Mr. Coleman
Biology
DNA- deoxyribonucleic acid
• 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.
• Chromosomes are coiled
DNA that is wrapped
around a protein called a
histone
Structure of Chromosomes
• Chromatid: each half of a chromosomes
• Centromere: where the two chromatids of
a chromosome join
• When a cell is not undergoing replication,
the chromosome is not so tightly coiled.
This is called chromatin.
Chromosomes in the cell
• Human and animal cells are categorized as sex
chromosomes or autosomes:
– Sex chromosomes: chromosomes that determine the
sex of the organism (XX, XY)
• Sperm and egg cell only have ½ the number of
chromosomes
– Haploid (n): 1 copy of each gene
– Autosomes: all other chromosomes
• Homologous chromosomes: each parent donates
one chromosome. This makes 2 copies of each
chromosome (homologous)
– Diploid (2n): 2 copies of each gene
EXAMPLE
• Humans have 46 chromosomes
– This is the total number and is called diploid
(2n)
– These 46 come from 23 pairs of chromosomes
(homologous chromosomes)
• 22 pairs are autosomes
• 1 pair are the sex chromosomes
– If you only look at half of the chromosomes,
this is the haploid number (1n)
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.
WHO CAME UP WITH THE
STRUCTURE OF DNA?
• Watson and Crick
– Used a big model (like
when you used the
model kits)
– Were stuck until they
were able to see the Xray photos created by
Franklin
– This gave them the
information needed to
determine a double
helix
• Franklin
– Did a procedure known
as X-ray diffraction
– Was able to determine
a photo of the DNA
molecule, but couldn’t
quite figure out how
the molecules bound
together
The Double Helix Molecule
• The DNA double
helix has two
strands twisted
together.
• (In the rest of this
unit we will look
at the structure of
one strand.)
One Strand of DNA
• The backbone of
the molecule is
alternating
phosphate and
deoxyribose, a
sugar, parts.
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
Nucleotides
One deoxyribose together with its
phosphate and base make a
nucleotide.
O
O -P O
O
Phosphate
Nitrogenous
base
O
C
C
C
O Deoxyribose
One Strand of DNA
nucleotide
• One strand of DNA
is a polymer of
nucleotides.
• One strand of DNA
has many millions
of nucleotides.
Four nitrogenous bases
DNA has four different bases:
• Cytosine
C
• Thymine T
• Adenine A
• Guanine 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?
N
N
C
N
N
C
C
C
O
• 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.
• (The bonds between
cytosine and guanine
are shown here.)
C
N
Hydrogen Bonds
N
C
N
C
C
C
N
O
Hydrogen Bonds, cont.
• When making
hydrogen bonds,
cytosine always pairs
up with guanine,
• And adenine always
pairs up with
thymine.
• (Adenine and thymine
are shown here.)
O
N
O
C
C
C C
N
C
Important:
• Adenine and Thymine always join
together in a double bond
A T
• Cytosine and Guanine always join
together in a triple bond
C G
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.
DNA Replication
• When cells reproduce, then need to be able
to pass the “cell template” to the new cell
they are creating
• In order to accomplish this, they need to
make an entirely new copy of the genes
within the nucleus.
– DNA Replication: the process by which DNA
in a cell is copied before it undergoes cell
division
DNA Replication
• Since DNA is in a double helix, it is first
necessary to separate the two strands from
each other
– This is done by an enzyme known as a helicase
– The helicases “unzip” the DNA double helix
– The point at which the helicases work is called
the replicating fork
DNA Replication
• Once the DNA is separated, complementary
base pairs bind to the now separated DNA
strands
– The enzymes called DNA polymerases are
used to facilitate this process
– This process is creating two identical strands
• One strand is from the original double helix
• The other strand is being newly created by the DNA
polymerases
DNA Replication
• When creating the new “daughter” strands
of DNA, there is a gap at the replicating
fork
– This area does not have enough room for the
DNA polymerases to work
– These “gaps” are filled in later by an enzyme
called DNA ligase
DNA Replication summary
• DNA replication creates two completely
identical double helix strands of DNA
• Each double helix contains 1 original strand
of the original DNA strand and 1 newly
created strand of DNA
– This is called semi-conservative replication
ERRORS IN REPLICATION
• For every billion base pairs that are created,
there is an error in replication (one base is
added to the strand or deleted that shouldn’t
be)
• The reason for this low error is because the
DNA polymerase has a repair function
– The DNA is “proofread” as it is created
– If an error is detected, the DNA
polymerase can repair the error
ERRORS IN REPLICATION
• Despite the high degree of accuracy in DNA replication,
errors do occur
• An error in replication is called a mutation
– Some errors do not have a significant impact on the
survival of the organism (the mutation is not expressed)
– Others can have serious effects depending on what
proteins the affected gene create
• Example: Cancer is a disease when a cell can’t stop
dividing . The stopping mechanism was damaged.
Uncontrolled cell division creates tumors and can
seriously affect the physiology of the organism.
TYPES OF ERRORS
• Point mutation: When one base is altered
– Insertion: When a base is accidentally added
to the DNA strand.
