by Ashley A Olvera 2 years ago
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3. The ribosome falls apart. All components of the ribosome separate from one another through the hydrolysis of two GTP.
3. Translocation - The empty tRNA moves to the E site and is released from the ribosome. The tRNA connected to the peptide chain is moved to the P site (the mRNA is shifted as well) and the A site is empty. The energy from GTP hydrolysis is used here. The cycle is then ready to be repeated.
3. The large ribosomal subunit attaches itself and the translation initiation complex is formed (this is due to the hydrolysis of GTP). When attached the tRNA is placed in the P site where the peptide chain can begin at the N-terminus with the amino acid MET.
Transcription is the most critical step for regulating gene expression because gene regulation can block transcription of a certain gene.
Gene Regulation at Transciption
Control elements in DNA bind transcription factors
Types of Control Elements
Distal Control Elements
Sequences in DNA called enhancer sequences that bind specific transcription factors (activators and repressors). Enhancer sequences may be upstream or downstream of a gene and close to or far from the gene they control.
A DNA-bending protein brings the bound activators/repressors closer to the promotor, and RNA polymerase II binds to the promoter, initiating trancription.
Proximal Control Elements
Sequences in DNA close to the promoter that bind general transcription factors
Transcription factors are proteins that bind at the promoter sequence on the DNA strand, increases the binding affinity for the RNA polymerase to recognize the promotor sequence and bind to it, and initiate transcription.
Types of Transcription Factors
Specific
Changes level of transcription
Types of Specific Transcription Factors
Repressors
Reduce levels of transcription down to background level
Activators
Increase levels of transcription above background level
General
Bring about low levels of transcription (background/basal)
Cell specific transcription: combinatorial control of gene expression to increase or decrease expression of different genes
Gene regulation occurs at the level of transcription.
Operons are a cluster of functionally related genes that can be under coordinated control of a single on-off “switch”. The "switch" is a segment of DNA called an operator which is usually positioned within the promoter.
The Lac operon uses both activators and repressors and is an example of both positive and negative regulation.
Glucose in bacterial cell
Adenyl cyclase active
cAMP levels high
CAP active
Adenyl cyclase inactive
cAMP levels low
CAP inactive
Lactose in bacterial cell
Not present
Repressor bound to operator
Lac operon is OFF
Present
Repressor bound to allolactose
Lac operon is ON
Types of Gene Regulation in Prokaryotes
Negative Regulation
Repressor proteins bind operator sequences to decrease expression down to basal level or stop transcription of a gene
Operon is OFF
Positive Regulation
Activator proteins bind operator sequences to increase expression above the basal level
Operon is ON
primase makes RNA primers complementary to the DNA parent strand sequence
then, DNA polymerase III will add nucleotides only to the 3' end
replication bubble is made at the Origin of Replication (ORI)
Topoisomerase relieve the tension caused by the unwinding of the DNA
Single-Stranded proteins keep the DNA from coming back together
helicase separates the two strands by breaking the hydrogen bonds between the strands
dispersive model
each strand contains parts of both old and newly synthesized DNA
semi-conservative model
the parental strands separate and makes its own new complimentary strand
conservative model
the two parental strands are used as templates, they stay together
guanine
cytosine
thymine
Adenine
connected by phosphodiester bonds
Forms: mRNA
Initiation Enzyme: RNA Polymerase
Forms: pre-mRNA then mRNA thru RNA Processing
Initiation Enzyme: RNA Polymerase II
Needs TRANSCRIPTION FACTORS that bind near the promoter before RNA Poly II can bind. TF recognize the TATA box in promoter sequence.
RNA Poly II binds to the promoter upstream the start site
TF + RNA Poly come together to form the translation initiation complex
Termination: 5' cap and 3' Poly A tail
Termination Enzymes: Ribonuclease: Cleavage PolyA polymerase: adds poly A tail, Uses ATP
RNA Processing: Removing of introns and the joining of exons
Spliceosomes: cut out the introns
Takes secretory pathways. The path taken by protein in a cell on synthesis to modification and then release out of the cell.
All protein synthesis starts on free ribosomes.
Polypeptide synthesis begins on free ribosome in cytosol
Synthesis stops temporarily from SRP binding to signal peptide
SRP is signal recognition particle that is made of RNA and protein.
SRP binds to receptor protein in ER membrane
SRP leaves, polypeptide synthesis resumes
Signal peptide is removed by an enzyme in receptor protein complex
Signal peptidase is the enzyme used to cleave the signal peptide.
Protein synthesis finishes inside the ER
Nucleus
Peroxisomes
Chloroplast
Mitochondria