DNA and RNA Structure and Replication

Study Notes

DNA and RNA are nucleic acids composed of nucleotides.
Each nucleotide consists of a nitrogenous base, a sugar (ribose for RNA and deoxyribose for DNA), and a phosphate group.
The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
In RNA, thymine is replaced by uracil (U).
Adenine pairs with thymine (or uracil in RNA) through two hydrogen bonds.
Guanine pairs with cytosine through three hydrogen bonds.
DNA consists of two antiparallel strands that wind into a double helix, held together by complementary base pairing.
RNA is typically single-stranded and does not form a double helix.

DNA replication occurs before cell division to ensure each daughter cell receives a complete and identical copy of the genome.
The process involves unwinding the DNA helix, synthesizing a complementary strand using DNA polymerase, and creating two identical DNA molecules, each with one original strand and one new strand (semiconservative replication).

DNA replication is bidirectional, meaning both strands are copied simultaneously.
DNA polymerase can only add nucleotides in the 5' to 3' direction.
The lagging strand, running in the 3' to 5' direction, is synthesized discontinuously in small fragments called Okazaki fragments.
Okazaki fragments are initiated by RNA primers, extended by DNA polymerase, and then joined together by DNA ligase.

Transcription is the process of synthesizing messenger RNA (mRNA) from a DNA template.
RNA polymerase binds to the promoter region of DNA to initiate transcription.
RNA polymerase separates the DNA strands and uses the template strand to synthesize a complementary RNA copy.
The RNA strand is antiparallel to the DNA template and substitutes uracil (U) for thymine (T).
The newly formed RNA strand is called pre-mRNA, which is then processed by splicing to remove introns and form mature mRNA.

Translation is the process of converting mRNA into a protein.
Ribosomes read the mRNA in codons, each representing a specific amino acid.
Transfer RNA (tRNA) molecules carry corresponding amino acids to the ribosome, matching their anticodon to the mRNA codon.
Amino acids are linked together in a chain, forming a polypeptide, which folds into a functional protein.
Stop codons signal the end of translation, and the newly formed protein is released.