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Section 12-3
Chapter 14
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Section 12-3
Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
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Translation
Section 12-3
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
The mRNA then enters the cytoplasm and
attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has
an anticodon whose bases are complementary
to a codon on the mRNA strand. The ribosome
positions the start codon to attract its
anticodon, which is part of the tRNA that binds
methionine. The ribosome also binds the next
codon and its anticodon.
Ribosome
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mRNA
Transfer RNA
Methionine
mRNA
Lysine
Start codon
Mutation - a change in the
structure or amount of the
genetic material of an organism
Nondisjunction - failure of
homologous chromosomes to
separate properly during meiosis.
Results in extra or missing
chromosomes
Polyploidy - an condition of
having more than two sets of
chromosomes – plants only
transcription factor - an enzyme
that is needed to begin and/or
continue genetic transcription
Intron - a nucleotide sequence that
is transcribed from DNA into mRNA
but is cut out (not translated).
Exon - nucleotide sequences that
are transcribed, joined together,
and then translated
Domain - in proteins, a functional
unit that has a distinctive pattern of
structural folding
1. What is the origin of genetic
differences among organisms?
• For the most part, genetic
differences among organisms
originate as some kind of genetic
mutation.
2.
What are the causes of mutation?
• Mutations occur naturally as accidental
changes to DNA or to chromosomes during the
cell cycle.
• Enzymes repair most DNA that is mismatched
during replication, but rarely, some DNA is not
repaired.
• The rate of mutation can be increased by some
environmental factors. Such factors, called
mutagens, include many forms of radiation and
some kinds of chemicals.
3.
What are the effects of mutation?
• A small change in DNA may affect one or
many amino acid(s) in the protein that
results from a gene.
• A mutation may have no effect, or may
harm or help in some way.
• The effect depends on where and when
the mutation occurs.
4.
What kinds of mutations are possible?
• Two main types: gene
mutations and chromosomal
mutations
What kinds of mutations are possible?
1. Gene mutations – (replication errors) a change in the sequence of
nucleotides in DNA
A point mutation is a change of a single nucleotide in a
sequence from one kind of base to another. (substitution)
A mutation is silent when it has no effect on a gene’s
function.
Point mutations are often silent because the genetic code
is redundant (each amino acid has multiple codons).
*A missense or replacement mutation results when a
codon is changed such that the new codon codes for a
different amino acid.
*A nonsense mutation results when a codon is changed to
a “stop” signal. In this case, the resulting string of amino
acids may be cut short, and the protein may fail to
function.
What kinds of mutations are possible?
Rarely, errors in replication can cause the insertion or
deletion of one or more nucleotides in a sequence.
An insertion or deletion can shift the reading frame, or
cause a frameshift mutation.
In frameshift mutations, the remaining sequence is incorrect.
Chromosomal Mutations (meiosis errors) – a change
chromosome structure or number
A. Errors in the exchange during crossing over in meiosis can cause
chromosomal mutations.
A deletion occurs when a piece of a chromosome is lost. At
the end of meiosis, one of the cells will lack the genes from
that missing piece. Such deletions are usually harmful.
A duplication occurs when a piece remains attached to its
homologous chromosome after meiosis. One chromosome
will then carry both alleles for each of the genes in that piece.
An inversion occurs when a piece reattaches to its original
chromosome, but in a reverse direction.
A translocation occurs when a chromosome piece ends up
in a completely different, nonhomologous chromosome.
Chromosomal Mutations
Section 12-4
Deletion
Duplication
Inversion
Translocation
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4.
What kinds of mutations are possible?
B. When homologous chromosomes fail to separate properly during
meiosis (nondisjunction), gametes have extra or missing
chromosomes.
Examples:
Autosomal - Down Syndrome - trisomy 21
•Mild to severe mental retardation
•Increased frequency of birth defects
•Increased susceptibility to disease
Sex Chromosome Disorders
•Turner’s syndrome (XO): sterile females, sex organs
fail to develop normally
•Klinefelter’s syndrome (XXY): sterile males, tall stature
The Y chromosome has genes necessary for the development
of male characteristics.
5.
What are the possible effects of mutation in
gametes and somatic cells?
• Only mutations that occur in gametes can be passed on
to offspring, but mutations in body cells affect only the
individual in which they occur.
• Certain genes control the normal growth, division, and
specialization of cells in bodies.
– Mutations in these genes can cause a normal somatic cell to
“lose control” and begin growing and dividing abnormally. The
group of cells that grows will become a tumor.
– If the tumor cells begin to invade other parts of the body, they
become a form of cancer.
• Note that although cancers result from somatic cell mutations,
not all somatic cell mutations cause cancer.