unit 11 part 2 Flashcards
rna splicing
- removes introns
- creates a continuous molecule with readable coding sequence
- carried out by the spliceosome
Spliceosome
- A variety of proteins and several small RNAs that recognize the splice sites
- The RNAs of the spliceosome also catalyze the splicing reaction
- Functions as a ribozyme (RNA enzyme)
- Ribozymes:
- catalytic rna molecule that functions as enzyme and splice rna
- not all biological catalysts are proteins
- Three properties of RNA enable itt o function as an enzyme:
- can form 3d structure becuase of its abilit to base-pair with itself
- some bases in rna contain functional groups that may participate in catalysis
- rna may hydrogen bond with other nucleic acid molecules
Some introns contain sequences that
mya regualte gene expression
alternative splicing
- Some genes can encode for more than one polypeptide, depending on which exons get included during splicing:
Splicing Results
- Proteins often have a modular architecture consisting of discrete regions called domains
- In many cases, different exons code for the different domains in a protein
- Exon shuffling may result in the evolution of new proteins
Understanding Translation
- Genetic information flows from mRNA to protein through the process of translation
- Translation creates a polypeptide from the mRNA information
transfer RNA (tRNA)
- tRNAs transfer amino acids to the growing polypeptide in a ribosome
- Each tRNA molecule enables translation of a specific mRNA codon into a certain amino acid
- Each carries a specific amino acid on one end
- Each has an anticodon on the other end
- Anticodon base-pairs with a complementary codon on mRNA
3D Structure of tRNA
- Single RNA strand that is~80 nt long
- Flattened into one plane to reveal tRNA base pairing, looks like a cloverleaf
- Hydrogen bonding twists tRNA into 3D molecule
- tRNA is roughly L-shaped with the 5’and3’ ends both located near one end of the structure
- The protruding 3’ end acts as an attachment site for an amino acid
Ribosomes
- Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
- The two ribosomal subunits (large and small):
- Made of proteins and ribosomal RNA (rRNA)
A site
holds the tRNA that carriers the next amino acid to be added to the chain
P site
holds the tRNA that carriers the growing polypeptide chain
E site
is the exit site where discharged tRNA leave the ribosome
- The three stages of translation:
- Initiation
- Elongation
- Termination
- Energy is required for some steps
Synthesizing a polypeptide - Initiation
- The start codon (AUG) signals the start of translation
- A small ribosomal subunit binds with mRNA and a special initiator tRNA
- The small subunit moves along the mRNA until it reaches the start codon (Met)
- Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
Synthesizing a polypeptide - Elongation
- During elongation, amino acids are added one by one to the C-terminus of the growing chain
- Each addition involves proteins called elongation factors
- Translation proceeds along the mRNA in a 5′ → 3′ direction
- The ribosome and mRNA move relative to each other, codon by codon
- Elongation occurs in three steps
codon recognition, peptide bond formation, and translocation
* Requires energy during the first and third steps
Synthesizing a polypeptide - Termination
- Elongation continues until a stop codon in the mRNA reaches the A site of the ribosome
- The A site accepts a protein called a release factor
Release factor
causes the addition of a water
molecule instead of an amino acid
* Hydrolysis, splits bond between tRNA and polypeptide
- energy is required
translation folding protein does what
- During translation, polypeptide chain spontaneously folds into 3D molecule
- A gene determines primary structure, and primary structure in turn determines shape
- Often translation is not sufficient to make a functional protein
Post Translational Modifications
- Polypeptide chains are modified after translation or targeted to specific sites in the cell
- Post-translational modifications may be required before the protein can begin doing its particular job in the cell
- Examples: protein cleavage, phosphorylation, acetylation
Ribosomes
- Ribosomes are identical and can switch from free to bound
- free ribosomes (cytosol)
- Free ribosomes mostly synthesize proteins that function in the cytosol
- bound ribosomes (attached to ER)
- Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell
- Polypeptide synthesis always begins in the
cytosol
- synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER
Polypeptides destined for the ER or for secretion are marked by a
signal peptide
- A signal-recognition particle (SRP)
Binds to the singal peptide on the N terminus
* The SRP escorts the ribosome to a receptor protein built into the ER membrane
* Polypeptide synthesis resumes, sending polypeptide into ER lumen
- Polyribosome (or polysome):
- Multiple ribosomes can translate a
single mRNA simultaneously - Polyribosomes enable a cell to make many copies of a polypeptide very quickly
Bacterial Processes
- A bacterial cell ensures a streamlined process by coupling transcription and translation
- Occuratsametime
- In this case the newly made protein can quickly diffuse to its site of function
Mutations
- Mutations are changes in the genetic information of a cell
- Mutations of one or a few nucleotides can affect protein structure and function
- If a mutation has an adverse effect on the phenotype of the organism
- Referred to as a genetic disorder or hereditary disease
Spontaneous mutations can occur during errors in
DNA replication, recombination, or repair
* Mutagens are physical or chemical agents that can cause mutations
- Point mutations:
changes in just one nucleotide pair of a gene
* The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein
* Point mutations within a gene can be divided into two general categories:
* Single nucleotide-pair substitutions
* Nucleotide-pair insertions or deletions
- Point mutations:
changes in just one nucleotide pair of a gene
* The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein
* Point mutations within a gene can be divided into two general categories:
* Single nucleotide-pair substitutions
* Nucleotide-pair insertions or deletions
- Silent mutations:
- No effect on the amino acid produced by a codon because of redundancy in the genetic code
- Due to Wobble
- Missense mutations:
- Still code for an amino acid, but not the correct amino acid
- Nonsense mutations:
- Change an amino acid codon into
a stop codon - Most lead to a nonfunctional protein
- Insertion or deletion (in/del)
of nucleotides often alters the reading frame, producing a frameshift mutation
* Depends if in/del is a multiple of three
- These mutations may have a disastrous effect on the resulting protein
