Genetics › DNA

DNA Structure Overview

Notes

DNA Structure Overview

Sections

DNA

  • Deoxyribunucleic acid
  • Linear polymer of nucleotide monomers
  • Stores genetic information
  • Fixed backbone of sugar-phosphate units (component of nucleotides)
  • 2 strands of complementary and antiparallel DNA come together to form a double helix.

Nucleotide

  • Monomer of DNA
  • 1 deoxyribose + 1 to 3 phosphate groups + 1 nitrogenous base
  • 4 nitrogenous bases: Adenine, Thymine, Cytosine, Guanosine

Purines

  • Adenine and Guanine
  • Double-ringed nucleotides

Pyrimidine

  • Thymine and Cytosine
  • Single-ring nucleotides

DNA STRUCTURE

  • 1 phosphate between nucleotides
  • Deoxyribose (or RNA ribose): nitrogenous base at C1, phosphate at C5, C3 has free -OH
  • 5' end is the end: free phosphate group
  • 3' end: free hydroxyl group

Base pairing

  • Adenine-Thymine
  • Guanine-Cytosine
  • Generates the two-stranded, double helix structure of DNA

Full-Length Text

  • Here we will learn the general features of DNA, the most essential macromolecule in a cell. DNA is the genetic code that contains all the functional instructions for any cell (and consequently, any organism).
  • Start a table.
  • Denote some key features of DNA:
    • It stands for deoxyribonucleic acid.
    • It's a linear polymer (a large molecule with repeating units) that comprises four types of monomers (the repeating units).
    • It stores a cell's genetic information.
    • DNA structure enables its function.
    • It has a fixed backbone of sugar-phosphate units that gives each DNA strand directionality.

Now, let's draw the general structure of a DNA monomer, which is called a nucleotide.

  • Draw a hexagon to represent a nitrogenous base.
  • Write that DNA comprises four nitrogenous bases.
  • Now add a pentagon bound to our hexagon to represent the sugar.
  • Next, indicate that, together, the nitrogenous base and sugar are called a nucleoside.
  • Write that in DNA, the sugar is deoxyribose, a five-carbon monosaccharide.
    • In RNA, the other type of nucleic acid, the sugar is ribose.
  • Now, draw the phosphate portion of DNA bound to the sugar.
  • Write that free nucleotides can have up to 3 phosphate groups; as we'll see they can lose one or two phosphate groups as an energy source in a reaction.
  • In DNA, nucleotides exist with only one phosphate group, as drawn here.

From our diagram, let's enumerate some key points.

  • Write that each nucleotide has three components:
    • A nitrogenous base, which is its variable unit.
    • A sugar, which determines the principle nucleic acid (DNA or RNA) the nucleotide makes.
    • A phosphate group (or groups), which enables bonds between monomers and also is a source of energy.

Now, let's draw the general structure of a single strand of DNA.

  • Redraw our nucleotide:
    • The nitrogenous base (labeled B).
    • Sugar (labeled D for deoxyribose (in DNA), again in RNA, this would be R for ribose).
    • Phosphate group (labeled P).
  • Now, draw two more nucleotides linked to the first nucleotide (and to each other) in a chain.
  • Delineate the nitrogenous bases, which form the sequence of DNA.
  • Delineate the sugar-phosphate backbone, where the monomers of DNA are linked together to form the DNA polymer
  • Use an arrow to indicate the strand's direction:
  • Show that the 5-prime end is the end with the free phosphate group.
    • It gets its name from the fact that the phosphate group is bound to carbon 5 of deoxyribose.
  • Show that the other end is called the 3-prime end.
    • The 3-prime end gets its name from the fact that the hydroxyl group is attached to carbon 3 of deoxyribose
  • Now indicate on our diagram that the 3-prime has a free hydroxyl group.

Now let's learn the four nitrogenous bases of DNA. These are the letters that write the genetic code.

  • Write that the four bases and their one letter codes are:
    • Adenine (A)
    • Cytosine (C)
    • Guanine (G)
    • Thymine (T)
  • Write that there are two types of bases: purines and pyrimidines.
  • Write that the purines are Adenine and Guanine; as we'll see, they are double-ringed nucleotides.
  • Write that the pyrimidines are Cytosine and Thymine; as we'll see they are single-ringed nucleotides.

We'll use shapes and colors to represent the bases (we learn their chemical structures, elsewhere).

  • For adenine (a purine), draw a hexagon and pentagon stuck together.
  • For cytosine (a pyrimidine), draw a hexagon.
  • For guanine (a purine), draw a hexagon and pentagon stuck to each other.
  • For thymine (a pyrimidine), draw a hexagon.
  • Each nitrogenous base of DNA has a "partner" with which it can form a base pair. Base pairing generates the two-stranded, double helix structure of DNA.
  • Write that base pairing is always purine to pyrimidine.
    • More specifically, write that A always pairs with T, and that C always pairs with G.
    • The nitrogenous bases in one chain form base pairs with the nitrogenous bases in the second chain.

Now let's draw the two-stranded structure of DNA.

  • DNA forms a double helix, which is its characteristic configuration.
  • Draw two lines in a double helix formation.
    • Indicate that that DNA nucleotides create this double-helix shape.
  • Now show that base pairs join the two sides of the helix.
  • Write that hydrogen bonds between base pairs hold the helix together.
  • Write that base pairing means that the DNA strands are complementary (one strand of DNA is exactly opposite in sequence to the other).
  • And that this allows for both DNA strands to be used as templates for replication.
  • Accordingly, indicate that one DNA strand goes from 5 prime to 3 prime, and the second DNA strand goes from 3 prime to 5 prime.

Lastly, let's consider how DNA determines the makeup of an organism.

  • Indicate that the central dogma of molecular biology simplifies this to:
    • DNA is transcribed to RNA.
    • RNA is translated to protein.
  • Thus, the genetic code of an organism determines the structure of its every component.

References:

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  2. McKee T. & McKee, J. Biochemistry: The Molecular Basis of Life, 6th ed., (Oxford University Press, 2015).
  3. Berg, J.M., Tymoczko, J.L. & Stryer, L. Biochemistry, 7th ed., (W.H. Freeman and Company, 2010).
  4. Bodansky, O. Biochemistry of Human Cancer, 248 (Academic Press, 1975).
  5. Cooper, G.M. & Hausman, R. E. The Cell: A Molecular Approach, 6th ed., 108-110, 172-175 (Sinauer Associates, 2013).
  6. Ulrich, K. Comparative Animal Biochemistry, 29 (Springer Science & Business Media, 1990).
  7. Bettelheim, F. A., Brown, W. H., Campbell, M. K., Farrell, S. O. & Torres, O. J. Introduction to General, Organic, and Biochemistry, 10th ed., 698 (Cengage Learning, 2013).