-Small, ~ 70-90 ribonucleotides long (remember, they are transcribed from certain DNA sequences)
-Single-stranded but folded into a particular 3D shape. The 3D shape is held together by intrachain hydrogen bonds between complementary base pairs. Like all RNA molecules, they have a 3'-OH terminus, but the opposite end terminates with a 5' monophosphate rather than a 5'-triphosphate, because tRNA molecules are cut from a large primary transcript.
see Figure of tRNA in text book (P. 274-275)
Note the D arm which contains dihydrouracil, the T psi C arm which contains a thymine, a pseudouridine, and a cytosine. Also note the presence of the extra arm. In class I tRNAs the extra arm is quite small, in class II RNAs the extra arm is long.
Note the 4 regions of molecule where you have intrastrand hydrogen bonding.
Note the location of the anticodon. The anticodon is a sequence of 3 bases that can form base pairs with a codon sequence in the mRNA.
No tRNA has anticodon complementary to stop codons UAG, UAA, or UGA
Theamino acid attachment siteis located at the 3'OH terminus
The amino acid corresponding to the particular mRNA codon that base pairs with the tRNA anticodon is covalently linked to this terminus. Amino acids will come off to form polypeptide chain during protein synthesis.
tRNAs are actually Amino acid carriers
With the help of a specific enzyme the tRNA recognizes and binds to a specific amino acid.
The enzymes are termed: aminoacyl tRNA synthetases
There is a different synthetase enzyme for every amino acid. A specific aminoacyl tRNA synthetase matches the amino acid with the anticodon; to do so , the enzyme must be able to distinguish one tRNA molecule from another. The necessary distinction is provided by an as yet ill-defined region encompassing many parts of the tRNA molecule called the recognition site.
At the opposite end of the molecule is a loop which contains the anticodon.
As the mRNA codons are read, only the appropriate tRNA can hydrogen bond with the codon by virtue of the anticodon.
C C G A A C mRNA codons
G G C U U G tRNA anticodons
tRNAs are initially made as part of a somewhat longer RNA transcript. First cut out, then completed by an enzyme that adds the three nucleotides CCA to the 3'end. Note that the 3' ribonucleotide base is ALWAYS adenine.
tRNA Terminology
charged - tRNA with amino acid attached
uncharged - tRNA, no amio acid
mischarged - incorrect amino acid
The different tRNA molecules and synthetases are designated by stating the name of the amino acid that can be linked to a particular tRNA molecule by a specific synthetase:
i.e. leucyl-tRNA synthetase attaches leucine to tRNA^Leu
Only the tRNA molecule, not its attached amino acid determines where the amino acid is added during protein synthesis. If you mischarge a tRNA it will put wrong amino acid in according to the anticodon.
[See Figure in text of how enzyme adds amino acid to the tRNA.]
Site where mRNA is translated into protein - brings together the tRNAs, mRNAs, and specific amino acids.
Ribosomes are composed of 2 subunits:
prokaryotic ribosome is a 70s ribosome. The eukaryotic ribosome is an 80s ribosome. The ribosomes and ribosomal subunits are described in terms of their approximate rate of sedimentation (in Svedberg units). The higher the mass of the particle the higher the s value and the greater the rate of sedimentation.
70s Prokaryotic Ribosome is composed of a 50s large ribosomal subunit and a 30s small ribosomal subunit.
The 50s subunit is composed of one, 5s rRNA, one, 23s rRNA, and approximately 32 different proteins.
The 30s subunit is composed of one, 16s rRNA and approximately 21 different proteins.
80s Eukaryotic Ribosome is composed of a 60s large ribsomal subunit and a 40s small ribosomal subunit.
The 60s subunit is composed of one, 5.8s rRNA, one, 28s rRNA, one, 5s rRNA, and approximately 45 different proteins.
The 40s subunit is composed of one, 18s rRNA and approximately 30 different proteins.
The ribosome serves as the site of protein synthesis. mRNAs, tRNAs, and amino acids are brought together. On the ribosome, the mRNA fits between the two subunits (the interactions are stabilized by interchain hydrogen bonding). The tRNAs occupy a site on the large ribosomal subunit.
The ribosome attaches to the mRNA at or near the 5'end. In prokaryotes there is a ribosome binding site near the 5'end of the mRNA. In eukaryotes, the ribosome first attaches at the 5'CAP (7-methyl guanosine).
The ribosome then moves along the mRNA in the 5' to 3' direction, one codon at a time.
New Topic:
FUNCTIONS:
1. Compartmentalization
Membranes enclose compartments --- plasma membrane encloses entire cell----nuclear and cytoplasmic membranes enclose various internal cellular spaces in which specialized activities take place.
2. Provides a selectively permeable membrane barrier
Movement of molecules either into or out of cell is regulated . Only very small uncharged molecules can readily diffuse i.e.. O2 and CO2
3. Provides for the transportation of solutes
The membrane contains the machinery for physically transporting substances from one side of the membrane to another --- even against a concentration gradient.
4. Ability to respond to external signals
Receptors on surface have a particular ligand.
When ligand binds to the receptor the signal must be transduced to the nucleus.
Signal Transduction
Hormones, growth factors, neurotransmitters can all serve as ligands.
5. Provides for intercellular interactions
cell-cell adhesion
cell-ECM adhesion
6. Serves as a site for biochemical activities
Organizes cellular activities by the association of multi-enzyme complexes
i.e. electron transport chains
7. Provides for energy transduction
Transduction refers to the conversion of one type of energy into a different form of energy.
Ex: the energy of sunlight is absorbed by membrane-bound photosynthetic pigments and converted to chemical energy contained in carbohydrates.
Conversely- the chemical energy in carbohydrates and fats can be converted to ATP.