Transcription and Translation Flashcards
Ribosomal subunits
Large subunit (LSU) = Contains the active site that catalyzes peptide bond formation
Small subunit (SSU) = Binds the anticodon end of the tRNAs
Differences between replication and transcription
replication transcription
Template double strands single strand
Substrate dNTP NTP
Primer Yes No
Enzyme DNA polymerase RNA polymerase
Product dsDNA ssRNA
Base pair A-T, G-C A-U, T-A, G-C
- enzyme responsible for the RNA synthesis
RNA Polymerase
- each transcriptable region is called
Operon
- DNA sequence that RNA-pol can bind
Promoter
Types of Regulatory Sequences
Promoter
Enhancers
Silencers
Insulators
Operators
Terminator
Prokaryotic Promoter
Pribnow Box
TRANSCRIPTION PROCESS: PROKARYOTES
INITIATION
- RNA polymerase enzyme and initiation factor binds at the promoter of DNA sequence and begin transcription
ELONGATION
- RNA polymerase, enzyme nucleoside triphosphate behaves as a substrate and polymerases the nucleotides of templates as a complementary strand
TERMINATION
- rho, a terminator factor replaces the initiation factor at the DNA sequence termination point
It helps the RNA polymerase holoenzyme recognize and bind to specific promoter sequences in the DNA, ensuring that transcription starts at the correct site
Sigma factor
refers to a region of DNA that regulates the expression of a gene
includes GC box, CAAT box, and TATA box (Hogness box)
cis-acting element
Steps in transcription
- Initiation
- Elongation
- Termination
Post-Transcriptional Modification
Splicing ->
-In eukaryotes, the structural gene has coding and non-coding regions. Introns are removed and coding regions or exons join together to form mature mRNA.
Capping ->
- 5′-end of hnRNA is capped by methyl guanosine triphosphate
Tailing ->
- 3′-end of hnRNA undergoes tailing by addition of 200-300 adenylate residues to form poly-A tail.
Prokaryotic vs. Eukaryotic ribosome
Prokaryotic ribosomes are smaller (also smaller svedberg coefficient) than eukaryotic ribosomes
Prokaryotic ribosomes contain 3 rRNA molecules: 16S rRNA in the small subunit and 23S rRNA and 5S rRNA in the large subunit
Eukaryotic ribosomes contain 4 rRNA molecules: 18S rRNA in the small subunit and 28S rRNA, 5.8S rRNA, and 5S rRNA in the large subunit
Prokaryotic ribosomes are located in the cytoplasm
Eukaryotic ribosomes are located in the cytoplasm as free ribosomes and some can be found in the rough endoplasmic
formylmethionine fMet-tRNA
methionine Met-tRNA
Steps in translation
- Initiation
- Elongation
- Termination
sites in the ribosome
Small subunit = A site and P site
Large subunit = A site, P site, and E site
Aminoacyl-tRNA binding site
Peptide bonding site
Exit site
Initiation of translation
- Initiation factors bind to small subunit and attract mRNA
- tRNA^fmet binds to AUG codon of mRNA in P site, forming initiation complex; IF3 is released.
- Large subunit binds to complex; IF1 and IF2 are released.
Subsequent aminoacyl tRNA is poised to enter the A site.
Elongation of Translation
- Second charged tRNA has entered A site, facilitated by EF-Tu; first elongation step commences.
- Peptide bond forms; uncharged tRNA moves to the E site and subsequently out of the ribosome
the mRNA has been translocated three bases to the left, causing the tRNA bearing the dipeptide to shift into the P site
- The first elongation step is complete, facilitated by EF-G.
- Third charged tRNA has entered A site, facilitated by EF-Tu; second elongation step begins
- Tripeptide formed; second elongation step completed; uncharged tRNA moves to E site.
- Polypeptide chain synthesized and exits the
ribosome.
Termination of Translation
- Termination codon enters A site; RF1 or RF2 stimulates hydrolysis of the polypeptide from peptidyl tRNA.
- Ribosomal subunits dissociate and mRNA is released; polypeptide folds into native 3-D conformation of protein; charged tRNA released.
also known as a polysome, is a group of ribosomes that are bound to a messenger RNA (mRNA) molecule
Polyribosomes
Stop codons
UAG
UGA
UAA
Start codon
Met (AUG)
Consequences of altering the nucleotide sequence:
Silent mutation = The change in the codon does not affect the amino acid due to the redundancy of the genetic code
Missense mutation = A single nucleotide change results in a codon that codes for a different amino acid
Nonsense mutation = A nucleotide change results in a premature stop codon (also called a termination codon), which truncates the protein, often resulting in a nonfunctional protein.
Characteristics of the Genetic Code
- Specificity = a particular codon always codes for the same amino acid.
- Universality = the specificity of the genetic code has been conserved from very early stages of evolution, with only slight differences in the manner in which the code is translated.
- Degeneracy = although each codon corresponds to a single amino acid, a given amino acid may have more than one triplet coding for it
- Nonoverlapping and commaless = the code is read from a fixed starting point as a continuous sequence of bases, taken three at a time.
Other mutations
- Trinucleotide repeat expansion:
* Occasionally, a sequence of three bases that is repeated in tandem will become amplified in number, so that too many copies of the triplet occur - Splice site mutations
* Mutations at splice sites can alter the way in which introns are removed from pre-mRNA molecules, producing aberrant proteins. - Frame-shift mutations
* If one or two nucleotides are either deleted from or added to the coding region of a message sequence the reading frame is altered.
TFIIF function
Binds tightly to polymerase II; Binds to TFIIB to prevent polymerase II from binding to nonspecific DNA sequences
TFIIB function
Binds to TBP and recruits polymerase II-TFIIF complex
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