DNA Replication: Steps, Locations, and Enzymes (USLO 5.1)

DNA replication is the process by which a cell duplicates its entire genome before cell division. This semi-conservative process ensures that each new cell receives a complete and identical set of genetic instructions.

Steps in DNA Replication:

1. Initiation: The process begins at specific sites called origins of replication. An enzyme called helicase unwinds the double-stranded DNA helix, breaking the hydrogen bonds between the bases. This creates a “replication fork,” a Y-shaped structure where replication will proceed.
2. Elongation:
   • A short RNA primer is synthesized by an enzyme called primase to provide a starting point for DNA synthesis.
   • DNA Polymerase III then binds to the primer and begins to synthesize a new strand of DNA by adding nucleotides that are complementary to the template strand.
   • The leading strand is synthesized continuously in the 5′ to 3′ direction, moving toward the replication fork.
   • The lagging strand is synthesized in the opposite direction, away from the replication fork. It is made in short, discontinuous segments called Okazaki fragments.
3. Termination:
   • DNA Polymerase I replaces the RNA primers with DNA nucleotides.
   • DNA ligase then seals the gaps between the Okazaki fragments on the lagging strand, forming a continuous DNA strand.
   • Topoisomerase (also known as gyrase in prokaryotes) relieves the tension caused by the unwinding of the DNA helix ahead of the replication fork.

Locations in Cells:

• Prokaryotic Cells: DNA replication occurs in the cytoplasm. Prokaryotes have a single, circular chromosome and typically have only one origin of replication.
• Eukaryotic Cells: DNA replication occurs in the nucleus. Eukaryotic cells have multiple, linear chromosomes and possess multiple origins of replication to ensure the entire genome is replicated in a timely manner.

Gene Expression: Transcription and Translation (USLO 5.2)

Gene expression is the process by which the information encoded in a gene is used to synthesize a functional product, typically a protein. It involves two main steps: transcription and translation.

• Transcription: This is the process of synthesizing an RNA molecule from a DNA template. It occurs in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells.
  1. Initiation: An enzyme called RNA polymerase binds to a specific DNA sequence called a promoter at the beginning of a gene.
  2. Elongation: RNA polymerase unwinds a small section of the DNA and synthesizes a complementary strand of messenger RNA (mRNA) in the 5′ to 3′ direction.
  3. Termination: The process ends when RNA polymerase reaches a terminator sequence, causing the enzyme to detach and release the newly formed mRNA molecule.
  • In eukaryotes, the mRNA molecule undergoes post-transcriptional modifications before leaving the nucleus, including the removal of non-coding regions (introns) through splicing, and the addition of a protective 5′ cap and a poly-A tail.
• Translation: This is the process of synthesizing a protein from an mRNA template. It takes place at ribosomes in the cytoplasm.
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