Genetics
DNA structure, replication,
transcription and translation

Review
• Amino acids and proteins:
• The bond between two amino acids is called a
peptide bond.
• Amino acids join together through condensation
reactions (water molecule is removed) and
separate via hydrolysis (water molecule is
added).
• Polypeptides, or chains of amino acids, are
known as proteins.

Proteins
• Primary structure: The sequence of amino acids
in a polypeptide.
• Secondary structure: The formation of patterns
within the polypeptide due to the folding caused
by hydrogen bonds. Examples: α-helix, β-
pleated sheet.
• Tertiary structure: the overall three-dimensional
shape of the protein.
• Quaternary structure: the way polypeptides fit
together when there is more than one chain.

DNA, the origin
• In 1928, Frederick Griffith showed that although a
deadly strain of bacteria could be made harmless by
heating it, some factor in that strain is still able to
change other harmless bacteria into deadly ones. He
called this the "transforming factor."

• In 1944, American biologist Oswald Avery and
his colleagues took Griffith's experiments one
step further: they treated the mixture with DNA-
destroying enzymes. This time the colonies
failed to transform. Avery concluded that DNA is
the genetic material of the cell.

4.1 DNA structure
• Nucleotides and nucleic acids:
• Nucleic acid is a polymer of nucleotides, that are
molecules that consist of 3 parts: a sugar, a
phosphate group and a nitrogen-containing ring
structure.
• RNA: ribonucleic acid, the sugar contained is
ribose
• DNA: deoxyribonucleic acid, the sugar within is
deoxyribose

DNA structure

DNA structure
• Nucleotides are joined to one another by
covalent bonds that connect the sugar of one
nucleotide to the phosphate group of the next.
• Nitrogenous bases: The bases that make up DNA
are: adenine (A), cytosine (C), quanine (G), and
thymine (T).
• In RNA uracil (U) is present instead of thymine.
• RNA is single stranded and DNA is double
stranded

DNA structure

• Scientists James Watson and Francis Crick
modeled DNA's structure with tin and wire.
Their early models failed to explain DNA's
chemical properties. Using X-ray
crystallography photos of DNA, Watson and
Crick created a new model in which two strands
of nucleotides wound about each other. This
formed a twisting shape called a double helix.

• The bases pair up between the
two intertwined sugar-
phosphate backbones, forming
the double helix discovered by
Watson and Crick. A pairs with
T, and G pairs with C.

DNA structure
• Summary:
• The nucleotides are held together by covalent bonds
• Sugar and phosphate molecules form a backbone
• The strands are held together by hydrogen bonds
between the nitrogenous bases.
• The strands are arranged in an anti-parallel fashion.
• 4 nitrogenous bases: adenine, thymine, guanine,
cytosine
• A is always paired with T, G is always paired with C
in a complimentary way.
• The sequence on one strand determines the
sequence on the other strand (whatever the
sequence of bases along one strand, the sequence of
bases on the other stand must be complementary to
it)

4.2 Transcription and translation
• Reproduction is one of the basic properties of
life.
• It involves the transmission of information from
parents to offspring = heredity.
• A gene is a unit of heredity that consists of a
sequence of DNA bases.
• Prokaryotic cells lack nuclei and many of the
organelles found in eukaryotes = DNA is found
in the cytoplasm.

• Eukaryotic cells have their DNA in the nucleus
in the form of a number of chromosomes.
• In order to pass heredity information to
daughter cells, a process of replication needs to
take place.

DNA replication
• When a cell divides, forming new cells, a complete
set of genetic instructions is generated for each new
cell
• Long before DNA was identified as the genetic
material, some people proposed that gene-copying
must be based on a template mechanism
• Watson and Crick's hypothesis was based on the
specific pairing rules of complementary bases. If you
know the sequence of bases on one strand of DNA,
you can determine the sequence on the other.

• Watson and Crick's
hypothesis was confirmed by
experiments performed in the
1950s.
• During DNA copying, the two
strands of the double helix
separate.
• Each single strand acts as a
"negative" for producing a
new, complementary strand

Steps of DNA replication
• 1. The original double helix
molecule.
• 2. Helicase enzyme breaks
the hydrogen bonds
between complementary
base pairs. This unzips the
double helix at a position
called the replication fork.
• 3. There is an abundant
supply of nucleotides in the
nucleus for the formation of
the new polynucleotides.

• 4. Nucleotides base pair to
the bases in the original
strands.
• 5. DNA polymerase joins
together the nucleotides
together with strong covalent
phosphodiester bonds To
form a new complementary
polynucleotide strand.
• 6. The double strand reforms
a double helix under the
influence of an enzyme.
• 7 Two copies of the DNA
molecule form behind the
replication fork. These are
the new daughter
chromosomes.

• DNA replication begins
at specific sites called
origins of replication.
The copying proceeds
outward in both
directions, creating
replication "bubbles”.
• This way of copying
DNA has important
implications

• A DNA molecule is copied precisely from one cell
generation to the next.
• In a unicellular organism this means that the total
genome is successfully copied into each new generation.
• In the multi-cellular organism all cells contain an exact
copy of the total genome (even though not fully
expressed).
• Genes (base sequences) are faithfully passed from one
generation to the next.
• With minor and rare modification the base sequences
copied by DNA replication and successfully passed on
through sexual reproduction. Your base sequences have
been copied for thousands of years.

DNA replication is semi-conservative
• Each of the original strands serves as a guide or
template for the creation of a new strand.
• The result is 2 DNA molecules composed of an
original and a copy strand.