KARYOTYPE:

The number size and shape of the chromosomes of a somatic cell arranged in a standard manner.

• position of centromere - arm length ratio
• secondary constrictions (nucleolar organisers)

The normal human karyotype has 46 chromosomes

• 23 derived from each parent
• sex is determined by X and y chromosomes
• Males are XY
• Females are XX
• The sex of an offspring is determined by the sex chromosome carried in the sperm

Chromosomes are described with the following categories:

Metacentric:

• centromere is median or near median
• chromosome has two well defined arms with a length ratio varying from 1:1 to 2.5:1

Acrocentric:

• centromere is close to one end of the chromosome
• one arm is substantially smaller than the other and the arm ratio ranges from 3:1 to 10:1

Telocentric:

• centromere is a strictly terminal entity and the chromosome is one armed

Chromosomes are always arranged with the short arm on top

• Short arm is labeled P (French for petit)
• Long arm is labeled Q

Standard nomenclature been developed for human chromosomes:

Paris nomenclature 1978

• Total number of chromosomes
• Sex chromosome constitution
• Description of abnormality i.e. 46,XX normal female 46,Y male

Human chromosomes are divided into 7 groups & sex chromosomes

• A 1-3 Large metacentric 1,2 or submetacentric
• B 4,5 Large submetacentric, all similar
• C 6-12, X Medium sized, submetacentric - difficult
• D 13-15 medium-sized acrocentric plus satellites
• E 16-18 short metacentric 16 or submetacentric 17,18
• F 19-20 Short metacentrics
• G 21,22,Y Short acrocentrics with satellites. Y no satellites.

Banded Human Karyotype

Difficult to distinguish between chromosomes without banding

Sources of tissue: blood-lymphocytes (mitogen phytohaemaggulatin), amniotic fluid, bone marrow, skin

Colchicine is used as a spindle arresting agent to accumulate metaphases

Differential staining of heterochromatic regions - assumed that the reagents differentially denature DNA reducing the amount of coiling

"Chromosomes & Karyotying" is installed in the network laboratories. It allows you to practice karyotyping, has a quiz and will be the basis for one of your practicals in BBT102.