L5 and L6- Morphology of Chromosomes and Karyotyping

Chromosomes

rod-shaped, filamentous bodies

present in the nucleus, which become visible during

cell division

Number of chromosomes

46 (23 pairs)

Chromosomes can be identified

• Their size

• Their shape (the position of the centromere)

• Banding patterns produced by specific stains (Giemsa)

Karyotype

A display of the chromosome pairs of a cell arranged by size and shape.

Karyotype is during

chromosome measurements are generally taken during mitotic metaphase.

Chromosome 1

largest chromosome

Chromosome 21

smallest chromosome

X chromosome

is relatively large (~156 Mb)

Y chromosome

is much smaller (~57 Mb)

p arm

short arm of chromosome

q arm

long arm of chromosome

Centromere

Area where the chromatids of a chromosome are attached

telomeres

ends of chromosomes

Centromeric index

ength of p arm ÷ total chromosome length × 100

metacentric chromosome

Chromosome in which the two chromosome arms are approximately the same length.

Submetacentric

centromere slightly off center

Acrocentric

centromere close to end , has satellite

satellite chromosome

small chromosomal segment

Satellite stalks

contain nucleolar organizer regions (NORs), the sites of ribosomal RNA genes and production of rRNA

Telocentric

the centromere is at or very near the end of the chromosome

chromosome by groups

Group A: 1-3, largest metacentric and submetacentric chromosomes

Group B: 4 & 5, large submetacentric

Group C 6-12 & X ; Very similar, medium-sized (arranged in order of decreasing size sub-metacentric)

Group D: 13-15, medium-sized acrocentric plus satellites

Group E 16-18 Smaller, metacentric and submetacentric

Group F, 19-20; even smaller, metacentric and submetacentric

Group G 21, 22 & Y; Smallest acrocentric with satellites. Y submetacentric

Banding pattern

result from dies showing a pattern of light and dark bands from karyotyping

dark stain

densely packed heterochromatin. (AT rich).

usually not transcribed; late s-pahse.

centromer, telomere, stalite dna regions.

light stain

less dense euchromatin (GC rich).

activity transcribe; replicated in eraly s phase

Facultative heterochromatin

exhibits variable levels of condensation, related to levels of transcription of resident genes.

Lyonization/X-inactivation

When one of the X chromosomes present is rendered inactive forever to ensure that the female does not have twice as many X chromosomes as the male.

by Xist non-coding RNA produced by XIC

constitutive heterochromatin

chromosomal regions that remain condensed in heterochromatin at most times in all cells

centromeres

the point on a chromosome by which it is attached to a spindle fiber during cell division.

of highly repetitive DNA and specific proteins that form the kinetochore,

Telomeres

Repeated DNA sequences at the ends of eukaryotic chromosomes

provide terminal stability to the chromosome and ensure its survival .

T-loop and D-loop,

this is formed when a telomere comes back around and invades the double stranded DNA. which help protect the chromosome end from being recognized as DNA damage.

G-quadruplex DNA

In areas of DNA that are guanine rich, 4 DNA strands can form a quadruplex structure that interferes with telomerase

idiogram

alignment of chromosomes based upon size and shape

Karyotyping

the process of sorting chromosomes into their matched pairs. An important technique for investigating chromosomal disorders

Colcemid

prevents spindle fibers from forming and stops mitosis at metaphase

hypotonic solution

Solute concentration is less than that inside the cell; cell gains water

Karyotyping Steps

1. cells are stained and prepared on a glass slide to see their chromosomes under a light microscope

2. photomicrograph images obtained of the chromosomes during mitotic metaphase

3. the images are cut out and separated, a process which can be done using scissors or using a computer

4. the images of each pair of chromosomes are placed in order by size and position of their centromeres

T-banding

A Giemsa staining technique that stains the telomeres (ends) and the centromeres of chromosomes.

Karyotype Nomenclature for normal chromosome

Total Chromosome Count: The first number indicates the total number of chromosomes in the cell. A normal human somatic cell contains 46 total chromosomes

Sex Chromosome Complement: The letters following the comma show the sex chromosomes.

XX designates a female.

XY designates a male

Karyotype Nomenclature for Sex Chromosome Abnormalities

The total chromosome count is listed first, followed by the sex chromosome complement, separated by a comma.

Turner Syndrome: 45,X (or 45,X0). Monosomy X; missing one sex chromosome, leaving a total of 45 chromosomes.

Triple X Syndrome: 47,XXX. Trisomy is X; an extra X chromosome in a female.

Mosaicism Example: 45, X/46, XY. Clones with different cell lines are separated by a forward slash (/).

Karyotype Nomenclature for Autosomal Chromosome Abnormalities

Total count, sex chromosomes, signs (+ or -), abnormality.

1. Numerical Abnormalities (Aneuploidy): trisomy 21 (Down Syndrome in a male): 47,XY,+21

2. Structural Abnormalities:

Deletion (Partial Monosomy): 46,XX,del(5)(p14.2)

Reciprocal Translocation: 46,XX,t(2;5)(q21;q31)

Robertsonian Translocation: 45,XY,der(13;14)(q10;q10)