The Cell Cycle

• What is a chromosome?

  A strip of tightly coiled DNA
• How many pairs of chromosomes does a normal human have?

  23
• What does a gene do?


  Code for a characteristic
• For what 3 reasons does mitosis occur?

  Growth, repair and asexual reproduction
• What occurs during the G1 phase of the cell cycle?

  1. Proteins are synthesised

  2. ATP is produced

  3. Key organelles (including centrioles) grow larger, then duplicate
• What is the G1 checkpoint known as in animals?

  The restriction point
• What is the G1 checkpoint known as in yeast?


  START
• Why is the G1 checkpoint important?

  The cell must be checked to make sure that it is ready for division, as once the chromosomes are duplicated the process cannot be reversed
• What factors are checked at the G1 checkpoint?

  1. Nutrients- are the necessary nutrients present in the cell and its environment

  2. Size- is the cell large enough for division?

  3. Molecular signals- Is the cell receiving positive cues (i.e. growth factors) from its neighbours?

  4. Mechanical signals- Is the cell attached to a support/crowded by neighbours

  5. DNA integrity- is any DNA damaged?
• What happens if the cell is rejected at the G1 checkpoint?

  It passes into a resting state called G0
• What are some reasons for passing into G0?

  Cell age/senescence, damage, lack of need for them to reproduce
• Is G0 temporary or permanent?

  It can be both
• Which cells spend all of their existence in G0?

  Neurons, although many cells spend much of their life in G0 performing functions
• What else, apart from carrying out functions, can occur in G0?

  Cell death
• What happens if a cell passes the G1 checkpoint?

  It passes into the S phase
• What occurs during the S phase?

  Each chromosome is duplicated in the nucleus
• Which stages of the cell cycle make up interphase?

  G1, S and G2
• What occurs during the G2 phase of cell division?


  The duplicated chromosomes are checked for damage, and repaired/replaced if possible and necessary
• What is checked at the G2 checkpoint?

  Cell size, DNA condition (integrity, i.e. is any damaged?, and replication, i.e. is it fully replicated?)
• What happens if a problem is detected at the G2 checkpoint?

  The cell will pause at the checkpoint to undergo repairs
• What happens if any problem is fixed at the G2 checkpoint?

  The cell re-enters the cell cycle, and proceeds with division as normal
• What happens if damage found at the G2 checkpoint is irreparable, and why is this important?

  The cell may undergo apoptosis, which is important in preventing cancer by stopping errors being passed to daughter cells
• What is apoptosis?


  Programmed cell death
• What happens if a cell passes the G2 checkpoint without any problems?

  It proceeds to mitosis
• How many chromatids is a chromosome made up of?

  Either 1, or two genetically identical ones
• What is a chromatid?

  A strand of DNA
• How many chromatids does each chromosome have during G1?

  1 chromatid
• How many chromatids does each chromosome have from S phase to the beginning of mitosis?

  2
• How do chromosomes condense?

  The DNA (in the form of chromatin fibres) is supercoiled around histone proteins
• What are the 4 stages of Mitosis?

  Prophase, Metaphase, Anaphase, Telophase
• What 6 things occur during Prophase in mitosis?

  1. Chromatin fibres begin to coil and condense to form chromosomes.

  2. Nucleolus disappears

  3. Nuclear envelope begins to break down, and fully disappears by the end of Prophase

  4. Microtubules begin to form spindle fibres linking the poles of the cell

  5. Centrioles migrate to opposite poles of the cell

  6. Spindle fibres attach to centromeres, begin to move chromosomes to cell centre
• What is a centromere?

  The point on a chromosome onto which a spindle fibre attaches
• What occurs in Metaphase in mitosis?

  Chromosomes are moved by the spindle fibres to form a plane in the cell's centre, called the Metaphase Plate, and are then held in position
• What occurs in Anaphase in mitosis?

  Centromeres holding pairs of chromatids together in chromosomes divide. Sister chromosomes separated and pulled to opposite poles of the cell by shortening spindle fibres
• What shape do chromatids tend to be in during Anaphase?

  A 'v' shape because of them being pulled by the spindle fibres
• What occurs during telophase during mitosis?

  Chromatids, which have now reached the poles of the cell, are referred to as chromatids. Each new set of chromosomes assembles at its respective pole, nuclear envelope and nucleolus reform. Chromosomes start to uncoil.
• When does cytokinesis start in both animals and plants?

  During telophase
• How does cytokinesis occur in animals?

  The cytoskeleton pulls the cell surface membrane inwards, causing it to invaginate (pinch in). This causes a cleavage furrow to form, until the cell surface membrane pinches off and two new cells are formed
• What is the difference between cytokinesis in plants and animals?

  In plants, it begins from the middle of the cell, as opposed to the outside as it does in animals
• How does cytokinesis occur in plant cells?

  Vesicles from the Golgi Apparatus line up in the same position as the metaphase plate, then fuse with each other and the cell surface membrane to form a new cell surface membrane in the middle of the cell called the cell plate. The cell plate then has new cellulose deposited along it.