chromosomes Flashcards
chromosome structure explained
consists of two identical strands called chromatids joined at the centromere
centromere divides the chromatids into short p and long q arms
telomeres at the tip
two different types of chromosome + definition
acrocentric- centromere is on one side
metacentric- centromere is in the middle
telomere function
maintain the structural integrity of chromosomes formed of highly conserved tandem repeat sequences
how is structure integrity maintained?
telomerase enzyme ensures replication, preventing the shortening of the DNA strand
what happens in cancers?
telomeres are over produced, preventing cell senesence
karyotype definition
a picture of a person’s chromosomes
G-banding explained
denature the proteins in the chromosome using trypsin, stain with Giemsa, a DNA binding dye which gives the chromosome a stripy pattern
active areas stain light
Other genetic techniques used to view chromosomes
FISH, Chromosome painting, CGH and array CGH
explain FISH
Uses DNA probe to anneal to its complementary target sequence where it is located on a metaphase spread
Used to detect chromosome abnormalities, such as Down’s syndrome
Three red dots shows that three probes bound to three chromosomes which signify Down’s syndrome
used when scientists are looking for a known sequence
explain chromosome painting
used when sequence is unknown
a mixture of probes specific for each chromosome are used, able to view translocations and identify which chromosomes they came from
CGH stands for
comparative genome hybridisation
explain CGH use
used when people do not look normal. try to detect regions of gene amplification or gene loss
Test DNA is labelled with green paint, normal DNA is labelled with red paint. Too much green shows gene amplification, too much red shows gene loss
Determination of the male sex
presence of Y chromosome leads to maleness regardless of the number of X chromosomes present
presence of SRY region on Y, the sex determining region, which encodes a transcription regulator
what happens if SRY gene is mutated?
XY female
SRY function
unregulated the gene transcription of genes involved in testis production from Wolffian ducts
testis Sertoli cells produce mullein inhibitory Factor, inhibiting female genitalia production
what happens if there is androgen insensitivity?
body does not react to testosterone, leading to an XY female despite SRY
Explain X-linked inactivation
Early in embryonic life, structurally abnormal X is preferentially inactivated, otherwise it is random.
Inactivation is not complete, some genes are known to escape inactivation
Inactivation is not permanent, reversed in development of germ cells
Barr body definition
inactivated X chromosome =, if an individual had 4 X chromosomes they would have 3 Barr bodies
abnormalities linked to X and Y
more Xs, more intellectually impaired
extra Y, more aggressive, increased criminal tendencies
2 different syndromes linked to sex chromosomes
Turner syndrome X
Klinefelter’s syndrome XXY
Explain Turner syndrome
intellectually normal, infertility, primary amenorrhoea, cardiac abnormalities and webbed neck
explain klinefelter’s
feminised male, moobs, fat around his and thighs, impaired IQ
three trisomies
Trisomy 21- Down’s
18- Edward’s
Patau- 13
explain nondisjunction of chromosomes
failure of separation of chromosomes during meiosis in gametes which results in cells with extra or less copies of a particular chromosome
explain down’s syndrome
tongue too big for their mouth, low muscle tone, single palmar crease, wide sandal gap, intellectual disability, septal defects, more prone to Alzheimers
explain Edward’s syndrome
clenched fingers they cannot release, rocker bottom feet, protuberant back of head, severe intellectual disability
explain patau syndrome
extra digits, propensity to develop cleft lip and cleft palate, severe intellectual disability, cyclopia
problem with Shh, responsible for mediating midline patterning
examples of chromosomal micro deletion syndromes
DiGeorge, WAGR, Williams
explain DiGeorge syndrome
deletion on the long arm q of chromosome 22 leading to cardiac abnormalities, immunodeficient, cleft palate, short stature
WAGR explained
Wilms Tumour Aniridia Genital Abnormalties Retardation
Renal tumour that occurs in children under 5, absence of iris, short arm of chromosome 11 deleted
Williams syndrome explained
intellectual disability, unable to form ideas of a conversation, raised calcium level, cardiac abnormalities, short stature, full cheeks, long arm of chromosome 7 deletion
chromosomal translocation definition
transfer of genetic material from one chromosome to another
two types of chromosomal translocation + definition
robertsonian - break points are close to centromeres of 2 acrocentric chromosomes, form of reciprocal
reciprocal- break in two chromosomes and their segments are exchanged
example of reciprocal translocation
Bcr-Abl
Abl is an oncogene, Bcr upstream of All prevents oncogene regulation leading to leukaemia
two types of robertsonian translocation
homologous acrocentric- both long ends join together from the same chromosome
non homologous afrocentric- both long ends from two different chromosomes join and short parts are lost
two different chromosome structures + definition
isochromosome- mirror image chromosome, loss of one arm, duplication of another arm
ring chromosome
delete the ends of one chromosome, forming a ring, normally removes telomeres
5 other chromosomal mutations
inversion, insertion, duplication, mosaicism, chimerism
two types of inversion + definition
pericentric- swapping of segment involving p and q arm and centromere
paracentirc- involves one arm
two breaks in one chromosome, fragments generated rotate 180 degrees and reinsert into the chromosome
mosaicism definition
presence in an individual of a tissue of 2 or more cell lines that differ in their genetic constitution but are derived from a single zygote
tends to result from non disjunction in an early mitotic division
chimerism definition
the presence of 2 or more genetically distinct cell lines derived from more than one zygote
two examples of chimeras
dispermic, 2 sperm, 2 eggs one embryo
blood- exchange of cells between non identical twins via placenta
normal chromosome complement defnition
22 pairs of autosomes
2 sex chromosomes
three numerical chromosome mutations
aneuploidy, monosomy, trisomy
aneuploidy definition
presence of an abnormal number of chromosomes in a cell
example of a monosomy
turners
