DNA Structure and Replication

  • Today's topics include: the structure of DNA and how it replicates.

Learning Objectives

  • Describe what DNA looks like

  • Talk about how DNA is arranged in eukaryotic (like plant and animal cells) and prokaryotic (like bacteria) cells

  • Go through the steps of DNA replication

  • Discuss why telomerase is important in replication

  • Explain how DNA gets repaired when it’s damaged

Structure of DNA

  • DNA is a double-stranded helix, meaning it twists into a spiral shape with a uniform thickness.

  • It has a sugar-phosphate backbone running along its edges.

  • The strands are made of two polynucleotides.

Nucleotide Structure

  • Nucleotides, the building blocks of DNA, consist of:

    • Nitrogenous Bases:

    • Purines: Adenine (A), Guanine (G)

    • Pyrimidines: Cytosine (C), Thymine (T)

  • The bases are stacked inside the DNA molecule.

  • Chargaff's Rules help us remember how bases pair up:

    • A pairs with T

    • G pairs with C

Base Pairing and DNA Dimensions

  • Specific grouping on nucleotides ensures pairing between bases:

    • A and T bond with 2 hydrogen bonds

    • G and C bond with 3 hydrogen bonds

  • This bonding keeps the DNA at a steady width.

Arrangement of DNA in Cells

  • Eukaryotic Cells: DNA is grouped into chromosomes located in the nucleus.

    • Chromatin: DNA coils around proteins called histones to form nucleosomes and compact into fibers.

  • Prokaryotic Cells: DNA is found in a region called the nucleoid as a single circular chromosome.

DNA Replication

  • The main job of DNA is to hold and pass down genetic info.

  • Genes get passed along when cells divide and during reproduction.

Semiconservative Model

  • Watson & Crick proposed that DNA replication follows the semiconservative model:

    • Each new DNA strand has one old (template) and one newly made strand.

    • This keeps half of the original DNA intact.

Process of DNA Replication

  • Starts at origins of replication, which are specific sequences.

  • Proteins help unzip the double helix, forming replication bubbles.

  • Replication happens in both directions from each origin.

  • Key steps of replication:

    • Helicase unwinds the DNA.

    • DNA polymerase adds new nucleotides to the growing strands at the 3’ end.

    • The leading strand is made continuously while the lagging strand is made in short segments called Okazaki fragments.

    • DNA ligase connects those Okazaki fragments to complete the lagging strand.

Orientation of DNA Strands

  • DNA strands have 3’ and 5’ ends, determined by the carbon atoms in the sugar.

  • This orientation is important since DNA polymerase can only work in the 5’ to 3’ direction.

Proofreading and Repair

  • DNA polymerases also help by checking new DNA to catch mistakes.

  • If errors happen, DNA repair systems kick in to fix them.

Summary

  • Grasping the structure and replication of DNA is key for understanding genetics and molecular biology.

  • Specific base pairing and the direction of DNA strands are