DNA Structure and Replication
DNA (deoxyribonucleic acid) is the genetic material that carries hereditary information in living organisms. Its structure and replication process are crucial for understanding how genetic information is stored and passed on.
Nucleic Acid Structure
DNA is composed of nucleotides, which are the building blocks of nucleic acids. Each nucleotide consists of three components:
- A phosphate group
- A pentose sugar (deoxyribose in DNA, ribose in RNA)
- A nitrogenous base
Vocabulary: Nucleotides are the monomers that make up the polymers known as nucleic acids or polynucleotides.
The nitrogenous bases in DNA are classified into two categories:
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Purines: Larger, double-ringed molecules
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Pyrimidines: Smaller, single-ringed molecules
Highlight: In RNA, uracil (U) replaces thymine as one of the pyrimidines.
DNA Structure
DNA molecules are composed of two polynucleotide strands that form a double helix structure. The key features of this structure include:
- The sides of the DNA "ladder" are made up of alternating phosphate and deoxyribose molecules.
- The nitrogenous bases form the "rungs" of the ladder, held together by hydrogen bonds.
- The two strands are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5').
Definition: Antiparallel refers to the orientation of the two DNA strands, where one strand runs from 5' to 3' and the other from 3' to 5'.
Semi-conservative Replication of DNA
The semi-conservative replication process in DNA ensures that genetic information is accurately copied and passed on to daughter cells. This process involves several steps:
- Unwinding of the DNA double helix by the enzyme helicase, which breaks the hydrogen bonds between base pairs.
- Formation of a replication fork, a Y-shaped structure where the two strands separate.
- Synthesis of a short RNA primer by the enzyme primase to mark the starting point for DNA synthesis.
- DNA polymerase uses the primer to synthesize new DNA strands in the 5' to 3' direction.
Example: The leading strand is synthesized continuously, while the lagging strand is made in short segments called Okazaki fragments.
- DNA ligase seals the gaps between Okazaki fragments to form a continuous double-stranded helix.
Highlight: The semi-conservative replication process in DNA ensures that each new DNA molecule contains one original strand and one newly synthesized strand.