Unit 1: DNA Lecture 2: DNA Function & DNA Replication

DNA Structure

• Who “discovered” the structure of DNA?

•  Experiments used to determine the structure of DNA:

  • Rosalind Franklin

    • Using X-ray crystallography (experiment published in 1953):

      • Identified sugar-phosphate backbones on the outside of helix

      • Nitrogenous bases on the inside

      • DNA has a consistent diameter of 2nm diameter

  • Erwin Chargaff

    • Composition of nucleotides (called “Chargaff’s rules” published in 1950)

      • Equal # of A and T, equal # of G and C

      • %A=%T and %G=%C

      • %ages vary by species

  • James Watson and Francis Crick

    • Constructed a model of a double helix that conformed/aligned with Franklin’s data, Chargaff’s data, and known nucleotide structure information

      • Used this model to deduce that a purine on one strand binds with a pyrimidine on other strand to produce a uniform thickness

      • A binds with T and a G binds with C; called Watson-Crick base-pairing (aka complementary base pairing)

      • Won Nobel Prize for solving DNA Structure

DNA Function

• What is the function of DNA?

1. Genetic “instructions” for making more DNA

   1. More DNA made using the process of DNA Replication (lecture 2)

2. Genetic “instructions” for making proteins

   1. DNA → RNA → Protein

      1. RNA made during process of Transcription (lecture 3)

      2. Protein made during process of Translation (lecture 4)

3. Genetic information passed from parent to offspring 

   1. Called inheritance

What aspect of DNA structure allows it to carry the coded (genetic) information?

Sequence of bases on each strands

• DNA has multiple functions in the cell:

1. Genetic “instructions” for making more DNA

• How do cells make copies of their DNA in order to pass it from parent to offspring?

  • DNA Replication

    • Purpose, advantages

    • How it happens

DNA Replication 

• Purpose:

  • Make more cells so each cell has its own DNA

  • More cells made because of

    • Organismal growth (we all start as a single-celled organism aka zygote)

    • Wound repair (damaged cells)

• Advantages

  • Ensures all the cells in a multicellular organism:

    • Carry the same genetic information

    • Have the genetic instructions (DNA) available for gamete (egg and sperm) development

• Components needed:

  • DNA (double-stranded) template strand

  • DNA Nucleotides (A, T, G, C) for making more DNA

  • Proteins/Enzymes:

    • Topoisomerase

    • Helicase

    • DNA polymerase

    • Ligase

DNA Replication - Brief

• How does it happen (overview)?

  • Two strands of parental DNA separate

    • Each strand becomes a template for the assembly of a complementary strand

    • Free nucleotides bind to the template strand

      • Abide by the base-pairing rules

    • Enzymes generate a new sugar-phosphate backbone

      • Phosphate of one nucleotide is linked to sugar group (-OH) of adjacent nucleotide

    • End result: 2 identical daughter molecules

      • DNA replication is known as the semiconservative model

• Semiconservative model (partially maintained)

  • Of the new double helix: 

    • 1 strand = newly formed “daughter” strand

    • 1 strand = old, original “parental” strand

      • Half of the DNA is made of the parental molecule and is therefore “conserved” in each DNA molecule

DNA Replication

• Happens very quickly

  • Speed of the process

    • E. coli = 4.6M DNA bp (-1h to copy genome)

    • Human = 6B DNA bp (46 chromosomes) (few hrs to copy)

• Why does it need to go fast?

  • Needs to make new cell (for some reason) and conditions are currently favorable

• To decrease time taken to replicate DNA, the cell:

  • Starts replication at multiple locations along the DNA

  • Replicate the DNA in opposite directions

• DNA Polymerase adds nucleotides in the 5’ to 3’ direction (using the 3’ end as the template strand)

DNA Replication - Specifics

• How does it happen (specifics)?

1. Proteins attach to DNA at each Origin of Replication (ORI) and separate the two strands

• Opening up the DNA causes “bubbles” to be formed

1. Topoisomerase: unwinding DNA

2. Helicase: pry apart helix/double-stranded DNA (breaks hydrogen bonds)

2. DNA Polymerase adds nucleotides to ( 3’ -OH starts at)

• Leading strand: strand continuously synthesized; works towards forking point; no interruption in replication

• Lagging strand: strand NOT continuously synthesized since new DNA only made away from fork; needs to wait for enough sequence to be available for DNA Pol to bind & replicate; smaller DNA fragments called Okazaki fragments

• Also has proof-reading abilities

  • Ensures proper complementary base was added (DNA repair)

    • Can also repair DNA damage caused by

      • Radiation (UV light, X-rays)

      • Toxic chemicals (tobacco smoke)

1. Nucleotides are added in a 5’ to 3’ direction

   1. 5’ Phosphate group (carbon 5 on the incoming nucleotide attaches to the -OH group (carbon 3) of the first nucleotide

   2. Forms covalent bonds between nucleotides

3. DNA ligase glues/links the Okazaki fragments together to make single strand on each side of DNA molecule