If a cytosine base is substituted for by a thymine base in a DNA segment during DNA replication, the sequence could code for the wrong amino acid during translation.
The change in the sequence of DNA is known as mutation.
When a base substitution occurs in the sequence of a DNA during replication, the genetic information in the region where the substitution occurred becomes compromised. 
During transcription, the process of encoding genetic information on DNA into RNA, the base substituted is also transcribed along. If cytosine is substituted by thymine, adenine is transcribed instead of guanine.
During the translation of the genetic codes, the wrong amino acids might end up being translated due to the substituted base, leading to a faulty protein. 
More on mutation can be found here: brainly.com/question/4347425
 
        
             
        
        
        
DNA  is made up of deoxyribose sugar  while mRNa is made up of ribose sugar. DNA has thymine as one of the two pyrimidines while mRNA has uracil as its pyrimidines base. DNA is present in the nucleus while mRNA diffuses into the cytoplasm after synthesis. DNA is double stranded while mRNA is single stranded.
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