Exam 5 Flashcards
DNA Repair
Errors in DNA replication or damage to DNA create mutations
Replication fairly accurate; error 1/100,000,000 bp replicated
DNA damage all of the time, if replicated may cause cancer
When DNA damaged 3 possible outcomes:
1. apoptosis
2. repair
3. Cancer or passage of mutation
Types of DNA Repair
In many modern species, three types of DNA repair peruse the genetic material
A - DNA Polymerase
B - 50+ genes involved in repair
1. Photoreactivation (not in humans)
2. Excision
3. Mismatch
UV Damage
UV light exposure results in covalent bonds between pyrimidine, primarily thymine
Generates thymine dimers
Results in kinking of DNA and disruption of DNA replication resulting in errors
This change in width causes the DNA polymerase to skip the thymine dimers
Repaired by excision repair
Excision Repair
Pyrimidine dimers and surrounding bases are removed and replaced
Humans have two types of excision repair
Nucleotide Excision Repair (NER)
Replaces up to 30 bases
Corrects mutations due to carcinogen and UV light exposure
Base Excision Repair (BER)
replaces up 1-5 bases
Corrects oxidative damage
Mismatch Repair
Corrects errors due to DNA replication
An enzyme recognizes a change in the DNA width, so it gets replaced by the correct nucleotide
Just changes one base pair
Failure of DNA Repair
Some damage/errors cannot be repaired
Mutations in DNA repair genes lead to an increase in mutations and lasting DNA damage
If repair genes are damaged, a disorder can result (p53)
Most autosomal dominant
Homozygous individuals are often severely affected
Heterozygous individuals have increased sensitivity to environmental mutations
P53 is a tumor-suppressor gene and it helps protect the organism from Gene Damage, by sending cells to apoptosis
Repair Disorders: Inherited Colon Cancer
HNPCC (hereditary nonpolyposis colon cancer) or Lynch, affects 1/200, 7 genes, 3% of colon cancer, increased risk of other cancers as well
Polyps are things inside of the small intestine, and you don’t want them there, the only way they can be found is through a colonoscopy (~age 45)
Clinical: early onset colon cancer (30s or 40s), no polyps, affects proximal colon (can’t be seen in a colonoscopy) Inheritance: Autosomal dominant
Molecular: breakdown of mismatch repair
Repair Disorders: Xeroderma Pigmentosum
Clinical: Painful blistering of skin when exposed to sun, increased risk of skin cancer and other cancers
Inheritance: Autosomal Recessive
Molecular: Defect in nucleotide excision repair, thymine dimers persist and block replication
Cytogenetics
Cytogenetics is a subdiscipline within genetics
Deals with chromosome variations
Excess genetic material has a milder effect than a deficit
Most large chromosomal abnormalities disrupt or halt prenatal development resulting in miscarriage
Portrait of a Chromosome
Chromosome consists primarily of DNA and protein (some RNA as well)
Distinguished by size and shape
Heterochromatin: Darker, condensed, not used for transcription
Euchromatin: Lighter, relaxed, used for transcription
Essential Parts: Telomeres, replication origins, and centromere
Centromeres
The largest constriction of the chromosome and where spindle fibers attach
Karyotype
A chromosome chart
Displays chromosomes arranged by size and structure
Humans have 24 chromosome types
22 autosomal types
2 sex types: X and Y
Useful at several levels
1. Confirm a clinical diagnosis
2. Reveal effects of environmental toxins
3. Clarify evolutionary relationships
Centromere Positions
At tip: Telocentric
Close to end: Acrocentric chromosome (13, 14, 15, 21, and 22)
P arms are called satellite or stalk and they contain the DNA that codes for rRNA (multiple copies)
Off-center: Submetacentric
At midpoint: Metacentric (ex. X)
Visualizing Chromosomes
Tissues is obtained from:
Fetal tissue:
Amniocentesis - amniotic fluid surrounding baby in the womb
Chorionic villi sampling - cells from placenta
Fetal cell sorting - blood sample from mom and isolate fetal cells
Chromosome microarray analysis
Adult tissue:
White blood cells
Skinlike cells from cheek swab
Chromosomes are extracted
Then stained with a combination of dyes and DNA probes
High Risk Pregnancy
Increased risk of miscarriage, chromosomal abnormality, or genetic disease
Advanced maternal age (35 or older at time pregnancy)
An ultrasound can also identify this
Pedigrees help identify at risk individuals
Maternal Serum Screening
Test performed on mom’s blood
A screening test
