Ribosomes

Ribosomes bound to the Endoplasmic Reticulum

This would be the 'Rough Endoplasmic Reticulum' (RER or rough ER)

Translating RNA to proteins.

The Ribosome is made of of two (2) subunits (oddly enough called a large subunit and a small subunit). Ribosomes are about 25 nm (nanometers) in diameter (a order of magnitude smaller than the wavelength of visible light). The subunits are made up of ribosomal RNA (rRNA) and ribosomal proteins. Ribosomes come in to basic varieties, this is just that they are bound to the Endoplasmic Reticulum or not. (Hence the name 'free ribosomes' for the ribosomes that "float" in the cytosol). First lets have a short review of the process to create proteins:

1. DNA is in the nucleus and is transcribed to make RNA by RNA polymerase.

2. The mRNA (messenger Ribonucleic Acid) migrates out of the nucleus. (NOTE: there are some other intermediate RNA's that maybe preprocessed).

3. The mRNA either is destroyed or it finds a ribosome and the Ribosome matches 3 Nucleic Acid patterns to line up and _join_ Amino Acids to form a protein (via tRNA).

Some definitions:

• mRNA - messenger RNA - moves from the nucleus to the cell cytoplasm where it either breaks down, or 'runs into' a ribosome.
• tRNA - transfer RNA - this has a 'anti-codon'. The ribosome matches the anti-codon of the tRNA to the codon of the mRNA.
• codon - is a 3 nucleic acid sequence that code for a specific amino acid.

How does RNA differ from DNA?

DNA is made up of 4 nucleic acids and there is one difference there is a Thymidine in DNA (T) and a Uricile (U) in RNA.

Nucleic Acids make up RNA and DNA. Amino Acids make up Proteins. If there is a T in a sequence it is DNA, if there are 'U's in a sequence you know it is RNA.

Here is a way to understand the codons:

            3 Letter   1 Letter       DNA codons for each Amino Acid
NAME    Abbreviation  Abbreviation

Alanine         Ala       A             GCA,GCC,GCG,GCT
Cysteine        Cys       C             TGC,TGT
Aspartic Acid   Asp       D             GAC,GAT
Glutamic Acid   Glu       E             GAA,GAG
Phenylalanine   Phe       F             TTC,TTT
Glycine         Gly       G             GGA,GGC,GGG,GGT
Histidine       His       H             CAC,CAT
Isoleucine      Ile       I             ATA,ATC,ATT
Lysine          Lys       K             AAA,AAG
Leucine         Leu       L             TTA,TTG,CTA,CTC,CTG,CTT
Methionine      Met       M             ATG
Asparagine      Asn       N             AAC,AAT
Proline         Pro       P             CCA,CCC,CCG,CCT
Glutamine       Gln       Q             CAA,CAG
Arginine        Arg       R             CGA,CGC,CGG,CGT,AGA,AGG
Serine          Ser       S             TCA,TCC,TCG,TCT,AGC,AGT
Threonine       Thr       T             ACA,ACC,ACG,ACT
Valine          Val       V             GTA,GTC,GTG,GTT
Tryptophan      Trp       W             TGG
Tyrosine        Tyr       Y             TAC,TAT 

Stop Codons               .             TAA,TAG,TGA

An example of the multiple combinations of DNA possible for a single peptide is an example of spelling my first name (without a termination codon): So to code for a bogus sequence, but it serves as a simple example. If we use the Amino Acid sequence 'MARK' there would be 16 combinations, other sequences of 4 letters would vary in the number of possibilities based on the number of codons that could code for a single amino acid. Some amino acids have upto 6 codons that will be translated into a single Amino Acid.

 M   A   R   K    M   A   R   K    M   A   R   K    M   A   R   K
MET Ala Arg Lys  MET Ala Arg Lys  MET Ala Arg Lys  MET Ala Arg Lys
===============  ===============  ===============  ===============
AUG-GCU-AGA-AAG  AUG-GCU-AGG-AAG  AUG-GCU-AGA-AAA  AUG-GCU-AGG-AAA
AUG-GCG-AGA-AAG  AUG-GCG-AGG-AAG  AUG-GCG-AGA-AAA  AUG-GCG-AGG-AAA
AUG-GCC-AGA-AAG  AUG-GCC-AGG-AAG  AUG-GCC-AGA-AAA  AUG-GCC-AGG-AAA
AUG-GCA-AGA-AAG  AUG-GCA-AGG-AAG  AUG-GCA-AGA-AAA  AUG-GCA-AGG-AAA

I will move the above information as I reorganize this course.

Back to Ribosomes

So for each codon the tRNA has a anti-codon, that will match the RNA sequence (AUG's anti-codon is UAC). And a Methionine would be added to the chain, then for GCU a Alanine would be added through a chemical bond to the Methionine. This all takes place at the Ribosome. The ribosome facilitates the tRNA's when the ribosome finds a sequence called the 'start codon' which is the sequences AUG, that codes for Methionine.

How is the Ribosome Created in the cell?

In Eukaryotes the information for the formation of the ribosomes is located in the nucleolus. (more later)...

Go to Chapter 2.2.2: 'Mitochondria/Chloroplasts'

Go to Chapter 4.2: 'Translation'

WWW Cell Biology Course Index

Go back to Chapter 2.2.1: 'Endoplasmic Reticulum'

Please send questions/comments/suggestions to: Mark Dalton at markwdalton@gmail.com.