DNA and RNA as hereditary material: chromosomes, plasmids and base pairing

What the exam asks

• Base-composition calculations (Chargaff’s rules): Given the percentage of one nitrogenous base in a double-stranded DNA molecule, calculate the percentages of the remaining three bases, or determine whether an unknown viral genome is single-stranded or double-stranded DNA or RNA.
• DNA stability and melting temperature ($T_m$): Predict or explain how a higher percentage of guanine–cytosine ($\text{G–C}$) base pairs increases the temperature required to separate (denature) two DNA strands.
• Genome organization: Compare prokaryotic circular chromosomes with eukaryotic linear chromosomes wrapped around histone proteins, including the role of plasmids in both prokaryotes and eukaryotes.
• Evolutionary conservation: Explain how universal purine–pyrimidine base pairing allows genetic material to be replicated accurately and transferred between species (such as in bacterial transformation or genetic engineering).

Core ideas

DNA and RNA as hereditary material

All cellular life (prokaryotes and eukaryotes) stores primary genetic information in DNA (deoxyribonucleic acid) and passes it to daughter cells during cell division. Some viruses, however, use RNA (ribonucleic acid) as their primary hereditary material (for example, retroviruses and many respiratory viruses). Whether stored in DNA or RNA, genetic information is encoded in the linear sequence of nucleotides.

Prokaryotic vs. eukaryotic chromosomes and plasmids

Cells package their DNA differently depending on their evolutionary lineage, though both prokaryotes and some eukaryotes can carry plasmids—small, extra-chromosomal, double-stranded circular DNA molecules that replicate independently of the main chromosome(s) and often carry genes that confer selective advantages (such as antibiotic resistance).