Gives an estimation of risk for chromosome abnormalities for a prengancy
Recommended for all pregnancies
Performed at approximately 15 weeks
Fetal Cell Sorting or Noninvasive Prenatal Testing (NIPT)
Fetal Cells are distinguished from maternal cells by a flourescence-activated cell sorter
Identifies cell-surface markers (ones that are different between the fetal cells and mother cells)
A new technique detects fetal mRNA in the bloodstream of the mother
Offered to all high risk moms
Performed anytime after 10 weeks
Abnormalities should be confirmed by amnio with karyotype of microarray
Tests for the common whole chromosome abnormalities
Starting to look at deletions and duplications
Amniocentesis
Is invasive
Detects about 1,000 of the more than 5,000 known chromosomal and biochemical problems
Ultrasound is used to follow needle’s movement
Uses a syringe to extra amniotic fluid from inside of the placenta, around week 15 and 16 (the syringe passes directly through the abdomen past the placenta)
Note: Advanced maternal age is when the ratio of chromosomal abnormalities to regular births is 1:500 or higher
Chorionic Villi Sampling
Is invasive
Performed during 10-12 week of pregnancy
Provides earlier results than amniocentesis
However, it does not detect metabolic problems And has a greater risk of spontaneous abortion
Sticks a catheter through the vagina into the amniotic sac to extract fluid
Chromosomal Abnormalities
Can be abnormal in chromosome amount or structure
Abnormal chromosomes account for at least 50% of spontaneous abortions
Chromosome Abnoramlity Vocab Words
Polypoidy: Extra chromosome setss
Aneuloidy: An extra or missing chromosome
Monosomy: One chromosome absent
Trisomy: One chromosome extra
Deletion: Part of a chromosome missing
Duplication: Part of a chromosome present twice
Translocation: Two chromosomes join long arms or exchange parts
Inversion: Segment of chromosome revered
Isochromosome: A chromosome with identical arms
Ring Chromosome: A chromosome that forms a ring due to deletions in telomeres, which causes ends to adhere
Polyploidy
Cell with extra chromosome set is polyploid
Triploid (3N) cells have three sets of each chromsome
Produced in two ways:
1. Fertilization of one by two sperm
2. Fusion of haploid and diploid gametes
Triploids account for 17% for all spontaneous abortions and 3% of stillbirths and newborn deaths because it is incapable of life
Aneuploidy
A normal chromosomal number is euploid
Cells with extra or missing chromosomes are aneuploid
Most autosomal aneuploids are spontaneously aborted (Only 18, 21, and 13 are viable most times)
Nondisjunction
The failure of chromosomes to separate normally during meiosis
Produces gamete with an extra chromosome and another with one missing chromosome
Nondisjunction during Meiosis 1 results in copies of both homologs in one gamete
Nondisjunction during Meiosis 2 results in copies of both sister chromatids in one gamete
Could result in genomic imprinting problems
Trisomy
Most autosomal aneuploids cease developing as embryos or fetuses
Most frequently seen trisomies in newborns are those of chromosome 21, 18, and 13
Trisomy 21
Result of Nondisjunction in maternal meiosis 1, not inheritance
Doesn’t allow for transmission to future generations
Most common trisomy among newborns
Short stature, heart problems, slanted eye fissures, protruding tongues, single palm crease
80% of people with Down syndrome were born from a mother younger than 35
Trisomy 18
Edwards Syndrome
Nondisjunction in maternal meiosis 2
Most common
Small at birth, heart problems, intellectual disability, rocker bottom foot
Trisomy 13
Patau Syndrome
Rare
Small at birth, seizures, intellectual disability, cleft lip and pallets, hearing abnormalities
Turner Syndrome
Called the XO Syndrome
Lacks a copy of the second sex chromosome (Nondisjunction of paternal)
Has to be a women
99% of affected fetuses result in abortion
Short height, delayed puberty, don’t complete sexual development and often infertile, hearing problems, thyroid problems
Klinefelter Syndrome
Called the XXY syndrome
Male
Don’t fully reach sexual maturation and usually infertile, bigger than peers, often taller, described as quirky personality
Deletions
ex. Cri du Chat (cries like cat)
46, XX, -5p
Cat-like-cry, intellectual disability, often don’t develop the ability to speak, smaller than peers
Deletions are not inherited, they are spontaneous and can occur in either the sperm or egg
Larger deletions are more likely to have a phenotype than smaller deletions
