Cell Cycle

• What is the cell cycle?


  The cell cycle is the series of events that take place in a cell leading to its division and duplication.
• What stages does the cell cycle consist of?

  Interphase, mitosis, cytokinesis.
• What is interphase?


  This is the resting phase of the cell cycle, in which cells are not continually dividing with periods of growth and cell function instead, eg. producing hormones and enzyme whilst preparing for cell division.
• What happens during interphase?


  • DNA is replicated and checked for errors in the nucleus.
  • Proteins synthesis occurs in the cytoplasm.
  • Mitochondria grow and divide, increasing in number in the cytoplasm.
  • Chloroplasts grow and divide (specific to plant and algae cytoplasm).
  • Normal metabolic processes occur.
• What is the mitotic phase?


  This is the stage in which mitosis occurs, and the cell divides into two diploid daughter cells with identical DNA.
• What is diploid cell?


  Cell with two sets of chromosomes.
• What is a haploid cell?


  Cell that contains only one set of chromosomes.
• What does the mitotic phase consist of?


  Prophase, metaphase, anaphase, and telophase (PMAT).
• What is the prophase stage?

  Chromosomes condense.

  • Chromatic fibres (made of protein, RNA, DNA) coil and condense to form visible chromosomes; the nucleolus then disappears, and the nuclear envelope begins to break down.
  • Centrosomes (containing centrioles in animal cells) move to opposite poles of the cells, and microtubules begin to emerge from the centrosomes.
  • This forms a mitotic spindle, with the nuclear envelope having broken down, the spindle fibres attach to the centromeres of the chromosomes, ensuring they are lined correctly in the centre.
• What does a chromosome consist of?


  Two sister chromatids, which are identical copies of DNA connected at a central region called the centromere.
• What is the process of metaphase?


  Chromosome alignment.

  • The chromosomes are lined up to form a metaphase plate, where they are held in place down the cell centre.
  • They are held in place by the spindle fibres pulling on each chromatid to create tension and maintain stability.
• What is the process of anaphase?


  Chromosome separation.

  • Protein complexes called cohesins holding chromatids together are broken down by the enzyme separase, allowing the the centromeres holding each pair of chromatids together to divide.
  • Each chromatid is separated from its pair and pulled to the poles by shortening spindle fibres, with the chromatids being dragged through cytosol and forming a v shape.
  • This process elongates the cell, as spindle fibres that aren't attached to chromatids push against each other, further the distance between each pole.
• What is the process of telophase?


  Reformation.

  • The chromatids reach the poles where they are now called chromosomes, uncoiling back into their thread form.
  • The nuclear envelope then reforms around each set of chromosomes, forming two distinct nuclei where the nucleolus reforms and normal nucleic function proceed.
  • The spindle fibres are then broken down and cytokinesis begins.
• What is cytokinesis?


  Cell division.

  • animals - a cleavage furrow forms around the middle of the cell and the cell membrane is pulled inwards by the cytoskeleton until it is close enough to fuse and form two cells.
  • plants - vesicles from the golgi apparatus assemble in the same place as where the metaphase plate forms and fuse with each other and the membrane, dividing the cell into two where a cell wall will them form along the new membrane.
• How are the three stages represented?

  G1 - growth; proteins which synthesise organelles are produced and organelles replicate, increasing cell size.

  S - synthesis; DNDA is replicated in the nucleus.

  G2 - growth; cell growth continues, increasing in size, increasing energy stores, and checking for errors in duplicated DNA.
• What is G0?


  This is when a cell leaves the cycle temporarily or permanently due to a host of reasons.
• What are the reason for G0?


  • Differentiation - a cell becomes specialised and can no longer divide so will not re-enter the cycle.
  • Damage - if the DNA is damaged it can no longer divide and will therefore enter cell arrest; with most cells only being able to divide a limited number of times regardless, and therefore becoming senescent.
  • Numbers - as you age the amount of G0 cells you have increases, mainly due to age related illness, and few that enter G0 can be stimulated back into the cycle and start dividing again.
• What senescent mean?


