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Modern Genetics
© Lisa Michalek
Chemical Basis For Genetics
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In the 1940’s and 1950’s experiments showed
that genes are made up of the chemical
compound DNA, or deoxyribonucleic acid.
DNA is a large complex molecule found in the
nucleus of the cell.
DNA is responsible for passing genetic
information from generation to generation.
DNA also controls the manufacture of enzymes.
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Enzymes control cellular activity.
Nucleotides
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The DNA molecule is made up of a long chain of
repeating nucleotide units.
Nucleotides contain nitrogenous bases, a sugar,
and a phosphate group.
The four different bases in DNA nucleotides are
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Adenine
Guanine
Cytosine
Thymine
Nitrogenous Bases of DNA
DNA Structure
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The structure of DNA was
discovered in the 1950’s by
James Watson and
Francis Crick.
According to Watson and Crick,
DNA molecules are shaped like
a twisted ladder.
The twisted ladder structure is
called a double helix.
DNA Structure
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The DNA double helix
has two strands or sides.
The strands are
connected at the rungs.
The sides of the ladder
consist of alternating
sugar and phosphate
molecules.
The rungs are pairs of
nitrogenous bases.
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The nitrogen bases are
attached to each other by
weak hydrogen bonds.
DNA Structure
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The four nitrogenous bases in DNA
bond (join) together in a certain
way known as base pairing.
Adenine (A) and Thymine (T)
bond together.
Guanine (G) and Cytosine (C)
bond together.
No other combinations are
possible.
DNA Replication
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During reproduction, DNA
makes exact copies of itself
(replicates).
The process begins with an
untwisting of the DNA helix.
The two strands “unzip”
leaving two strands of
nucleotides.
DNA Replication
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Each strand is a pattern
for the new nucleotide
strand.
Free nucleotides present
in the cytoplasm pair
with the free nitrogenous
bases of both strands.
Two identical DNA
molecules that are
identical to the original
are produced.
RNA - Ribonucleic Acid
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RNA also controls cellular activities.
RNA contains the sugar “ribose.”
RNA is single stranded.
RNA also has 4 nitrogen bases:
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Adenine
Guanine
Cytosine
Uracil instead of Thymine.
RNA - Ribonucleic Acid
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RNA has different functions than DNA:
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Messenger RNA - mRNA – is a single, uncoiled strand
that carries the genetic code information of DNA from
the nucleus to the ribosomes in the cytoplasm.
Transfer RNA - tRNA – is a single folded strand that
reads the message for protein formation carried by
mRNA. tRNA then transfers amino acids to form amino
acids.
Ribosomal RNA - rRNA – is a form of RNA that makes
up most of the ribosomes in a cell.
Mutations
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A mutation is any change or mistake in the genes or
chromosomes of an organism that can be inherited.
These changes usually produce new characteristics.
To be inherited, these changes must occur in the
gamete or sex cell.
As a result of fertilization, the changed gene or
chromosome in the sex cell is passed on to the new
organism.
If they occur in other cells, they can be passed on
only to other body cells.
Mutations
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Mutations occur as random chance events and may
occur naturally within the cell or be caused by forces
outside the cell called mutagenic agents.
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X rays, ultraviolet light, radioactive substances, cosmic
rays, and chemicals such as formaldehyde and benzene,
are mutagenic agents.
Asbestos fibers and drugs, such as LSD, marijuana,
cigarettes and alcohol, are also known to cause changes
in genes and chromosomes.
Unborn children can be injured when their pregnant
mothers are exposed to mutagenic agents.
Human Genetic Diseases
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Most genetic diseases in humans are inherited
as recessive genes and involve mistakes in
enzyme formation.
When there is a mistake in enzyme formation,
the person’s metabolism is incorrect.
The presence of many genetic disorders can be
detected either before or after birth.
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Screening (chemical analysis of body fluids)
karyotyping (detecting chromosome abnormalities by
matching homologous chromosomes)
amniocentesis (removing a small amount of amniotic
fluid from around the fetus) are tests used to detect
(identify) genetic diseases
Genetic Engineering
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Today, scientist can deliberately
remove genes from one organism
and add them to the genetic material
of another organism.
Genetic Engineering will alter (change) the DNA
of a cell.
The changed DNA is called recombinant DNA.
The cell that received the recombinant DNA
receives new traits, such as, ability to prevent a
certain disease.
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