When DNA Changes – Chap. 17 Left: A normal fruit fly (drosophila). Middle: A fruit fly with its legs jutting from its head; a mutation induced by radiation. Right: A disastrous effect of mutations on the human body. The boy is a Chernobyl nuclear plant accident victim.

DNA is stable bases (A,T,C,G) are protected inside of sugar- phosphate backbone tight wrapping around proteins called histones (eukaryotes) and coiling do not allow for direct access to DNA complimentary base pairing to fix any accidental “changes”

mutations can be beneficial, neutral, or harmful for the organism mutations are random there are lots of places for mutations to occur; however, not all mutations matter for evolution –somatic mutations occur in non-reproductive cells and won’t be passed onto offspring DNA is not stable

only mutations that matter to large-scale evolution are those that can be passed on to offspring –these occur in reproductive cells like eggs and sperm and are called germline mutations

chromosomal defects are surprisingly frequent events –about 20% of all human conceptions have them –many, if not most, irregular chromosomes are the result of a failure of meiosis in the production of sperm and ova (egg) most are so severe as to cause spontaneous abortions early in pregnancies mutations in the DNA molecules that make up chromosomes are not usually detected

Frequency of Mutations in humans and other mammals, mutations occur at the rate of about 1/50 million nucleotides added to the chain –with 3 billion base pairs in a human cell, that means that each new cell contains about 120 new mutations. however, not that big of a deal because of synonomous codons and “junk DNA” for eukaryotes this often happens during interphase where all DNA is replicated into two copies before mitosis or meiosis

Damaging DNA mutagens are agents that cause mutations – a failure to repair DNA radiation—forms of energy (gamma rays, x- rays, UV) that penetrate the cells and damage DNA chemical—damage DNA through chemical reactions

Three levels of mutational change Level 1. Point or Single-Base Mutations Involves changes in one or only a few nucleotides in DNA. 3 Types: I. (Base) Substitutions (Fig in text) – substitutions of a single base pair. Three outcomes are... Silent mutations – has no effect on the structure of the resulting protein since it still codes for the same a.a. because of synonymous codons

Neutral mutations – changes an a.a. in the protein but has no effect its performance “Drastic” mutations -- has serious effects on future structure and performance of proteins. Ex) Sickle cell – a codon for an amino acid in hemoglobin mutates from GAA (glutamic acid) to GUA (valine) Ex) Tay Sacs disease NOVA. One Wrong Letter

II. Chain-Termination Mutations – a new stop codon is produced or a stop codon is mutated into a codon other than a stop III. Frameshift Mutations (Fig. 17.6) result from the deletions and additions of bases into a nucleotide sequence. These cause shifts in the reading of codons during transcription and result in an abnormal protein

Level 2. Chromosomal Mutations major changes in chromosomal structure rather than simple DNA sequence changes a complete breaking, misalignment, and rejoining of the double helix in one or more chromosomes

3 Types: Deletions – entire sections of the chromosome are left out Inversions – a middle fragment may flip over and then rejoin

Translocations – two nonhomologous chromosomes break, swap segments, and fuse together (ex. human #9 and #22 resulting in a type of lukemia)

Level 3. Transpositions certain segments of DNA frequently “jump” (sometimes called "jumping genes“) to new locations in the same DNA molecule or a different one not caused by mutagens but are considered mutagens themselves because they cause the mutation could an organism “purposely” evolve? discovered by Barbara Clintock (nobel prize 1983)

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