10/11/2012 § 4 Comments
The second part of protein synthesis is translation, which is a little bit harder than transcription (because transcription is basically DNA replication but with RNA nucleotides). Translation happens between tRNA, ribosomes, and the mRNA strand.
The structure of tRNA molecules
tRNA are structured so that they can hold specific amino acids. Along with specific structures and loops, the structure of tRNA is what allows for there to be twenty different tRNA molecules for the twenty different amino acids available [in the world]. ATP is used to attach the amino acids to the tRNA molecule and a high-energy bond is what holds the two together. This bond later provides energy to link the amino acid to the elongating polypeptide chain in translation. All tRNA molecules have:
- a triplet of bases that are like an ID; these are called anticodons and are made up of a loop of seven bases (three of the bases on this loop make up the anticodon). The anticodon is an identifier and is complementary to the codon (also three bases) that can be found on the mRNA.
- some sections that are double stranded because of base pairing between purines and pyrimidines
- two extra loops
- a section with the base sequence CCA, which is the site that allows an amino acid to attach to the tRNA by the enzyme tRNA activating enzyme, which recognize which tRNA molecule is which by the shape and chemical properties
The structure of ribosomes
Translation happens on ribosomes, which are composed of a mix of rRNA (ribosomal RNA) and proteins. RIbosomes are crucial in the process of translation and are made up of two subunits, a large one and a small one. The surface of the ribosome holds a binding site for the mRNA strand and three binding sites for the tRNA to bind to. Two tRNA molecules can bind to the ribosome at a time. One of the binding sites (site “E”, the exit site) are used for detaching the tRNA that is finished with its part of elongating the polypeptide chain (during translation). The other two sites are the “P” (peptidyl) and “A” (aminoacyl) sites. There are free ribosomes that can be found all around the cytoplasm that do translation to produce proteins for within the cell and there are also bound ribosomes that are found on the membranes of the endoplasmic reticulum that do translation to produce proteins to be secreted from the cell or for lysosomes. More than one ribosome can do translation on a single strand of mRNA at a time.
Protein Synthesis 101: Translation
Like transcription, the three parts to translation are: initiation, elongation, and termination.
During initiation, a tRNA molecule with the anticodon complementary to the starting codon (AUG – therefore the starting anticodon must be UAC) binds to the peptidyl site of the small subunit of the ribosome. The small subunit then binds to the 5′ end of the mRNA strand and moves along it until it comes across the starting codon AUG. Here, the large subunit of the ribosome binds to the small unit and the next tRNA (with an anticodon complementary to the codon at the aminoacyl site) binds to the whole unit. Translation begins.
During elongation, when the two tRNA molecules are bound to their respective P and A sites, the large subunit slides over the small subunit in order to detach the polypeptide from the previous tRNA (the one that was in the P site) and attaches it to the tRNA molecule in the A site using the peptide link from the amino acid in that tRNA. The small subunit slides over to follow the large subunit, placing the now empty-handed tRNA into the E site, where it is detached and can being translation somewhere else after attaching again to its respective amino acid. Note that translation always occurs in a 5′ to 3′ direction, where initiation began at the 5′ end and the amino acids are added at the 3′ end.
During termination, the polypeptide chain continues to elongate and the entire unit moves down the mRNA strand (in a 5′ to 3′ direction) until it comes across the stopping codon on the mRNA, to which no tRNA molecule has a complementary anticodon. Similarly to elongation, the large subunit slides over the small subunit, detaching from it the finished polypeptide chain. The whole unit then separates: the tRNA, large subunit, small subunit, and mRNA strand. Easy?
- Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. (5)
- Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. (5)
- Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together. (8)
- Outline the structure of the nucleosomes in eukaryotic chromosomes. (4)
- State a role for each of four different named enzymes in DNA replication. (6)
- Explain the process of DNA replication. (8)
- Explain how the process of DNA replication depends on the structure of DNA. (9)
- Describe the genetic code. (6)
- Discuss the relationship between genes and polypeptides. (5)
- Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. (4)
- Compare the processes of DNA replication and transcription. (9)
- Distinguish between RNA and DNA. (3)
- Describe the roles of mRNA, tRNA and ribosomes in translation. (6)
- Outline the structure of tRNA. (5)
- Outline the structure of a ribosome. (4)
- Explain the process of translation. (9)
- Compare DNA transcription with translation. (4)
DATA BASED QUESTIONS
Page 73, interpreting electron micrographs
The electron micrographs in the figures [that are only found in our textbook] show transcription, translation, and DNA replication. Deduce, with reasons, which process is occurring in each electron micrograph.
The electron micrograph on the left depicts DNA replication because if you look closely, the purple units show the many different enzymes working on replication. There are more subunits (enzymes) on this micrograph than in the right micrograph. These purple dots show the enzymes DNA polymerase I, DNA polymerase III, RNA primase, helicase, and ligase – all of which are working on the leading and lagging strands of replication.
The electron micrograph in the middle depicts transcription specifically because of the bubble we see forming along the strand. When RNA polymerase unzips the DNA helix into two strands, the gap between the two strands creates a bubble for the RNA polymerase enzyme to add nucleotides (on the 3′ end) to the growing mRNA strand.
This leaves the electron micrograph on the left to depict translation, which can be shown through the elongating polypeptide chains that are already folding into their secondary structures even as more amino acids are being added to the chain. It is also translation because there is only a single strand (mRNA) from which translation is occurring, not two strands.