Unit 2 - The cell cycle, mitosis, meiosis Flashcards
What is the cell cycle?
The sequence of events that takes place in a cell to enable growth and repair
What are the two main phases of the cell cycle?
- Interphase (longest phase ~95% of time)
- Mitotic phase
What does a cell do during interphase?
- Cells carry out their major functions
- Protein synthesis occurs in the cytoplasm
- Organelles grow and divide in the cytoplasm
- Normal metabolic processes (some continue throughout cell division e.g. respiration)
- DNA is replicated and checked for errors in the nucleus
What are the 3 stages of interphase?
- G1 - growth phase 1
- S - synthesis phase
- G2 - growth phase 2
What occurs in G1 of interphase?
- Proteins from which organelles are synthesised are produced
- Organelles are replicated
- The cell increases in size
What occurs in G2 of interphase?
- Cell continues growth
- Energy stores are increased
- Cell double checks duplicated chromosomes for errors and makes any necessary repairs
What occurs in S of interphase?
Chromosomes are duplicated in the nucleus
What is G0 of the cell cycle?
Phase when cells leave the cell cycle either temporarily or permanently.
A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing again e.g., lymphocyctes in an immune response.
Why might a cell leave the cell cycle?
Phase when cells leave the cell cycle either temporarily or permanently because:
- The cell has differentiated and is no longer able to divide
- The DNA of the cell is damaged so the cell is no longer viable and therefore becomes senescent.
- Age. The number of senescent cells increases with age and has been linked with age related diseases.
What occurs in the G1 checkpoint?
- Checks that chemicals needed for replication (growth factors) are present
- Checks for any damage to DNA before S phase
- Checks for cell size
- Checks that the nutrients required for replication are present
What occurs in the G2 checkpoint?
- Checks for cell size
- Checks for DNA replication
- Checks for any errors in the DNA and repairs mistakes
What occurs in the spindle assembly checkpoint?
- Checks if spindle fibres are connected to chromosomes
- Checks that the chromosomes have aligned ready for metaphase
*Also known as metaphase checkpoint - mitosis cannot proceed unti this checkpoint is passed.
What are the two parts to the mitotic stage of the cell cycle?
- Mitosis
- Cytokinesis
What are the two types of cell division?
Mitosis and Meiosis
When does meiosis occur?
The production of gametes
What occurs in mitosis?
The division of the nucleus into two genetically identical nuclei via a 5 step process:
- Early prophase
- Late prophase
- Metaphase
- Anaphase
- Telophase
What occurs in cytokinesis?
- The cytoplasm divides and the two identical daughter cells are produced
- Starts in late telophase
- Characterised by the formation of a cleavage furrow, which pinches the cell in two, in aminal cells.
Why is mitosis important?
- It ensures the two daughter cells produced are genetically identical
- Each cell produced has an exact copy of the DNA present in the parent cell, and the same number of chromosomes
- Growth, repair and replacement of cells in multicellular organisms
- Assexual reproduction by Eukaryotic organisms
Define chromatid
- Two identical copies of DNA (a chromosome)
- Two chromatids are held together at a centromere
Define chromatin
Uncondensed DNA which is in complex with histone proteins
Define chromosomes
Structures of condensed and coiled DNA molecoles in the form of chromatin
Define sister chromatid.
Two identical DNA molecules joined by a common centromere
Define centromere.
- Region at which two sister chromatids are held together
- Area of constriction of cells chromosomes
- Point at which the chromosomes short arm (P) and long arm (Q) are separated.
- Point where mitotic spindle fibers attach to pull sister chromatids apart during cell division.
Define centrioles.
Component of the cytoskeleton made of microtubles and associated protiens, involved in the development of spindle fibres.
Define spindle fibres
A structure made of microtubles and associated proteins that assemble from the centrosome to provide the structure that moves chromosomes.
Define homologous pairs
- A pair of chromosomes, one maternal and one paternal
- Contain the same genes on the same loci
- Do not necessarily have the same alleles
How is DNA packaged?
- Wrapped around histone proteins to form chromatin
- Coiled around more proteins to form chromosomes
When is DNA found as visible chromosomes?
Only during cell division.
Why does DNA form chromosomes for cell division?
Helps protect DNA from damage when it is being moved and replicated
Why does DNA wrap around histones?
- Makes it more compact
- Makes it easy to access
What are the 4 stages of mitosis in order?
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens in prophase of mitosis?
- Chromatin fibres become more tightly coiled, condensing into discrete chromosomes visible with a light microscope
- Nuclear membrane begins to break down and nucleolus disappears
- Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins
- The mitotic spindle begins to form from centrosomes and the microtubles from them. N.b. shorter, radial arrays of microtubles are called asters ‘stars’.
