5.2
DNA as a Genetic Template
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Q1: What are the two key structural features of DNA that enable heredity?
The Watson-Crick model identified two critical structural features: DNA's double-stranded nature and its four nucleotide bases—adenine, guanine, cytosine, and thymine. These bases pair specifically between purines and pyrimidines, allowing each DNA strand to serve as a template for its complementary partner during replication.
Q2: How are the two strands of DNA oriented relative to each other?
The two DNA strands are antiparallel, meaning the 3' end of one strand faces the 5' end of the other strand. This antiparallel arrangement allows each strand to act as a template for its partner, enabling the production of two new complementary DNA strands during chromosome replication.
Q3: What does the Watson-Crick model reveal about DNA's three-dimensional shape?
The Watson-Crick model, proposed in 1953, revealed that DNA is composed of two strands of nucleotides that twist around each other to form a right-handed helix. This helical structure, combined with complementary base pairing, provides the molecular basis for accurate DNA replication and hereditary information transfer.
Q4: How do nucleotide sequences relate to protein synthesis and genetic variation?
Nucleotide sequences are organized into three-letter codons that code for one of 20 amino acids during translation. Different permutations of codons form different genes, much like how word combinations create different books. This genetic code allows organisms to produce diverse proteins and explains how genome differences result in genotypically and phenotypically distinct organisms.
Q5: What did the Meselson and Stahl experiment prove about DNA replication?
The Meselson and Stahl experiment used heavy nitrogen isotopes to track DNA replication in E. coli. Results showed that after one generation in light nitrogen medium, DNA contained one heavy strand and one light strand, validating the semi-conservative replication model predicted by the Watson-Crick model.
Q6: What is a genome and how does it determine organism characteristics?
A genome is the complete set of genetic information within an organism, encompassing all genes and the RNA molecules and proteins produced over its lifetime. Differences between genomes result in genotypically and phenotypically distinct organisms and species, making the genome the fundamental blueprint for biological diversity.
Q7: How do the four nucleotide bases function as a genetic alphabet?
The four nucleotide bases—adenine, guanine, cytosine, and thymine—form a four-letter alphabet. When arranged into three-letter codons, they encode amino acids; different codon combinations create genes, analogous to how letters form words and sentences. This genetic code system enables the storage and expression of hereditary information.