Chromosomes

Learning Objectives

By the end of this class you should understand:

The purpose of a karyotype

The reason mitosis must be stimulated during a karyotype

The notation used to describe chromosomal abnormalities

The purpose of banding a chromosome

The two ways to karyotype a fetus

The names and types of chromosomal abnormalities

The major risk factors in chromosomal abnormalities

Why many developmental disorders occur as a result of chromosomal abnormality

Chromosomes

Remember that chromosomes are the packages that our DNA is organized into

Normally spread out Only condensed

during mitosis & meiosis

Chromatin & Centromere

Chromosomes are made of a material called chromatin A mixture of DNA and protective

protein The core in the center is

referred to as the centromere Not always in the center!

Sometimes off to one side Divides the chromosome into two

arms which may be uneven (long arm and short arm)

Why the X?

Remember chromosomes are only compacted during mitosis Mitosis doubles the

chromosome from a I to an X

A visual image of the chromosome is only possible at this stage

This visual stain is called a karyotype

Blood Karyotype

Actual Karyotype Result

Different Banding Stains

Karyotyping Tissue

Why karyotype at all?

What tissues can be karyotyped?

What is an important application of karyotyping?

Fetal Karyotyping!

Another Procedure

Man I thought the needle was bad!

The needle method of extracting amniotic fluid to test the fetus's DNA is called amniocentesis

The catheterization method is called chorionic villus sampling or CVS Currently amniocentesis

is more common and considered safer

What To Look For

Chromosomal abnormalities come in two varieties: The wrong number of

complete sets of chromosomes (polyploidy)

Haploid, diploid, triploid, tetraploid....

Missing or extra chromosomes (aneuploidy)

Extra X, missing X, etc....

Triploidy

Polyploidy

Often caused by completely simultaneous fertilization of egg by two sperm

Invariably fatal though baby may be born disfigured but will die soon

Aneuploidy

Many varieties of aneuploidy

Typically caused by a nondisjunction in meiosis of one gamete Extra

chromosome in one sperm, missing in other

Chromosomal notation

(Number of chromosomes)(Which chromosomes are missing or extra)(any deletions or translocations) Often includes sex chromosomes listed

Triploidy: 69,XXY Down Syndrome: 47,+21 for an extra #21 46,del(5p): deletion on the 5th chromosome on

the short arm (p)

Aneuploidy Varieties

Monosomy Only one of a particular

chromosome Turner Syndrome (one X)

Trisomy Three of a particular

chromosomes Down Syndrome (three #21)

Autosomal Trisomies

Patau Syndrome 47,+13 Fatal within 1 month

Edwards Syndrome 47,+18 Fatal within 2-4 months

Down Syndrome 47,+21 Reduced life expectancy but not fatal

Sex-linked Aneuploidy Turner Syndrome

Monosomy: 45,X Viable female, but short and

sterile Klinefelter Syndrome

Trisomy: 47,XXY Viable male, but feminine and

sterile XXY Syndrome

Trisomy: 47,XYY Few or no phenotypic variations

What about the others?

Other chromosomal defects end in miscarriage Imagine reading a choose your own adventure

book with a page missing Imagine reading a choose your own adventure

book with duplicated pages! Chromosomal information is more than raw

blueprints, during fetal development there is a strict order to when genes activate, and an extra chromosome screws it up

Maternal Age

Maternal age is the largest risk factor in chromosomal abnormalities Over 1% of births to

women over 40 have chromosomal abnormalities

Reason(s) still not fully understood

Translocation

Chromosomes sometimes have an incorrect crossing over process that changes how they are attached This is termed

translocation Often but not

always fatal

Deletion

Chunks of chromosomes may fail to be copied

Resulting syndromes have deletion patterns and often result in developmental issues

Copy Number Variants

Many portions of our genome are not directly coding for protein but instead consist of long strings of repeated letters

These repeated patterns may be erroneously shortened or lengthened This is a copy number variant

Increases risk for many diseases

Copy Number Variants

Have a good weekend!