Genetics and Evolution - Molecular Basis of Inheritance

DNA as Genetic Material: The structure of $DNA$ is a double helix proposed by Watson and Crick. It consists of two polynucleotide chains where the backbone is constituted by sugar-phosphate, and the bases project inside. The two chains have anti-parallel polarity ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).

Chargaff's Rule: In a double-stranded $DNA$, the ratio between Adenine ($A$) and Thymine ($T$), and Guanine ($G$) and Cytosine ($C$) are constant and equal to one. Thus, $A+G = T+C$.

Nucleosome Structure: In eukaryotes, $DNA$ is wrapped around a positively charged histone octamer to form a nucleosome. A typical nucleosome contains $200 \text{ bp}$ of $DNA$ helix.

Central Dogma: Proposed by Francis Crick, it states that genetic information flows from $DNA \rightarrow RNA \rightarrow \text{Protein}$. In some viruses, reverse flow occurs via Reverse Transcription ($RNA \rightarrow DNA$).

Semiconservative Replication: DNA replication is semiconservative, meaning each daughter $DNA$ molecule retains one parental strand and one newly synthesized strand, as proven by Meselson and Stahl using $^{15}N$.

Transcription Unit: A transcription unit in $DNA$ is defined by three regions: a Promoter, the Structural gene, and a Terminator. The strand with $3' \rightarrow 5'$ polarity acts as the template strand.

Genetic Code: The code is a triplet ($64$ codons). $AUG$ has dual functions: it codes for Methionine ($met$) and acts as an initiator codon. $UAA, UAG, UGA$ are stop (nonsense) codons.

Lac Operon: A polycistronic structural gene is regulated by a common promoter and regulatory genes. In the Lac Operon, the $i$ gene codes for the repressor, and structural genes ($z, y, a$) code for $\beta$-galactosidase, permease, and transacetylase respectively.