Including Mutations & Non-disjunction Mistakes in Meiosis Including Mutations & Non-disjunction

What is a Gene? A gene is the basic unit of heredity in a living organism Genes hold the information to build and maintain cells and pass genetic traits to offspring.

What is a Gene? A gene is holds the information for the production of a particular protein. A gene is the basic instruction: a sequence DNA

What is a Gene? Remember that DNA is made up of four nucleotide bases: A, T, G, C Each gene is a string of hundreds of base pairs in a particular sequence. An allele is one variant of that instruction.

Mutations Mutations are changes in the genetic makeup of a cell They are changes in the sequence of base pairs Mutations can change the protein that the gene codes for.

Mutations Can involve large regions of a chromosome or just a single nucleotide pair

Causes of Gene Mutations Mutations can occur spontaneously during DNA replication Many mutations occur as a result of exposure to mutagens, or mutation causing agents in the environment

Mutagens X-rays UV light radioactivity chemicals cigarette smoke

Mutations Results of a mutation on the organisms may be: Deleterious Positive no effect

Mutations Species have evolved as a result of mutations that have given an organism a positive benefit that has been naturally selected Effects of a mutation may not surface immediately

Alterations of Chromosome Structure There are several different mutations that alter the chromosome structure: Deletions Duplication Substitutions Inversions Translocations Point mutations (insertion, deletions, substitutions of a single base pair)

Alteration of Chromosome Number Aneuploidy – incorrect number of chromosomes Polyploidy – more than 2 pairs of each chromosome

Deletions With a deletion, a segment of a chromosome is lost Or a single base pair is lost

Duplications With a duplication, the same linear stretch of DNA within a chromosome is repeated, often several to many times in the same chromosome or in a different one

Inversions With an inversion, a segment of DNA within a chromosome may flip upside down but remain in place

Translocation With a translocation, a stretch of one chromosome’s DNA moves to another location in the same chromosome or a different one

Base-pair substitutions The replacement of one nucleotide and its partner with another pair of nucleotides

Insertions and Deletions Additions or losses of one or more nucleotide pairs in a gene Usually have a more deleterious effect than substitutions http://www.larasig.com/node/1806

Insertions and Deletions Because mRNA is read as a series of nucleotide triplets during translation, the insertion or deletion may alter the reading frame (triplet grouping) of the gene All the nucleotides that are downstream of the deletion or insertion will be improperly grouped into codons

Alteration of Chromosome Number Sometimes the movement of chromosomes during meiosis goes wrong When this happens the gamete may end up with an unusual number of chromosomes This is called nondisjunction

Nondisjunction The members of a pair of homologous chromosomes do not move apart properly during meiosis I Or The sister chromatids fail to separate during meiosis II In these cases, one gamete receives two of a chromosome and another gamete receives no copy

Nondisjunction If either of the aberrant gametes unites with a normal one, the offspring will have an abnormal chromosome number, known as aneuploidy If the chromosome is present in triplicate in the zygote, (the cell has a total of 2n + 1 chromosomes) then the cell is trisomic for that chromosome trisomy 21 = Down syndrome

Nondisjunction If a chromosome is missing (cell has 2n-1 chromosomes) the cell is monosomic for that chromosome Some organisms have more than two complete sets of chromosome sets and are termed polyploidy 3n = triploidy 4n = tetraploidy

Syndromes Associated with Nondisjunction XXY Klinefelter syndrome have low levels of testosterone causing small testicles and the inability to make sperm. They also have learning disabilities and behavior problems such as shyness and immaturity XXX Triple X syndrome Individuals show no significant clinical manifestations, although there is usually some degree of mental retardation. X Turner syndrome is characterized by retarded growth that leads to a small stature and frequent infertility. Trisomy 21 Down syndrome Mental retardation and characteristic physical features

Animation of nondisjunction in meiosis I Animation of nondisjunction in meiosis II

That’s all Folks