DNA mutation and repair II

Depurination

• Sugar-base bond is spontaneously broken

• Base is lost, usually purines, and nucleotide is left empty (called apurinic site)

What would happen to apurinic site during

DNA replication?

A random base (usually adenine) is inserted, causing a mutation.

Deamination

An amino group of C or A is spontaneously removed (converted to another base)

Causes of spontaneous damage include

Depurination, deamination, oxidative damage, and transposons (aka jumping genes)

Oxidative damage

Normal process of aerobic cellular respiration creates extremely reactive atoms called free radicals

Free radicals

An atom or group of atoms that has/have an unpaired electron

Cancer and aging

Thought to be major mutagen in our cells

Transposons (aka jumping genes)

Mobile pieces of DNA abundantly found in all living things (nearly 45% of human genome)

 Transposons (aka jumping genes) (2)

Cut or copy themselves and then insert randomly in the host genome

Control transposase = control movemen⃗

Methylation and mRNA destruction

Some external agents (chemical and physical) can induce DNA damage:

Base analogs, alkylating agents, intercalating agents, UV light and low energy radiation, high-energy radiation (ionizing radiation), and viruses

Base analogs

Chemicals that resemble normal nucleotides and can substitute for them during DNA replication. However, they exhibit abnormal base-pairing properties

Example of a base analog

5-bromouracil resembles thymine

Alkylating agents

These chemicals add an alkyl group (CH₃ or CH₃CH₂)  to amino or ketone groups in nucleotides

Alkylated nucleotides

exhibit abnormal base pairing 

Example of alkylated nucleotides

Ethyl guanine pairs with T

Intercalating agents

 Flat, multiple-ringed molecules that tightly wedge themselves between the bases of FNA - distorts its 3-D structure

Intercalating agents (2)

Causes insertions or deletions in the DNA (unlike all others discussed)

UV light and low energy radiation

All electromagnetic radiation have wavelengths shorter than visible light (-380 nm) are very energetic

Pyrimidine dimers (usually two thymines)

UV light causes adjacent pyrimidine bases to fuse with one another

Pyrimidine dimers (2)

Prevents DNA pol from replicating normally

High-energy radiation (ionizing radiation) Mutates DNA in different ways:

It causes electrons to be released from various molecules in the cell producing free radicals, it directly breaks phosphodiester bonds in the DNA strands (causes double - stranded breaks), and it creates thymine dimers

Viruses

Retroviruses have the ability to randomly insert themselves into our genome 

Example from Viruses

Retroviral gene therapy and leukemia

Other viruses produce proteins that directly inhibit

DNA replication, monitoring, or repair mechanisms

Ames test

Used to test if a new chemical has ability to mutate DNA (cause cancer)

Set-up

 Uses bacterial strain that can’t make its own histidine (won’t grow without it)

Results from Ames Test

Results H₂O control = very few colonies (spontan)

Most types of DNA damage can be fixed by the cell

Must be fixed prior to DNA replication

Exceptions that cannot be fixed by the cell

Transposons and retrovirus (can’t be removed)