  Specialised cells that no longer divide.
• How is the cell cycle controlled?


  Cells only divide when the correct size is achieved through growth, replicated DNA is error free, and chromosomes are in the correct place during mitosis; all of which are checked at checkpoints.
• What are cell checkpoints?


  These act as control mechanisms that monitor and verify whether processes at each stage of a cycle are successfully completed.
• What is the G1 checkpoint?


  This occurs at the end of the G1 phase and if the cell meets requirements it is triggered to begin the S phase, and if not the G0 phase.
• What is the G2 checkpoint?


  This occurs at the end of the G2 phase and checks that the DNA has replicated without errors, if it passes molecular processes are initiated that signal mitosis to proceed, and if not it will enter G0.
• What is the spindle assembly checkpoint?


  This occurs during metaphase, checking that all chromosomes are successfully attached to spindle and have aligned.
• What are checkpoint kinases?


  These are enzymes that catalyse the addition of a phosphate group to a protein (phosphorylation), changing the tertiary of checkpoint proteins (cyclins) and causing them to be activated at certain points in the cell cycle. When the enzyme binds to the correct checkpoint protein, it form a cyclin-dependant kinase complex (CDK), which activates the protein through this phosphorylation, with the cyclin being broken down when not needed and signalling the cell to move onto the next stage.
• What are checkpoint proteins called?


  Cyclins, that are activated by kinases and broken down when no longer needed, signalling the cell to move onto the next stage in the cell cycle.
• What are checkpoint proteins?


  These are enzymes that catalyse the addition of a phosphate group to a protein (phosphorylation), changing the tertiary of cyclins and causing them to be activated at certain points in the cell cycle. When the enzyme binds to the correct checkpoint protein, it form a cyclin-dependant kinase complex (CDK), which activates the protein through this phosphorylation.
• What happens when there is an overexpression of cyclins?


  These results from mutation, with the abnormal amount disturbing cell regulation and causing uncontrolled cell division. This can lead to tumour and cancer formation, requiring treatment in which CDK activity is reduced, stopping cell division.
• How does treatment for cancer effect healthy cells?


  The prevention of DNA replication and division has effect on our healthy cells, causing issues such as hair loss, as hair cells reproduce rapidly and are therefore vulnerable; in 85% of cancer cells there is a high level of telomeres which enable cells to continue to divide more than normal, causing cancer growth.
• What are stem cells?


  Undifferentiated cells that have the ability to develop into different types of specialized cells in the body.
• What are embryonic stem cells?


  Stem cells produced at the blastocyst stage, an early stage of the embryo, differentiating into any type of cell.
• What are adult (tissue) stem cells?


  Stem cells produced throughout the entirety of an organisms life, differentiating into a limited number of cells specific to their organ of origin; as when they are being specialised, only the relevant gene will be translated from the DNA.
• Where can stem cells be found?


  Brain, cell surface, skin, breast, muscles, intestines, bone marrow.
• Where can plant stem cells be found?

  The meristem (growing roots and shoots) and between phloem and xylem tissue.
• How can stem cells be used in growth?


  Stem cells can be transplanted anywhere in the body that require healthy growth, where they will adapt to the cell, and this initial growth stimulating healthy new growth.
• What are examples of where stem cells can be used in medical terms?


  Spinal injury, type 1 diabetes, burns, heart disease, neurodegenerative disease.
• What are the different types of potency in stem cells?

  Totipotent, pluripotent, multipotent, and unipotent.
• What is potency?


  This is the varying ability of stem cells to differentiate into specialised cell types, those that can generate more cell types having greater potency.
• What is totipotent?


  Cells that have the potential to form any cell type, and has the potential to generate an entire, viable organism.
• What is pluripotent?


  Cells that have the potential to develop into almost any cell type, differentiating into any of the three primary germ layer, mesoderm, endoderm, and ectoderm; these can form neurons, muscles, blood, and skin cells.
• What is multipotent?


  Cells that have the potential to develop into a limited number of cell types within a particular tissue or organ, producing closely related cells and often found in body organs, replacing aged or ill cells.