Mehthylases are those enzymes which catalyze the transference of only a methyl group (-CH3) from a donor to the target molecule. Since we are talking about DNA Methylases these are those enzymes in which the target or recipient molecule happens to be a DNA or Nucleotide in a DNA sequence. A class of enzymes that alter the supercoiling of double-stranded DNA. (In supercoiling the DNA molecule coils up like a telephone cord, which shortens the molecule.) The topoisomerases act by transiently cutting one or both strands of the DNA. The regulation of DNA supercoiling is essential to DNA transcription and replication, the DNA helix must unwind to permit the proper function of the enzymatic machinery involved in these processes.

1. Presentation on DNA Methylases &
Topoisomerases
Submitted To – Dr. Munish Sharma
Submitted By – Abhishek Kumar (CUHP19BOT01)

2. DNA Methylases
 Mehthylases are those enzymes which catalyze the transference
of only a methyl group (-CH3) from a donor to the target molecule.
 Since we are talking about DNA Methylases these are those
enzymes in which the target or recipient molecule happens to be a
DNA or Nucleotide in a DNA sequence.
Methylation normally occurs on Cytosine & Adenine residue in
DNA sequences.
 The process of transfer of methyl group to its substrate is called
Methylation.

3. There is a molecule known as S – Adenosyl Mehionine known as
SAM. This is a molecule which happens to be a universal donor
for all types of DNA Methyl transferases.
DNA methyl transferase (DNMT) mediates transference of a
methyl group from the donor to the target nucleotide, at the
same time the donor gets converted from S-Adenosyl
Methionine to S – Adenosyl Homocystine.

4.  DNA Methylation regulates genes or silence genes without
changing DNA sequences, as a part of epigenetic regulation.
 In Bacterial system, methylation play a major role in
preventing their genome from degradation by restriction
enzymes.
 Some common examples of methyltransferases are – DNA
Adenyl Methyltransferase (DAM) , O-methyltransferase etc.

5. Classification of Methyltransferases
Three different patterns of DNA Methylation has been observed so
far. The targets of methylation include –
Fifth position carbon of Cytosine (m5C), fourth position nitrogen
of cytosine (m4C) and sixth position nitrogen of Adenine (m6A)

6.  m6A - generates N6 Methyladenosine
 m5C – generates N5 Methylcytosine
 m4C – generates N4 Methylcytosine

7. TOPOISOMERASES
 A class of enzymes that alter the supercoiling of double-
stranded DNA. (In supercoiling the DNA molecule coils up like a
telephone cord, which shortens the molecule.) The
topoisomerases act by transiently cutting one or both strands of
the DNA.
 The regulation of DNA supercoiling is essential to DNA
transcription and replication, the DNA helix must unwind to permit
the proper function of the enzymatic machinery involved in these
processes.
 Topoisomerases serve to maintain both the transcription and
replication of DNA.

8. Role of Topoisomerases
 The winding problem of DNA arises due to the
intertwined nature of its double-helical structure.
 During DNA replication and transcription, DNA
becomes overwound ahead of a replication fork.
 If left unabated, this torsion would eventually
stop the ability of RNA & DNA polymerase
involved in these processes to continue down the
DNA strand.
 Topoisomerases bind to either single-stranded
or double-stranded DNA and cut the phosphate
backbone of the DNA.
 This intermediate break allows the DNA to be

9. Types of Topoisomerase
 Topoisomerases I : breaks only one strand
 Topoisomerases II : breaks both strands

10.  Type I topoisomerases are enzymes that cut one of the
two strands of double-stranded DNA, relax the strand, and
reanneal the strand.

11.  Type II topoisomerases cut both strands of the DNA helix
simultaneously. They use the hydrolysis of ATP. These
enzymes change the linking number of circular DNA by ±2