cytogenetics

human genetics lecture 4

cytogenetics

the study of the structure and function of chromosomes and their role in genetics

trisomy

three copies of a single chromosome

monosomy

one copy of a single chromosome

histones

proteins that DNA wraps around

nucleosome

bead of DNA wrapped around a histone

core DNA

DNA in contact with the histones

linker DNA

DNA between two nucleosomes

heterochromatin

compact chromatin, making DNA transcriptionally inactive

euchromatin

loosely packed chromatin, allowing DNA to be transcribed

chromatin

coiled and compacted mass of DNA

histone octamer

8 histone units in each nucleosome: 2 each of H2A, H2B, H3, and H4

“p” arm of a chromosome

short arm

“q” arm of a chromosome

long arm

telomerase

enzyme helping build and rebuild telomeres

centromere

region of a chromosome that joins sister chromatids and serves as an attachment site of microtubule fibers during cell division

satellite DNA

tandem repetitive sequences within the centromere

cen-proteins

proteins bound to the satellite DNA of the centromere that are essential for the attachment of spindle fibers during cell division

metacentric

a chromosome that has a centrally placed centromere

submetacentric

a chromosome whose centromere is placed closer to one end than the other

acrocentric

a chromosome whose centromere is placed very close to, but not at, one end

telomere

repeating sequence of DNA located at the ends of chromosomes to act as a buffer zone against gene erosion

TTAGGG

telomere sequence

T-loop

loop of DNA at the telomeres

autosomes

chromosomes other than the sex chromosomes, chromosomes 1 to 22 in humans

sex chromosomes

in humans, the X and Y chromosomes that are involved in sex determination

karyotype

the complete set of chromosomes from a cell that has been photographed during cell division and arranged according to size and centromere position

G-banded chromosome

chromosome stained with Giemsa stain

dark bands on G-banded chromosomes

high concentrations of adenine and thymine

gene address

chromosome number, arm, region, band / subregion

information obtained from karyotypes

• number of chromosomes

• sex chromosome content

• presence or absence of individual chromosomes

• nature and extent of large structural abnormalities

amniocentesis

method of sampling the fluid surrounding the developing fetus to extract fetal cells

Chorionic Villus Sampling (CVS)

method of sampling fetal chorionic cells by inserting a catheter through the vagina or abdominal wall into the uterus

triploidy

chromosomal number that is three times the haploid number, having three copies of all autosomes and three sex chromosomes

tetraploidy

chromosomal number that is four times the haploid number, having four copies of all autosomes and four sex chromosomes

cause of aneuploidy

non-disjunction events, failure of homologous chromosomes to separate properly during meiosis

autosomal monosomy

lethal condition, eliminated early in development

autosomal trisomy

relatively common, most result in spontaneous abortion but three types can result in live births

types of autosomal trisomy that can result in live births

• Patau syndrome (trisomy 13)

• Edwards syndrome (trisomy 18)

• Down syndrome (trisomy 21)

trisomy 13

Patau syndrome

trisomy 18

Edwards syndrome

trisomy 21

Down syndrome

leading risk factor for trisomy

maternal age

why maternal age is a risk factor for autosomal trisomy

• meiosis is not completed until ovulation, intracellular events may increase risk of non-disjunction

• maternal selection, embryo-uterine interactions that normally abort abnormal embryos become less effective

Klinefelter syndrome

XXY, tall, small testes, gynecomastia

Turner syndrome

X, short, widened neck (edema)

chromosome structural abnormalities

• deletions

• duplications

• inversions

• translocations

deletions

involve a loss of chromosome material

cri-du-chat syndrome

deletion of the p arm of chromosome 5, associated with an array of congenital malformations such as infant cry that resembles a meowing cat

DiGeorge syndrome

22q11.2 deletion

fluorescent in situ hybridization (FISH)

probes are designed to be complementary to a target sequence and are labeled with a fluorescent tag, allowing it to be seen where the probe is binding

duplications

when any part of the genetic material is present more than once in the genome, caused by unequal crossing

inversions

occur when a segment of DNA is turned around 180 degrees within a chromosome, no loss of genetic material

pericentric inversion

inversion including centromere

paracentric inversion

inversion not including centromere

translocations

involve exchange of chromosome parts, can result in genetically-imbalanced and aneuploid gametes

Robertsonian translocation

when an acrocentric chromosome’s q arm translocates onto another acrocentric chromosome, making one very large chromosome and one very small one

leukemia & XX male

translocations

Philadelphia chromosome

chromosome 22 with part of chromosome 9 translocated onto it, present in leukemia cells