10a. Explain the molecular basis of heritable traits, and how information in DNA is ultimately expressed as a protein.
DNA indirectly controls traits, because a trait is directly the result of the particular version of a protein that an individual can produce, and the type of protein that can be produced is determined by that individual’s version of the corresponding gene (which is made of DNA). We use the term "gene expression" to refer to the molecular process of building a particular protein using the code that’s stored (in DNA form) as a gene.
- What are the processes involved in gene expression?
- In what order do these processes occur?
- What specifically occurs in these processes?
Gene expression consists of two serial processes, occurring in the following order:
- Transcription is the only part of gene expression that directly involves the gene. The DNA nucleotide sequence of a gene is “read” by the cell, and that sequence is used as a code for how to construct a complementary sequence of RNA nucleotides. That RNA sequence is known as messenger RNA (mRNA), because the code originally in the gene is now carried as a message by the mRNA to a ribosome. DNA’s job is done until the gene is read again in a later round of gene expression.
- Translation is the part of gene expression during which the polypeptide (protein) is actually constructed. A polypeptide is a sequence of amino acids. The code in mRNA (produced by transcription earlier) instructs the cell’s ribosomes to build the polypeptide by conveying information about how many amino acids to string together, which amino acids to use, and how those amino acids should be ordered in the sequence. As a ribosome reads each codon (a sequence of three nucleotides) in the mRNA, an amino acid of the correct type is delivered to the ribosome by a type of RNA called transfer RNA (tRNA). Amino acids are added in this way, one-by-one, until the polypeptide is complete.
A completed polypeptide (produced by translation) assumes a particular shape, depending on its particular sequence of amino acids. Because of that particular shape, that polypeptide will have a particular function, and it will therefore give the individual a particular trait.
Review this section to be sure you understand the difference between transcription and translation, what occurs in each, and the order in which these processes occur. Pay attention to the fact that gene expression is the molecular mechanism by which genes indirectly determine phenotypes.
Unit 10 Vocabulary
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.
- Amino Acid
- Gene Expression