Chromosomes / Karyotyping

Chromosomes

Structures found inside the nucleus, condensed and visible during cell division; consists of 2 identical sister chromatid threads at the start of mitosis

1

1

p arm

Short arm (top)

centromere

Center region of a chromosome

q arm

Long arm (bottom)

telomere

End tips of a chromosome

Chromosomes

Identified by size, shape (position of centromere), and banding patterns

Sex Chromosomes

X and Y chromosomes (Females have two large X chromosomes)

Meiosis

Cell division that produces sex cells, halves chromosome count, and randomly places one chromosome from each pair into a sex cell

Karyotype

Complete set of chromosomes photographed during cell division and arranged in a standard sequence

Karyotyping

Analysis of chromosomes used to identify missing, excess, and abnormal chromosomes

Autosomes

Chromosomes 1–22

Trisomy 18

Edward's syndrome

Trisomy 21

Down syndrome

Samples for Karyotyping

1. WBC (lymphocytes) 2. Skin cells (fibroblast) 3. Amniotic fluid (amniocytes) 4. Chorionic villus cells (placental cells)

G Banding

Treat metaphase spreads with trypsin (digests chromosomal protein) and stain with Giemsa

Q Banding

Treat metaphase spreads with quinacrine mustard to observe a fluorescent banding pattern

C Banding

Chemically treat metaphase spreads to extract DNA from arms (not centromere) and stain with Giemsa

R Banding

Heat metaphase spreads at high temps for partial DNA denaturation and stain with Giemsa

Polyploidy

Chromosomal number that is a multiple of the normal haploid set

Triploidy

Most common form of polyploidy, found in 15% to 18% of all miscarriages

Aneuploidy

Addition or deletion of individual chromosomes from the normal diploid set of 46

Nondisjunction

Most common cause of aneuploidy

Trisomy

Condition in which one chromosome is present in three copies

Autosomal Trisomy

Most are lethal during prenatal development, accounting for up to 50% of chromosomal abnormalities

Monosomy

Condition in which one member of a chromosomal pair is missing

Autosomal Monosomy

Aneuploidy during gamete formation produces equal numbers of monosomic and trisomic gametes