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Gene Expression at the Molecular Level - Coggle Diagram
Gene Expression at the
Molecular Level
Gene Expression
Central dogma of gene expression
Transcription
This messenger RNA (mRNA) specifies the amino
acid sequence of a polypeptide
Produces an RNA transcript (RNA copy) of a gene
Translation
Process of synthesizing specific polypeptide on a
ribosome using the mRNA template
Transcription
Gene
An organized unit of DNA sequences that enables
a segment of DNA to be transcribed into RNA
Other genes code for RNA
Transfer RNA (tRNA) - translates mRNA into amino acids
Ribosomal RNA (rRNA) - part of ribosomes
stages of transcription
Initiation
Recognition
In bacteria, sigma factor (a protein) causes RNA polymerase to recognize and bind to the promoter region
Stage completed when DNA strands separate
near promoter to form an open complex
Elongation
RNA polymerase synthesizes RNA
DNA strand used as a template for RNA synthesis:
template strand (3’ to 5’) and the Coding strand: opposite DNA strand (5’ to 3’)
Uracil substituted for thymine with respect to coding
strand
Termination
RNA polymerase reaches termination sequence
Causes both the polymerase and newly-made RNA
transcript to dissociate from DNA
RNA Processing in Eukaryotes
Bacterial mRNAs can be translated immediately
Eukaryotic mRNAs are made in a longer pre-mRNA
form that requires processing into mature mRNA
Introns which are transcribed but not translated
Exons which are coding sequence found in mature mRNA
Splicing which is removal of introns
Spliceosome removes introns precisely and it is Composed of snRNPs (small nuclear RNA + proteins)
Alternative splicing, splicing can occur more
than one way to produce different products
rRNA and tRNA are self-splicing
and they are They are ribozymes
Other modifications which is addition of tails and caps
Capping
Modified guanosine covalently attached to 5’ end
Needed for mRNA to exit nucleus and bind ribosome
Poly A tail
100-200 adenine nucleotides added to 3’ end
Increases stability and lifespan of in cytosol
Not encoded in gene sequence
Stages of Translation
Initiation
mRNA, first tRNA and ribosomal subunits assemble
Requires help of ribosomal initiation factors
Requires input of energy (GTP hydrolysis)
Initiation in bacteria
mRNA binds to small ribosomal subunit facilitated
by ribosomal-binding sequence
Initiator tRNA recognizes start codon in mRNA
At the end, the initiator tRNA is in the P site
Large ribosomal subunit associates
eukaryotic initiation
Instead of a ribosomal-binding sequence, mRNAs
have guanosine cap at 5’ end
Position of start codon more variable
Elongation
Synthesis from start codon to stop codon
1.Aminoacyl tRNA brings a new amino acid to
the A site
Binding occurs due to codon / anticodon
recognition
Peptide bond is formed between the amino acid at the A site and the growing polypeptide chain
Movement or translocation of the ribosome
toward the 3’ end of the mRNA by one codon
Termination
Complex disassembles at stop codon releasing the
completed polypeptide
When a stop codon is found in the A site,
translation ends
3 stop codons – UAA, UAG, UGA
Recognized by release factors
Completed polypeptide is attached to a tRNA in
the P site and stop codon in the A site
Bond between polypeptide and tRNA hydrolyzed to
release polypeptide
Ribosomal subunits and release factors disassociate
Release factor binds to stop codon at the A site