Genetics Exam 2

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Cytogenics: The study of the chromosomal basis of disease
Euploid: normal number of chromosomes, a multiple of 23 in humans
Aneuploid: missing or extra chromosomes
Which is more common, missing chromosomes or having extra?: generally do not survive if missing an entire chromosome, more likely to have extra copies
How are chromosomes differentiated from one another?: banding pattern, location of centromere (length of arms)
Metacentric: centromere is in the middle, arm lengths are equal
Acrocentric: centromere is near an end, one set of arms is much longer than the other
Submetacentric: centromere is closer to the middle than to the tip
What letter is used to describe the short arm on a chromosome?: p for petite
What letter is used to describe the long arm on a chromosome?: q (comes after p)
If a gene is on the long arm of chromosome 13, how would you describe it?: it is on 13q
Besides chromosome number and arm, how are genes described in regards to location?: Each band on a chromosome is numbered, and each band has regions.
Where is the gene 13q13?: Chromosome 13, long arm, band 1, region 3
How is a karyotype described?: The number of chromosomes and type of sex chromosomes.
How would you describe a normal female karyotype? A normal male?: -46XX
-46XY
Monosomy: When a chromosome is missing it homologous mate
Trisomy: When there is one extra of a particular chromosome
How does aneuploidy occur in autosomal cells?: There is a mistake in dividing chromosomes during meiosis I or II
(nondisjunction)
Nondisjunction: the failure of chromosomes to disjoin normally during meiosis. One gamete will get both copies of a chromosome, while the other will not get any.
Who is affected most by nondisjunction?: Older women, beginning around age 35
Trisomy 21: Down Syndrome
What is the male and female karyotypes of Down Syndrome?: male: 47XY+21
female: 47XX+21
What is the frequency of Down Syndrome?: 1/1000 live births
What is the general phenotype of Down Syndrome?: upslanted palpebral fissure, flat maxillary region, short neck, hypotonia, inc risk of leukemia, congenital heart defects, GI obstructions, varying degrees of MR, inc risk of Alzheimers Disease
Mosaicism: Not every cell is affected, some cells may be euploid.
Anticipitory Guidance: The types of tests you should run and things you should be looking for in a patient with a particular diagnosis.
Translocation: A piece of a chromosome breaks off and attaches to another chromosome.
Are all cases of Down Syndrome caused by nondisjunction?: No, about 5% are caused by translocation of a piece of a chromosome 21 attaching to chromosome 14
Trisomy 18: Edwards Syndrome
Male and female karyotypes of Edwards Syndrome: male: 47XY+18
female: 47XX+18
What is the frequency of Edwards Syndrome?: 1/6000 live births (goes up with maternal age)
What is phenotype of Edwards Syndrome?: Elfin features, SGA, clenched hands, with outside fingers overlapping middle, heart defects (Ventricular Septal Defect)
What is survival rate of Edwards Syndrome?: Usually lethal in first year of life if baby survives at all, due to heart defects.
Trisomy 13: Patau Syndrome
What is the frequency of Patau Syndrome: 1/10000 births
What is the common phenotype in Patau Syndrome?: Cleft palate, atrial septal defect, inguinal hernia,mental retardation
What is the survival rate of Patau Syndrome?: very rare to survive past one year
13,18,21 Test: test to look for trisomies in pregnant women of advanced maternal age
How does aneuploidy of sex chromosomes occur?: Nondisjunction of one of the sex chromosomes
What is the frequency of sex chromosome aneuploidy in males and females?: males 1/400
females 1/650
Why are consequences of sex chromosome aneuploidy generally less severe than autosomal aneuploidy?: x-inactivation
When is sex chromosome aneuploidy incompatible with life?: A complete absence of X chromosome material.
Monosomy of X chromosome: Turner Syndrome
(XO)
What is the karyotype of Turner Syndrome?: 45X
What is the frequency of Turner Syndrome: 1/2500 - 1/5000
(many are not diagnosed until late)
What is the typical phenotype of Turner Syndrome?: -not usually fatal
short in stature, webbed neck, no secondary sex characteristics, may have heart defects or mild MR, Usually not diagnosed until puberty
Why is Turner Syndrome variable?: Mosaicism
Can Turner Syndrome be treated?: Yes, with hormone replacements, if diagnosed early.
What if XXY? (xx egg with normal y sperm): Klinefelter Syndrome
What is the frequency of Klinefelter Syndrome?: 1/1000, but can be very mild
What is the phenotype of Klinefelter Syndrome?: tall, very long limbs, gynecomastia, small testes, infertility, some level of learning disability, not usually diagnosed early
What are some complications of Klinefelter?: osteoporosis
breast cancer
What occurs with other aneuploidies involving too many X's and one Y?: similar to klinefelter, but increasing amount of mental retardation as number of X's goes up.
