The Nature of Genes
Early ideas to explain how genes work came from studying human diseases. |
Archibald Garrod proposed that patients with the disease alkaptonuria lacked a particular enzyme. |
Beadle and Tatum studied Neurospora crassa. They looked for fungal cells lacking specific enzymes. |
Beadle and Tatum results was that each mutated enzyme disrupted one key enzyme in the metabolic pathway. |
Prokaryotic Transcription
Prokaryotic Transcription: Single RNA polymerase - Initiation of mRNA synthesis does not require a primer |
Prokaryotic Transcription requires a Promoter, Start Site, and a termination site. |
Transcription occurs in three major stages: - Initiation - Elongation - Termination |
Initiation: RNA polymerase binds to the promoter |
Promoter: Forms a recognition and binding site for the RNA polymerase. - Found upstream of the start site. - Not transcribed. - Asymetrical: indicate site of initiation and direction of termination. |
Elongation: RNA transcript grows in the 5'-to-3' direction as ribonucleotides are added. - Transcription bubble: contains RNA polymerase, DNA template, and growing RNA transcript. - After the transcription bubble passes, the now-transcribed DNA is rewound as it leaves the bubble. |
Termination: Marked by sequence that signals "stop" to polymerase. - Causes the formation of phosphodiester bonds to cease. - RNA-DNA hybrid within the transcription bubble dissociates. - RNA polymerase releases the DNA. - DNA rewinds. |
Hairpin in RNA causes RNA polymerase to pause |
U:A base pairs weaken the DNA/RNA bonding. |
Prokaryotic transcription is coupled to translation - mRNA begins to be translated before transcription is finished. |
Frameshift mutations
- Addition or deletion of a single base - Much more profound consequences - Alter reading frame downstream - Triplet repeat expansion mutation
|
Hunting disease Repeat unit is expanded in the disease allele relative to the normal |
|
|
Transcription and Translation
The Central Dogma |
Information only flows from: DNA-->RNA-->protein First described by Francis Crick. |
Transcription |
DNA--> RNA - DNA-directed synthesis of RNA - Only template strand of DNA used - T in DNA replaced by U in RNA. - mRNA used to direct synthesis of polypeptides. |
Translation |
- Synthesis of polypeptides. - Takes place at ribosome. - Requires several kinds of RNA. |
RNA |
All synthesized from DNA template by transcription - Messenger RNA (mRNA). - Ribosomal RNA (rRNA). - Transfer RNA (tRNA). - Small nuclear RNA (snRNA) - Signal recognition particle RNA (SRP RNA). - Micro-RNA (miRNA). |
Eukaryotic Transcription
3 different RNA polymerase!! |
RNA polymerase I: Transcribes rRNA. |
RNA polymerase II: treanscribes mRNA and some snRNA. |
RNA polymerase III: transcribes tRNA and some other small RNAs |
Each RNA polymerase recognizes it own promoter. |
Initiation of transcription: Requires a series of transcrption factors (helper). - Transcription factors: Necessary to get the RNA polymerase II enzyme to a promoter* and to initiate gene expression. |
Elongation: RNA transcribed from the DNA template. |
Termination not as well defined. |
Initiation of trancription - Transcription factors bind to a promoter region and recruit RNA polymerase. - Forms the initation complex.
Protein Targeting
In eukaryotes, translation may occur in the cytoplasm or the rough endoplasmic reticulum (RER). |
Signal sequences at the beginning of the polypeptide sequence bind to the signal recognition (SRP). |
- The signal sequence and SRP are recognized by RER receptor proteins - Docking holds ribosome to RER - Beginning of the protein-trafficking pathway
|
Mutation: Altered Genes
Point mutations |
alter a single base |
Base substitution |
substitute one base for another |
Siletn mutation |
same amino acid inserted |
Missense mutation |
changes amino acid inserted - Transitions - Transversions |
Nonsense mutations |
changed to stop codon |
|
|
The Genetic Code
Francis Crick and Sydney Brenner determined how the order of nucleotides in DNA encoded amino acid order. - They introduced single nucleotide insertions or deletions and looked for mutations (Frameshift mutations) |
A Codon is a block of three DNA nucleotides corresponding to an amino acid, |
Spaced Codons: Codon sequence in a gene punctuated. |
Unspaced Codons: codons adjacent to each other. - Marshall Nirenberg identified the codons that specify each amino acid. |
Stop Codon: 3 codons (UAA, UGA, UAG) used to terminate translation |
Start Codon: Codon (AUG) used to signify the start of translation |
Code is degenerate: Some amino acids are specified by more that one codon. |
Code practically unicersal: Strongest evidence that all living things share common ancestry. - Advanced in genetic engineering. - Mitochondria and cloroplast have some differences in "stop" signals. |
mRNA modifications
In eukaryotes the primary transcript must be modified to become mature mRNA |
Addition of a 5' cap Protects nucleotides from getting lost, from degradation. - Involved in translation initiation. |
Addition of a 3' poly-A tail Created by poly-A polymerase, protection from degradation - Puts whole string of A's (AAA) to protect! |
Removal of noncoding sequences (introns): Pre-mRNA splicing done by spliceosome. - Cut it out to get rid of it!!! |
tRNA charging reaction
Each aminoacyl-tRNA synthetase recognizes only 1 amino acid but several tRNAs. |
Charged tRNA has an amino acid added using the energy from ATP. -Can undergo peptide bond formation without additional energy. |
Ribosomes do not verify amino acid attached to tRNA. |
The ribosome has multiple tRNA binding sites: |
P site: binds the tRNA attached to the growing peptide chain |
A site: binds the tRNA carrying the next amino acid. |
E site: binds the tRNA that carried the last amino acid, tRNA exits ribosome. |
The ribosome has two primary functions - Decode the mRNA. - Form peptide bonds. |
Peptidyl transferase: - Enzymatic component of the ribosome. - Forms peptide bonds between amino acids. |
Chromosomal mutations
Chang the structure of a chromosome |
Deletions: part of chromosome is lost |
Duplication: part of chromosome is copied |
Inversion: part of chromosome in reverse order |
Translocation: part of chromosome is moved to a new location |
|
|
Eukaryotic pre-mRNA splicing
Introns |
non-coding sequences |
Exons |
sequences that will be translated |
Small ribonucleoprotein particles (snRNPs "snurps") |
Looks for introns and exons and recognizes it. |
Spliceosomes |
responsible for removing introns |
snRNPs cluster with other proteins to form spliceosome
tRNA and Ribosomes
tRNA moleules carry amino acids to the ribosome for incorporation into a polypeptide. - Aminoacyl-tRNA synthetase add amino acids to the acceptor stem of tRNA. - Anticodon loop contains 3 nucleotides complementary to mRNA codons. |
Translation
Process by which the mRNA transcript is read by the ribosomes and used to make a polypeptide. Occurs in 3 main stages: - Initiation - Elongation - Termination |
There are some important differences between translation in prokaryotes and eukaryotes. |
In prokaryotes, initiation complex includes Initiator tRNA charged with N-formylmethionine {[nl}}- Small ribosomal subunit - mRNA strand |
- Ribosome binding sequence (RBS) of mRNA positions small subunit correctly. - Large subunit now added. - Initiator tRNA bound to P site with A site empty.
|
Initiations in eukaryotes similar except: - Initiating amino acid is methionine. - Lack of an RBS – small subunit binds to 5′ cap of mRNA. |
Elongation adds amino acids - 2nd charged tRNA can bind to empty A site - Requires elongation factor called EF-Tu to bind to tRNA and GTP - Peptide bond can then form. - Addition of successive amino acids occurs as a cycle. |
- There are fewer tRNAs than codons - Wobble pairing allows less stringent pairing between the 3′ base of the codon and the 5′ base of the anticodon - This allows fewer tRNAs to accommodate all codons
|
Termination - Elongation continues until the ribosome encounters a stop codon - Stop codons are recognized by release factors which release the polypeptide from the ribosome |
|
Created By
Metadata
Favourited By
Comments
No comments yet. Add yours below!
Add a Comment
Related Cheat Sheets