Mitosis and Meiosis Flashcards
What cells spend a long time in G0?
Liver
What stages does interphase consist of?
1st Gap or growth (G1) phase: normal cell activities and increase in cell organelles results in doubling of cell size
Synthesis (S) phase:
DNA is synthesised – 2 copies of each chromosome (sister chromatids) are produced and the centrioles replicate
2nd Gap or growth (G2) phase
The cell grows in size and each cell now has 2 pairs of centrioles then the mitotic phase starts again
What events occur in prophase?
In this stage the chromatin which is usually loose condenses and chromosomes form. These chromosomes are visible as two sister chromatids joined by a centromere.
The nucleolus shrinks & disappears and the cytoplasmic microtubules of the cytoskeleton disassemble and then reassemble to form the spindle
One pair of centrioles migrates to each pole of the spindle and the nuclear membrane disappears
What events occur in metaphase?
Chromosomes arrange on the equator of the spindle
Homologous chromosomes don’t associate (this ensures that each daughter cell gets an exact copy of parent cell’s DNA)
What events occur in anaphase?
The chromatids separate and migrate led by the centromeres, to opposite poles of the cell
Chromatids eventually reach the poles to make two daughter chromosomes
What events occur in telophase?
The chromosomes unravel and the nuclear membranes reform (nucleoli reform)
Although this is all encased in one cell membrane at this point
How does cytokinesis occur?
Firstly a cleavage furrow forms and deepens until the cytoplasm is halved which is accomplished by a ring of actin filaments attached to the inner aspect of the cell membrane
Control of the cell cycle ensures that…
Damaged DNA is not replicated
DNA replication must be complete & occur just once per cell cycle
Chromosomes must be positioned on spindle correctly
The phases of cycles must be synchronous
Control of the cell cycle occurs on two levels:
Intracellular
Via checkpoints at the end of G1, G2 and M phases these require cyclin dependent protein kinases (Cdks) & cyclins
Intercellular
Via mitogens/ growth factors (stimulate cell growth) and survival factors (promote cell survival and inhibit apoptosis)
What regulates Cyclin dependent protein kinases (Cdks)?
Cyclins
What are growth factors and what do they do?
These are signaling molecules which bind to cell surface receptors and activate pathways which control genes.
They have multiple effects, but they are mitogenic (stimulate mitogenesis or cell division)
They act by stimulating cells to enter G1 from G0 and originate from neighbouring cells (this maintains the correct number of cells in an organ)
Which signalling pathways do growth factors activate?
RTK’s, Ras and MAPK pathways which all result in the activation of Myc
Myc increases the expression of G1 cyclin kinases and this stimulates DNA synthesis
How does the cell keep proliferation in check?
Excessive Myc production activates Arf and Arf binds to the P53 inhibitory protein Mdm2 which renders P53 active and this produces cell cycle arrest and apoptosis
When do cells undergo apoptosis?
When they are:
damaged beyond repair
infected with a virus
undergoing stressful conditions (starvation)
What are the three stages of meiosis?
Meiotic S phase (chromosome duplication) Meiosis I (chromosome segregation) = segregation of homologous chromosomes (+ crossing over) Meiosis II (chromosome segregation) = segregation of non homologous chromosomes (like mitosis)
What happens in prophase 1 of meiosis? What are the 5 stages?
Leptotene: Long slender chromosomes visible.
Zygotene: Homologous chromosomes begin to pair up (synapsis)
Pachytene: Synapsis complete and Crossing over occurs forming chiasmata.
Diplotene: Chromosomes move apart a little. Each bivalent remains attached at 1 or more chiasmata
Diakenesis: Transition to metaphase, 4 chromatids in each bivalent visible
What occurs in metaphase 1 of meiosis?
The nuclear envelope disappears and the bivalent chromosomes move to equator of spindle
The centromeres of each bivalent chromosome orientate to opposite poles
What occurs in anaphase and telophase 1 of meiosis?
Anaphase I: The homologous chromosomes separate and migrate to opposite poles of the spindle until one chromosome with 2 chromatids arrives at each pole
Telophase I: The cytoplasmic division starts and the spindle breaks down then the nuclear envelope & nucleoli form
What happens in the second meiotic division of meiosis?
Prophase II
centrioles replicate & migrate to poles
new spindle forms at right angles to previous one
Non DNA replication!
Metaphase II
Chromosomes move to the equator of the spindle
Chromatids orientate to opposite poles
Anaphase II
Chromatids separate and move apart to poles
Telophase II
The spindle disappears and nuclear envelope & nucleoli form
Chromosomes become thread like
Cytoplasmic division
What happen to the two X chromosomes?
Females inherit two X chromosome, however one of them is randomly inactivated during embryonic development
This phenomenon is known as X chromosome inactivation (so there is only one active copy of X linked genes)
The inactive X chromosome is condensed as Barr body (this get reactivated in germ cells)
X chromosome inactivation involves methylation of the DNA and expression of antisense transcript called XIST (X-Inactive-Specific-Transcript)
Is it always the same X chromosome that is inactivated?
Females have a mosaic of two types of cells: half from active X of the father and half from active X of the mother
Different alleles present on the X chromosomes after fertilisation will result in a mosaic (different cells will express different alleles)
This is what happens in the Tortoiseshell cat
