Question

QUESTION 32

1. You are a researcher studying the genetic basis of colon cancer. You have been working with a colon cancer cell line to determine the expression levels of different genes that might contribute to cancer formation. You obtain the DNA methylation status of five genes of interest (the data are shown in the table below). The plus (+) sign indicates the level of DNA methylation; more plus signs correlates with increased methylation levels.

   Gene	Methylation levels
   1	++
   2	+++++
   3	+++
   4	++
   5	+

   
   Based on the information shown above, which gene would you predict to have the highest rate of transcription?

   	A.	gene 5
   	B.	gene 2
   	C.	gene 1
   	D.	gene 3
   	E.	gene 4

Answer 1

1.Gene 5 have highest rate of transcription. (option A).

Reason :

• First coming to what is DNA methylation : as the name indicates,it is the addition of methyl group to a DNA molecule.
• It involves the covalent transfer of methyl group to the C - 5 position of cytosine ring of DNA by a specific enzyme called DNMTs(DNA methyltransferases).
• Not only cytosine ,adenine can also be methylated,but cytosine methylation is most prominently seen in both prokaryotes and eukaryotes.
• 60 - 70% of CpG islands( region having large number of cytosine - guanine dinucleotide repeats) are found in promoter region of humans.
• So how does this DNA methylation affects the transcription: DNA methylation is linked with transcription repression.(i.e. DNA methylation decreases the rate of transcription).

DNA methylation affects the transcription in 2 ways ;

1. DNA methylation affects the binding of transcriptional proteins to the genes,which are essential for transcription,hence lags the process.
2. Methylated DNA may bind with the MBDs(methyl CpG binding domain proteins),thereby they attracts the histone deacetylases and other chromatin remodelling proteins.This will leads to modifications of histones thus forming compact inactive chromatin or Heterochromatin.This is another way of effecting the transcription rate.     Here in this question gene 5 is less DNA methylated compared to the others,so it has the chance of having higher rate of transcription.