Protein Synthesis

Slide 10

DNA passes on genetic information so that proteins can be synthesized in the cell.   Proteins control many different chemical processes in the cell – such as energy production, intra & extracellular transportation, enzyme synthesis, hormone synthesis, and normal cell maintenance.  Slide 10 summarizes the process from DNA replication to RNA synthesis, also known as transcription, and translation, also known as protein synthesis.  Notice that once RNA is made in the nucleus of the cell, it leaves and enters the cytoplasm, where most cellular proteins are made.

In the transcription process, DNA’s genetic code or information is transcribed to make messenger RNA (mRNA).  The DNA nucleotide language (A, T, G, C) is transferred or transcribed to an RNA language (A, U, G, C).  Once RNA is made the molecule will pass on information to make a protein consisting of amino acids.  This part of the process is known as translation; here the mRNA nucleotide language is translated to an amino acid language – two completely different types of molecules. 

Slide 11

Transcription requires 4 important components:

• RNA polymerase enzyme - catalyzes the addition of one complementary nucleotide one at a time to make a single stranded RNA molecule; each type of RNA has a different polymerase enzyme
• Promoter site - a site on the DNA molecule to signal where RNA synthesis begins; it precedes the coding region
• Transcription factors - assist with polymerase binding; factors are specialized proteins
• Nucleotides - complementary to the DNA strand

In slide 11 a double stranded DNA molecule is pictured.  One of the strands is termed the coding strand and the other is the template strand.  Notice the region in red at the 5’ end of the coding strand – this signifies the promoter site.  Promoter sites tend to be A-T rich regions, where transcription factors & polymerase bind.  The mRNA polymerase will then catalyze the formation of a single-stranded mRNA that is complementary & anti-parallel to the template strand of the DNA and has the “same” nucleotide sequence as the coding strand with the exception that the RNA has uracil as a complement to adenine rather than thymine found in DNA.  RNA is always made in the 5’ to 3’ direction.  The presence of mRNA in a cell indicates that a gene is active!  Pharmaceutical researchers are using this fact to determine if certain drugs will cause a particular response or protein to be produced in a cell and will probe for mRNA production.

Slide 12

Once the mRNA is made, it will undergo a process known as splicing, as seen in slide 12.  Enzymes known as spliceosomes will cleave intron regions from exon regions of the mRNA.  Intron regions are nucleotide sequences that do not code for protein – they are called non-coding or nonsense regions.  Exon regions contain a nucleotide sequence that holds the code for a protein.  The spliceosomes remove the introns & splice together the exon regions to form a functional copy of mRNA.  It is not known why humans have non-functional regions or introns.  Some suspect that through evolution, proteins were modified, so the entire sequence is not needed.  Others say that depending upon how the DNA is activated & transcribed, some of those nucleotides are used in other genes to code for different proteins.

The mRNA is then transported out of the nucleus and into the cytoplasm for translation or protein synthesis to occur.  Translation requires 3 different types of RNA - messenger, ribosomal and transfer:

• mRNA serves as the template for protein synthesis
• rRNA serves as the site for protein synthesis; also known as the “assembly factory”
• tRNA decodes mRNA and transports a single amino acid for the growing protein chain

There are 20 different types of amino acids that make up all human proteins.  There are 1 or more tRNA’s for each amino acid but tRNA’s are specific for only 1 type of amino acid. 

Slide 13

Slide 13 summarizes the transcription and translation process.  In the upper left of the slide the transcription process is seen which occurs in the nucleus.  After mRNA has been synthesized it is transported into the cytoplasm.  Once in the cytoplasm it will meet with rRNA and tRNA.  The rRNA serves as the “location” for mRNA & tRNA to bind.  tRNA will decode mRNA to determine the sequence of amino acids in the protein that the gene on DNA coded for.  tRNA determines which amino acid to bring in by decoding or translating a codon on mRNA.  A codon is a 3 nucleotide sequence that codes for a particular amino acid.

Slide 14 will help to define a codon.  In the upper left of the slide   mRNA, designated as a red line, contains the codon, AGU, being “read” by tRNA.  tRNA contains an anticodon that is complementary and antiparallel to the mRNA codon sequence, thus it is translating the nucleotide language to an amino acid language.  Notice that the mRNA is decoded from the 5’ to 3’ direction. 

Slide 14

How do we know what the nucleotide language translates into?  There is a genetic code or amino acid dictionary that was discovered in the 1960’s that indicates which amino acid is coded for with each codon as seen in the bottom half of slide 14.

The AGU codon is decoded by taking the first letter & locating it in the column on the left.  The next letter, G, is located in the top row on the far right.  The last letter U, is located on the column on the right where it intersects with the A & G box.  The amino acid that this tRNA will bring in is serine, abbreviated as Ser.  Notice in the upper left diagram that tRNA actually binds to serine to transport it to the growing amino acid chain or protein (polypeptide).

Also notice there are some codons (UAA, UAG, UGA) that code for STOP.  These are called termination codons.  When one of these codons is encountered, it signals for protein synthesis to stop.  Once protein synthesis is complete, the protein can be utilized by the cell or it can be further processed and packaged for transport or export out of the cell to be utilized by another cell.

For an animated view of the translation process, visit the web site http://www.ncc.gmu.edu/dna/ANIMPROT.htm.  This animation shows more detail than presented here and will help to illustrate how mRNA, rRNA, tRNA all work together to produce a protein.

We have covered the components of DNA, how it holds a code for a gene in its sequence of nucleotides, DNA replication, and how it passes on its gene information to make RNA and ultimately proteins.