Gene expression Flashcards
Transcription
An RNA molecule complementary to the template DNA strand is synthesized. Messenger RNA molecules contain information that specifies the amino acid sequences of polypeptide chains.
Translation
A polypeptide chain specified by the mRNA is synthesized. Each sequence of the three nucleotide bases in the mRNA constitutes a codon, which specifies one amino acid in the polypeptide chain, or a start or stop signal. Translation requires tRNA and cell machinery, including ribosomes.
Compare the structures of DNA and RNA
In RNA each subunit contains the sugar ribose, a base (uracil, adenine, G and C) and three phosphates. Like DNA, RNA subunits covalently joined by 5’->3’ linkages to form alternating sugar phosphate.
Why the genetic code is redundant and virtually universal?
It is redundant because some amino acids are specified by more than one codon. Only a few minor variations are exceptions to the genetic code found in all organism.
Differences between bacterial and eukaryotic mRNAs
Eukaryotic mRNA molecules are more complicated than those of bacteria. After transcription a 5’ cap is added to the 5’ end of eukaryotic mRNA. It also has a poly-A tail added to the 3’ end. Both modifications may be involved in the longer lifespan of eukaryotic mRNA.
Introns and Exons
In many eukaryotic genes, the coding regions, called exons, are interrupted by non coding regions called introns. Both are transcribed, but introns are later removed from the original pre-mRNA and the exons are spliced together to produce polypeptide-coding sequence.
Transfer RNAs (tRNAs)
They are the decoding molecules in the translation process. Each tRNA molecule is specific for only one amino acid. One part of the tRNA molecule contains an anticodon that is complementary to a codon of mRNA. Attached to one end of the tRNA molecule is the amino acid specified by the complementary mRNA codon.
Ribosomes in polypeptide synthesis
Each ribosome is made of a larger and a small subunit; each one contains ribosomal RNA and many proteins. Ribosomes couple the tRNAs to their proper codons on the mRNA, catalyze the formation of peptide bonds between amino acids and translocate the mRNA so that the next codon can be read.
Polypeptide synthesis: Initiation
Initiation factors bind to the small ribosomal subunit, which then binds to mRNA in the region of AUG, the start codon, and then to the large ribosomal unit.
Polypeptide synthesis: Elongation
It is a cyclic process in which amino acids are added one by one to the growing polypeptide chain. Elongation proceeds in the 5´->3’ direction along the mRNA.
Polypeptide synthesis: Termination
The final stage of translation, occurs when the ribosome reaches one of the three stop codons. The A site binds to a release factor, which triggers the release of the completed polypeptide chain and dissociation of the translation complex.
Polyribosome in bacterial cells
Unlike the processes in eukaryotic cells, transcription and translation are coupled in bacterial cells. Bacterial ribosomes bind to the 5’ end of the growing mRNA and initiate translation before the message is fully synthesized. As many as 15 ribosomes may bind to a single mRNA, forming a polyribosome.
Base-pair substitution
It is a mutations that may alter or destroy the function of a protein if a codon changes so that it specifies a different amino acid (missense mutation) or becomes a stop codon (a nonsense mutation).
Frameshift mutation
The insertion or deletion of on or two base pairs in a gene. They are changes in the codon sequences as they affect two bases.
Transposons
Movable DNA sequences that jump into the middle of a gene.
