An overview of the Central Dogma is shown in the image above. Answers to the letters can be seen from a previous session.
Inside the nucleus, the processes - DNA replication and Transcription happen.
DNA Replication is the process of creating an exact copy of the DNA strand. It starts with the unwinding of the strands as seen in the image below. The bases are then added and proofread by a DNA polymerase. When everything is copied exactly, there will be two DNA strands composed of the original strand and a new strand.
The image below shows the next process in the Central Dogma. It is Transcription. During Transcription, the DNA is transcribed into an mRNA strand. The mRNA strand contains the genetic information to produce a protein. The mRNA is different from the DNA because it does not have the Thymine base, instead it is replaced by Uracil.
The only process of the Central Dogma not happening inside the nucleus is the Translation. Translation stars when the mRNA is released to the cytoplasm towards a ribosome. The ribosome will be attached to the mRNA and proceed to translate the codon with the tRNA matching the anti-codon and bringing the amino acids. The sites E, P, and A are where the tRNAs are placed when bringing the amino acids. The amino acid sequence finished during translation is further processed in the endoplasmic reticulum.
This is the overview of the Central Dogma - DNA Replication, Transcription, and Translation.
ANSWER to the question: Nitrogen bases in DNA are used as a template to create a messenger molecule.