What happens in early metaphase (prometaphase) of mitosis?
- The nuclear envelope fragments
- Microtubles extending from each chromosome can now invade the nuclear area
- The chromosomes have become even more condensed
- Each of the two chromatids of each chromosome now has a kinetochore, a specialised protein structure at the centromere.
- Some of the microtuble attach to the kinetochores becoming ‘kinetochore microtubles’ which jerk the chromosomes back an forth
- Non-kinetochore fibres interact with those from the opposite pole of the spindle.
What happens in (late) metaphase of mitosis?
- Centrosomes are now at opposite poles of the cell
- Chromosomes are moved by spindle fibres to line up along the equatorial plate of the cell known as the ‘metaphase plate’
- Each chromosome has the kinetochore of the sister chromatid attached to the kinetochore microtuble from the opposite pole.
What happens in anaphase of mitosis?
- Shortest phase lasting only a few minutes
- Cohesion protiens are cleaved allowing the sister chromatids to be pulled apart by spindle fibres
- Daughter chromosomes move towards the poles of the cell as their kinetochore microtubles shorten
- Chromosomes move centromere first at ~1μm/min
- The cell elongates as the non-kinetochore microtubles lengthen
- Results in two poles of cell having equivalent, and complete, collections of chromosomes.
What happens in telophase of mitosis?
- Two daughter nuclei form in the cell - nuclear membrane forms from fragments of parents and portions of endomembrane system
- Nucleoli reappears
- Chromatids now called chromosomes which become less condensed and indistinct
- Remaining spindle fibres are depolymerized
- Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.
What happens in cytokinesis of animal cells?
- A cleavage furrow forms
- Microfilaments form a ring around the edge of the plasma membrane pulling the membrane inwards pinching the cell in two
- The plasma membrane fuses in the middle to form two cells
What happens in the cytokinesis of plants?
- Vesicles carry cell membrane and cell wall components assemble along the plane of division
- The cell membrane first forms as vesicles fuse
- Cell wall forms alongside
What occurs in prophase I of meiosis?
- Chromosomes begin to condense, and homologs loosly pair along their lengths, aligned gene by gene
- Synapsis - Pair homologs become physically connected to each other along their lengths by a ‘zipper-like’ protein structure
- Crossing over, genetic rearrangement between non-sister chromatids, leaving at chiasmata‘x-shaped’ regoins at the point of crossover
- Centrosome movement, spindle formation and nuclear membrane disintegration
- Bivalents are formed
What occurs in metaphase I of meiosis?
- Pairs of homologous pairs of chromosomes line up along the metaphase plate, with one chromosome in each pair facing each pole
- Both chromatids of one homolog are attached to kinetochore microtubles from one pole; those of the other homolog are attached to microtubles from the opposite pole
What occurs in anaphase I of meiosis?
- Breakdown of protiens responsible for sister chromatid cohesion along chromatid arms allows homologs to separate
- Spindle fibres pull homologous chromosomes to opposite poles
- Random assortment of alleles as pairs move apart
- Sister chromatid cohesion persists at the centrome, causing chromatids to move as a unit toward the same pole
What occurs in telophase I of meiosis?
- Starts with each half of the cell having a complete haploid set of duplicated chromosomes
- Each is composed of two sister chromatids; one or both chromatids include regiods of non-sister chromatid DNA
- Cytokinesis occurs simultaneously forming two haploid daughter cells (which are unidentical)
- Nuclear envelope reforms
What occurs in prophase II of meiosis?
- Chromosomes, which still consist of two chromatids, recondense
- Nuclear membrane disintegrates
- A spindle apparatus forms from centrosomes
- Chromatids start to move towards the metaphase plate II
What occurs in metaphase II of meiosis?
- Pairs of chromatids line up along the (equatorial) metaphase plate as in mitosis
- Because of crossing over in meiosis I, the two sister chromatids are not genetically identical
- Random assortment of homologous chromosomes at the metaphase plate is the second mechanism that introduces variation into the gametes = Independent assortment
What occurs in anaphase II of meiosis?
- Breakdown of proteins that hold the sister chromatids together at the centromere
- Allows chromatids to be pulled apart by spindle fibres
- The chromatids move towards opposite poles as individual chromosomes
What occurs in telophase II of meiosis?
- Nuclei form, the chromosomes begin decondensing, and cytokinesis continues
- Mitotic division of one parent cell to produce four haploid daughter cells which are genetically distinct from each other
