The cell cycle Flashcards
Define ‘the cell cycle’
The fundamental mechanism of all living organisms to reproduce themselves and to pass down their genetic material to the next generation.
Describe the cell cycle
o Duplication of cell contents -> DNA, organelles, cytoplasm
o Division into new progeny cells.
• Unicellular organisms such as bacteria or yeast: - each cell cycle gives rise to 2 new organisms.
• Multicellular organisms, such as humans:
o Single fertilised egg (zygote) must undergo many rounds of the cell cycle to make a new fully-grown organism.
o Must also constantly replace any cells that die during the lifetime of the organism.
Describe the phases in the cell cycle
• G1 = Gap 1 phase: growth and preparation for S phase.
• S = Synthesis phase: chromosome duplication, DNA synthesis.
• G2 = Gap 2 phase: growth and preparation for M phase.
• Interphase = G1 + S + G2
• M = Mitotic phase: mitosis + cytokinesis (cell division).
• Between S and M two phases G1 and G2 cells prepare for next stage, G0 non-dividing stage.
• Many cells in our body are in G0 and not all can re-enter the cell cycle
o Cell cycle re-entry not possible (e.g. nerve cells)
o Maintained in Go unless stimulated to divide (e.g. hepatocytes-liver cells).
o Constantly in the cell cycle (e.g. epithelial cells of the gut, haematopoietic cells [blood forming cells- stem cells that continue to replicate as fresh blood is needed continuously] in the bone marrow).
Describe mitosis
- Prophase - Chromosomes condense, centrosomes move to opposite poles, mitotic spindle forms.
- Prometaphase - Breakdown of nuclear envelope, chromosomes attach to mitotic spindle.
- Metaphase - Centrosome are at opposite poles, chromosomes are at their most condensed and line up at the equator of the mitotic spindle.
- Anaphase - Sister chromatids separate synchronously, each new daughter chromosome moving to the opposite spindle pole.
- Telophase - Chromosome arrives at the spindle poles, chromosomes decondense, nuclear envelope reforms.
- Cytokinesis: cytoplasmic division - At the position of the metaphase plane. Contractile ring of actin and myosin II constrict the cell into two daughter cells.
Describe the mitotic spindle
“Bipolar” array of microtubules – they have a plus (growing) end and a minus end (shrinking).
• Start to assemble during prophase from the centrosomes at each pole.
• Attach to the chromosomes via the kinetochore (a large protein structure assembled on the centromere).
• Pull apart the sister chromatids.
• 3 types of spindle microtubules:
• Astral microtubules
• Kinetochore microtubules
• Interpolar microtubules
Define Kinetochore, Centromere and Chromatids
- Kinetochore: a protein structure formed on a chromatid, where the spindle fibers attach to pull the chromatids apart during cell division.
- Centromere: a part of the chromosome connected to the spindle fibre.
- Chromatids: the two chromosomes that have been replicated and linked through the centromere.
Describe the centrosome cycle
- Microtubule-organising centre in somatic animal cells.
- Centrosome consists of a pair of centrioles surrounded by pericentriolar matrix (a cloud of amorphous material (doesn’t have a fixed form).
- Duplicated during interphase.
- Migrate to opposite poles in preparation for M phase.
Describe cytokinesis
Cytokinesis
• Final step in the cell cycle.
• Divides the cytoplasm into two daughter cells.
• Contractile ring
• Cytoskeletal structure composed of actin and myosin bundles.
• Accumulates between the poles of the mitotic spindle beneath the plasma membrane.
• Ring contracts and forms an indentation or cleavage furrow, dividing the cell in two.
Describe cell organelle division
- All cell organelles must be redistributed between the 2 new daughter cells.
- Cell organelles cannot spontaneously regenerate so must be already present in the new daughter cells.
Compare and contrast Meiosis and Mitosis
As shown on table
Describe Meiosis
• Meiosis is a specialised cell division that starts with one diploid cells and ends with 4 haploid cells.
• Purpose is to produce gametes: sperm and egg (sex cells).
• One round of DNA replication during S phase and two rounds of cell division.
o Meiosis I: homologous chromosomes line up on the spindle and separate to opposite spindle poles.
o Meiosis II: sister chromatids line up on the spindle and separate to opposite spindle pole.
• Recombination occurs between homologous chromosomes.
• Nondisjunction- failure of homologous chromosomes to separate from one another, either at meiotic division I or meiotic division II.
• Autosomes- usually fatal, exceptions are:
o Trisomy 21 (Down’s Syndrome)
o Trisomy 18 (Edward’s syndrome)
o Trisomy 13 (Patau Syndrome)
• No viable autosomal monosomies – cells with 1 autosome bottom right pic will not survive.
• Sex chromosomes:
o XO (Turner’s syndrome)
o XXX (Triple X syndrome)
o XXY (Klinefelter’s syndrome)
Describe the regulation of the cell cycle
- Entry into the cell cycle must be strictly controlled.
- Each phase must occur only once per cell cycle.
- Phases must be in the correct order: G1-S-G2-M
- Phases must be nonoverlapping.
Describe the checkpoints in the cell cycle
On image
Describe cell cycle regulators
These include CDK and cyclins
Describe cell cycle control in yeast
On image
