MUTATIONS

What causes a snake to be born with two heads?

Are Mutations common? As scientists learn to read the instructions in our genes, they are discovering that much of our DNA is riddled with errors. We each inherit hundreds of genetic mutations from our parents, as they did from their forebears.

In addition, the DNA in our own cells undergoes an estimated 30 new mutations during our lifetime, either through mistakes during DNA copying or cell division or, more often, because of damage from the environment.

When are Mutations Harmful? During copying, bits of our DNA may be deleted, inserted, broken, or substituted. Most mutations affect only the parts of DNA that do not contain instructions for making a gene, so we need not worry about them.

Problems arise only when an error in DNA alters a message that tells certain cells to manufacture a certain protein. Such messages are spelled out in varying sequences of the four chemical bases that make up DNA: adenine (A), thymine (T), guanine (G), and cytosine (C).

Mutations Mutations occur as random chance events and cause changes in the DNA sequence If mutations occur in gametes, they can be passed on to offspring Mutations may be caused by exposure to toxins or radiation (mutagens).

Some mutations cause disease, yet others may lead to variations in the organism that make it better adapted for their environment

Kinds of Mutations

There are 2 kinds of Mutations: Chromosomal Mutations Gene Mutations

Chromosomal Mutations

1. Chromosomal Mutations An abnormal change in the structure of all or part of a chromosome, OR in the number of chromosomes an organism has Ex: normal humans have 46 chromosomes Humans with Down Syndrome have 47

Trisomy 21

Down syndrome (Trisomy 21) Extra 21 chromosome Effects 1/700 Alters child’s phenotype– characteristic facial features, short stature Usually some degree of mental retardation

Gene Mutations

2. Gene Mutation A change that affects a gene on a chromosome Causes the cell to produce abnormal proteins There are 2 types of gene mutations:

Point Mutation Frame Shift Mutation

Point Mutations

Point Mutation A gene mutation involving only a single nucleotide Ex: ACA mutates to read ACT mRNA codes ACT to be UGA which is a stop codon! This will cause the necessary protein to not be made

Point Mutation:

Frame-Shift Mutations

Frame Shift Mutation This is the insertion or deletion of one or more nucleotides Ex: THE CAT ATE In a frame shift mutation it would read THE ATA TE which is meaningless! Which letter was deleted? C

Genetic Disorders

There are 3 different types of Human Genetic Disorders Sex-linked Disorders Autosomal Genetic Disorders Chromosomal Genetic Disorders

Sex-linked Traits

Sex-linked Traits A trait that is controlled by a gene found on the sex chromosome

Why are Males more likely to inherit sex-linked genetic disorders? On the other hand, men are more likely to inherit sex-linked disorders because they only have one X They lack the protection of having 2 X’s as females do.

Each male child of a mother who carries the defect has a 50 % risk of inheriting the faulty gene and the disorder. Each female child has a 50% chance of being a carrier like her mother.

The two sex-linked disorders we will be discussing are: Color Blindness Hemophilia

Color Blindness

1. Color Blindness Color blindness is a condition in which the individual cannot perceive certain colors Found mostly in males Females can carry the gene for color blindness and are called carriers Carriers have the gene for color blindness on one X chromosome

This is what a strawberry patch would look like to a color blind person

Hemophilia

2. Hemophilia Hemophilia is a genetic disorder that affects the ability of blood to form an effective clot and thereby results in prolonged bleeding. Occurs almost exclusively in males.

Vessel damage, blood loss Coagulation, scab forms                                                                                                                  Platelets form Coagulation, scab forms

Why Blood Clots A. Normal Blood Clot:                                                     B. Person with Hemophilia

Chances for Hemophilia

Autosomal Genetic Disorders

What are Autosomal Genetic Disorders? Humans have 23 pairs of chromosomes 22 pairs are called Autosomes The last pair are called sex chromosomes because they determine your gender Autosomal genetic disorders therefore are mutations that occur on the first 22 pairs of autosomes

An Example of an Autosomal Genetic Disorder is: Sickle Cell Anemia

Sickle-Cell Anemia

Sickle Cell Anemia

1. Sickle-cell Anemia The genetic defect that causes sickle cell anemia affects hemoglobin, a component of red blood cells. What causes this defect is a mutation when making a certain amino acid

How to Detect Genetic Disorders

There are 2 ways to detect genetic disorders during pregnancy Amniocentesis Karyotype

Amniocentesis

1. Amniocentesis A long needle is inserted into the amniotic sac of a pregnant woman to obtain a sample of amniotic fluid The cells of the fetus that are contained within the fluid are then examined to see if there are any abnormalities

Karyotyping

2. Karyotyping A sample of blood cells is taken Chromosomes are separated out, viewed with a microscope and photographed. The photograph is then rearranged to show the paired chromosomes. Size, shape and banding pattern are used to pair up the chromosomes.

Notice the differences in these chromosomes: Differences in length Differences in banding patterns Differences in centromere locations (top, middle)