Protein Synthesis Notes: Biology
The flow of genetic information is DNA RNAProtein. This is called the “central dogma”
of Biology.
The “genetic info” stored in DNA is the instructions for making protein.
Protein synthesis includes the processes of Transcription and Translation.
Transcription
Transcription occurs in the nucleus of the cell. DNA is used as a template to produce
messenger RNA (mRNA). RNA polymerase enzyme assembles complementary base pairs to
form single-stranded mRNA: A—U (uracil), T—A, C—G, G—C
Messenger RNA is edited to make each unique protein the cell needs. This mRNA
processing allows millions of different proteins to be made from the same DNA strand.
Parts of the mRNA that are not needed and that stay in the nucleus are called introns. The
remaining segments of mRNA that will be used to code for the proteins are called exons.
These pieces are spliced together to form the primary mRNA transcript, which exit the
nucleus to head to the ribosomes where the protein will be assembled.
Translation
Translation occurs at the ribosomes.
Ribosomes can be attached to the rough
endoplasmic reticulum (RER) or be free in
the cytoplasm. Ribosomes are made of
ribosomal RNA (rRNA) and protein.
Messenger RNA contains the genetic code
specifying the amino acids needed to
assemble the polypeptide that will form the
protein.
The mRNA is “read” at the ribosomes in
groups of 3 nucleotides called “codons”.
The amino acids coded for by the mRNA are gathered by transfer RNAs (tRNA) that have an
“anticodon” complementary to the codons for each amino acid.
The process of translation begins with the START codon AUG (methionine) and ends with a
STOP codon. The “stop” codon signals that the polypeptide is complete.
The resulting polypeptide may be modified at the Golgi apparatus, and then must fold into
its specific 3-D shape before it becomes functional.
Protein Synthesis in Prokaryotes vs. Eukaryotes
Prokaryotes do not have a nucleus, but do have ribosomes for protein synthesis.
In prokaryotes (bacteria), the mRNA is not edited before being used to make protein.
The processes of transcription and translation occur at the same time in the cytoplasm.
Eukaryotes edit the mRNA before it is used to make protein. The processes of
transcription and translation are separated.
Transcription takes place in the nucleus, while translation takes place at the ribosomes.