Locus: Every gene occupies a fixed, specific position on a particular chromosome, known as its locus. This precise mapping ensures that during processes like meiosis, homologous chromosomes can align and exchange genetic material accurately.
Alleles: A gene can exist in different forms called alleles, which have slightly different nucleotide sequences. Although they represent variations of the same trait, alleles always occupy the same locus on homologous chromosomes.
| Feature | Exons | Introns |
|---|---|---|
| Coding Status | Coding regions that specify amino acids | Non-coding regions within a gene |
| Presence in mRNA | Retained in mature mRNA after splicing | Removed during the splicing process |
| Function | Directly determine protein primary structure | May play roles in gene regulation |
Transcription: The entire gene, including both introns and exons, is first copied into a molecule called pre-mRNA. This ensures that the complete genetic instruction set is captured before refinement begins.
Splicing: Before the mRNA leaves the nucleus for translation, a process called splicing occurs. During splicing, the non-coding introns are enzymatically removed, and the coding exons are joined together to form a continuous sequence of mature mRNA.
Translation: The mature mRNA is then read by ribosomes, where the triplet code is translated into a specific sequence of amino acids. This step converts the digital information stored in DNA into the physical structure of a functional protein.
Terminology Precision: Always distinguish clearly between a 'gene' (the instruction) and an 'allele' (the version). Examiners often test whether students understand that alleles of the same gene must occupy the same locus.
Code Characteristics: Be prepared to define and explain the significance of the terms degenerate, universal, and non-overlapping. For example, if asked about mutations, explain how the degenerate nature of the code might result in a 'silent' mutation where the amino acid remains unchanged.
Splicing Context: Remember that splicing is a characteristic of eukaryotic cells. Prokaryotic genes generally do not contain introns, so their mRNA does not undergo this specific refinement process before translation.
Mathematical Logic: If given a sequence of bases, divide by three to find the number of amino acids, but remember to account for 'stop' codons which signal the end of translation but do not code for an amino acid.
