The Cell Cycle Flashcards
How is the cell cycle useful in uni- and multicellular organisms?
UNICELLULAR ORGANISMS (eg. bacteria, yeast): -Each cell cycle gives rise to two new organisms.
MULTICELLULAR ORGANISMS:
- a single-celled zygote must undergo many rounds of the cell cycle to make a new fully grown organism
- the organism must also constantly replace any cells that die during the lifetime of that organism
What are the 3 different situations with cell cycle re-entry?
- cell cycle re-entry in NOT possible (eg. nerve cells)
- cells are maintained in G0 until stimulated to divide (eg. hepatocytes)
- cells are constantly in the cell cycle (eg. epithelial cells of the gut, haematopoietic cells in bone marrow)
What are the different phases of the cell cycle in eukaryotes?
-GAP PHASE 1 (G1): growth and preparation for S phase
- SYNTHESIS PHASE (S): chromosome duplication
- GAP PHASE 2 (G2): growth and preparation for M phase
(these three phases are all under Interphase)
- MITOTIC PHASE (M): mitosis + cytokenisis
Describe the M Phase in detail.
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 & 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 de-condense
- Nuclear envelope reforms
CYTOKINESIS:
- Cytoplasm divides
Describe the mitotic spindle, and how it works.
It’s a bipolar array of microtubules.
It starts to assemble during prophase from the centrosomes at each pole. It attaches to the chromosomes via the kinetochore. It pulls apart the sister chromatids.
What are the three types of spindle microtubules?
- ASTRAL MICROTUBULES: they anchor the spindle poles to the cell membrane
- KINETOCHORE MICROTUBULES: they help in lining up the chromosomes
- INTERPOLAR MICROTUBULES: they interdigitate with each other from opposite poles - extending across the equator - to provide stability to the bipolar spindle
What is the kinetochore?
It is a protein structure formed on the centromere, where the spindle fibres attach to pull the chromatids apart during cell division.
What is the centromere?
It is a part of the chromosome connected to the spindle fibre.
What are chromatids?
They are the two chromosomes that have been replicated and are linked through the centromere.
What are centrosomes?
They’re the microtubule organising centres (MTOCs) in somatic cells. They consist of a pair of centrioles surrounded by a pericentriolar matrix (a cloud of amorphous material).
It’s duplicated during interphase, and they migrate to opposite poles in preparation for the M phase.
Describe Cytokinesis.
It’s the final step in the cell cycle. It divides the cytoplasm into two daughter cells.
The contractile ring is a cytoskeletal structure composed of actin and myosin bundles. It accumulates between the poles of the mitotic spindle beneath the plasma membrane. The ring contracts and forms an indentation/cleavage furrow, dividing the cell into two.
All the cell’s organelles must be redistributed between the two daughter cells, as organelles cannot spontaneously regenerate.
List some difference between mitosis and meiosis.
MITOSIS:
- two cells made at the end
- diploid (2n) DNA
- the cell divides once
- no recombination between homologous chromosomes (no exchange of DNA sequences between tightly linked chromosomes)
MEIOSIS:
- four cells made at the end
- haploid (n) DNA
- the cell divides twice
- homologous recombination occurs (“chiasm” structure, allows exchanged of DNA between mother and father chromosomes)
Describe Meiosis.
Meiosis is a specialised cell division that starts with 1 diploid cell & ends with 4 haploid cells
Its purpose is to produce gametes: sperm & egg.
One round of DNA replication during S phase & 2 rounds of cell division
MEIOSIS I: homologous chromosomes line up on the spindle & separate to opposite spindle poles
MEIOSIS II: sister chromatids line up on the spindle & separate to opposite spindle poles
Recombination occurs between homologous chromosomes.
What happens when meiosis goes wrong?
Nondisjunction is the failure of homologous chromosomes to separate from one another either at meiotic I or meiotic II division.
In autosomes, it’s usually fatal, exceptions being:
- Trisomy 21 (Down’s syndrome)
- Trisomy 18 (Edward’s syndrome)
- Trisomy 13 (Patan syndrome)
In sex chromosomes:
- XO (Turner’s syndrome)
- XXX (Triple X syndrome)
- XXY (Klinefelter’s syndrome)
How does the cell cycle have to be regulated?
- Each phase must occur only once per cell cycle
- Phases must be in the correct order, G1-S1-G2-M
- Phases must be non-overlapping
