Things Can Go Wrong With DNA and Chromosomes Genetic Disorders Things Can Go Wrong With DNA and Chromosomes

I. Overview of DNA Structure A. Review 1. A gene is a segment of DNA that codes for a particular protein 2. Proteins determine the physical traits of an organism 3. In humans, DNA is organized into 23 pairs of homologous chromosomes B. DNA Structure 1. The basic building block of DNA is a nucleotide 2. Nucleotide chains are held together to form a double helix 3. Nucleotides are represented using the letters A,T,C,G

One nucleotide

II. DNA Based Disorders A. Dysfunctional Genes (mutation) 1. A gene can be missing some of its nucleotides or some of the nucleotides present may be in the wrong order The protein made from that gene may not work properly B. Missing Genes 1. A person can be born missing most or all of a gene No protein is produced and the trait cannot be expressed properly

C. Examples of DNA Based Disorders 1. Sickle-cell anemia Caused by a substitution of one nucleotide in the allele for normal hemoglobin Red blood cells become sickle-shaped in stressed conditions and die in 10 to 20 days (normal blood cells live for 120 days) leading to anemia These tend to become stuck in capillaries, leading to tissue death Most common among individuals with African decent, because it provides a resistance to Malaria Normal Red Blood Cell Sickle Cell

Huntington’s Disease Produced by a single dominant allele A gradual change of the nervous system occurs around the age of 30 to 45 Marked by a painful, progressive loss of muscle control and mental function until death occurs Approximately 6/100,000 affected

III. Chromosomal Based Disorders Causes 1. Nondisjunction during meiosis I, homologous chromosomes do not separate properly Results in an extra copy of a chromosome in one cell, and a loss of that chromosome from another Results After fertilization, the resulting person will have an abnormal number of chromosomes A monosomic cell is missing one chromosome A trisomic cell has one extra chromosome

C. Nondisjunction of Sex Chromosomes Genes critical to all embryonic development are on the X chromosome absence of an X chromosome is lethal absence of a Y chromosome is not lethal Individuals can survive if they have an extra sex chromosome Affects approximately 1 birth in every 1000

D. Disorders involving Sex Chromosomes 1. Klinefelter Syndrome  males with extra X-chromosomes  can be XXY or XXXY Affects 1/500 births abnormal development of the testis, leading to infertility Affected individuals are often tall and produce relatively small amounts of testosterone

 females with extra X-chromosomes (XXX, or XXXX) 2. Triple X  females with extra X-chromosomes (XXX, or XXXX) Affects 1/1000 female births Affected individuals are usually taller than average and have slender builds normal development of sexual traits and are fertile 3. Turner Syndrome  Females with only one X-chromosome Affects 1/2500 live female births only 1 in 40 affected zygotes develops to term Individuals are short in stature, generally lack prominent female secondary sexual characteristics and may be sterile Health problems such as heart disease, kidney problems, diabetes and thyroid problems.

E. Nondisjunction of Autosomal Chromosomes Can affect chromosomes #1 - #22 There only 3 trisomies that result in a baby that can survive for a time after birth – trisomy in chromosomes 13, 18 or 21. Most frequently, the baby dies in utero. The life expectancy of a baby born with trisomy 13 or 18 is days to weeks. F. Disorders Involving Autosomal Chromosomes 1. Down Syndrome  one extra chromosome 21 -written as (47,+21) 1 in 800 live births Life expectancy has improved but heart problems persist lower than average mentality decreased muscle tone, a flat face, eyes slanting up, irregular shaped ears, large tongue relative to the mouth.

IV. Diagnosing Gene Disorders A. Karyotype - a display of all the chromosomes in the nucleus 1. The process: Harvested cells in metaphase are treated and stained Chromosomes are observed under the microscope A photograph is taken and enlarged Chromosomes are arranged in homologous pairs Abnormalities are identified

B. Prenatal Diagnosis Carrier Identification Tests CVS (chorionic villus sampling) – a needle is inserted into the mother’s abdomen to test for DNA, gene, and chromosome disorders. Ultrasound – detects major physical deformations Amniocentesis – fluid is removed from the sac around the embryo to detect chromosomal abnormalities Fetal blood sampling – needle is used to removed cells from the fetus to test for blood disorders

Preventing Genetic Disorders Blood Test Simple blood test in males and females can screen for more than 400 genetic mutations Approximately $400 and results are available in less than a week In Vitro Fertilization Doctors screen embryos after fertilization and only implant those that appear free of disease