Clinical Aspects of the Clinical Aspects of the Cell CycleCell Cycle

Clinical aspects of MitosisClinical aspects of Mitosis

Crossing over does NOT occur in somatic Crossing over does NOT occur in somatic cells (as homologous chromosomes do cells (as homologous chromosomes do not usually pair in mitosis).not usually pair in mitosis).

If it occurs (somatic crossing over) it If it occurs (somatic crossing over) it produces produces cancer.cancer.

Apoptosis ApoptosisApoptosis is is programmed cell deathprogrammed cell death and involves a and involves a

sequence of cellular events involving:sequence of cellular events involving: fragmenting of the nucleus, fragmenting of the nucleus, blistering of the plasma membrane, andblistering of the plasma membrane, and engulfing of cell fragments by macrophages and/or engulfing of cell fragments by macrophages and/or

neighboring cells.neighboring cells. Apoptosis and cell division are Apoptosis and cell division are balancingbalancing processes processes

that maintain the normal level of that maintain the normal level of somaticsomatic (body) (body) cellscells..

Cell death is a Cell death is a normal normal and necessary part of and necessary part of development: for example the human embryo must development: for example the human embryo must eliminate webbing found between fingers and toes.eliminate webbing found between fingers and toes.

Death by apoptosis Death by apoptosis prevents a tumorprevents a tumor from from developing.developing.

The Cell Cycle and Cancer Cancer Cancer is a is a cellular cellular

growth disordergrowth disorder that that results from mutation of the results from mutation of the genes that regulate the cell genes that regulate the cell cycle;cycle; i.e., cancer results i.e., cancer results from the loss of control and a from the loss of control and a disruption of the cell cycle.disruption of the cell cycle.

Carcinogenesis;Carcinogenesis; the the development of cancerdevelopment of cancer is is gradual—it may take gradual—it may take decades before a cell has decades before a cell has the characteristics of a the characteristics of a cancer cell.cancer cell.

Origin of Cancer A A DNA repair systemDNA repair system corrects mutations during corrects mutations during

replication; replication; mutations in genesmutations in genes (as proto- (as proto-oncogenes or tumour suppressor genes) encoding oncogenes or tumour suppressor genes) encoding the various repair enzymes can cause cancer.the various repair enzymes can cause cancer.

Clinical aspect of MeiosisClinical aspect of Meiosis Normally in meiosis I and II “Disjunction” Normally in meiosis I and II “Disjunction”

occures between homologous pair of occures between homologous pair of chromosomes (Meiosis I) or sister chromosomes (Meiosis I) or sister chromatids of a chromosome (Meiosis II) chromatids of a chromosome (Meiosis II) reulting in gametes (sperms or ova) with reulting in gametes (sperms or ova) with 23 chromosome 23 chromosome each.each.

If non-disjunction occurs, abnormal If non-disjunction occurs, abnormal gametes are produced with 24 and 22 gametes are produced with 24 and 22 chromosomes chromosomes each. each.

Fertilization between normal gamete (with 23 Fertilization between normal gamete (with 23 chromosome) with one containing 24 chromosome) with one containing 24 chromosomes results with a fetus with 47 chromosomes results with a fetus with 47 chromosomes (trisomy) as Down syndrome chromosomes (trisomy) as Down syndrome or trisomy 21.or trisomy 21.

Fertilization between normal gamete and one Fertilization between normal gamete and one containing 22 chromosomes results in containing 22 chromosomes results in monosomy (a fetus with 45 chromosomes monosomy (a fetus with 45 chromosomes which is a lethal condition except in which is a lethal condition except in Monosomy X; Turner syndrome).Monosomy X; Turner syndrome).

Down syndrome (Trisomy 21)Down syndrome (Trisomy 21)

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

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