Rucete ✏ AP Biology In a Nutshell
13. DNA, RNA, and DNA Replication
This chapter explains the structure and function of nucleic acids—DNA and RNA—as the carriers of genetic information, and how DNA replicates in a semiconservative manner to maintain continuity across generations.
DNA and RNA Structure
• Both DNA and RNA are made of nucleotides: – A nitrogenous base – A five-carbon sugar – A phosphate group
• DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T)
• RNA bases: adenine (A), guanine (G), cytosine (C), uracil (U)
• A & G are purines (double-ring); C, T, & U are pyrimidines (single-ring).
• Base-pairing rules: – DNA: A–T (2 H-bonds), G–C (3 H-bonds) – RNA: A–U (2 H-bonds), G–C (3 H-bonds)
• DNA sugar: deoxyribose (no OH on 2′ carbon); RNA sugar: ribose (OH on 2′ carbon)
• DNA is double-stranded and forms an antiparallel double helix. RNA is usually single-stranded but can fold into 3D shapes.
Genetic Information and Storage
• DNA carries genetic information in prokaryotes and eukaryotes. Some viruses use RNA as their genetic material.
• Prokaryotic DNA: circular chromosomes in the nucleoid region.
• Eukaryotic DNA: linear chromosomes in the nucleus.
• Both can have plasmids (small circular DNA)—extranuclear genetic material.
DNA Replication Overview
• Purpose: maintain continuity of genetic information across generations.
• Semiconservative: each daughter DNA has one original strand and one new strand.
Steps in DNA Replication
1. Helicase: unwinds the double helix at the origin of replication.
2. Topoisomerase: prevents supercoiling by nicking and resealing DNA strands.
3. RNA Polymerase: lays down an RNA primer to start DNA synthesis.
4. DNA Polymerase: adds nucleotides to the 3′ end of the RNA primer in the 5′ to 3′ direction.
5. Ligase: joins Okazaki fragments on the lagging strand.
Leading vs. Lagging Strand
• DNA strands are antiparallel → replication is different on each strand.
– Leading strand: synthesized continuously in the same direction as the replication fork.
– Lagging strand: synthesized discontinuously in short Okazaki fragments, which are joined by ligase.
In a Nutshell
DNA and RNA are nucleic acids that store and transmit genetic information. DNA is double-stranded and more stable than RNA. DNA replication is semiconservative and relies on enzymes like helicase, DNA polymerase, and ligase to ensure accurate copying. Understanding these processes is essential for grasping molecular genetics and heredity.