosomes (composed of sister chromatids) move towards the center of the call, center area is called equatorial plate because it is midway between the poles of the cell
• chromosomes appear as dark, thick filamentous structures that are attached to spindle fibres
• even though they are most visible at this stage, it is still very hard to count the number of chromosomes in most cells because the chromosomes are entangled
• chromatids can become intertwined during metaphase

Anaphase

• the THIRD phase of MITOSIS
• centromeres divide and the sister chromatids, now referred to as chromosomes, move to opposite poles of the cell
• if mitosis happens correctly, the same number and type of chromosomes will be found at each pole
• occasionally segments of the chromatids will break apart, and may reattach in anaphase

Telophase

• the FOURTH and LAST phase of MITOSIS
• the chromosomes reach the opposite poles of the cell and begin to lengthen
• the spindle fibres dissolve and a nuclear membrane forms around each mass of chromatin
• telophase is followed by cytokinesis, the division of cytoplasm

Cytokinesis

• once the chromosomes have reached opposite sides, the cytoplasm starts to divide
• cytokinesis appears to be quite distinct from nuclear division
• animal cell – a furrow develops, pinching off the cell into two parts, this marks the end of cell division
• plant cells – the separation is accomplished by a cell plate the forms between the two chromatin masses, cell plate will develop into a new cell wall, eventually sealing off the contents of the new cells from each other

• Cell Plate. Forms between the daughter nuclei and grows laterally until it divides the cytoplasm.
• Centrosome: a site near nucleus, from which microtubules radiate outward in all directions
• Centrosome= Microtubule organizing centre (MTOC), made of pair of centrioles, at right angles to each other
• Gets duplicated in S phase. Then separate until they reach opposite ends of the pole, microtubules extend from them.
• NO centrosome/ centrioles in flowering plants, microtubules comes from multiple MTOC around cell; when nuclear envelope break, it moves into former nuclear region.
• Recent data shows that chromosomes  “walk” to poles over stationary  microtubules.
• CDK= Cyclin-dependent kinases. Regulators of cell division; directly control cell cycle
• Protein kinases, enzymes add phosphates groups to target proteins. CDK enzymes called “cyclin dependent” because they are “switched on” only when combined with another protein called a cyclin.
• Cyclin levels rise and fall during cell cycle -> therefore enzyme activity of the CDKs  do to (even though the concentration of CDK proteins remain constant)
• G1- S checkpoint (Cyclin E- CDK2)                G2-M (cyclin B- CDK 1) – can’t progress until ready for mitosis
• Contact inhibition: stabilize cell growth in fully developed organs/ tissues.
• Cellular Senenscence: loss of proliferative ability over time. “Hayflick factors” : DNA damage/ telomere shortening.
• Accumulation of random damage to cell’s DNa sequence, or chromosome structure; gets older and diminish function
• Telomeres; repetitive DNA added to ends of chromosomes, by telomerase. DNA replication unable to replicate entire; some of it is lose; once teleomeres diminish to min. length cell stop dividing; you DIE!
• Senescence= important anti-tumour mechanism; increasing telomerase would cause cells to divide out of control, less telomerase= more resistant to cancer.
• Cancer occurs when cell lose control and divide rapidly. Cancer cells can move throughout body to effect other parts= Metastsis
• Mutation of CDK system and other proteins= turn into Oncogenes encode altered version of these products.
• Apoptsis: “programmed cell death” can result from external or internal factors. Good for cells suffering from: dna damage, viral infection, mutations
• Caspase: code for cell death
• Binary Fission: splitting into two parts- produce two daughter cells from original parent (cytoplasmic growth, DNA replication etc.
• Bacterial Chromosomes: circular DNA molecules; founds in prokaryotes; DNA replication takes up majority of time for cell cycles- then cytoplasm quickly divides.
• Replication of bacterial chromosome commences at a specific region called origin of replication; in middle of cell where enzymes for DNA replication are located.
• Ori gets replicated; moves origins toward ends of cells two poles; as replication proceeds.
• Cytoplasmic division then occur, cell wall cuts the two replicated molecules in half.