Duplications
Refers to the presence of an extra genetic segment on a chromosome
Also spontaneous, and also more likely to have a phenotype if it is a larger duplication
Duplications in Chromosome 15
Can be type 1 (least duplicated material), type 2, or type 3(most duplicated material)
Type 1 and type 2 yield no symptoms while type 3 has symptoms
Symptoms include seizures, mental retardation, poor muscle tone, epicanthic folds, small size, developmental delay, scoliosis, learning disabilities, and autistic features like poor speech, hand flapping, lack of eye contact, and need for routine
Translocation
In a translocation, two nonhomologous chromosomes exchange segments
There are two major types:
- Robertsonian - involves acrocentric chromosomes (13, 14, 15, 21), two short arms are lost and two long arms fuse to make one chromosome resulting in 45 total chromosomes, produces unbalanced chromosome
Is a cause of Down syndrome - Reciprocal - pieces of two chromosomes break and rearrange (not acrocentric), can result in balanced or unbalanced
Can be balanced (all material there) or unbalanced (piece is lost resulting in not balanced)
Unbalanced more likely to result in phenotype
People with balanced translocation are at risk for their offspring to inherit unbalanced translocations
Inversion
An inversion is a chromosome segment that is flipped in orientation
Paracentric inversion - two cuts happen at a place on the chromosome that is not the centromere
Pericentric inversion - inversion occurs at the centromere, which moves the position of centromere
Isochromosome
Happens with metacentric chromosomes
Can result in turner’s syndrome
Can cut metacentric chromosomes at the wrong spot resulting in the deletion or duplication of chromosomes
Ring Chromosome
May arise when telomeres are lost and sticky chromosome ends fuse
Genes can be lost or disrupted causing symptoms, not likely for life
Genetics and Cancer
Cancer arises due to alterations in genes
It takes years to develop as it accumulates mutations
10% due to single gene inheritance
Inherited from parent
Affects every cell in body
Most are due to changes in the gene at the cellular level that increase susceptibility but still require environmental input
Cancer Causing Genes
Oncogenes:
Cancer causing gene, cause cancer when inappropriately activated, typically have a function in embryo but turned off afterwards in normal functioning, more than 100 oncogenes
Similar to autosomal dominant traits
Tumor suppressor genes:
Normal function is to prevent cancer, fix DNA, send cells to apoptosis, deletion or inactivation causes cancer, controls cell cycle checkpoints, more than 30 genes
Usually if one copy is inactive or deleted we can still function normally
In addition, changes in gene expression accompany cancer (epigenetics)
Cell Cycle Control
Cancer is due to a cell cycle disruption, where cancerous cells divide more often and quickly than the cell it originated from
Timing, rate, and number of cell divisions depend on:
Protein growth factors, signaling molecules from outside the cell, transcription factors
Cancerous cells arise often - but most of the time the immune system or cell cycle checkpoints will destroy them
Checkpoints control the cell cycle
Ensure that mitotic events occur in the correct sequence
DNA Damage Checkpoint in S phase, the Apoptosis Checkpoint between G2 and prophase, and Spindle assembly checkpoint during anaphase all play a role in cancer
Cancer Cells take on a stem cell style look
Telomeres and Telomerase
Loss of control of telomere length may also contribute to cancer
Telomerase: enzyme that adds telomere sequence to the end of the chromosomes
Normal, specialized cells have telomerase turned off to limit cell division
Cancer cells have to express telomerase to divide infinitely
Activation of telomerase is not enough to cause cancer
Somatic Mutations vs. Germline Mutations
Somatic Mutations:
Occur sporadically in nonsense cells
result from a single dominant mutation or two recessive mutations in the same gene
Cancer susceptibility not passed on to offspring
Germline Mutations:
Cancer susceptibility is passed on to offspring
Requires second somatic mutation
Rare but results in early onset of cancer
Characteristics of Cancer Cells
Divide continually and quicker and than normal cells
Contain heritable mutations - cancer cell divides to produce cancer cells
Transplantable
Dedifferentiated: lose their specialized identity
Have a different appearance
