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Protein Synthesis and Gene Expression. The theory that genetic information flows from DNA to RNA to protein is often termed the “central dogma” of gene expression During gene expression, DNA is copied into an RNA molecule in a process called transcription .
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Protein Synthesis and Gene Expression • The theory that genetic information flows from DNA to RNA to protein is often termed the “central dogma” of gene expression • During gene expression, DNA is copied into an RNA molecule in a process called transcription. • Transcription occurs in the nucleus and involves a special type of RNA molecule called messenger RNA (mRNA). • The mRNA nucleotide sequence directs the synthesis of a polypeptide (a chain of amino acids) with the aid of another RNA molecule—transfer RNA (tRNA)—called translation.
The Genetic Code • In a gene, each set of three bases is known as a codon. • The mRNA codon determines the genetic code rather than the nucleotide sequence of the original DNA strand. Three characteristics are present: • Redundancy – more than one codon can code for an amino acid. • There are three codons that are used as “start” or “stop” signals in the transcription process • Continuous – the genetic code is read as a series of three-letter codons • Universal – the genetic code is known amongst the entire scientific community
Transcription • The information in a DNA is copied into a mRNA – which carries information from DNA in the nucleus to the protein synthesis machinery in the cytoplasm of the cell. • The sense strand carries one strand of the double-stranded DNA • The anti-sense strand is not transcribed. • RNA polymerase catalyzes the synthesis of RNA
Translation • To create proteins the cell needs it must translate the codons along a stretch of mRNA into amino acid sequences. • Once the mRNA reaches the cytoplasm, the translator and protein synthesis equipment work together to assemble proteins. • tRNA links each mRNA codon to its specific amino acid. • One lobe contains the anticodon, which is a stretch of three nucleotides that is complementary to the mRNA codon. • The ribosome is the site of this action, just as RNA polymerase was the site of mRNA synthesis. • Ribosomes bring together the mRNA strand, the tRNA molecules carrying the amino acids, and the enzymes needed to build the polypeptides.
The ribosome matches the base sequence on the mRNA in sets of three bases (called codons) to tRNA molecules that have the three complementary bases in their anticodon regions. *Again, the base pairing rule is important in this recognition (A binds to U and C binds to G). *The ribosome moves along the mRNA, matching 3 base pairs at a time and adding the amino acids to the polypeptide chain. *When the ribosome reaches one of the "stop" codes, the ribosome releases both the polypeptide and the mRNA.
