Intro to gene expression (central dogma)
Overview: Gene expression
Genes specify functional products (such as proteins)
How does the DNA sequence of a gene specify a particular protein?
- In transcription, the DNA sequence of a gene is copied to make an RNA molecule. This step is called transcription because it involves rewriting, or transcribing, the DNA sequence in a similar RNA "alphabet." In eukaryotes, the RNA molecule must undergo processing to become a mature messenger RNA (mRNA).
- In translation, the sequence of the mRNA is decoded to specify the amino acid sequence of a polypeptide. The name translation reflects that the nucleotide sequence of the mRNA sequence must be translated into the completely different "language" of amino acids.
Transcription and RNA processing: Eukaryotes vs. bacteria
The genetic code
Steps of translation
- Matching tRNA binds to exposed codon in rightmost slot of ribosome.
- Chain of amino acids is transferred from tRNA in middle slot of ribosome onto the amino acid of the tRNA in the rightmost slot. This has the effect of adding the amino acid to the end of the amino acid chain.
- The ribosome shifts one codon over. The tRNA formerly in the middle slot moves to the leftmost slot and exits the ribosome. The tRNA formerly in the right slot moves into the middle slot and continues to hold the amino acid chain. A new codon is exposed in the rightmost slot for a new tRNA to bind to.
What happens next?
- DNA is divided up into functional units called genes, which may specify polypeptides (proteins and protein subunits) or functional RNAs (such as tRNAs and rRNAs).
- Information from a gene is used to build a functional product in a process called gene expression.
- A gene that encodes a polypeptide is expressed in two steps. In this process, information flows from DNA RNA protein, a directional relationship known as the central dogma of molecular biology.
- Transcription: One strand of the gene's DNA is copied into RNA. In eukaryotes, the RNA transcript must undergo additional processing steps in order to become a mature messenger RNA (mRNA).
- Translation: The nucleotide sequence of the mRNA is decoded to specify the amino acid sequence of a polypeptide. This process occurs inside a ribosome and requires adapter molecules called tRNAs.
- During translation, the nucleotides of the mRNA are read in groups of three called codons. Each codon specifies a particular amino acid or a stop signal. This set of relationships is known as the genetic code.