What occurs with XXX?: generally mild, not usually any symptoms except for maybe dysmenorrhea.
What occurs with XYY?: Males with few problems, tend to be taller, minor behavioral problems, like ADHD
What is Fragile X Syndrome?: part of the X chromosome looks like it is falling off. Phenotype: Large head, elongated face, ears, prominent jaw, mild to severe MR, more prominent in Males than Females
What gene is found on the fragile X region?: Gene FMR1
What is the role of inheritance in fragile X?: more frequently passed from mother to son than from father to daughter, the frequency and severity increases over generations (anticipation)
What else is on the fragile X locus?: A Trinucleotide repeat of CGG (some form of repeat is on all X chromosomes at this location)
5-50 repeats of CGG: Normal
50-200 repeats of CGG: premutation
200-4000 repeats of CGG: affected
What is one theory on why the number of TNR's increases over generations?: The DNA polymerase makes more copies each time because as it is going along, the C and G on the same strand are attracted to eachother, and the strand begins to fold over, cause the DNA poly. to stutter.
What is one other theory?: C and G have strong Hydrogen bonds, so it is harder for DNA polymerase to break apart and go through.
T/F Repeat sequences of DNA behave the same way random sequences do.: FALSE
What is the general problem in metabolism disorders?: There is a mutation causing a problem of an enzyme, so metabolism cannot go correctly. If you intervene early, you may be able to correct with diet.
PKU: phenylketonuria
-cannot break down phenylalanine, so there is an excess build up. This can cause mild to severe MR.
How do you treat PKU?: Test newborns right away
put on severely restrictive diet immediately. (no breast milk). No high protein, no dairy. Can eat potatoes, most vegetables, "medical foods".
Does patient have to stay on diet for life?: No, some can go off diet in teens/adulthood and be fine. Some show loss of concentration. If pregnant, must go back on diet.
Why must a pregnant PKU patient go back on restrictive diet?: excess phenylalanine is a teratogen
Classic Galactosemia: cannot break down galactose
-failure to thrive, developmental delay, hepatic insufficiency, cataracts
What is the treatment for classic galactosemia?: Avoidance of dairy products
What is the usual function of Lysosomes: enzymes degrade molecules inside lysosomes so that the enzymes do not destroy the cell
Features of lysosomal disorders: molecules are not broken down correctly. Leads to cardiovascular, joint, hearing, and visual problems.
2 examples of lysosomal disorders: Hurler Syndrome
MCAD deficiency
Hurler Syndrome, clinical features: Coarse facial features, crouched stance, thickened digits, protrubed abd, along with other defects assoc. with lysosomal disorders.
Hurler Syndrome, what problem, can it be treated?: mutation of alpha-1 iduronidase.
Can be treated by administering this enzyme.
MCAD Deficiency: Medium Chain Acyl Coenzyme A Deficiency. First seen in children after period of fasting, which is generally when they are sick
What happens with MCAD?: build up of fatty acids, causing damage to major organs, leading to cerebral edema and liver/kidney problems. Can be fatal if not dx and tx during first 2 years of life.
What is MCAD death often mistaken for?: SIDS
At what stage (how many cells) does separation of embryonic cells not produce identical twins? Why?: Around 16 cell stage. Compaction begins, all of the cells are touching (gap jcns), "conversation" b/n the cells
What is the end result of compaction?: There is a tight ball of cells, 1 or 2 in the middle form the inner cell mass, while the cells on the surface give rise to the trophoblast.
What does the ICM give rise to?: embryo and amnion
What does the trophoblast give rise to?: fetal placenta and syncytium which implants into endometrium
What happens if only paternal chromosomes are present?: only trophoblast will form
What happens of only maternal chromosomes are present?: only ICM will form.
What two types of tissues does the trophoblasts differentiate into?: Syncytium - multiple nuclei, lots of cytoplasm (no telophase occurs)
Chorion Membrane - cellular portion of fetal placenta
cytotrophoblasts: normal trophoblast cell, what forms chorion membrane
Chorionic villi: Finger like projections in the chorion that form as pregnancy proceeds
epiblast: single layer/plate of tissue formed by the ICM, it gives rise to all the tissues in the body and the amnion
What do the cells at the edge of the trophoblast plate begin doing?: Dividing upward instead of horizontally, forming a dome over the flat sheet, forming amnion.
Gastrolation: Epiblast forms 3 layers total (besides amnion), that give rise to all tissues in body (germ layers)
What are the three germ layers, from amnion inward?: ectoderm
mesoderm
endoderm
What tissues does the ectoderm give rise to?: skin, hair, nerves
What tissues does the mesoderm give rise to?: it splits into 2 linings and forms the muscles, bone, and cardiovascular system
What does the endoderm give rise to?: GI tract
when is an embryo considered a fetus?: around 8 weeks
notocord: a specific part of the mesoderm that runs down the middle and induces other cells to become certain tissues.
