If the sequence of the template strand in a transcription unit is...

The mRNA is synthesized in the 5' $\rightarrow$ 3' direction and is complementary and antiparallel to the template DNA strand.

Given template strand: 3' - TAC GAC TAT GCG ATC - 5'

In RNA, thymine (T) is replaced by uracil (U). The complementary base pairing rules are:
T pairs with A
A pairs with U
C pairs with G
G pairs with C

Matching the bases from the 3' end of the template gives the mRNA sequence from the 5' end:
TAC $\rightarrow$ AUG
GAC $\rightarrow$ CUG
TAT $\rightarrow$ AUA
GCG $\rightarrow$ CGC
ATC $\rightarrow$ UAG

Thus, the newly transcribed mRNA sequence is 5' - AUG CUG AUA CGC UAG - 3'.

Answer: 5' - AUG CUG AUA CGC UAG - 3'