Biochemistry Flashcards
What is the initiation codon for all proteins?
GUU valine
CAA glutamine
UAU tyrosine
AUG methionine
AUG methionine
AUG (methionine) is the universal initiation codon for protein synthesis in eukaryotes (ATG for DNA). Stop codons are UAA, UAG, and UGA (for RNA) which terminate protein synthesis. Protein synthesis begins with transcription of messenger RNA from DNA. The mature messenger RNA is then transported into the cytoplasm where the codon AUG signals for the initiation of protein synthesis. The initiation complex is formed via ribosomal subunits, tRNA (transfer RNA), mRNA (messenger RNA), initiation factors and GTP (for energy). The 80s (70s for prokaryotes) ribosomal subunit contains a P site (peptidyl) and an A site (acceptor). The transfer RNA that is bound to methionine (AUG) binds to the P site; and a second tRNA with the proper anticodon sequence and associated amino acid binds to the A site. The amino acids carried by the initial two tRNAs are then bound together via peptidyl transferase and elongation factors. The ribosome then advances three base pairs (one codon) and releases the first tRNA. The P site is now occupied by the second tRNA, which has vacated the A site. The tRNA that contains the appropriate anticodon that matches the codon of the mRNA will then bind to the A site along with its respective amino acid. The new amino acid will bind to the first two amino acids, creating a polypeptide chain. This sequence continues until a stop codon is encountered by the ribosome on the mRNA. At this point releasing factors and ATP causes dissociation of the ribosomal complex and the protein and mRNA are released. The mRNA is decoded from the 5’ end to the 3’ end. Proteins are synthesized from the N terminal to the C terminal. While it elongates, the amino acids of the polypeptide chain interact, causing folding /looping of the growing protein.
Which 3 of the following are considered types of mutations? (Select 3)
Silent
Point
Recombination
Frame-shift
Master
Blank
Silent
Point
Frame-shift
Recombination refers to a process in which genetic elements from two different genomes are combined together into one unit. A point mutation results from the change of a single base. Point mutations can lead to missense mutations, silent mutations, and nonsense mutations. Missense mutations are caused by the alteration of a single nucleotide, resulting in a codon that now codes for a different amino acid. Silent mutations occur when a single base is changed but the actual codon still codes for the same amino acid. For instance, ACC is altered to CGG both codons code for arginine. Nonsense mutations are the result of a changed nucleotide that leads to a stop codon. For example, CAG is altered to UAG. CAG codes for the amino acid lysine, but UAG is a stop codon, causing premature completion of a protein. Deletion or insertion of a single or multiple bases usually results in frame-shift mutations and may lead to a complete loss of ability to produce necessary proteins.
Which enzyme catalyzes the addition of nucleotide units during DNA replication?
DNA helicase
DNA topoisomerase
DNA polymerase
DNA primase
DNA polymerase
DNA polymerase (there are several) is responsible for the addition of nucleotides during DNA replication. Polymerase joins the new base to an old base via a phosphodiester bond elongating the strand from the 5’ end to the 3’ end. Polymerases are very accurate and proofread the new strand. If a base is incorrect, the enzyme will remove it.
Helicases and topoisomerases are responsible for unwinding the helix and separating the strands to expose the template for replication, resulting in a replication fork.
Primase adds the RNA primer that is necessary to initiate the replication sequence. Without a primer, replication cannot occur. After replication is complete the RNA primers are excised by exonucleases, and DNA polymerase I adds DNA to the now vacated spots. All gaps are filled in via DNA ligase resulting in the formation of two identical DNA strands.
Premature death of living cells and tissue that is caused by factors external to the cell or tissue is known as which of the following terms?
Autolysis
Phagocytosis
Apoptosis
Necrosis
Apoptosis and necrosis are two different methods of cell death. Apoptosis refers to the process of programmed cell death; necrosis refers to premature death of living cells and tissue. There are several characteristic features that help differentiate between apoptosis and necrosis, summarized below.
- Apoptosis typically involves a natural process of cell death, while necrosis is caused by factors external to the cell (such as infection, trauma, or toxins)
- Apoptosis is usually regarded as beneficial to the tissue, while necrosis is detrimental to the tissue
- