CONCEPT 6. The cell cycle

• What is the purpose and necessity of cell division?

  Purpose: Growth, repair, reproduction. Necessity: Replace damaged cells, allow organisms to grow and develop.
• What are the three steps in the prokaryotic cell cycle?

  Gap phase, replication and division
• What is the difference between binary fission and mitosis?

  Binary fission occurs in prokaryotes, mitosis occurs in eukaryotes
• Where does chromosomal replication occur?

  At the origin of replication (OriC). Since prokaryotes only have a single DNA molecule, there is only one origin of replication. Eukaryotes have a large amount of DNA to be replicated and thus have multiple origins or replication.
• What is the role of initiator proteins?

  To initiate the separation of DNA strands, binding to OriC
• What is the role of helicase?

  Unwind DNA and form a replication fork
• How is the rebinding of separated DNA molecules prevented?

  Single stranded binding proteins (SSB)
• What is the role of Type 1 topoisomerase/DNA gyrase?

  Splicing supercoiled DNA.
• Where does DNA replication stop?

  When the replication fork reaches the termination site
• What is the result of binary fission?

  The production of two daughter cells, each consisting of one parental strand and one newly synthesised strand
• What is the benefits of the semi-conservative model of DNA replication?

  Chances of mutation are reduced as the parent strand can be used for comparison and errors can be repaired and fixed
• What is the role of FtsZ?

  Pinches a prokaryotic cell inwards during cell division
• What initiates DNA replication in bacteria?

  The accumulation of Dna-A at the Dna-A binding boxes of the Ori-C. Once the threshold point is reached, DNA replication is initiated.
• What are the features of DNA replication?

  Semi-conservative, bidirectional, and semi-discontinuous.
• Why is DNA replication in the 3' to 5' direction discontinuous?

  DNA can only be read in the 5' to 3' direction, thus on the lagging strand, RNA primers must be placed across the strand by DNA primase, allowing small sections of DNA to be read by DNA polymerase III. This results in the formation of Okazaki fragments.
• What are Okazaki fragments?

  Short DNA fragments synthesized on the lagging strand during DNA replication.
• How are the Okazaki fragments on the lagging strand joined together?

  RNaseH removes the RNA primers and DNA polymerase I fills in the empty spaces between Okazaki fragments. The DNA fragments are joined together by ligase, forming a continuous strand of DNA.
• What are the two stages of the eukaryotic cell cycle?

  • Interphase: comprised of G1 phase, S phase and G2 phase
  • Mitotic phase: comprised of mitosis and cytokinesis
• What occurs in G1 phase?

  Cell growth and metabolic activity.
• What occurs in S phase?

  DNA replication
• What occurs in G2 phase?

  Cell growth and preparation for cell division.
• How is the progression of the cell cycle regulated?

  Cyclins and CDKs are present at each restriction point. They are able to phosphorylate target molecules which allow the progression of the cell cycle but if certain conditions are not met e.g DNA damage, incomplete replication, unattached chromosomes to spindle apparatus then progression is stopped
• What are the steps in mitosis?

  Prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis.
• What occurs in prophase?

  • Chromatin condenses into x-shaped chromosomes
  • Centrosomes migrate to opposite sides of the cell
  • Microtubule rods extend from the centrosomes forming the spindle apparatus
• What occurs in prometaphase?

  • Nuclear envelope dissolves
  • Kinetochores form on both sides of the centromere
  • Microtubules fasten to each kinetochore, attaching sister chromatids to opposite poles
• What occurs in metaphase?

  Spindle apparatus arranges the chromosomes into a fixed row along the cell's equator
• What occurs in anaphase?

  • Microtubules shorten, pulling apart sister chromatids
  • Microtubules elongate the cell
• What occurs in telophase?

  • Spindle apparatus disbands
  • Chromosomes land on opposite poles and loosen into chromatin
  • Two nuclear envelopes form
• What occurs in cytokinesis?

  • The cell's cytoplasm and organelles are distributed between the two daughter cells and the cell is divided
• What divides the cell in animals and plants?

  Animals: Contractile rings

  Plants: Formation of cell plate from fused vesicles
• What is the role of DNA polymerase I?

  Removes original RNA primers on the lagging strand, fills in the gaps and fixes any errors made by DNA polymerase III
• What is the role of DNA polymerase III?

  Primary replicative polyermase, synthesising the leading strand and Okazaki fragments