• 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

• The normal DNA sequence produces a mRNA that provides instructions for the correct series of amino acids in a protein
Correct order

• 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

• 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

• 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

• 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

• 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

• 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

• 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 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

• 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

• 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

• 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)
HO CH
2
H
3
C
O
H
H
O
N
N
H
O
N
HO CH
2
H
O
H
N
O
N
N
H
N
3
H H H

• 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
reverse transcriptase protease
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