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Section 2: DNA Structure and Replication

Unit 6: Microgenetics Chapters 13,14.4

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Objectives

  • Describe the basic structure of DNA, including the role of hydrogen bonding.
  • Explain the basic process of DNA replication.
  • Explain how the process of DNA replication results in the transmission and conservation of the genetic code.

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Warm-up:

  • List two things you know about DNA.

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DNA Structure

  • Chromosomes are made up of a single strand of DNA that is wrapped around proteins.
  • The chromosomes, and therefore the DNA, are found in the nucleus of eukaryotic cells.

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DNA Structure

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  • Nucleus: A membrane-bound organelle in eukaryotic cells functioning to maintain the integrity of the genetic material and, through the expression of that material, controlling and regulating cellular activities.

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DNA Structure

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  • DNA is a type of nucleic acid, which is a biological macromolecule.
    • Nucleic Acid: A biological macromolecule composed of the elements C, H, N, O, and P that carries genetic information.

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DNA Structure

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  • Deoxyribonucleic Acid (DNA):a biological macromolecule that encodes the genetic information for living organisms and is capable of self-replication and the synthesis of ribonucleic acid (RNA).
    • The monomers of DNA are nucleotides that are made of sugar, phosphates, and nitrogenous bases.

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DNA Structure

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  • Nucleotide: the monomers of nucleic acids that when combined make up the polymers DNA and RNA.

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DNA Structure

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  • The sugar and phosphate molecules make up the back-bone of a DNA molecule.
  • Each sugar molecule is bonded to a nitrogenous base.

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DNA Structure

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  • In DNA, the four possible nitrogenous bases are:
    • Adenine
    • Thymine
    • Cytosine
    • Guanine

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DNA Structure

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  • The nitrogenous bases will form hydrogen bonds with each other to link two different nucleotides together.
    • Adenine always pairs with thymine.
    • Guanine always pairs with cytosine.

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DNA Structure

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  • Hydrogen Bonds: a weak chemical bond formed between the slightly positive hydrogen atoms of one molecule and the slightly negative charged atoms of a different molecule.

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DNA Structure

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  • When the nucleotides are connected, the structure of DNA takes on the shape of a double-helix.
    • This was first discovered by scientists James Watson and Francis Crick, working off the discoveries of scientist Rosalind Franklin.

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Check for Understanding:

Which nitrogenous bases will pair up with the following DNA strand?

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DNA Replication

  • During interphase, the DNA has to replicate before cell division can occur.
    • DNA Replication: The process in which DNA makes a duplicate copy of itself.

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DNA Replication

  • In order for replication to start, the hydrogen bonds holding the nitrogenous bases together must be broken.
    • An enzyme called helicase will break the hydrogen bonds to open up the DNA molecule.

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DNA Replication

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  • Enzyme: A protein that increases the rate of a chemical reaction without being changed by the reaction.
    • Helicase allows room for the next enzyme, DNA polymerase, to read each strand of DNA and put together the corresponding base pairs.

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DNA Replication

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  • When DNA replicates, each strand of the original molecule serves as a template to build a new strand.
    • Each new DNA molecule now has one strand from the original molecule and one newly built strand.
      • This is called semi-conservative replication.

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DNA Replication

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  • Semiconservative Replication: The process in which the DNA molecule uncoils and separates into two strands. Each original strand becomes a template on which a new strand is constructed, resulting in two DNA molecules identical to the original DNA molecule.

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Check for Understanding:

What are the different roles of enzymes in DNA replication?

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DNA Mutations

  • Sometimes DNA replication doesn’t occur exactly as it should and a mistake, called a mutation, occurs.
    • Gene Mutation: A permanent alteration that changes a segment of DNA within a gene.

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DNA Mutations

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  • There are several different types of mutations that can occur during DNA replication:
    • Point Mutation: A single-base substitution causing the replacement of a single-base nucleotide with another nucleotide.
      • Original: ATCGTTA
      • Mutation: ATGGTTA

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DNA Mutations

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    • Deletion Mutation: A genetic mutation in which one or more nucleotides are removed from a section of DNA.
      • Original: ATCGTTA
      • Mutation: ATGTTA

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DNA Mutations

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    • Insertion Mutation: A genetic mutation in which one or more nucleotides are inserted into a genetic sequence.
      • Original: ATCGTTA
      • Mutation: ATCCGTTA

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DNA Mutations

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  • Mutations in the DNA occur frequently and randomly.
    • Most of these mutations are repaired.
    • Mutations that occur in an adult’s somatic cells cannot be passed onto offspring, but mutations that occur in the gametes can!

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DNA Mutations

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  • Mutated DNA can have many possible effects on the protein that it is supposed to make.
    • Beneficial
    • Harmful
    • Neutral

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Discussion:

What are some ways humans might increase their chances of having a mutation in their DNA?