We already know that DNA is a double molecule.  Let's see what that looks like:

DNA looks like a ladder. 

 

  • The Rungs (steps) of the ladder are the Nitrogenous Bases (A, T, C, G).

  • The Backbone (legs) of the ladder is made up of Phosphate and Sugar Molecules.

  • The specific Sugar molecule is Deoxyribose - the D in DNA.

  • The molecule is a Double Helix - it spirals around itself.

Two sugar-phosphate backbones spiral around each other, forming the vertical structure of DNA.

 

The Sugar-phosphate backbones connect by the bases sticking out from their sugar molecules.

The Nucleotide

Nucelic Acids are macromolecules, and are composed of smaller subunits.  These subunits are called Nucleotides.  A Nucleotide is composed of three parts: a Phosphate Group, a Sugar and a Nitrogenous Base.

 

The Phosphate Group and Sugar are referred to as the Backbone of the molecule (the legs of the ladder), and the Nitrogenous Bases are the cross-pieces (rungs of the ladder).

The Sugar in DNA is Deoxyribose, and it attaches to both a Phosphate

and a Nitrogenous Base

There are four Nitrogenous Bases in the DNA molecule:

  1. Adenine

  2. Thymine

  3. Guanine

  4. Cytosine

 

Adenine ALWAYS pairs with thymine,

and guanine ALWAYS pairs with cytosine.

The Sugar Phosphate Backbone

The Nitrogenous Base is the part of the DNA that contains the instructions for building proteins.

The Phosphate joins to the sugar molecules

More About DNA Structure:

DNA Transcription

How DNA makes copies

 

The central dogma of molecular biology explains that DNA codes for RNA, which codes for proteins. We need to remember that DNA has to serve two distinct purposes: it not only has to contain a code of instructions, it also needs to be able to copy itself perfectly to pass this information along to future generations of cells.  This section will explore how the DNA molecule achieves this other goal.

Let's review what we know:

We have learned:

  •  there are two types of Nucleic Acids (DNA & RNA)

    • DNA always remains inside the nucleus of the cell

    • RNA can leave the nucleus and enter the Cytoplasm

  • Ribosomes are the organelle that manufacture proteins

    • Ribosomes are located in the Cytoplasm

  • Proteins are made up of Amino Acids strung together like beads on a string

    • Proteins serve many purposes in the cell

      • Enzymes: Facilitate chemical reactions in the cell (speed them up, control what happens)

      • Hormones: Communicate instructions between cells

      • Structural: Building blocks of the cell

  • DNA is a double stranded molecule

    • Double Molecule is twisted like a spiral staircase (Double Helix)

    • Made up of Nucleotides

    • Instructions are coded in Nitrogenous Bases

    • Four Bases

      • Adenine

      • Thymine

      • Cytosine

      • Guanine

    • Chargaff's Rule:  For the DNA in a cell

      • amount of "A" equals the amount of "T"

      • amount of "C" = the amount of "G"

  • Both DNA and RNA play a central role in manufacturing Proteins.

We already know that DNA is a double molecule.  To make a copy, the molecule unzips itself into two complimentary strands, and then uses these as templates to rebuild itself.  In this way, the molecule makes an exact copy of itself.

DNA looks like a ladder that is twisted. 

 

  • The Rungs (steps) of the ladder are the Nitrogenous Bases (A, T, C, G).

  • The Backbone (legs) of the ladder is made up of Phosphate and Sugar Molecules.

  • The specific Sugar molecule is Deoxyribose - the D in DNA.

  • The molecule is a Double Helix - it spirals around itself.

Replication

Once the DNA Double Helix molecule unzips, each individual leg can be used to generate the other side of the molecule.  Just look at the illustration to the right - the split molecule can only accept complimentary bases to match with the unpaired bases on the old strand.  This ensures that the two resulting DNA strands are exact duplicates of each other.

Adenine ALWAYS pairs with thymine,

and guanine ALWAYS pairs with cytosine.

Semi-Conservative Replication

The original strand here is blue.

The new red strand is built off of

the blue template.  The bases can

only pair according to the rules we've

already seen.

The original strand here is red. 

The new blue strand is built off of the red template.  The bases can only pair according to the rules we've already seen.

Semi-conservative replication references the fact that part of the original molecule remains in both duplicated molecules that result.

 

This is an elegant method to make a copy.  By using the original molecule as a template, and having bases that only fit together one way, the cell is able to ensure that all resulting copies are identical with the original molecule.

Semi-Conservative Replication