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Biology 105
Chapter 12: DNA - The Carrier of
Genetic Information
Pages 263-281
Student Outcomes
• Summarize the discovery of the DNA
structure in the 1940-50’s
• Sketch how nucleotides link together to
form a DNA strand
• Summarize the process of how DNA
replicates.
Student Outcomes
• Explain the connections between
telomerase and cell aging and cancer.
DNA as genetic material
• Transformation - process of changing the
genetic makeup of an organism by an
another organism.
• Viruses are now used to introduce
changes in DNA of the host
DNA structure
In 1953 - Watson and Crick proposed the
double helix model of DNA
DNA is made of a nucleotide:
deoxyribose - sugar
phosphate
1 of 4 nitrogenous bases
Nitrogenous bases
Two Types
Purines
Adenine (A)
Guanine (G)
Pyrimidines
Thymine (T)
Cytosine (C)
The ratio of Purines to Pyrimidines is equal
DNA Replication
• Known as semi-conservative replication.
DNA strands break at the hydrogen
bonds(unzips). Each separate strand
becomes a template for the other.
• This type of replication allows mutations
to continue once they occur.
DNA replication continued
• Each strand must untwist and unzip. Controlled
by enzymes known as DNA helicases.
• SSB’s then stabilize the DNA so that it doesn’t
recoil. (single-strand binding proteins)
• DNA polymerases catalyze the linking of the
new nucleotides.
DNA replication continued
An RNA primer is needed at the beginning
point of DNA replication. This primer is
synthesized by DNA primase
The RNA primer is later filled in with DNA
Telomeres
• End caps of chromosomes
• These are short noncoding DNA repeats
• Each time DNA replicates (a cell duplicates
•
itself) part of the telomere does not replicate
and is lost.
DNA can replicate many times. However,
eventually important and essential coding DNA
may be lost
Cell Aging
• Telomerase is a DNA replication enzyme which
can lengthen the Telomeres. Somatic cells do
not have this but germ cells do.
• Shortening of telomeres contribute to cell aging
and cell death
• Cancer cells have the DNA Telomerase which
allows many uncontrolled replications.
Chromosomal mutations
Occur in either mitosis or meiosis
4 types:
Deletion
Inversion
Translocation
Nondisjunction
Gene Mutations
• Detectible and heritable change in the
genetic material - not caused by genetic
recombination!
• Occurs at the gene level.
• Caused by Mutagens - physical or
chemical agent
Gene Mutations
• Base-Pair Substitution Mutation
AT is replaced with a GC
Three types:
Missense-the change results in a different codon
Nonsense-the change results in an ending codon
UAC, UAA, UGA
Neutral - change does not result in a different
amino acid
Gene Mutations
• Frameshift mutation
– The addition or deletion of a base pair of a
gene