Disorders of Replication

Nondisjunction
Where chromosomes do not separate properly during meiosis; you’ll end up with a wrong number of chromosomes in a particular gamete
Can occur during Meiosis I (homologous chromosomes do not separate, end up with too few or too many chromosomes)
Can occur during Meiosis II (sister chromatids do not separate)
Aneuploidy: abnormal number of chromosomes is found in a cell
Trisomy: 3 copies of a chromosome instead of 2 in a cell
Monosomic: 1 copy of chromosome instead of 2 in a cell
Polyploidy:
More than 2 complete sets of chromosomes
Triploidy: 3 sets of chromosomes
Tetraploidy: 4 sets of chromosomes (particularly lilies)
Polyploidy occurs in some plants
Alterations of Chromosome Structure (during meiosis)
Chromosomes can break and reform
Changes in the chromosome structure
Changes can be:
Lethal: which means that the embryo is not going to be able to develop normally and will die
Sub-lethal, cause serious problems to the organism but it doesn’t actually kill the developing embryo
Not serious, but phenotype of cell changes
Mutation by Base Substitution (animation)
A mutation occurs by base substitution when an incorrect base is incorporated into DNA. Some base substitutions occur because purines and pyrimidines exist in 2 structural forms
The most common form results in base-pairing between adenine and thymine and between guanine and cytosine
However, the hydrogen atoms can move to form a base with altered hydrogen bonding properties, creating a tautomeric shift. When a tautomeric shift occurs in adenine, the adenine can bond to cytosine
A tautomeric shift in thyme allows it to bond to guanine
The error in DNA replication is passed on to the cell’s progeny. The change in a single nucleotide in the DNA results in a change in the corresponding nucleotide in messenger RNA
The change in the codon can result in a different amino acid being incorporated into the protein
Figure 15.14 Alteration of chromosome structure
A deletion removes a chromosomal segment
A duplication repeats a segment
An inversion reverses a segment within a chromosome
A translocation moves a segment from one chromosome to another, nonhomologous one. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return
Extranuclear Inheritance
Not all of the cellular genetic material is found in the cell nucleus
Mitochondria and some plant plastids contain DNA
In humans,