W3 Proteins structure and synthesis Flashcards
What are the main functions of proteins? (4)
Carrying out enzymatic functions
Generating movements of cells
Carrying signal from cell to cell and within individual cells
Providing support to cells and tissues
What are proteins made up of?
Linear chains of amino acids arranged in a 3D structure
What does the function of a protein depend on?
A protein’s amino acid sequence
What are ISD’s- Intrinsically disordered proteins? (not mandatory)
They are proteins that have lost their native, functional folding, resulting in an insoluble aggregation in many organs, causing many diseases.
How many amino acids are there?
20
What do all amino acids have in common? (structure)
What is different?
C, COOH, H, NH2 and different R group (specific for each amino acid)
What are the 3 phases of (DNA) transcription called?
Initiation
Elongation
Termination
What occurs during initiation?
(transcription)
- RNA polymerase binds to a specific sequence of DNA (promoter).
- RNA polymerase separates the double strands and uses a single strand as a template.
- 2 initial RNA nucleotides are joined together to begin RNA strand synthesis.
What occurs during elongation (transcription)?
RNA polymerase moves along the DNA coding sequence in the 5’ to 3’ direction. It melts duplex DNA whilst it adds ribonucleotides to the growing RNA
What happens during Termination?
(transcription)
The RNA polymerase encounters a stop codon, this releases the RNA strand
The newly RNA transcript is a precursor of mRNA
How can a protein and a peptide be classified separately?
Peptide= Short chain (<50 amino acids)
Protein= Longer chain (>50 amino acids), also referred to as polypeptides
What are the 3 steps of RNA processing?
- Capping at the 5’ end
- Polyadenylation at the 3’ end
- Splicing
What is capping at the 5’ end?
What linkage?
A 5’ cap (7-methylguanate) is added to the ribonucleotide at 5’ by an usual 5’-5’ triphosphate linkage
What is polyadenylation?
A repetition of poly-A is added to the 3’ end
What are the 3 steps of translation?
What are the steps? (simple)
Initiation- The ribosome gets together with the mRNA and the first tRNA
Elongation- Amino acids are brought to the ribosome by tRNAs and linked together to form a chain
Termination- The finished polypeptide is released to go and do its job in the cell.
What happens during splicing?
A large assembly of RNA and protein (splicesome) excises (cuts out) introns and exons are joined together.
How are mRNA sequences read?
What is the set of 3 bases called?
From 5’ to 3’ in sets of three, from a specified starting point in the mRNA.
Codons are nonoverlapping and the message has no gaps (alterations in the codon reading frame would alter the protein – Several diseases)
What is the structure of a protein?
Primary structure:
- The unique sequence of amino acids in a polypeptide chain.
Secondary structure:
- Beta pleated sheets and Alpha helix
Tertiary structure:
- 3D folding including ionic, disulphide and hydrogen bonds
Quaternary structure:
- 2 or more polypeptide chain subunits in a closely packed arrangement
Why does transcription occur in the nucleus?
DNA is too large to leave the nucleus (RNA can pass through the nuclear pores)
What occurs in termination? (transcription)
- The RNA polymerase encounters a specific termination sequence (stop site).
This releases the RNA strand - The newly RNA transcript is a precursor of mRNA (or pre-mRNA)
TAC GCA TGA CTA ACT
What RNA complementary strand will be made from the following DNA
sequence?
AUG CGU ACU GAU UGA
Why does RNA processing occur?
✓ increase the stability of mRNA molecules (preventing its degradation),
✓ facilitate its migration to the cytosol
What is translation and where does it occur?
Translation – (RNA to protein) is the conversion of the information from RNA (4 nucleotides) into protein (20 amino acids). It occurs in the cytoplasm on the ribosomes
How many combinations of 3 nucleotides are possible for 20 amino acids?
Since RNA is made of 4 different nucleotides,64 (4^3) combinations of three nucleotides are
possible for 20 amino acids. Some amino acids being specified by more than one codon
tRNA structure?
What is the function of tRNA?
What are their 2 important structures?
- Short RNA folded into a 3D structure
- Adaptors between codons of mRNAs and
the corresponding amino acid
➢ anticodon, 3 nucleotides that bind the
complementary mRNA codon
➢ 3ʹ end (acceptor stem) attached to the
specific amino acid that matches the codon
tRNA for info:
- Some amino acids are linked to >1 tRNA
- Many tRNAs can pair with >1 codon (“wobble” base-pairing – non-perfect complementarity)
What are the functions of ribosomes?
-Synthesise proteins
- In eukaryotes, ribosomes (80S) consist of a
small (40S) and a large subunit (60S)
- Each subunit contains three binding sites
for tRNA molecules A, P and the E site
- During translation, tRNAs move through
sites A to P to E,
What is the
A site?
P site?
E site?
➢ A site, where the incoming charged tRNA bind to mRNA
➢ P site, binds to the tRNA holding the growing
polypeptide chain of amino acids
➢ E site, where the spent tRNA is ejected
What is the process of initiation in Translation? (detailed- 5 steps)
- tRNA carrying methionine (linked to the
start codon) binds the small subunit - Ribosomes bind to the mRNA 5’ end by recognising the 5’ GTP cap (RNA processing)
- Ribosomes slide along the mRNA (direction 5’-to-3’) until the start codon (AUG), which
matches the anticodon of the linked tRNA - The large subunit attaches to the small one so that the tRNA is aligned on site P,
completing the formation of an 80S initiation complex
What is the process of Elongation in Translation? (detailed- 4 steps)
- A charged tRNA carrying the next amino acid binds to A site on the ribosome and interacts with the mRNA codon
- The C-term of the polypeptide chain forms a peptide bond to the NH2
group of the amino acid at A site - The ribosome translocates along the mRNA to the next codon
- The spent tRNA is ejected and A site is open to bind the next charged tRNA
What is the process of Termination in Translation? (detailed- 4 steps)
- Translation halts at a stop codon (UAA, UAG, and UGA)
- The stop codons are not recognised by a tRNA and do not specify
an amino acid - Release factors bind to any stop codons that reaches the A site
- The ribosome releases the mRNA and dissociates into its two subunits
