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Flashcards in Intro to Cytogenetics Deck (41):

Somatic division

-results in duplication of the contents of the cell including the DNA followed by cytokinesis or separation of the materials into two new daughter cells
-with respect to the DNA content the original cell is dipoid (2N)
-after the DNA replicates the DNA content becomes 4N but the original constitution (2N) is reestablished following division
-five steps: interphase, prophase, metaphase, anaphase, and telophase



-the DNA replicates so the 2N cell becomes 4N
-the newly relpicated chromatids are held together and appear as one structure, so there is no increase in the number of chromosomes although the DNA content has doubled



-the chromosomes shorten and thicken and can be visualized by light microscopy



-centromeres divide and the chromosomes separate



-the final step in cell division occurs in telophase
-the net result is 2 daughter cells that should be copies of the original parent cell
-each daughter cell is 2N



-germ cells undergo a different type of cell division
-occurs in gonads
-has two different events recombination of linked alleles leads to the reassortment of the genes
-the hallmark of meiosis is reduction division which is a reduction by half of the original number of chromosomes
-this is accomplished by two consecutive cell divisions, but only a single DNA duplication
-thus a cell begins as 2N but the final product is N



-when portions of two homologues break and exchange places
-this has been visualized and the event is known as crossing-over. If you look at the allelic distribution of genes within the families you will see a patchwork of different alleles attesting to the recombination in meiosis



-an exchange between homologous chromosomes resulting in a reassortment of the genes/allelespresent on each chromosome


Steps of meiosis

1) A dipoid cell with 2C DNA replicates giving rise to a 2N cell with a 4C DNA content
2) At metaphase I, the homologous chromosome pair and in the first division (anaphase I) the centromeres remain together and the homologs separate such that one chromosome of each pair is pulled to the opposite pole of the cell. This is reduction division the crucial step in meiosis
3) The 2N cell has split into 2 daughter cells- both with half the total number of chromosomes so they are haploid or N. However, each chromosome in the daughter cells is a duplex (each chromosome has 2 chromatids, so the total DNA content of the cell is 2C
4) A second division must occur. This division is similar to a mitotic division without DNA replication
5) In each daughter cell, the chromosomes line up, the centromeres divide, and the chromosomes are pulled to opposite poles
6) Each daughter cell receives one chromosome, but there are NO pairs of chromosomes. Each chromosome is unique. These daughter cells have half the original total of chromosomes and half the original DNA content


Meiotic Nondisjunction

-failure of chromosome or chromatids to disjoin propertly can occur
-in meisos nondisjunction can occur at either the first or second division and the net result is different



-the presence of 2 chromosomes
-isodomy: 2 chromosoms from the same source -> duplication of 1 chromosome
-heterodisomy: 2 different chromosome


Meiotic Nondisjunction I

-happens in Anaphase I
-if nondisjunction at meiosis I occurs, A1 and A2 may NOT separate. This will give rise to a cell with both A chromosomes and the other with No A chromosomes
-assuming the second division occurs correctly, the net result will be 4 cells with a chromosomal imbalance
-two different copies of A = heterodisomic for A
-missing A = nullosomic for A


Meiotic Nondisjunction II

-non disjunction happens in meiosis II
-there is just two cells with chromosomal imbalacne
-one cell disomic and one nullosomic



=how gametes are formed
-spermatogenesis- occurs in males and produce sperm
-an original germ cell replicates via mitosis to generate a large population of spermatogonia which are 2N. At some point one of these cells will enter meiotic division. The DNA replicates giving a primary spermatocyte. This cell divides and following reduction division becomes a secondary spermatocyte. The second meiotic division results in haploid spermatids which will mature into sperm. Each primary spermatocyte gives rise to 4 spermatids. This is a classic meiotic division
-oogenesis occurs in females and produces the eggs



-oogonia replicate via mitosis. One of these cells will enter meiosos and become a primary oocyte
-the first meiotic division results in two unequal daughter cells
-both daughters receive equal halves of the DNA but one daughter receives the majority of the cytoplasm. This is the secondary oocyte. The daughter with little cytoplasm is known as the first polar body. This cell usually degrades, though it can go through a second division giving rise to secondary polar bodies
-2nd oocyte goes through second meiotic division two daughter cells one has more cytoplasm which is the egg cell
-only one functional gamete


Timing of oogenesis

-begins in fetus
-by 3rd month gestation the primary oocytes are present
-these cells reach dictyotene (prophase 1) by birth and remain so until ovulation up to 50 years later
-at ovulation, the oocyte completes meiosis I becoming a secondary oocyte
-meiosis will only be completed if fertilization occurs
-penetration of the sperm head stimulates the final division and release of the second polar body
-it is then possible for the male and female pronuclei to fusion creating a zygote


