Lecture 22-Cytogenetics 1 Flashcards
Euchromatin is largely composed of ______ sequences vs. heterochromatin which is largely composed of _______ sequences.
- coding
- non-coding
Heterochromatin is largely found where?
- centromeres
- distal end of Y
Each centromere has how many bps?
171
Are all centromeres the same?
no, they can differ from chromosome to chromosome
Chromosomes 13, 14, 15, 21, 22 all have what structure?
acrocentric
Describe an acrocentric chromosome.
- no euchromatin on P arm
- has many repeats of rRNA genes so it can be very large or non-existant
- these genes are redundant among acrocentric chromosomes
If someone has trisomy they have ____ chromosomes.
47
If someone has monosomy they have ____ chromosomes.
45
What are 3 ways to get cells from a fetus in order of lowest to highest rate of miscarriage?
- amniocentesis
- CVS
- PUBS (percutaneous umbilical blood sample)
When is peripheral blood not used for diagnosis of congenital abnormalities?
- PUBS
- Acquired abnormalities like leukemia
Phytohemagglutinin (PHA)
mitogen to stimulate WBCs in peripheral blood to divide so they can get to metaphase for analysis
What is bone marrow tissue used for with respect to diagnostic testing?
leukemia
When is a skin biopsy usually used? What is it’s caveat?
- to compare karyotypes between skin and blood
- takes a long time to growth them in culture and therefore before preparation can be done
If you draw someones blood you could expect their WBCs to be in what phase?
- G0
What are the chromatid counts (per chromosome) at the end of each phase of the cell cycle?
- G1: 1
- S: 2
- G2: 2
- M: 1
What happens during prometahpase?
nuclear envelope disappears
What is the difference between the nuclear spindle attachments between prophase and prophase I?
- prophase: one chromosome/spindle
- prophase I: homologous chromosomes/spindle
colcemid
spindle fiber poison that freezes cells in metaphase
What do you need to do with cell samples prior to Giemsa staining?
- trypsin digest of some chromosomal proteins
When does independent assortment take place?
- meiosis 1
reduction division
- occurs during meiosis 1
- is the halving of chromosomes from 46–>23
What is the law of independent assortment?
largely maternal chromosomes and largely paternal chromosomes can be grouped together in the same gamete
At least 1 _____ is required for regular orientation of the chromosomes on the spindle during meiosis I.
chiasma
Other than mixing of genes, chiasmata do what?
- stabilize chromosomes on the spindles and decrease the risk of nondisjunction
Where are chiasmata not seen?
on acrocentric chromosomes
How do the number of meiotic divisions differ between males and females?
men have more divisions that are subdivisions of prophase 1. This is where crossing over occurs
Female gamete synthesis stops where first? When does it resume?
- prophase 1, resumes in the 1st half of the menstrual cycle
- meiosis completes at fertilization
What is the difference in outcomes of meiosis in females vs. males?
- females have 1 gamete made/meiosis
- males make 4/meiosis
Constitutional abnormalities (cytogenic abnormalities present at CONCEPTION) are associated with ____ (2)
- birth defects
- miscarriages
What is the most common and clinically significant type of chromosome disorder? What is it’s frequency?
- aneuploidy
- 3-4%
How does aneuploidy arise?
- nondisjunction
How can you tell which parent gave the aneuploidy in trisomies?
- you can’t tell
Majority of aneuploidies are from _____, when?
the mom during meiosis 1, nondisjunction is more common in females
Downs syndrome accounts for what percent of chromosomal abnormalities at birth?
50%
Kids with downs are at increased risks for what 2 things?
- leukemias
- congenital heart defects
Do you need 47 chromosomes to have a trisomy 21?
no, you can have a translocation so the third copy of 21 is attached to another chromosome
Describe the viability of Trisomy 18 and 13.
- 5% live to 1 year old
Which trisomy is associated with midline defects?
- Trisomy 13
What are the phenotypes associated with autosomal abnormalities? (4)
- developmental delay/MR
- facial features more characteristic fo the syndrom than of family members
- growth delay
- congenital malformations
Why do the phenotypes associated with sex chromosome abnormalities tend to be milder than autosomal abnormalities?
- fewer genes on sex chromosomes and most have to do sex development
Which sex chromosome abnormalities do not result in infertility? Are they pretty harmful to those affected?
- XXX, XYY
- fairly benign: just the typical abnormal heigh, lower IQ
What can aberrant crossover between sex chromosomes cause? How?
- sex reversal: if the SRY gene gets translocated it results in sex reversal
