Transcript 103 Lecture Ch22b

Genetic Mutations
• A mutation alters the nucleotide sequence in DNA, which can
cause a change in the amino acid structure of the corresponding
protein, possibly destroying its function
• Mutations have a variety of causes, such as UV rays, X rays,
chemicals (mutagens), viruses and mistakes during replication
• A mutation in DNA produces one or more incorrect codons in
the corresponding mRNA
• This leads to a protein that incorporates one or more incorrect
amino acids
• Defective proteins, such as enzymes, can lead to cancer or
genetic diseases
Normal DNA Sequence
• The normal DNA sequence produces a mRNA that provides
instructions for the correct series of amino acids in a protein
Correct order
Substitution Mutation
• The substitution of a base in DNA changes a codon in the mRNA
• A different codon can lead to the placement of an incorrect amino
acid in the polypeptide
• An incorrect amino acid may alter or destroy protein function
Incorrect order
Wrong amino acid
Frameshift Mutation
• In a frameshift mutation, an extra base is added to or
deleted from the normal DNA sequence.
• All the codons in mRNA, and the amino acid sequence, are
incorrect from the point of the base change on
• This almost always leads to destruction of protein function
Incorrect amino acids
Genetic Diseases and Cancer
• Mutations in reproductive cells can cause genetic diseases
• Some genetic diseases are dominant, requiring mutation in
only one copy of the gene
• Most genetic diseases are recessive, requiring mutation in both
copies of the gene
• Mutations in somatic (non-reproductive) cells can lead to
uncontrolled growth, or cancer
• However, the cell has mechanisms to protect against mutation
- during replication, the new DNA is proofread, and most
mistakes are corrected
- mutations that remain after proofreading may be corrected by
other DNA repair mechanisms
- mutated DNA that can not be repaired is usually recognized,
and cell death is triggered
Some Genetic Diseases
Recombinant DNA
• Recombinant DNA combines a DNA fragment from one
organism with the DNA in another organism
• Prokaryots have small circular pieces of DNA called plasmids
in addition to the genomic DNA
- plasmids contain genes for various proteins and can replicate
- plasmids can be shared between bacteria
• Restriction enzymes are used to cleave a gene from a foreign
DNA and open DNA plasmids in bacteria, such as E. coli
- restriction enzymes are used by bacteria as defensive weapons
- the cleaved DNA has sticky ends that match each other
• The DNA fragments are mixed with the E. coli plasmids, the
ends are joined by a ligase, and the recombinant plasmids are
absorbed by new E. coli
• The new gene in the altered DNA produces the desired protein
Preparation of Recombinant DNA
Products of Recombinant DNA
• Recombinant DNA is used to produce many therapeutic proteins
• One that is very useful is insulin, which previously had to be
obtained from cadavers, and is now readily available
DNA Fingerprinting
• In DNA fingerprinting (Southern transfer) restriction
enzymes cut a DNA sample into smaller fragments (RFLPs)
• The fragments are sorted by size using gel electrophoresis
• A radioactive isotope in the gel that adheres to certain base
sequences in the fragments produces a pattern on x-ray film,
which is the “fingerprint”
• The “fingerprint” is unique to each individual DNA
• DNA fingerprinting is used in forensics and genetic screening
and also in mapping genomes
Polymerase Chain Reaction (PCR)
• A polymerase chain reaction
(PCR) produces multiple
copies of a DNA in a short
time
• Sample DNA strands are
separated by heating
• Separated strands are mixed
with enzymes and nucleotides
to form complementary strands
• The cycle is repeated many
times to produce a large
sample of the DNA
Viruses
• Viruses are small particles of DNA or RNA, usually with a
protein coat, that require a host cell to replicate
• When the DNA or RNA enters a host cell a viral infection occurs
• Viruses hijack cellular materials and enzymes for replication
Viral Diseases
Reverse Transcription
• In reverse transcription a retrovirus, which contains viral
RNA, but no viral DNA, enters a cell
• The viral RNA uses the enzyme reverse transcriptase to
produce a viral DNA strand
• The viral DNA strand forms a complementary DNA strand
using the nucleotides and enzymes in the host cell
• The new viral DNA (a provirus) is incorporated into the host
DNA, which is used to synthesize the proteins and viral RNA
needed to make new virus particles
• Once all the parts are assembled, the new virus particles are
formed as they emerge from the cell, using a part of the host
cell membrane to close themselves off
Diagram of Reverse Transcription
HIV Virus and AIDS
• AIDS (acquired immune deficiency syndrome) is a devastating
disease that does not yet have either a cure or a vaccine
• AIDS is caused by the HIV-1 (human immunodeficiency virus)
• The HIV-1 virus is a retrovirus that infects T4 lymphocyte cells
• As the T4 level decreases, the immune system fails to destroy
harmful organisms
• AIDS is associated with a variety of opportunistic infections,
such as pneumonia and Kaposi’s sarcoma, a type of skin cancer
AIDS Treatment (Nucleoside Analogs)
• One type of AIDS treatment prevents reverse transcription of
the viral DNA
• When altered nucleosides such as AZT and ddI are
incorporated into viral DNA, the virus is unable to replicate
Azidothymine (AZT)
Dideoxyinosine (ddI)
O
H
H3C
HO CH2
O
N
N
O
H
N
O
HO
CH2
N
O
H
H
H
H
N3
H
H
H
N
N
AIDS Treatment (Protease Inhibitors)
• Another type of AIDS treatment involves protease inhibitors such
as saquinavir, indinavir, and ritonavir
• Protease inhibitors modify the active site of the protease enzyme,
which prevents the synthesis of viral proteins
Inhibited by
AZT, ddI
reverse
transcriptase
Inhibited by
protease inhibitors
protease
Viral RNA  Viral DNA  Viral proteins