From DNA To Proteins Flashcards
Bp
“base pairs”
so when describing DNA sizes it refers to both strands
kDa
KiloDaltons
One Dalton is the mass of an H atom, or 1/12 of a C 6
atom.
S
Svedberg unit, and refers to the mass and shape of cellular organelles.
High S means
Larger mass
S values are not additive
e.g. bacterial ribosome (70S) consists of 50S and 30S subunits
Human DNA is
Double-stranded with a complementary chain
Present in cells at all time
Human RNA is single stranded, and any duoble stranding is usually with itself
Types of RNA mRNA, rRNA, tRNA with different functions
Many mRNA species only accumulate following cell stimulation
mRNA
Messenger RNA (mRNA) is printed as a long linear transcript
It then processed to the mature form (in proximity of the nuclear membrane)
It has a 5’CAP and a 3’ Poly A tail
Ribosomes are abundant in eukaryotic cytoplasm.
What forms ribosomes?
Four main types of rRNA combine with proteins to form 80S ribosomes.
How are all the types of RNA linked?
Ribosome (rRNA) prints transcript (mRNA) with help from transfer RNA (tRNA)
tRNA
tRNA carry amino acids to ribosomes, and check that they are incorporated in the right poistion
Each tRNA only carries one aa»_space;> at least 20 tRNA types
Very small molecules
At the anticodon, a triplet sequence pairs with mRNA
»> right aa for the right triplet ( = CODON)
The global picture
Slide 18
Regions of the gene
Slide 20
tRNA binds to a codon on the mRNA
tRNA contains an amino acid at its 3’ end corresponding to the codon on mRNA to which the anticodon of the tRNA can base pair.
The code is :
• Degenerate , but unambiguous
– Many amino acids specified by more than one codon – But each codon specifies only one aminoacid
• Almost universal
– All organisms use the same code – Fewer that 10 exceptions
• Non-overlapping and without punctuation – Codons do not overlap
– Each nucleotide is only read once
Factors initiating gene expression
• Proteins called “transcription factors” find their way into specific sequences 5’ of the 1st exon (region called “promoter”)
• A “transcription complex” forms around the TATA box 5’ of 1st exon
• Helix opens, DNA strand separation.
• RNA Pol II starts building mRNA
• Etc (as described before)
TATA box
In molecular biology, the TATA box (also called the Goldberg–Hogness box)[1] is a sequence of DNA found in the core promoter region of genes
Factors turning off expression
• Activation of repressors (inhibitors of RNA polymerase binding)
• Each step of RNA transcription or processing finds no longer actively produced transcription and processing proteins
• Complexes do not form anymore for lack of phosphorylation
• Enzymes no longer activated
• RNA stability
• Many other unknown mechanisms
Process of DNA TRANSCRIPTION AND THE PRODUCTION OF PROTEINS
• Proteins called transcription factors (proteins which bind to promoter regions) find their way to specific sequences on the 5’ of the 1st exon (region call the promoter)
• Promoter has a specific sequence of nucleotides - they do not code for proteins but instead act as binding sites - found at the 5’ end.
• A ‘transcription complex’ forms around the TATA box (reads thymine, adenine, thymine, adenine etc.) on the 5’ of the 1st exon:
Steps of transcription and protein production
- Topoisomerase unwinds the double helix by relieving the supercoils. DNA helicase then separates the DNA apart exposing the nucleotides. SSB’s coat the single DNA strands to prevent DNA re- annealing
- Free mRNA nucleotides line up next to their
complementary bases on the template strand/ antisense strand of DNA ( U-T & C-G). - RNA polymerase (specifically RNA polymerase 2) joins
the mRNA nucleotides (catalysing phosphodiester bonds between them) to form and antiparallel mRNA strand ( with a 5’CAP head and a 3’Poly A tail) - starting at a promoter (specific sequence that RNA polymerase binds to - initiation of transcription. Transcription is stopped at the stop codon) - mRNA leaves the nucleus and attaches to an 80s
ribosome - At ribosome the mRNA (bases on mRNA are read in 3 - codon) sequence is used as a template to bind to complementary tRNA molecules at their anticodon (3 bases complementary to codon on mRNA). Ribosome reads mRNA codon by codon, one codon will code for a particular amino acid. This amino acid is brought by a specific tRNA molecule (carried on it 3’ end) since tRNA molecules are attached to specific amino acids. BASES ARE READ 5’ TO 3’
- Enzymes remove amino acid from tRNA and amino acids are linked together by a
peptide bond (created by a condensation reaction), creating a polypeptide chain - a protein
How is the ribosome involved in translation
- Ribosome recognises mRNA from its CAP on the 5’ end
- Ribosome is responsible for translation
Promoter region
is what RNA polymerase recognises and where it starts
mRNA primary transcript
mRNA strand is a complementary cop of original DNA
Silencing genes
• DNA can be chemically altered to the methylated form i.e turned on or off (silenced)
• In a macrophage, where immunoglobulins are not produced, but still has normal DNA, the gene for producing immunoglobulins will be in the heterochromatin (can remember as h = hiding i.e. not active) state -
since its not needed to be synthesised thus in the heterochromatin state no transcription of these gene can occur
• Whereas in a B cell, that needs to produce immunoglobulins, that gene will be in the euchromatin state - so it can be transcripted and thus immunoglobulins (proteins) can be synthesised
Factors running off gene expression
• Activation of repressors (inhibitors of RNA polymerase binding)
• Enzymes no longer activated
• Transcription and processing proteins required for RNA transcription and or processing are no longer produced
