First, mRNA is formed through transcription. During transcription, the two DNA strands in the
helix unzip to allow RNA polymerase to make a complementary copy of the gene, called mRNA
(messenger RNA). mRNA is made in the 5’ to 3’ direction by readingthe DNA strand in the 3’ to
5’ direction. A newly transcribed mRNA is called a pre-mRNA, as it is not yet ready for export from
the nucleus and translation to protein. For the mRNA to mature, it needs several modifications,
called posttranslational modifications.
Posttranslational modifications in eukaryotes include the addition of a 5’ GTP cap, which is
important for targeting the mRNA for nuclear export, preventing degradation, and promoting
translation. A poly(A) tail is alsoadded to the 3’ end of the mRNA;thisa stringofadenosinesthat,
like the 5’ GTP cap, are important for nuclear export, protection from degradation, and
translation. Some parts of the sequence that do not translate into proteins, called introns, must
also be removed through a process called splicing. The parts of the gene that encodes mature
mRNA are called exons. In some cases, exons are selectively included or excluded, resulting in
several different sequences of proteins from the same gene in an organism. This process is called
alternative splicing.
Once the mRNA is mature, it is shuttled out of the nucleus to the cytoplasm for the next step,
translation. Proteins are made during translation. In this process, the mRNA enters the ribosome
where transfer RNA (tRNA) translates the message to make proteins. tRNA recognizes mRNA
sequences and translates them to amino acid sequences. The bulk of ribosomes are also made of
specialized RNA, called rRNA, and proteins. The function of ribosomes is to bring together mRNA
and complementary tRNAs so that proteins form. In eukaryotes, ribosomes are found both in the
cytoplasm and on rough endoplasmic reticulum; thus, translation can occur at either of these
sites. In prokaryotes, translation occurs in the cytoplasm.
Translation occurs in three steps: initiation, elongation, and termination. During initiation, an
mRNA sequence, called the start codon, signals the start of translation. Codons are groups of
three base pairs, the sequence of which encodes either a specific amino acid or start or stop
sequence. The RNA-amino acid code is universal for all life on Earth; in other words, the same
sequence ofcodons encodes the same aminoacids for any form oflife, and thisprovidesevidence
of a common ancestor. During elongation, tRNA read the mRNA codon through RNA interactions,
and this in turn signals the attached amino acid to be transferred to the growing protein chain,
also called a polypeptide chain. The mRNA continues to elongate until it reaches the stop codon,
which signals termination of translation. At the end of translation, a newly formed protein is
released.
Gene expression in prokaryotes is carried out in a similar manner to eukaryotes, with a few
fundamental differences. Prokaryotes do not have a nucleus, so transcription and translation can
occur simultaneously. They do have some forms of posttranslational modifications, but they are
different than those mentioned for eukaryotes. There are also no introns known in prokaryotes,
so alternative splicing does not occur.