DNA: The Molecule of Life
What is a DNA
??
Deoxyribonucleic Acid (DNA)
• genetic material of all cellular organisms and
most viruses
• used to encode genetic information for all life on
Earth.
DNA



organized into
chromosomes
Eukaryotes store it
inside the cell
nucleus
prokaryotes in the
cytoplasm.






In normal human cells,
DNA is arranged in 23
pairs of chromosomes
22 pairs autosomes
the 23rd pair are the sex
chromosomes
XX (female) or XY (male)
not all chromosomes are
equal
Ex. #1 – 4220 genes
#18 – 300-400 genes
General Structure of Nucleic Acid



DNA is a long chain
of nucleotides.
Each nucleotide is
composed of a
base, a sugar
(deoxyribose), and a
phosphate group.
The phosphate joins
the sugars in a DNA
chain
Types of Nucleotides








4 types of nucleotides
only nitrogenous base is different
A – adenine
G – guanine
C - cytosine
T - thymine.
A bonds with T (two hydrogen
bonds)
G bonds with C (three hydrogen
bonds).






Purines: Adenine & Guanine
double ring structure
Pyrmidines: Cytosine &
Thymine
single ring structure
the number of thymine always
equal number of adenine
number of cytosine always equal
to guanine.
Discovery of DNA Structure

In 1950’s scientists knew that DNA:
 existed
 was
made of nucleotides sticking together like
string
 would have to be over 1 m long in each cell,
so must have been compressed somehow
 several scientists were working/competing to
discover the structure of DNA
 Greatest Genetics Discoveries 1/3 - YouTube
James D. Watson
Francis Crick
Rosalind Franklin
Maurice Wilkins





Franklin used x-ray crystallography to determine
DNA had a spiral shape
the photos she took were taken by Wilkins
(without her knowledge or permission) and
shown to Watson & Crick
Watson & Crick were able to discover the shape
of DNA in 1953.
Watson, Crick & Wilkins were awarded the
Nobel Prize in 1962.
As Franklin had died 4 years earlier, she was not
included and her work was ignored for years.
The Double Helix









‘sides’ made of deoxyribose sugar & phosphate
molecules
‘rungs’ are pair of nitrogen bases joined by hydrogen
bonds
3.5 billion base pairs & 30,000 genes on 46 human
chromosomes (in each cell in body)
each series of 3 base pairs (codon) code for 1 amino
acid
genes are a series of codons that create a chains of
amino acids
amino acids make up proteins
therefore, a gene is the code for a protein
proteins code for all your genetic traits
pg 195 “Reading The Code”
Packaging of DNA (deoxyribonucleic acid)

In eukaryotic cells, DNA is found in the nucleus.
 Most
of the time, DNA exists as long, invisible, tangled
fibres called chromatin
 As a cell prepares to divide, chromatin thickens and
shortens into chromosomes.



Each chromosome is made of many small segments called genes.
Humans have 23 pairs of chromosomes (46 total).
 Each human cell has 2 m of chromosomes (if stretched out).
 Each species has a different number of chromosomes.
DNA in chromatin is tightly coiled around bead-shaped proteins
called histones.
DNA (deoxyribonucleic acid)
Like thread wrapped around a spool to keep
it organized and protected, chromatin
consists of DNA wrapped in coils around
protein “beads” called histones.