H14
Proteins and DNA
DNA is one of the most important compounds known and its importance and uses are
increasing rapidly. Proteins that make up so much of living organisms are made using
the genetic information stored in the DNA molecule.
Proteins
Naturally occurring condensation polymers of amino acids.
There are two main types of protein
 Fibrous proteins including
o Keratin
o Collagen
o Fibroin
 Globular proteins including
o Insulin
o Haemoglobin
o Myoglobin
Amino acids
Compounds that contain both an amine (-NH2) group and a
carboxylic acid (-COOH) functional group.
There are 20 amino acids commonly found in proteins and
these fall into three groups:
 Non-polar R groups
 Polar R groups capable of being charged
 Polar R groups that do not tend to become charged.
Zwitterions
Amino acids exist primarily as dipolar ions because the acidic
–COOH group loses a proton to the basic –NH2 group.
Amino acids are consequently crystalline solids with relatively
high melting points and they are soluble in water.
Dipeptide
Compound formed when two amino acids react together.
Water is also formed from this condensation reaction.
Peptide bond
The amide linkage ( - CO – NH - ) between two amino acids
that combine to form a dipeptide.
Determination of
protein structure
The chemist needs to determine the
 Identity of all the amino acids present
 Relative molar amounts in which these amino acids are
present
 The sequence in which the amino acids are linked
together.
Tests for proteins
and/ or amino acids
These tests indicate that a protein or amino acids are present:
 Ninhydrin turns purple when heated with either a protein
or amino acid.
 Biuret turns purple when a few drops of copper sulfate
are added to an alkaline solution of protein or
polypeptide. Biuret does not give a positive result with
amino acids.
Chromatography
Method of separating substances based on their differential
distribution between two phases, one stationary and the other
mobile.
Electrophoresis
Method of separating substances based on the different signs
of their charge and on their different mobilities.
The speed at which particles move depends on their size and
charge.
Neutral molecules do not migrate during electrophoresis,
positively charged ions move in one direction and negatively
charged ions move in the opposite direction.
Small ions move faster than large ions (given the same
charge)
Doubly charged ions move faster than singly charged ions of
the same size.
Isoelectric point
The pH at which an amino acid exists as an over-all neutral
zwitterion.
Chromatography and
electrophoresis
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Both separate mixtures of amino acids
Chromatography separates amino acids based on their
solubilities in polar and non-polar solvents. The degree of
separation can be improved by changing the solvent.
Electrophoresis separates amino acids based on their
charges and size. Altering the pH of the solution can vary
the charge on the amino acid.
Electrophoresis gives more effective separation than
chromatography but involves the use of more expensive
equipment.
DNA
Abbreviation for deoxyribonucleic acid.
 Acid because of the many phosphoric acid groups
 Nucleic because the molecule occurs in the nucleus of
cells
 Deoxyribo- because the molecule contains the sugar
deoxyribose.
DNA is a condensation polymer in which the monomer units
are nucleotides.
DNA occurs as a double helix in which the two helixes are
held together by hydrogen bonds.
The hydrogen bonds holding the two helixes together easily
break and reform.
Nucleotide
Consists of a sugar molecule (deoxyribose in DNA) attached
to a phosphate unit and to a base.
Four bases are used in DNA
 Adenine
 Guanine
 Cytosine
 Thymine
The bases form complementary pairs in the DNA molecule
 Adenine with thymine
 Guanine with cytosine
Genes
DNA carries the genetic code that allows living things to
function and reproduce themselves.
Genes are sections of DNA molecules. Each DNA molecule
contains thousands of genes.
The sequence of bases along the section determines the
sequence of amino acids in the protein that the gene
synthesises.
Exons are coding sequences along the DNA strand (genes).
Introns are the non-coding sequences and vary significantly
from one person to another. Introns are used for DNA analysis
by forensic chemists.
Steps in DNA
analysis
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Uses for DNA
analysis
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Separate the DNA from other material in the sample
Make multiple copies of selected segments of the DNA in
intron regions by using the polymerase chain reaction.
Determine the length of these copied segments by
electrophoresis. Rather than recording the number of base
pairs in each fragment the practice is to record the
number of repeat units, called short tandem repeats
(STRs)
Compare the samples to see if they match by comparing
the DNA profile (the number of STRs on each DNA
molecule for 10 different segments from identical
positions).
Identify the person who produced a biological sample at a
crime scene.
Identify the father of a child in disputed paternity cases
Establish familial links