Describe the overall subunit that make up a DNA strand into a gene sequence

DNA is the chemical that makes up chromosomes. It is a polymer consisting of monomers linked together in a long chain. The monomers are known as nucleotides.  Distinct sections of chromosomes are known as genes. When these are later copied or transcribed by messenger RNA, spliced and processed into mature messenger RNA, the genetic code can be translated at the ribosomes in the cytoplasm into a protein needed by the cell.

Each DNA subunit is called a nucleotide and consists of a phosphate group, linked to a five carbon sugar known as deoxyribose.  Linked to the sugar is one of four nitrogenous bases--adenine, guanine, cytosine or thymine. 

DNA is a double helix which means there are two complementary strands which are held together at their nitrogenous bases by hydrogen bonds. Base pairs form according to the rules- adenine pairs to thymine and cytosine to guanine. The two strands are antiparallel to each other. 

Messenger RNA copies either strand of the DNA molecule, with the assistance of enzymes that help to unwind the DNA and help to join the growing polynucleotide chain. As DNA is copied by mRNA, the same base pairing rules apply with one exception. RNA lacks the base thymine and instead substitutes the base uracil. For example, if the DNA code reads: ATCG, the complementary mRNA code reads; UAGC.  The base uracil is subsituted for thymine which is not present in RNA.

Genes can be hundreds or thousands of nucleotides long depending on what gene is being transcribed.  Once all nucleotides are transcribed into mRNA, all of the necessary genetic information known as exons are spliced together to be sent from the nucleus to the cell's protein factory--a ribosome where translation of the code occurs resulting in the formation of a functional protein.

Every triplet of mRNA is called a codon consisting of three possible bases. Each triplet is actually the code for a particular amino acid to be joined in a growing chain called a polypeptide chain that is being assembled at the ribosome. Eventually, a stop codon is reached and protein synthesis stops. The polypeptide undergoes processing including folding into a functional shape to become a protein molecule. 

The flow of information is DNA--RNA--protein synthesis. Genes are encoded by DNA which is found in the nucleus packaged as chromosomes. I have included a link showing the pathway for gene expression which results in the formation of a functional protein.