7.3 (DNA Unit) Flashcards
Outline the process of translation initiation.
Initiation involves the assembly of translation machinery.
- The small subunit of the ribosome binds to the 5’ end of the mRNA.
- The anticodon (UAC) of an initiator tRNA (carrying methionine) binds to the start codon (AUG) on the mRNA by hydrogen bonds.
- Anticodon to codon binding follows complementary base pair rules.
- The initiator tRNA assists in the binding of the large subunit of the ribosome to the small subunit.
- The initiator tRNA is in the P site of the large subunit.
Outline the process of translation elongation, including codon recognition, bond formation and translocation.
Once the translation machinery has been assembled, the polypeptide chain can be synthesized which involves a repeated cycle of events.
- A charged tRNA binds at the A site.
- To bind to the A site, the anticodon of the tRNA must be complementary to the codon on the mRNA.
- A peptide bond forms between the amino acids at the P and A sites (the energy required is provided by the charged tRNA).
- The tRNA at the P site detaches from its amino acid.
- The ribosome moves one codon along the mrna (5’ to 3)’.
- The tRNA with the growing polypeptide chain is now in the P site.
- The tRNA with no amino acid is now in the E site and exits the ribosome (to be recharged by tRNA activating enzyme).
- A newly charged tRNA can now enter the A site.
State the direction of movement of the ribosome along the mRNA molecule
5’ to 3’ direction
Outline the process of translation termination, including the role of the stop codon.
- The ribosome moves down the mRNA until there is a stop codon in the A site.
- No tRNA molecules can bind to stop codons.
- Proteins called release factors bind to the A site.
- This causes: the polypeptide chain to be released from the tRNA in the P site. The ribosome separates from the mRNA and splits into large and small subunits.
State the difference between free and bound ribosomes.
Free ribosomes synthesize proteins for use primarily within the cell while bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
List destinations of proteins synthesized on free ribosomes.
The cytoplasm, mitochondria and chloroplasts
List destinations of proteins synthesized on bound ribosomes.
The ER, the Golgi apparatus, lysosomes, the plasma membrane or outside the cell.
Outline how a ribosome becomes bound to the endoplasmic reticulum.
(Whether the ribosome is free in the cytosol or bound to the ER depends on the presence of a signal sequence on the polypeptide being translated. It is the first part of the polypeptide translated). As the signal sequence is created it becomes bound to a signal recognition protein that stops the translation until it can bind to a receptor on the surface of the ER. Once this happens, translation begins again with the polypeptide moving into the lumen of the ER as it is created.
Compare the timing and location of transcription and translation between prokaryotes and eukaryotes.
(In eukaryotes, cellular functions are compartmentalized whereas in prokaryotes they are not.) Once transcription is complete in eukaryotes, the transcript is modified in several ways before exiting the nucleus. Thus there is a delay between transcription and translation due to compartmentalization. In prokaryotes, as soon as the mRNA is transcribed, translation begins.
Describe the primary structure of a protein, including the type of bonding involved.
The primary structure of a protein is the number and sequence of amino acids that make up the linear polypeptide. The number and sequence of amino acids is determined by the base sequence of the gene. The primary structure will determine how the polypeptide will fold-up following translation. The shape of a protein determines its function.
Describe the secondary structure of a protein, including the type of bonding involved.
Secondary structure represents the first level of protein folding. It involves hydrogen bonds forming between polar C(double bond) O and N-H groups within the polypeptide backbone. These groups are positioned on either side of each peptide bond.
Identify the alpha-helix and beta-pleated sheet in images of protein structure.
There are two types of folding that contribute to secondary structure: (1) Alpha helix- the polypeptide forms a helix with hydrogen bonds between the turns of the helix. (2) Beta pleated sheets - if sections of the polypeptide align parallel to each other, hydrogen bonds form between them. In both cases, hydrogen bonds stabilize the structure. Alpha helices and beta-pleated sheets are found in nearly all proteins.
Describe the tertiary structure of a protein, including the types of R group interactions involved.
The tertiary structure is the final, three dimensional shape of a polypeptide. R groups determine the way that the polypeptide chain folds up. There are four types of interactions between R groups that hold the tertiary structure: (1) Hydrogen bonds form between amino acids with polar R groups. (2) Ionic bonds form between amino acids with oppositely charged R groups. (3) Disulfide bridges (covalent bonds) can form between two amino acids containing sulfur in the R group. (4) Hydrophobic interactions form between amino acids with non-polar R groups.
Explain how the chemical characteristics of R groups in the polypeptide chain affect protein folding.
If surrounded by water, amino acids with hydrophobic (non-polar) R groups tend to be located in the center of the protein and those with hydrophilic (polar or charged) R groups tend to be on the outside
Outline the quaternary structure of protein folding.
Some proteins are composed of more than one polypeptide. Intermolecular bonds (including hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions) hold different polypeptide chains together. The number and positioning of polypeptide chains is called the quaternary structure. For example, the structural protein collagen is formed by three polypeptide chains winding around each other.
