DNA Replication: Initiation, Elongation, and Termination

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DNA replication involves several steps:

  1. Initiation: Initiator proteins bind to the origin of replication.
  2. Unwinding: DNA helicase unwinds the DNA strands.
  3. Single-strand binding: Proteins bind to exposed single-stranded DNA to keep them apart.
  4. Topoisomerase: Moves ahead of replication fork and removes torque by making a double-stranded break in one DNA double helix and passing another segment of helix through it.
  5. Primase: Lays down an RNA primer (10-12 nt long) that provides a 3’OH group to start synthesizing DNA.
  6. Elongation: DNA polymerase III synthesizes DNA on both strands in the 5’-3’ direction. The leading strand is replicated continuously, while the lagging strand is replicated discontinuously.
  7. Removal of RNA primers: DNA polymerase I removes RNA primers and replaces them with DNA.
  8. Ligation: DNA ligase recognizes nicked DNA strands and joins them together.
  9. Termination: In bacteria, termination is mediated by terminator proteins or replication forks meeting. In eukaryotes, termination involves multiple types of topoisomerase, nucleosome disassembly and assembly, and coordination with the cell cycle.

Additionally, DNA replication in eukaryotes is regulated by various factors, including licensing factors, Cdks, and the completion of mitosis. Different DNA polymerases with specialized functions are involved, and telomerase is responsible for replicating chromosome ends.

Deviation from Mendel's ratios can have biological significance, and interactions among alleles of a gene can result in various patterns such as lethality, incomplete dominance, codominance, incomplete penetrance, variable expressivity, and pleiotropy. Interactions between genes, such as epistasis, can also influence traits.

Other inheritance patterns include mitochondrial inheritance, maternal effect, sex-limited traits, sex-influenced traits, autosomal recessive and dominant inheritance, and X-linked recessive, dominant, and Y-linked inheritance.

Furthermore, DNA from different individuals may exhibit variations in length of DNA fragments (RFPs) and variations at a single nucleotide (SNPs).

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