Mr. Coleman
Biology
DNA
• DNA is often called
the blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for
many reasons, e.g.,
• its central
importance to all
life on Earth,
• medical benefits
such as cures for
diseases,
• better food crops.
Chromosomes and DNA
• Our genes are on
our chromosomes.
• Chromosomes are
made up of a
chemical called
DNA.
The Shape of the Molecule
• DNA is a very
long polymer.
• The basic shape
is like a twisted
ladder or zipper.
• This is called a
double helix.
One Strand of DNA
• Nucleotides consist
of:
– 1 sugar
– 1 phosphate
– 1 nitrogen base
• One strand of DNA
has many millions
of nucleotides.
nucleotide
One Strand of DNA
• The backbone of
the molecule is
alternating
phosphate and
deoxyribose
sugar
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
Four nitrogenous bases
DNA has four different bases:
• Cytosine
C
• Thymine T
• Adenine A
• Guanine G
Important:
• Adenine and Thymine always join
together
A T
• Cytosine and Guanine always join
together
C G
Two Kinds of Bases in DNA
• Pyrimidines are
single ring bases.
• Purines are double
ring bases.
N C
O C
N
C
N C
N
N C
C
C
N
N C
N C
Thymine and Cytosine are
pyrimidines
• Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
N
O
C
C C
N
N
C
C
thymine
O
C
C
C
N
C
cytosine
Adenine and Guanine are
purines
• Adenine and guanine each have two
rings of carbon and nitrogen atoms.
O
N
N
C
N
C
N
C
C
C
C
N
N
N
C
Adenine
N
C
N
C
Guanine
N
C
Two Stranded DNA
• Remember, DNA
has two strands
that fit together
something like a
zipper.
• The teeth are the
nitrogenous bases
but why do they
stick together?
Hydrogen Bonds
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are
weak but there are
millions and millions
of them in a single
molecule of DNA.
DNA by the numbers
• Each cell has about 2 m
of DNA.
• The average human has
75 trillion cells.
• The average human has
enough DNA to go from
the earth to the sun
more than 400 times.
• DNA has a diameter of
only 0.000000002 m.
The earth is 150 billion m
or 93 million miles from
the sun.
I. DNA Replication
A. When a cell divides, it
needs to make a copy of
its DNA so that both of
the new cells have the
right amount of DNA
I. DNA Replication
B. The steps of DNA Replication
1. Separation of Strands- Enzyme unzips the
DNA (breaks the hydrogen bonds)
2. Free Base Pairing- Free nucleotides pair up
with the exposed nitrogen bases on the two half
DNA strands
3. Base Bonding- Another enzymes zips the
hydrogen bonds together
C. Two identical strands created!
Genes & Proteins
• Genes are where the ribosomes get the
information to build different proteins
– A gene is one section of the DNA strand
• Different proteins make different traits
– Eye color, height, hair color, etc
• Proteins are made of amino acids
– The gene tells the ribosome what order to put the amino
acids
– If they get out of order the wrong protein is produced.
Protein Synthesis
(Making Proteins)
• Step 1: Transcription- Making a one sided copy
of the DNA message called mRNA
– Enzymes read the DNA in order to produce mRNA
– RNA is made from ribose sugars and only has one side
– RNA is small enough to leave the nucleus (DNA is not because
it has two sides) This is why we call it messenger RNA—that’s
what the m stands for!
– In RNA uracil replaces thymine (U instead of T)
• See anmation
Protein Synthesis
(Making Proteins)
• Step 2: Translation-using mRNA and tRNA to
link amino acids together to make proteins
– Takes place in the ribosome
– Ribosomes read mRNA and match it with the
appropriate tRNA molecule
• The tRNA carry the amino acids to the ribosome
• See anmation
I now give you the Protein
Synthesis Rap…
Mutations
• Mutation- Any permanent change in the order of
nucleotides in the DNA
– Causes different proteins to be made
– Some mutations can be good, but many are
harmful
– Things that cause mutations:
• UV Rays
• Chemicals
• Smoking, Drinking, poor eating, etc
• Some happen naturally
• Etc