How many subunits are ribosomes made up of?
2
When are ribosomal subunits joined?
When synthesising a protein
What does the ‘S’ of a 70S ribosome stand for?
Svedberg unit = sedimentation coefficient
What is a sedimentation coefficient?
A measure of how quickly a molecule sediments
Bigger molecules sediment _______ and therefore have a ________ S value
Quicker
Higher
What is the structure of a ribosomal subunit?
RNA core with proteins attached to the outside
What is the number of proteins attached to the outside of the RNA core of a ribosome dependent on?
The length of RNA
Roughly how many ribosomes are there in a cell?
1,000,000
Describe the process of ribosome synthesis.
- rRNA is synthesised in the nucleolus
- Ribosomal proteins are synthesised in the cytoplasm
- Ribosomal subunits are produced by assembling the rRNA and the ribosomal proteins in the nucleolus
- Individal subunits leave the nucleus are pair off to form whole ribosomes during protein synthesis
Where are the genes encoding for rRNA found?
On 10 chromosomes in close proximity to each other found in the nucleolus
What is the main function of ribosomes?
Translation
Describe the process of translation.
- mRNA binds to a small subunit
- The subunits moves along the mRNA until it reaches the start codon
- The large subunit binds
- The ribosome matches the tRNA anticodon sequence with the mRNA codon sequence
- Each time a new tRNA is incorporated in the anticodon sequence the amino acid attached to it is added the the elongating polypeptide
- When the mRNA subunit reaches the stop codon the release factor will bind to the mRNA
- The polypeptide chain is released
- The two subunits dissocaite
What is a typical start codon?
AUG
What is a typical stop codon?
TAA
TAG
TGA
What three sites are found within the large subunit?
E - site
P - site
A - site
What happens at the E-site in the large subunit?
Uncharged tRNA exits the ribosome
What happens at the P-site in the large subunit?
Peptidyl tRNA is formed
What happens at the A-site in the large subunit?
Aminoacyl tRNA enters = charged tRNA
What does ribozyme do?
Catalyses the formation of peptide bonds between adjacent amino acids
What are polyribosomes?
Multiples ribosomes bound to the same piece of mRNA
Why might proteins require degradation?
Misfolded
Aged
Damaged
What component of the cell is responsible for the degradation of proteins?
Proteasomes
What is the structure of a proteasome?
Hollow tube of many subunits containing ATP dependent proteases
Cap structures at either end act as gateways and have ATPase activity = this unfolds target proteins
How long is a substrate retained in the proteasome tube for?
Until it is broken down into small peptides
