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NadiaA

Cell Cycle and Cell Division
LO:
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Understand the molecular mechanisms involved in cell cycle regulation
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Cell cycle
is divided into two phases called (i) Interphase – a period of preparation for cell division, and (ii)
Mitosis (M phase) – the actual period of cell division
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Unicellular reveal genetic info on cell cycle
 Unicellular organisms are easy to genetically manipulate e
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yeast
 Yeast S pombe is a typical eukaryote and is rod shaped; it has all the stages of cell division
that is common among eukaryotes: G1,S,G2, M
Whereas, yeast S
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g
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e
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g
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Multicellular organisms reveal biochemistry of cell cycle
 Study of early embryonic frog cells
 They are large cells- 1mm diameter;
 Thus easy to inject samples
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 Have a large cytosol
Rapid cell divisions called ’cleavage divisions’
Has S and M phase
No DETECTABLE G1 and G2

Role of chemicals in cell cycle
Cell fusion experiment 1
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Fuse S and G1 phase
DNA synthesis initiates in G1 nucleus
Thus there is something in S that causes DNA synthesis and that these factors can freely
diffuse between the nucleus and cytoplasm… (we now know these are cdks!)

Cell fusion experiment 2
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Fuse M phase nucleus with G1
G1 nucleus immediately enters M phase
Chromosomes were not replicated thus there is no S phase
Although the spindle formed and chromatin condensed and nuclear envelope did break
down
Thus something in the M phase indices interphase cells to divide

The cell cycle control system
o
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Regulates progression to next stages in cell division
There are 3 main transition points- see diagram
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g
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4 million combinations
Egg + sperm = zygote = 223 x 223 = 70 trillion combinations
Haploid (n) - carry single copy of chromosome e
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gametes

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Diploid (2n) - carry double copy of chromosome – one from each parent, there are alleles
e
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23 pairs in humans, egg and sperm fuse forming a diploid zygote
Zygotes form genetically distinct offspring- they are their own person
Meiosis reduces chromosome number and produces germ cells i
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Sperm or egg

Non-disjunction
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Non-disjunction happens when karyotyping is not correct, it can happen in meiosis 1 or
meiosis 2
Examples of non-disjunction are below-

(Turner syndrome-affects only girls and women, results when a sex chromosome (the X
chromosome) is missing or partially missing
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Trisomy- an abnormality characterized by the presence of an additional chromosome to the normal
diploid number
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Cohesin ((complimentary slide))
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Cohesin- large protein complex
It contains SMC proteins (structural and maintenance of chromosomes)
S phase in mitosis and meiosis generate sister chromatid pairs which are linked by cohesin along their
entire length
Cohesin is enriched around the centromere
It is deposited in many areas along the length of each sister chromatid as DNA replication occurs
Cohesin generally dissociates in prophase or anaphase
The metaphase to anaphase transition is the point of ‘no return’ (cell division is confirmed) and it is
thought that cohesin dissociation is a trigger

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