DNA transcription:

DNA copied gene by gene onto an RNA.
Length of DNA to be transcribed is called transcription unit
Each transcription unit has a sequence just "above" towards 3'(upstream) called the promoter that tells RNA polymerase where to start the transcription

This promoter is usually comprised of A,T nitrogenous bases called the TATA box (sequence is TATAWAW, W = A or T)

RNA Polymerase binds at the promoter(TATA box) and goes along the strands, splitting them
It uses one strand as a template as it goes upstream
This is similar to how DNA polymerase replicates DNA but instead produces the RNA version of the nitrogenous bases
It re zips the DNA back as it goes along until it reaches a section of the DNA called the termination box which causes it to pull off.

The RNA that is produced(pre-mRNA) undergoes some changes before it leaves the nucleus
A special type of guanine(G) is added to the 5' end of the pre- mRNA called the 5' cap
On the 3' side, another enzyme added about 250 adenine(A) onto the 3' end called the poly A tail.
These help the mRNA to not degrade, make it easier to leave the nucleus and attach to other organelles.

The pre-mRNA also has non-coding parts of it(introns) removed and only the coding parts(exons) are spliced back together.
The process of removing the introns is very complicated.
The spliced mRNA is called mature mRNA.