Spermatogenesis vs. Oogenesis

-primary spermatocytes produced throughout reproductive life
-games produced continually
-4 equal gametes per original primary gametocyte

-primary oocytes all present at birth
-gametes produced once a month
-1 gamete per original primary gametocyte



-2 copies of one type of chromosome



-they have 1 copy of two different types of chromosomes an X and Y chromosome


Pseudoautosomal region

-a region on the short arms of the X and Y chromosomes that engaged in recombination


Sex Determination

-female development is default
-females lack TDF/SRY so ovaries will develop followed by differentiation of the Mullerian ducts, which will result in the internal female reproductive organs, and regression of Wolffian ducts
-if TDF/SRY present and active testes develop, Mullerian ducts degrade and androgen is produced and that stimulates Wolffian ducts
-due to genes on the X Y and autosomes
-occurs very early in development
-loss of one sex chromosome later in life is clinically irrelevant


Lyon Hypothesis

-1 X is inactivated in somatic cells of females
-Barr Bodies= total number is the equal to total number of X chromosomes minus 1
-determination of of a normal female there must be 2 active X chromosomes
-inactivation happens after critical point (3-7 days)
-inactivation is random but once established is not reversible in somatic tissues
-results in dosage compensation



-if female heterozygous then some cells with inactivate maternal X and some will inactivate paternal X
-like calico cat


Non-random inactivation

-a deletion or damage to one of the X chromosomes can lead to a change in the inactivation patterns
-a damaged X may be preferentially inactivated, skewing the distribution such that the alleles on the other X chromosome are always expressed
-clinical problems if the only active X has mutant allele on it


Mechanism of X inactivation

-epigenetic mechanism
-methylation initiated at XIST (X inactivation center) and spreads along the length of X chromosome
-several sites appear to escape inactivation include the pseudoautosomal region
-process irreversible since the inactive X must be reactivated at meiosis so that all gametes will have an active X



-the science that combines the methods and findings of cytology and genetics
-the study of heredity at the cellular level


Chromosomal Basis of Inheritance

-46 chromosomes in a somatic cell (23 pairs) 22 autosome pairs, 1 pair of sex chromosomes
-members of a pair are homologous chromosomes
-one homologue was inherited from each parent
-genes are located along the length of the chromosomes


Clinical Role of Cytogenetics

-identify chromosomal anomalies that may be associated with disease
-contribute to the diagnosis and treatment of patient
-individuals of all age groups
-many different disease
-survey the cellular genetic constitution of an individual with a single assay


Effects of chromosomal abnormalities

-change in phenotype
-fetal loss
-genetic disease


Fetal loss

-1 in 12 conceptuses with chromosomal abnormality but 6/1000 are live born
-15% of recognized pregancies end in spontaneous abortion -> 80% in the first trimester
-of the spontaneous losses, 60% are chromosomal
-of the chromosomal losses, 52% are autosomal trisomies



-0.6% of newborns have a chromosome anomaly
-features of a known chromosomal disorder (trisomy 13)
-ambiguous genitalia
-multiple congenital anomalies


Children and adult

-not all chromosome abnormalities manifest early in life
-features of a known chromosomal disorder
-family history of a chromosomal disorder
-mental retardation
-some malignancies


What are we looking for in cytogenetics

-a change in the total number of chromosomes
-a change in the size or shape of one or more chromosomes
-important to detect change and know exactly which chromosome is affected and where on that chromosome the defect lies


How Do we identify chromosomal abnormalities

-blood- easiest and less painful
-bone marrow- oncology studies
-tissue- often used when a karyotype is needed for a deceased individual
-amniotic fluid and chorionic villi are prenatal specimens


p arm

-the shorter of the two arms of chromosome


q arm

-the longer of the two arms of chromosome



-when the centromere is approximately equidistant from both ends



-when the centromere is closer to one end than the other



-chromosome has modified short arms with stalks containing only multiple copies of rRNA genes that are capped by a modified telomere termed a satellite


Banding Pattern

-chromosomes are stained with Giemsa or Wright's stain which are positively charged dyes that bind to the negative DNA
-mild trypsinization of the chromosomes prior to staining apparently weakens the DNA-protein interactions yielding a defined pattern of alternating light and dark regions after the stain is applied
-each pair of chromosomes has a unique band pattern that has been schematically represented in an ideogram


Chromosomal polymorphism

-the presence of two or more alternative structural forms for a chromosome within a population
-these are inherited as Mendelian characters and can be traced through pedigrees
-the variation is usually not associated with specific clinical anomalies or a particular disease