Question

16.   Is the relationship between frequency of recombination and the distance on a chromosome between two genes always linear? Why?

17.   If a proliferating cell is exposed to UV radiation causing DNA damage, at what checkpoint would the cell cycle be halted? What is the mechanism behind this control?

18.   What do we mean by proofreading activity of a DNA polymerase?

19. What is a tumor suppressor gene? Explain and describe the role of Rb as an example.

20. What is an oncogene? How does a proto-oncogene turn into oncogene? Give two examples of two oncogenes that are frequently mutated in human cancers.

Answer 1

16) When genes are found on different chromosomes or far apart on the same chromosome, they assort independently and are said to be unlinked.

When genes are close together on the same chromosome, they are said to be linked. That means the alleles, or gene versions, already together on one chromosome will be inherited as a unit more frequently than not. We can see if two genes are linked, and how tightly, by using data from genetic crosses to calculate the recombination frequency. The more recombination frequency the more their apart and not linear and less apart less recombination frequency and more linear.

17) The exposure of proliferating cell to UV rays cause the DNA damage and DNA replication is halted. The DNA is damaged and breaks into single strands some time dimers are formed in the DNA strand or a mismatch pair of DNA mismatch occurs due to high radiation of UV rays which may lead to cancers. Generally the UV rays affects at DNA replication phase that is synthesis phase and mismatch mechanism occurs.

18) Of thousand of bases pairs attached to complementary strand of DNA ate 3’ end are may be a mismatch by adding wrongs base pairs to that of parent strand when such mismatch occurs the DNA elongation pauses and DNA polymerases reverses its activity on the strand removing the mismatch pair on the strand.

19) Tumor suppressing genes which suppress the growth of oncogenes .Tumor suppressor genes act on specific gens in cell making the cell cycle stop. Retinoblastoma acts as tumor suppressor gene which binds to E2F transcription factor inhibiting the production of enzymes and proteins once Rb release form E2F factor E2F factor binds to the transcription site and begins synthesis of proteins.

20) Oncogene is a gene tht has ability to cause cancer due to abnormal function and gene expression produced rapid cell division and protein synthesis due to loss of cell death. Proto-oncogene is the gene which favors the development and growth of oncogene due to increased mutation and cell growth and later converts to oncogene

Example: ras genes have assocated witrh many of human tumors, Burkitt lymphoma is a type of cancer found in B lymphocytes cells.