Molecular Genetics: DNA Replication and Mutagenesis

DNA Replication and Mutagenesis

Overview of DNA Replication

• DNA replication is part of the cell cycle in eukaryotic cells
• It occurs during the S phase of the cell cycle
• The process involves the unwinding of DNA, synthesis of new strands, and the formation of replication forks

Structure of the Human Genome

• The human genome consists of 23 pairs of chromosomes
• The total number of nucleotides is approximately 3.2 x 10^10
• Only 1% of the genome encodes proteins (20,000 genes)
• Regulatory regions of genes make up about 4% of the genome
• Other unique sequences account for about 44% of the genome
• Repeated sequences, including LINES, SINES, and satellite DNA, make up about 51% of the genome

Initiation of Replication

• Replication begins at the origin of replication
• The double helix is unwound, and the bases are exposed
• DNA polymerase synthesizes new strands using the parental strands as templates
• The process is semi-conservative, resulting in two new DNA molecules, each containing one parental strand and one daughter strand

The Replication Fork

• The replication fork is the region where the DNA is unwound and replication occurs
• Helicase unwinds the DNA, and single-strand binding proteins stabilize the single-stranded DNA
• Topoisomerase breaks a phosphodiester bond in one parental strand to prevent supercoiling
• The leading strand is synthesized continuously, while the lagging strand is synthesized in short, discontinuous segments called Okazaki fragments

Role of Enzymes in DNA Replication

• Helicase: unwinds DNA strands
• Topoisomerase: releases supercoils in DNA
• Single-strand binding protein: stabilizes single-stranded DNA
• Primase: makes RNA primer for DNA synthesis
• DNA polymerase: synthesizes DNA
• Exonuclease: removes RNA primer
• DNA ligase: joins adjacent Okazaki fragments

Fidelity of DNA Replication

• DNA polymerase has a proof-reading mechanism to correct errors during DNA synthesis
• The enzyme also has a 3'-5' exonuclease activity to remove incorrect nucleotides

Origins of Replication

• There are multiple origins of replication per eukaryotic chromosome
• Bacterial chromosomes are circular and have a single origin of replication

Chromosome End Problem

• The end of the chromosome is lost during DNA replication due to the inability of DNA polymerase to synthesize DNA at the very end of the chromosome
• Telomeres, specialized structures at the end of chromosomes, consist of hundreds of repeats of the sequence 5' TTAGGG 3'
• Telomeres shorten with every round of DNA replication, leading to cellular senescence

Types of Mutation

• Point mutation: a single base is changed
• Silent mutation: a change that does not affect the encoded amino acid
• Missense mutation: a change that affects the encoded amino acid
• Nonsense mutation: a change that results in a stop codon
• Small scale insertions or deletions: indels
• Large scale changes to chromosomes: inversions, deletions, duplications, translocations

Cause of Mutagenesis

• Spontaneous errors in DNA replication
• Replication slippage associated with repetitive regions
• DNA damage caused by spontaneous deamination, UV radiation, ionizing radiation, and alkylating agents
• Most DNA damage is repaired through direct repair, nucleotide excision repair, or homologous recombination

Chemotherapy Drugs

• Methotrexate: inhibits the synthesis of thymine
• Doxorubicin: binds to DNA and prevents the action of topoisomerase
• 5-fluorouracil: inhibits the synthesis of thymine

This quiz covers the fundamentals of molecular and cellular genetics, including DNA replication, mutagenesis, and the chromosome end-problem. Understand the chemistry and structure of DNA/RNA and appreciate the clinical importance of DNA mutations.