6 Translation and Gene Expression
Follow how genetic information is expressed through RNA and protein production, and see how gene products contribute to traits alongside other genes and the environment.
From DNA to a gene product
A gene is a stretch of DNA whose information can be used to make a functional product. For a protein-coding gene, cells first copy relevant DNA information into messenger RNA (mRNA) through transcription. The mRNA is then used to assemble a chain of amino acids through . This flow is commonly summarized as .
is when, where, and how much a gene is used. Genes may direct the production of proteins or functional RNA molecules.
The and codons
The connects sequences in mRNA to amino acids or signals. A reads mRNA in groups of three nucleotides called codons. For example, AUG usually marks the start of and specifies methionine. The codons UAA, UAG, and UGA signal to stop.
There are 64 possible codons but only 20 common amino acids, so more than one can specify the same amino acid. This property is called redundancy, or degeneracy. Each nevertheless has only one meaning in a given . The code is nearly universal across organisms, with a few exceptions.
How ribosomes build polypeptides
happens on ribosomes, which are made of ribosomal RNA and proteins. A moves along an mRNA molecule in the -to- direction, reading codons in order.
Initiation: The assembles on the mRNA at a start site. A transfer RNA () carrying methionine pairs with the start ,
AUG.Elongation: Each carries a particular amino acid and has an anticodon that pairs with a matching mRNA . The links the amino acids with peptide bonds and advances to the next .
Termination: When a stop enters the , ends and the newly made polypeptide is released.
The polypeptide folds into a particular shape and may be modified or joined with other polypeptides. Its final structure helps determine what the protein can do. Some proteins act as enzymes; others provide structure, carry substances, or help regulate cell processes.
From gene products to
Gene products contribute to traits, but a does not necessarily result from one gene alone. A gene may encode an enzyme that helps produce pigment. A difference in the gene’s DNA sequence could change the enzyme’s structure or activity, affecting pigment production and therefore coloration. A DNA difference does not always change a protein or a visible .
Many traits are influenced by multiple genes, and environmental conditions can affect how they develop. also affects when, where, and how much a gene is used. As a result, an organism’s reflects gene products and their interactions with other genes, cells, and the environment.
Takeaway: Gene products can help shape traits, but the relationship between genes and observable features is not simply one gene to one .