DNA is made up of Adenine, Thymine, Guanine, and Cytosine. RNA has these same bases, except in RNA, there is no Thymine. Instead, Uracil is found.  

Thymine is exclusively present in DNA. Uracil replaces thymine in RNA; thus, RNA contains the following four nitrogenous bases: adenine, guanine, cytosine, and uracil.

RNA’s nucleotide components consist of a phosphate group, ribose, and nitrogenous bases. RNA’s nitrogenous bases include the purines adenine and guanine, and the pyrimidines cytosine and uracil. Thymine is a pyrimidine found exclusively in DNA. 

The genetic code is considered both degenerative and unambiguous. A codon will only code for one amino acid, making the code unambiguous. In contrast, multiple codons can code for the same amino acid, making it degenerative. For example, UGU will always code for cysteine, but UGC also codes for cysteine.

Nearly every living organism uses the same genetic code. tRNA anticodons are complementary to mRNA codons; they are not the same code.

As amino acids are added to a polypeptide during translation, tRNA molecules will enter the A site of the ribosome. The tRNA is then transferred to the P site, where a peptide bond is formed between the amino acid residue and the amino acid chain. Finally, the tRNA moves to the E site to release its tRNA and exit the ribosome.

The ribosome contains three sites: the A, P, and E sites.

The A site is where activation occurs, starting translation. This is where a tRNA molecule enters the ribosome and matches its anticodon to the mRNA codon.

The tRNA then shifts over to the P site to attach the amino acid. The ribosome facilitates the formation of a peptide bond, adding the amino acid to the chain.

At the E site, the empty tRNA exits the ribosome and dissociates from mRNA.

DNA contains genetic information that is transcribed into mRNA. This process is known as transcription, and occurs in the nucleus. After modification in the nucleus, mRNA exits the nucleus and enters the cell cytoplasm. In a process called translation, mRNA (in conjuction with tRNA and a ribosome) is used as a template to join amino acids to form specific polypeptides.

In summary, DNA is transcribed into RNA, which is translated into protein.

During initiation, the first tRNA molecule will bring the first amino acid to the ribosome. Although the following amino acids will enter at the A site, the first amino acid is positioned in the middle P site. The large ribosomal subunit will then attach, and translation can begin.

During the subsequent elongation phase of translation, tRNA/amino acid complexes will enter the ribosome at the A site, transfer to the P site, and then exit through the E site. Only the first complex will begin in the P site, during initiation.

While one might quickly calculate that 90 nucleotides/3 would yield a 30 amino acid sequence, it is important to remember that while the first 3 nucleotides will encode for an amino acid to start translation (methionine), the last 3 nucleotides do not. They simply stop translation and signal for the growing polypeptide chain to be released from the ribosome-tRNA translation complex without actually adding another amino acid to the end of the chain.

The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes, and a modified methionine (f-Met) in prokaryotes. The most common start codon is 5' AUG 3'. The start codon is preceded by a untranslated region which includes the ribosome binding site in prokaryotes. 

5' UAA 3', 5' UGA 3', 5' UAG 3' are all stop codons.