Protein Synthesis Notes

Proteins are composed of amino acids.
There are 20 different amino acids.
Each amino acid has the same general structural formula.
The large variety of proteins occurs because of the different arrangement and variable number of amino acids in each protein.
The structural details for a protein are stored in DNA.
Each protein is made of a long chain of amino acids that twist and fold into a functional 3D shape.
Proteins have diverse structures and functions.
The digestive system uses enzymes to chemically break proteins in the food we eat into individual amino acids.
The amino acids are then absorbed into the bloodstream and transported to cells where they are used as raw materials for the synthesis of new proteins.
During protein synthesis, amino acids are linked together in the correct sequence to form a specific protein by ribosomes.
A section of DNA called a gene holds a set of instructions for the sequence of amino acids in a protein.
Each protein has a different gene and is composed of a unique sequence of amino acids.

RNA is similar to DNA except it has only one strand.
The nucleotides contain the sugar ribose and the base uracil (U) in place of thymine (T).

There are three types of RNA required for protein synthesis:
- messenger RNA (mRNA)
- transfer RNA (tRNA)
- ribosomal RNA (rRNA)

The information from a section of DNA (the gene) is transcribed (copied) into mRNA.
The mRNA carries the information out of the nucleus through the nuclear pores into the cytoplasm.
Ribosomes translate the information contained on the mRNA into the correct sequence of amino acids forming a particular protein.
Thus there are two steps to protein synthesis:
- Transcription
- Translation

Information on one of the DNA strands of the gene is copied to make mRNA (a strand of RNA nucleotides joined together).
Each of the bases on the DNA strand (adenine, thymine, guanine and cytosine) is paired with each complementary RNA nucleotide to produce mRNA.
It’s not an exact copy.

The mRNA molecule carrying the sequence information leaves the nucleus and links up with a ribosome, rRNA, in the cytoplasm.
The ribosome reads and translates this information.
Transfer RNA molecules in the cytoplasm, tRNA, carry amino acids to the ribosome (rRNA) to be added to the growing polypeptide chain.
The correct amino acid sequence is determined by the order of bases on the mRNA.
Peptide bonds link the amino acids at a rate of 15 peptide bonds per second!

DNA (transcription) → mRNA (translation) → new protein
- Transcription occurs in the nucleus.
- Translation occurs in ribosomes (cytoplasm).
The information from a section of DNA (the gene) is transcribed onto mRNA.
The mRNA carries this information out of the nucleus, through the nuclear pores, into the cytoplasm.
Ribosomes translate the information contained on the mRNA, into the correct sequence of amino acids, forming a specific protein.

Ribosomes attached to the rough endoplasmic reticulum (RER) synthesise proteins which may be exported from the cell.
Free ribosomes within the cytoplasm synthesise proteins used in the cytosol.
The RER transports the proteins to the Golgi complex where they are processed and packaged into secretory vesicles.
The secretory vesicles deliver the proteins to the cell membrane, where they are discharged by exocytosis out of the cell.

Transcription: A segment of DNA unravels and forms mRNA against the DNA template.
mRNA emerges from the nucleus via a pore in the nuclear membrane.
Translation: mRNA is read by ribosomes on rough endoplasmic reticulum.
Protein moves through rough endoplasmic reticulum.
Protein moves from RER to Golgi apparatus.
Golgi apparatus form secretory vesicles containing the protein to be secreted.
Transport vesicle secreting its contents into the extra cellular fluid outside the cell membrane.