Express different cell surface proteins/antigens
Lack contact inhibition (continue to proliferate after taking up space)
Induce angiogenesis: formation of local blood vessels
Invasive: squeeze into any space available
Metastasize: move to new location in body
Origins of Cancer Cells
Cancer can begin at the cellular level in at least four ways:
- Activation of stem cells that produce cancer cells
- Dedifferentiation
- Increase in proportion of a tissue that consists of stem cells or progenitor cells
- Faulty tissue repair
Cancer from Stem Cells
Early stem cells can become cancerous while a totipotent stem cell or a progenitor cell
Cancer due to Dedifferentiation
A specialized cell become a progenitor cell, then to a cancer like cell
Cancer due to Shifting Balance of Cell Types in Tissues
In a normal tissue we have a small amount of stem cells naturally spaced far away (6% of tissue)
A mutation can occur and cause some of the cells to dedifferentiate back into stem cells, this can then become an abnormal growth or a tumor
Cancer Due to Uncontrolled Tissue Repair
Acute injuries cause a resting stem cell to become activated, so it can heal the minor injury
In chronic injuries there becomes a build up of activated stem cells causing cancer
Oncogenes
Proto-oncogenes are normal versions of genes that promote cell division
Expression at the wrong time or in the wrong cell type leads to cell division and cancer
Overexpression of a normal function:
Viruses integrated next to a proto-oncogene can cause transcription when the virus is transcribed
Moving a proto-oncogene next to a highly transcribed gene can lead to overexpression of the proto-oncogene
Fusion Proteins
Oncogenes are activated when a proto-oncogene moves next to another gene
The gene pair is transcribed together
The double gene product is a fusion protein
It activates or lifts control of cell division
Chronic Myelogenous Leukemia
A gene on chromosome 9 called abl and a gene on chromosome 22 called bcr are involved
9 and 22 swap genetic information and fuses the abl-cbr gene on chromosome 22 (tip of 9 on 22 called Philedelphia)
BCR-ABL oncoprotein is a tyrosine kinase that excessively stimulates cell division
Understanding cellular changes allowed development of a new drug called Gleevac
Before the development of medication, chemotherapy was used
the BCR-ABL oncoprotein usually binds to a substrate, atp fits into the BCR-ABL which has phosphorous bonded to it, then the phosphorous binds to tyrosine
The Gleevac prevents ATP from fitting into the BCR-ABL protein, which keeps the tyrosine normal
Acute Promyelocytic Leukeia
Transfusion between 15 and 17
Combination of retinoic acid cell surface receptor and an oncogene
Instead of chemotherapy, vitamin A has to be suggested
Her-2/neu
Product of an oncogene
Excessive levels in approximately 25% of breast cancer patients
Too many receptors on cell
Too many signals to divide
Monoclonal antibody drug, herceptin binds to the HER2 receptors which blocks division
Tumor Suppressor Genes
Cancer can be caused by loss of genes that inhibit cell division
Tumor suppressor genes normally stop a cell from dividing
Mutations of both copies of a tumor suppressor gene is usually required to allow cell division
Genes can also be lost by deletion or silenced by promoter hypermethylation
Retinoblastoma
Tumor in retina
A rare childhood cancer
The RB gene is on chromosome 13
The RB protein binds transcription factors so taht they cannot activate genes that carry out mitosis
Normally halts the cell cycle at G1
Study of RB was the origin of the “two-hit” hypothesis of cancer causation
Two-Hit Hypothesis
Two mutations or deletions are required
One in each copy of the RB gene
Sporadic Cases (non-inherited)
Retinoblastoma is a result of two somatic mutations
Familial Cases (inherited)
Individuals harbor one germline mutant allele for the RB gene in each of their cells
This is followed by a somatic mutation in the normal allele
p53 Gene
The p53 gene is the “guardian of the genome”
Li Fraumeni Syndrome
Every cell in the body has a mutation in the p53 gene
Cancer Diagnosis and Treatment
Most often, discovery of cancer follows a screening test
Oldest treatment is surgery which removes the tumor
Radiation and chemotherapy non-selectively destroy rapidly dividing cells
Other drugs help patients tolerate the side effects
New Types of Cancer Drugs:
Herceptin, gleevac, all are developed to hit specialized cancer, others are being made to prevent angiogenesis, micro RNAs are also being used
