Genetic Mutations A mutation alters the nucleotide sequence in DNA, which can cause a change in the...

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)

N

N

N

N

O

H

O

H H

H H

CH2HO

OCH2HO

H H

HN3

N

N

O

O

HH3C

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 Inhibited by AZT, ddI protease inhibitors

reverse

transcriptase protease Viral RNA Viral DNA Viral proteins