Structure of nucleic acids and replication of DNA

Structure of Nucleic Acids

Nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Building blocks of nucleic acids: nucleotides.
Each nucleotide consists of a phosphate group, pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.
Nitrogenous bases in DNA: Adenine (A), Guanine (G), Thymine (T), Cytosine (C); in RNA: Adenine (A), Guanine (G), Uracil (U), Cytosine (C).
Base-pairing rules: A with T in DNA, A with U in RNA, C with G in both.
DNA structure: double-stranded helix, backbone made of alternating sugar and phosphate groups, bases inside forming pairs (A-T, C-G).
RNA structure: single-stranded, can fold into various shapes.

Replication of DNA

A crucial process for cell division.
Major steps: Initiation, elongation, termination.
Initiation: Proteins break the hydrogen bonds between the base pairs, unzipping the DNA double helix and exposing the bases. This forms a ‘replication fork’.
Elongation: Each strand serves as a template. DNA polymerase attaches free DNA nucleotides to the exposed bases following the base pairing rules to form new strands.
Termination: Two new identical DNA molecules are produced, each with one old strand (parental) and one new strand, a process known as semi-conservative replication.
DNA replication is bidirectional and antiparallel: It occurs in two directions from the replication origin, and new strands are formed in a 5’ to 3’ direction.
Okazaki fragments are short pieces of DNA formed on the lagging strand during DNA replication, later joined together by DNA ligase.