Gene Expression and DNA Replication

DNA Replication

• DNA replication is a semiconservative process, where each parent strand serves as a template for a new strand, resulting in two new DNA molecules, each with one old and one new strand.
• The process occurs in the 5' to 3' direction.
• DNA replication requires a template strand, deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP), DNA polymerase, and a source of chemical energy.

Replication Fork Formation

• DNA helicase unwinds and separates double-stranded DNA, breaking hydrogen bonds between nucleotide pairs, forming a replication fork.
• The leading strand is oriented 5' to 3', while the lagging strand is oriented 3' to 5'.

Primer Binding

• A short RNA piece called a primer binds to the 3' end of the leading strand, serving as the starting point for replication.
• Primers are generated by the enzyme DNA primase.

Elongation

• DNA polymerase synthesizes new DNA molecules by adding nucleotides to the leading and lagging strands.
• In bacteria, polymerase III is the main replication enzyme, while polymerases I, II, IV, and V are responsible for error checking and repair.
• In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication.
• The leading strand is replicated continuously, while the lagging strand is replicated in short, discontinuous pieces called Okazaki fragments.

Termination

• An enzyme called exonuclease removes RNA primers from the original strands, replacing them with appropriate bases.
• DNA ligase joins Okazaki fragments together, forming a single, unified strand.
• Telomeres act as protective caps at the end of chromosomes, preventing nearby chromosomes from fusing.

Gene Expression

• Gene expression is the process of converting a gene's DNA sequence into the structures and functions of a cell.
• Non-protein coding genes are not translated into protein.
• Genetic information is transferred to a daughter cell through DNA replication and expressed by transcription followed by translation.

Transcription

• Transcription is the first step in gene expression, involving the copying of a gene's DNA sequence to make an RNA molecule.
• The process involves three stages: initiation, elongation, and termination.
• In eukaryotes, RNA molecules must be processed after transcription, involving splicing, 5' capping, and poly-A tailing.

Translation

• Translation refers to the process of creating proteins from an mRNA template.
• The sequence of nucleotides on the RNA is translated into the amino acid sequence of proteins, carried out by ribosomes.
• The process requires multiple enzymes and energy from ATP and GTP.