What happens if you take out the notocord?: development stops
neurectoderm: special layer of ectoderm that is influenced by notocord to become brain and spinal cord (aka neural tube)
Transcription Factors: Tell which genes get transcribed, and which don't
Signalling molecules: bind to other cells, telling them which part of their genome to express, which not.
AER: Apical Ectodermal Ridge- a specific area of ectoderm on distal part of limb bud that is responsible for outgrowth of limb
What happens if you remove AER?: outgrowth of limb stops
ZPA: Zone of Polarizing Activity- another part of ectoderm (on posterior side) that is responsible for limb polarization, the pattern of the digits.
What happens if you take out ZPA?: AER goes away and limb development stops
What happens if you add a second ZPA on anterior side?: you get 2 sets of digits in a mirror image to eachother.
Diffusable Morphogen: Signal that influences the cells around it to form certain tissues depending on how far it is from signal.
Retinoic Acid (RA): a known teratogen, used in expiraments to determine signalling molecule of ZPA
Is RA the morphogen used in limb formation?: No, but it mimics its effects
What happens if you cut off a limb of a salamander?: a cap of ectoderm will form over the stub, and the tissue under the cap dedifferentiates into a blastema, which will eventually reform limb from the spot it was cut off.
does limb regrowth occur in humans?: not in adults, but can occur as an embryo.
What happens if you put RA on a blastema?: the whole limb will regrow at that point from pectoral girdle out, no matter where amputation occured.
T/F. Secreted ligands are secreted from a cell and act extracellularly on receptor of target cell, which then sends signals into target cell to activate Transcription Factors: True
What is a Transcription Factor?: An intracellular protein that associates with DNA and modulates the transcription of new genes
What are some examples of secreted ligand genes?: Fibroblast Growth Factor (FGF)
Hedgehog Family, e.g. sonic headgehog (shh)
What happens if disruption in secreted ligand?: birth defects, can be fatal to embryo, one example is holoprosencephaly, a mutation in Shh.
What is an example of a transcription factor?: Hox (homeotic) family of genes.
What is the homoedomain?: region of 60 a.a's on each Hox gene that codes for DNA binding proteins wich bind to DNA at special areas and turn on or off genes.
In what organism is Hox gene mutations seen very easily? Why?: Drosophila, which have one chromosome with several Hox genes that go in order from head to tail development
What do hox genes do in adults?: Maintains stem cells
Where has stem cell research made the most progress?: bone marrow stem cells, which are very plastic, they have been able to make more hemopoetic cells (blood cells)
What is the theory on why the female donated hearts, when transplanted into males, begin to show male chromosomes?: When cardiac muscle is injured, it may send out factors that recruit other stem cells to come and differentiate into myocytes.
What is the gene mutation model of cancer?: There is some genetic mutation that leads to bad cell cycle control, something goes wrong with cell cycle check points
hyperplasia: increased cell growth with no obvious changes
dysplasia: increased cell growth will obvious cell changes (nucleus, membrane, etc)
metastasis/malignancy: when abnormal cell growth spreads and has invasiveness
Monoclonal: derived from one cell that goes wrong ... all cancers are said to be monoclonal
How are cancers classified?: What type of tissue it started in.
Carcinoma: tumor in epithelium
sarcoma: tumor of connective tissue/muscle
lymphoma: tumor of lymph tissue
Cancer cells lack contact _____________ , which is present in normal cells. What is result?: INHIBITION, it does not grow in sheets, but in a ball or mass
Angiogenesis: tissue emits growth factors that encourage blood vessel growth into tumor, so that cells in the middle of tumor will not die.
CDK: cyclin dependent kinase, it will bind with cyclin when available
MPF: Mitosis Promoting Factor, what is formed when CDK binds with cyclin, the cell will go into M phase when it is formed
protooncogene: normal genes found for normal growth factors
oncogene: mutated protooncogene. Growth Factor is constantly being produced, and cancer occurs
Tumor Supressor: another family of factors that stop cells from growing if there are chromosomal abnormalities. If repair is possible, cell arrests until repair is made, if not, TSF will instruct cell into apoptosis
p53: widely studied TSF, if mutated, suppression will not happen, cancer will occur. 70% of cancers have mutated p53
Other things that are seen in cancer cells . . .: often more than one mutation is seen for full fledged cancer
-aneuploid cells
Senesence: cell aging, normal cells go through cell cycle for a certain amount of time, aging, and then natural cell death
protected chromosomes: at the end of each chromosome is a telomere, which is a repeating sequence of DNA, telomere shortens with each cell division
What happens when the telomere is so short that it cannot fold over?: it looks like a broken chromosome, and TSF is activated, stopping division.
telomerase: enzyme that rebuilds telomere, keeping it long, so that cell never enters senesence.
What cells have telomerase?: Cancer cells, placental cells, inner mast cells.

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