Clinical Aspects of the Cell Cycle

1. Clinical Aspects of the Cell Cycle

2. Clinical aspects of Mitosis • Crossing over does NOT occur in somatic cells (as homologous chromosomes do not usually pair in mitosis). • If it occurs (somatic crossing over) it produces cancer.

3. Apoptosis • Apoptosis is programmed cell death and involves a sequence of cellular events involving: • fragmenting of the nucleus, • blistering of the plasma membrane, and • engulfing of cell fragments by macrophages and/or neighboring cells. • Apoptosis and cell division are balancing processes that maintain the normal level of somatic (body) cells. • Cell death is a normal and necessary part of development: for example the human embryo must eliminate webbing found between fingers and toes. • Death by apoptosis prevents a tumor from developing.

5. The Cell Cycle and Cancer • Cancer is a cellular growth disorder that results from mutation of the genes that regulate the cell cycle;i.e., cancer results from the loss of control and a disruption of the cell cycle. • Carcinogenesis;the development of cancer is gradual—it may take decades before a cell has the characteristics of a cancer cell.

6. Origin of Cancer • A DNA repair system corrects mutations during replication; mutations in genes (as proto-oncogenes or tumour suppressor genes) encoding the various repair enzymes can cause cancer.

7. Clinical aspect of Meiosis • Normally in meiosis I and II “Disjunction” occures between homologous pair of chromosomes (Meiosis I) or sister chromatids of a chromosome (Meiosis II) reulting in gametes (sperms or ova) with 23 chromosome each. • If non-disjunction occurs, abnormal gametes are produced with 24 and 22 chromosomes each.

9. Fertilization between normal gamete (with 23 chromosome) with one containing 24 chromosomes results with a fetus with 47 chromosomes (trisomy) as Down syndrome or trisomy 21. • Fertilization between normal gamete and one containing 22 chromosomes results in monosomy (a fetus with 45 chromosomes which is a lethal condition except in Monosomy X; Turner syndrome).

10. Down syndrome (Trisomy 21)

11. Turner Syndrome (45,X)Klienfelter Syndrome (47,XXY)

12. Breaks or unequal crossing over of chromosomes during meiosis may lead to many structural chromosomal abnormalities as translocation, deletion, duplication, inversion, ring chromosome, isochromosome and chromosome fragile sites. All these errors lead to severe congenital anomalies in the fetus.

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