Gene expression
What is gene expression?
Gene expression is the process by which DNA directs the synthesis of proteins theough translation and transcription
What is transcription?
The synthesis of RNA using info in DNA
What is translation?
The synthesis of a polypeptide using information in mRNA
How are proteins made in the cell?
Proteins are made by linking amino acids together on ribosomes using the instructions provided by mRNA during translation
How is the specific mRNA for a protein made?
mRNA is made during transcription, when the DNA sequence of a gene is copied into RNA
Why do cells regulate gene expression?
To save emergy and materials
respond to cells needs
create different cell types in multicellular organisms
What determines whether a gene is on or off or highly or weakly expressed?
Gene expression is controlled by:
signal molecules
transcription factors
regulartory DNA sequences and DNA accessibilty
How does transcription and translation differ between prokaryotes and eukaryotes?
In prokaryotes, translation can begin before transcription is finished
In eukaryotes, the nuclear envelope seperates transcription (in nucleus) from translation (in cytoplasm)
What are the different types of mRNA?
Polycistronic (prokaryotes only) contains the coding sequence for many polypeptides
Monocistronic (both eukaryotes and prokaryotes) contains the codinf sequence for one polypeptide
Which DNA strand is used during transcription and why?
DNA template strand
It is used to build an RNA molecule that matches the opposite coding strand.
How does RNA polymerase know where to start transcription?
RNA polymerase recognises and binds to a specific DNA sequence called a promoter.
What are the 3 stages of transcription?
Initiation: RNA polymerase binds to promoter
Elongation: RNA polymerase synthesises RNA
Termination: RNA polymerase stops and releases RNA transcript
Which protein binds tor TATA box in eukaryotes?
The TATA binding protein (TBP)
What does phosphorylation of the CTD (carboxyl terminal doman) of RNA Polymerase II allow?
It releases RNA Pol I from the PIC so transcription can begin
How many DNA bases are unwound at a time during elongation?
10-20 bases
How fast does transcription occur in eukaryotes?
About 40-50 nucleotides per seconds
To which end of the RNA are nucleotides added
3' end
How does transcription termination differ between bacteria and eukaryotes?
Bacteria: RNA polymerase stops at a terminator sequence, mRNA is ready immediately
Eukaryotes: RNA Pol II transcribes pash the polyadenylation signal; cleavage occurs 10-35 nt downstream
What is polycistrinic mRNA and where is it found?
mRNA that codes for multiple proteins only found in prokaryotes
What is monocistronic mRNA?
mRNA that encodes for a signle polypeptide.
what is a promoter?
a DNA sequence where RNA polymerase binds to start transcription
Why do strong and weak promoters differ in transcription frequency?
Strong promoters have sequenced that attract RNA polymerase efficiently -> frequent transcription
Weak promoter bind less efficiently -> infrequent transcrption
What are the two main modifications added to eukaryotic pre-mRNA ends?
5' end
Poly-A-tail
What is the purpose of splicing?
To remove introns and join exons into a continuous coding sequence.
What is a spliceosome?
A large RNA protein complex that removed introns
What is alternative splicing?
Different combinations of exons are joined to produce different mRNAs from one gene
Why are introns important evolutionarily?
Contain regulatory sequences
Enable alternative splicing
Allow exon shuffling
Why can prokaryotes befin translation before transcription ends?
Because they lack a nucleus