– Deletion: When a base is accidentally removed
from the DNA strand
– Substitution: When one base is accidentally
replaced by another
• All of these can cause a frame-shift
mutation which alters protein synthesis
Eukaryotic vs. Prokaryotic
• In prokaryotic cells (bacteria), there is only
1 circular chromosome
– DNA replication is a little different from
eukaryotic cells
• Two replicating forks are created and
proceed in opposite directions
• Replication continues until the forks
meet and you now have 2 new,
identical, chromosomes
PROTEIN SYNTHESIS
• In the cell, proteins have many uses:
– Works as an enzyme
– Is one of the components of many organelles
and structural aspects of the cell
• Therefore, the ability to create proteins is
essential for proper cell function
• The process to create proteins is called
protein synthesis
PROTEIN SYNTHESIS
• The blueprint for all proteins exists in DNA
• The flow of information from DNA to the
creation of proteins goes as follows:
DNA  mRNA  tRNA/rRNA (ribosomes)
transcription
translation
COMPONENTS OF
PROTEIN SYNTHESIS
• In addition to the DNA template, there are 3
other molecules necessary in protein
synthesis:
– mRNA (messenger RNA)
– tRNA (transfer RNA)
– rRNA (ribosomal RNA)
• Each type of molecule is necessary for a
step in protein synthesis
TRANSCRIPTION
DNA is copied into a complementary strand
of mRNA.
WHY?
• DNA cannot leave the nucleus. Proteins are
made in the cytoplasm. mRNA serves as a
“messenger” and carries the protein
building instructions to the ribosomes in the
cytoplasm.
TRANSCRIPTION
• To transcribe means to write something
down
• In the first phase of protein synthesis, the
DNA code is “written down” onto a form of
ribonucleic acid known as messenger RNA
(mRNA)
– mRNA is a single strand assembly of
nucleotides
TRANSCRIPTION
• We already discussed the base pairing that
occurs between nucleotides:
• A=T (double bond)
• C=G (triple bond)
• The way that DNA is “written down” is
similar to the way that DNA is replicated
– A segment of DNA is opened using RNA
polymerase enzymes
– Complementary nucleotides bind to the DNA
template to create the mRNA
TRANSCRIPTION
• There is one substitution:
– Thymine in DNA is replaced by
Uracil (U) in mRNA
– Therefore, the binding in mRNA
goes as follows:
• A=U (double bond)
• G= C (triple bond)
PROCESS OF
TRANSCRIPTION
• Transcription starts at a specific DNA site.
This is called the promoter.
– This is where the RNA polymerase binds
• The RNA polymerase then adds
complementary bases to the DNA strand
– Again, T is replaced by U
• Finally, the RNA polymerase hits a DNA
site that causes transcription to end. This is
called the termination signal
TRY THIS
• If you have the following DNA sequence,
what would be the mRNA strand created:
CGATTTGCTACA
CODONS
• What does the mRNA code tell you?
– A sequence of 3 bases makes up a codon
– The codon codes for a specific amino acid
• The handout tells you what amino acids
each codon codes for
TRANSLATION
PROTEINS
Structural and functional components of
organisms.
• Composed of amino acids
• order of nucleotides in DNA determines
order of amino acids in a protein
• One gene codes for one protein
TRANSLATION
• To translate something means to convert it
from one language/code to another
• In translation, you take the mRNA strand
and use it to create the amino acid sequence
to make a protein.
• Translation uses:
– tRNA (transfer): carries amino acids to the
ribosome.
– rRNA (ribosomal): composes the ribosome.
TRANSLATION
GENETIC CODE
The “language” that translates the sequence of
nitrogen bases in DNA (mRNA) into the amino
acids of a protein.
• Codon = three nucleotides on mRNA
• Anticodon = three nucleotides on tRNA
• One codon specifies one amino acid
• Some codons are redundant (code for the same
amino acid)
• The genetic code is universal to all organisms
AMINO ACID
DNA:
TAC CTT
GTG CAT
GGG ATC
mRNA AUG GAA
CAC GUA
CCC UAG
A.A
MET G.A
HIS VAL
PRO STOP
AMINO ACIDS
IMPORTANT CODONS
• AUG = start translation
(Met)
• UAA, UAG, UGA= stop
translation
TRANSLATION
1. mRNA binds to the ribosome.
2. Ribosome searches for start codon (AUG)
3. tRNA brings correct amino acid
(methionine) to the ribosome.
– Each tRNA carries one type of amino acid.
– The anticodon (three nitrogen bases on tRNA)
must complement codon for amino acid to be
added to protein chain
TRANSLATION
4. ribosome reads next codon
5. tRNA’s continue lining up amino acids
according to codons
6. peptide bonds link amino acids together
7. ribosome reaches STOP codon
• Amino acid chain is released
PROTEIN SYNTHESIS
• This is a molecule of messenger RNA.
• It was made in the nucleus by transcription from a
DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
PROTEIN SYNTHESIS
• A ribosome on the rough endoplasmic
reticulum attaches to the mRNA molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first
three bases (codon) on the mRNA.
anticodo
n
The three unpaired bases
(anticodon) on the tRNA link up
with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into
place, bringing a second amino acid.
Its anticodon links up with the second
codon on the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Peptide bond
A peptide bond forms between the
two amino acids.
A U G G G C U U AAA G C A G U G C A C G U U
The first tRNA molecule releases its
amino acid and moves off into the
cytoplasm.
A U G G G C U U AAA G C A G U G C A C G U U
The ribosome moves along the mRNA
to the next codon.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule
brings the next amino acid
into place.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule
brings the next amino acid
into place.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination
(stop) codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U