MEIOSIS & Abnormal Meiosis Topics: 3.3, 10.1 MEIOSIS & Abnormal Meiosis

Meiosis Animations McGraw Hill http://highered.mheducation.com/sites/0072437316/student_view0/chapter12/animations.html# http://www.johnkyrk.com/meiosis.html

Karyogram A picture of the chromosomes of an individual arranged by size, and type. Karyograms can be used to determine the sex of an unborn baby and to determine if there are any chromosomal abnormalities.

Normal Male Karyogram

Normal Female Karyogram

What’s wrong here? This individual has an extra 18th chromosome, which results in a condition called Edward’s Syndrome.

Prenatal Testing A pregnant woman can have a prenatal testing performed in order to create a karyotype for her baby. In an amniocentesis, some amniotic fluid is extracted. The fluid contains cells from the baby, from which a karyotype can be made. In chorionic villus sampling, tissue samples are taken from the chorionic villi.

Prenatal Testing Amniocentesis Chorionic Villus Sampling

How does an individual get an extra chromosome? Abnormal meiosis In (normal) meiosis, special diploid cells undergo 2 divisions to produce 4 haploid gametes (sperm or egg cells).

Normal Meiosis Diploid Cell Meiosis I Meiosis II 46 23 Diploid Cell Meiosis I Meiosis II 4 gametes (sperm or egg cells)

Nondisjuction When there is an error that occurs in meiosis The chromosomes fail to separate properly. It can happen in either meiosis I or meiosis II As a result, some gametes will be missing a chromosome, and some will have an extra chromosome.

If one of these abnormal gametes participates in fertilization, a zygote with an abnormal chromosome number is produced. Since the zygote is the first cell of the new organism, when this cell undergoes cell division, all the new cells will be identical to it. Therefore, all the cells in the organism will have an abnormal chromosome number.

If the error occurs in meiosis I, all 4 of the gametes will be abnormal If the error occurs in meiosis II, 2 of the gametes will be normal, and the other 2 gametes will be abnormal. In the example below, the parent diploid cell has 4 chromosomes. The 4 haploid gametes should each have 2 chromosomes.

Nondisjunction Diorders In most cases, the zygote will die because there is either too much or not enough information in each cell. But in some cases the zygote will survive and a person with a nondisjunction disorder is born.

Down Syndrome People with Down syndrome have an extra chromosome for the chromosome pair 21. People with Down syndrome may have a mental disability – although a wide range of mental abilities is possible.

Turner Syndrome Normal females have two X chromosomes: XX Females with Turner’s syndrome have only one X chromosome instead of two. These women do not mature sexually (they can’t have babies) This is the only nondisjunction disorder in which the individual is missing a chromosome and still lives!

Klinefelter’s Syndrome Normal males have one X chromosome and one Y chromosome: XY Males with Klinefelter syndrome have two X chromosomes and one Y chromosome. These males produce high levels of female sex hormones and are usually sterile (because of a low sperm count)

Edward’s Syndrome/Trisomy 18 An extra chromosome 18 Almost every organ system in the body is affected. Most fetuses die in utero. The ones that do make it to term usually do not live more than a few months. About 1% live to the age of 10

Patau’s Syndrome/ Trisomy 13 An extra chromosome 13 Results in serious developmental problems including brain, kidney, heart, and eye defects. Children rarely live more than a few months.

XYY These are males with an extra Y chromosome. Individuals are somewhat taller than average and often have below normal intelligence. At one time, it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time.

XXX / Trisomy X Females with an extra X chromosome Individuals are health and fertile – usually cannot be distinguished from normal females except by karyotypes.

Chromosomal Mutations Mutations of large segments of DNA on the chromosome

Recall…. In prophase I of Meiosis, homologous chromosomes pair up and crossing over may occur.

TRANSLOCATION Fragments of DNA from 2 different (nonhomologous chromosomes) exchange places

Translocation con’t…. Disrupts the normal structure of the genes When the chromosome is transcribed, the 2 unrelated sequences on a single chromosome will create a new protein with altered function. Known to cause some types of leukemia

Transposable Elements “jumping genes” Fragments of DNA that consistently move from location to location on a chromosome. If it lands in a coding region of a gene, it will disrupt the transcription of a gene

Inversion The reversal of a segment of DNA within a chromosome Genetic material not gained or lost However, may disrupt a gene sequence.

Cri-du-chat deletion of part of chromosome #5 High-pitched cry (identified as a cat-like cry) Low birth weight Poor muscle tone small head size small jaw wide spaced eyes Round face

Cri-du-chat Epicanthal folds Low set ears Feeding difficulties Delays in walking Hyperactivity Scoliosis Language difficulties Mental retardation Organ defects

Causes of Mutations Spontaneous Mutations Not caused by environmental conditions; Errors made in DNA replication ex: accidently inserting an incorrect base.

Causes of Mutations Mutagenic agents Induced Mutations Agents that can cause a mutation Ex: UV radiation, X rays, certain chemicals (pesticides) Induced Mutations Mutations caused by a chemical agent or radiation Could lead to certain cancers ie: melanoma (skin cancer