CLASSIFICATION OF GENETIC DISEASES

1.single gene defects

2- Mitochondrial disorders

3.polygenic or multifactorial

4.Chromosomal disorders

5.somatic cell genetic diseases

Chromosomes is a combination of Greek words chroma (=color) and soma (=body).

Very simply, chromosomes can be considered as being made up of genes.

Dr. Joe Hin Tjio stands beside photographs of his remarkable chromosome studies at the first International Human Genetics Congress in Copenhagen in 1956.

The correct chromosome complement in human was established in 1956, and the first chromosomal disorders (Down's, Turner's and Klinefelter's syndromes) were first defined in 1959.

The study of chromosomes and cell division is referred to as cytogenetics.

The 46 human chromosomes are called the human karyotype.The chromosomes are differentiated into 22 autosomal pairs and 2 sex chromosomes

A schematic representation of the banding pattern of a karyotype is called an

ideogram

Centromere

Short arm "p"

Long arm "q"

Telomere

Sister chromatides

CHROMOSOME STRUCTURE

Centromeresconsistofseveralhundredkilobases ofrepetitiveDNAandareresponsibleforthemovementofchromosomesatcelldivision

Telomeresplayacrucialroleinsealingtheendsofchromosomesandmaintainingtheirstructuralintegrity.TheyconsistofmanytandemrepeatsofaTTAGGGsequence.

CLASSIFICATION Chromosomes are classified according to:

1- Their size2- Position of the centromere3- Light and dark banding pattern

1.Metacentric

The centromere is near the middle of the chromosome.

2. Submetacentic

The centromere is somewhere between the middle and the tip.

3. Acrocentric

The centromere is near the tip.

They have small mass of chromatin known as satellites attached to their short arm by a narrow stalk.

Metacentric Submetacentic Acrocentric

Chromosomes are grouped according to their size and centromere site:

Group A (1-3)Group B (4-5)Group C (6-12 & X)Group D (13-15)Group E (16-18)Group F (19-20)Group G (21-22 & Y)

Determine the type of each chromosomes in the following karyotype?

Group-A (1,2,3) Large size Metacentric (1 & 3) , Submetacentric (2)Group-B (4,5) Large size SubmetacentricGroup-C (6,7,8,9,10,11,12,X) Medium size SubmetacentricGroup-D (13,14,15) Medium size AcrocentricGroup-E (16,17,18) Short size metacentric (16) and Submetacentric(17,18)Group-F (19,20) Short size MetacentricGroup-G (21,22,Y) Short size Acrocentric (Y has no satellite)

Heterochromatin:

–More condensed–Gene poor (high AT content)–Stains darker

Euchromatin:

–Less condensed–Gene rich (higher GC content)–Stains lighter

Each chromosome has:Long arm (q) and Short arm (p)

Each arm is divided into regions starting from the centromere to the telomere.

Example: the two regions adjacent to the centromere are labeled as “1” in each arm, the next more distal regions as “2” and so on. Arm region

Arm region bands subbands

Each region is divided into bands

Each band are again divided into sub-bands

same location of the region and band “3”and sub-band “1”Arm region bands subbands

long arm of chromosome X(Xq), region “1” , band “1”and sub-band “1”.

Example:“chromosome X q 11.1” location means,

While “chromosome X q 13.1” location means

ptel: telomere of short arm

qtel: telomere of long arm

cen: centromere location

Determine the following chromosome locations:

Xp 22.31Xq 26.2Xp 11.4Xq 13.3XptelXcen

CHROMOSOME IDENTIFICATION(KARYOTYPE TECHNIQUE)

Chromosome can be prepared from any tissue with a nucleated cell such as:

SkinBone marrow

Amniotic fluid

But it is usually prepared from peripheral blood.

In blood, chromosomes are present in the white blood cell (lymphocyte) because they contain a nucleus. There are no chromosomes in the red blood cells and platelets, as they contain no nucleuses.

Separate Cells from plasma

Culture media is added into the blood lymphocyte to stimulate lymphocyte to divide and go into mitosis for about 3 days at 370C.

Colchicine is added to block cell division at metaphase by preventing spindle formation (The chromosomes are better visualized at this phase)

Hypotonic buffer is added to lyses the cell (break the cell) and release the chromosomes

There are different staining methods of which each will give different color of chromosomes under the microscope, these include:

G – Staining(Giemsa), which is the commonest.C- Staining(Centromere)R – StainingQ - Staining

Plain staining

G – Staining (Giemsa)

R – Staining

C- Staining (Centromere)

culture

colchicine

stain

cell lysis

microscope

Fluorescence In Situ Hybridization (FISH)

WHAT IS FISH?

Fluorescence in situ hybridization (FISH) uses fluorescent molecules to vividly paint genes or chromosomes.

This technique is particularly useful for gene mapping and for identifying chromosomal abnormalities.

HOW DOES FISH WORK?

FISH involves the preparation of short sequences of single-stranded DNA, called probes, which are complementary to the DNA sequences needed to paint and examine.

These probes hybridize, or bind, to the complementary DNA and, because they are labeled with fluorescent tags, allow you to see the location of those sequences of DNA

Unlike most other techniques used to study chromosomes, which require that the cells be actively dividing, FISH can also be performed on non-dividing cells (interphase), making it a highly versatile procedure.

WHAT IS FISH USED FOR?

There are different types of FISH probes, each of which has a different application:

This is an image of probe DNA as viewed through a florescent microscope. The red dots indicate trisomy 21 (Down's Syndrome)

1- Locus specific probes hybridize to a particular region of a chromosome. The probe is prepared from a piece of the gene and observe if the probe hybridizes to the gene (normal) or not (absent or deleted).

2- Centromeric repeat probes are generated from repetitive sequences found at the centromeres of chromosomes.

This can be used to determine whether an individual has the correct number of chromosomes or, for example, whether a person has an extra copy of a chromosome

3- Telomeric probe: A complete set of telomeric probes has been developed for all 24 chromosomes. This is useful for identifying tiny subtelomeric abnormalities such as deletions and translocations in children with mental retardation.

This full color image of the chromosomes allows distinguishing between the chromosomes based on their colors, rather than based on their dark and light banding patterns, viewed in black and white through traditional karyotyping

4- Whole chromosome probes are actually collections of smaller probes, each of which hybridizes to a different sequence along the length of the same chromosome. Using these libraries of probes, it is possible to paint an entire chromosome and generate a spectral karyotype.

Whole chromosome probes are particularly useful for examining chromosomalabnormalities, for example, when a piece of one chromosome is translocated to the end of anotherchromosome