Understanding DNA and Its Functions

• Location of DNA

  • DNA is typically found in the nucleus of a cell.

  • Exception: During mitosis, DNA moves around and is not in the nucleus.

  • Chromatin: Form of DNA when it's not densely packed as chromosomes.

Protein Synthesis Overview

• Processes Involved:

  • Transcription: Converts DNA to RNA.

  • Translation: Converts RNA to protein.

Transcription

• Role of RNA Polymerase:

  • Unwinds DNA and breaks hydrogen bonds.

  • Pulls in RNA nucleotides in a complementary manner.

  • Officially known as DNA-dependent RNA polymerase.

DNA Replication

• Definition:

  • Process where DNA makes a copy of itself.

  • Occurs immediately prior to mitosis.

  • Essential for cell division and maintaining the genome.

Significance of DNA Replication

• Must occur after a cell passes a specific checkpoint in the cell cycle.

• A factory analogy: Requires its own set of instructions (complete DNA) for new cells to function properly.

Key Enzymes in DNA Replication

• DNA Polymerase:

  • Main enzyme involved in synthesizing new DNA strands.

  • Uses DNA nucleotides: A, T, C, G (complementary base pairing).

• Helicase:

  • Unwinds the DNA double helix by breaking hydrogen bonds.

• Single-Stranded Binding Proteins (SSB):

  • Stabilize unwound DNA strands to prevent them from rejoining.

• Topoisomerase:

  • Relieves torsional stress from unwinding DNA.

Steps of DNA Replication

• Replication Fork: Area where DNA is unwound.

• Directionality:

  • DNA polymerase synthesizes DNA only in the 5' to 3' direction.

  • Can only read the template strand in the 3' to 5' direction.

Starting the Replication

• Primase (a type of RNA polymerase) synthesizes a short RNA primer to provide a free 3' hydroxyl group for DNA polymerase to attach to.

Leading and Lagging Strands

• Leading Strand:

  • Continuous synthesis as helicase unwinds DNA.

• Lagging Strand:

  • Synthesized in shorter segments called Okazaki fragments.

  • Requires repeated primase action for each new fragment.

• Okazaki Fragments:

  • Short, newly synthesized DNA segments on the lagging strand.

Final Steps of Replication

• RNA primers are eventually removed and replaced with DNA by DNA polymerase.

• DNA Ligase:

  • Seals gaps in the DNA backbone by forming covalent bonds.

Fidelity and Proofreading

• DNA polymerase is both accurate and fast.

• Capable of proofreading and correcting errors during DNA synthesis.

  • If an error is detected, it can remove mismatched nucleotides before continuing.

Significance of the Process

• Importance of accurate DNA replication to prevent mutations that could lead to diseases like cancer.

Summary of DNA Replication

• Semiconservative Nature:

  • Each newly synthesized double helix consists of one original strand and one newly synthesized strand.

• Origin of Replication:

  • Marks where replication starts, continues through elongation until termination at the end of DNA.

Additional Terminology

• Replication Bubble: Area where DNA is being unwound and replicated.

• Friction Ridges (Fingerprints): Serve a purpose that remains somewhat speculative, possibly aiding in tactile sensation.

• Normal turnover of skin cells: Around 50,000 skin cells per minute, resulting in high rates of cell replenishment.

Cellular Repair and Health

• Understanding processes like DNA transcription, translation, and replication is crucial in fields like molecular biology, genetics, and medicine because these processes are fundamental to life.