The central dogma of molecular biology is a simple phrase describing the flow of information in cells:
DNA makes RNA makes protein.

Or, a more detailed version of the phrase would be:
DNA is transcribed into RNA strands. Messenger RNA strands are then translated into proteins.

The central dogma path is largely one-way. While there are some special cases where RNA can be transcribed back into DNA, proteins are never translated back into nucleic acids.

The first portion of the central dogma, "DNA makes RNA" is a process called transcription. DNA is transcribed into RNA. This means that a strand of DNA is read by an enzyme called RNA polymerase, and a new strand of RNA is created from the DNA template.

But, DNA is double-stranded and RNA is only single-stranded. So how does the RNA polymerase know which strand to use?

DNA has a promoter region slightly upstream from where the RNA polymerase needs to bind. This promoter region allows the polymerase to find the correct place to begin transcription. When transcribing, the RNA polymerase moves from 3' to 5', so the polymerase will use the DNA strand with the 3' end nearest the promoter region.

The DNA strand used for transcription is called the template strand. The other strand is called the non-template strand.

When DNA is transcribed into RNA, what changes?

In the previous section, we saw that DNA is double-stranded while RNA is only single-stranded. Another difference is the switch of nucleic acids thymine and uracil.

For every thymine (T) that was present in a DNA strand, RNA polymerase will instead use uracil (U). So each time RNA polymerase sees an adenine (A) on the template strand, rather than complementing the A with a T, it will use U.

How does translation work?

Every 3 nucleotides codes for one amino acid. Each set of 3 nucleotides is called a codon. A tRNA reads the mRNA's codons and adds the appropriate amino acids. The chart below shows all the possible nucleotide combinations and for which amino acids they code.

Notice that there are several codons that code for the same amino acid. There are 64 possible nucleotide combinations, but only 20 amino acids. Often, it is the third nucleotide in a codon that can be changed without changing the amino acid. AUG is a special codon, which codes for methionine, but also acts as a start codon, telling the polymerase where to begin. UAA, UAG, and UGA are all stop codons which do not code for any amino acid. Because there is no amino acid to match those codons, translation ends when a stop codon is read.

Translate the following codons into amino acids. Use the chart to help you. Answer by typing the one-letter amino acid abbreviation. For example, enter R for Arginine.