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Molecular Genetics
DNA, or deoxyribonucleic acid, is the
hereditary material in humans and
almost all other organisms. Nearly
every cell in a person’s body has the
same DNA. Most DNA is located in
the cell nucleus, but a small amount
of DNA can also be found in the
mitochondria.
Genes are sections of DNA molecules. DNA controls
the cellular activities by controlling the production of
enzymes.
Mrs. Degl
Nucleotides
DNA molecules are
very large and each
is made up of
thousands of
repeating units called
nucleotides.
A DNA nucleotide is composed of three parts:
1. A Phosphate Group
2. One 5 carbon sugar molecule, called Deoxyribose
3. One Nitrogenous Base
Mrs. Degl
Nitrogenous
There are 4 different nitrogenous
Bases
bases:
1. Adenine
2. Cytosine
3. Guanine
4. Thymine
Therefore, there are 4 different types of
nucleotides, depending on which nitrogenous
base is present.
Mrs. Degl
Watson & Crick Model of DNA
In 1953, James Watson and Francis Crick developed a
model of the DNA Molecule. They based their model on their
own knowledge and the knowledge of other scientists who
were looking into “The Secret of Life”.
•The DNA molecule consists of two
long chains of nucleotides attached
in a ladder like fashion.
• The sides of the ladder are the
alternating phosphate and
deoxyribose molecules.
•The inside of the ladder consists of
pairs of nitrogenous bases bonded
together by hydrogen bonds.
Mrs. Degl
The two chains are twisted together into a spiral, called a
Double Helix.
Mrs. Degl
The four bases bond together in only one way:
Adenine pairs with Thymine
Guanine pairs with Cytosine
Because the two strands
bond together in only one
way, the two strands are
always complementary. If
you know the order of the
bases on one strand, then
you know the order on the
other strand.
Mrs. Degl
DNA Replication
DNA can make exact copies of itself. This is called
replication. This happens in Mitosis and Meiosis.
•In replication, the double stranded helix unwinds, which
is called unzipping. For this to happen, the bonds
holding the nitrogen bases together have to break.
•Free nucleotides from the cytoplasm now enter the
nucleus and bond to their complementary base on the
DNA strand.
•Replication produces 2 copies that are identical to the
original DNA molecule.
•Special enzymes carry out this procedure.
Mrs. Degl
RNA
• RNA is ribonucleic acid
• The 5 carbon sugar is ribose,
instead of deoxyribose
• The nitrogenous base is
Uracil, instead of Thymine
• RNA is only one strand, not
a double helix.
There are 3 types of RNA:
1. mRNA (messenger)
2. tRNA (transfer)
3. rRNA (ribosomal)
Mrs. Degl
Mrs. Degl
mRNA
•mRNA is synthesized
in the nucleus.
•A genetic code
contains the
information for the
sequence of amino
acids in a particular
protein.
•This code is present
in mRNA molecules
and is three bases
long, and is specific.
This sequence is
known as a codon.
Mrs. Degl
tRNA
•tRNA are found the cytoplasm.
•They carry amino acid molecules to the
ribosomes, which are the sited for protein
synthesis.
•Ribosomes are made of rRNA and proteins.
•There are 20 different kids of amino acids in
cells, and there is a different form of tRNA for
each amino acid.
•Each kind of tRNA has a 3 nucleotide sequence,
called an anticodon, which is complementary to a
codon on the mRNA.
Mrs. Degl
Protein Synthesis
The synthesis of proteins takes two steps: transcription and
translation. Transcription takes the information encoded in
DNA and encodes it into mRNA, which heads out of the cell’s
nucleus and into the cytoplasm, attached to tRNA. During
translation, the mRNA works with a ribosome and tRNA to
synthesize proteins. The resulting chain of amino acids
is a polypeptide. The polypeptide can either be one
chain or many chains.
Mrs. Degl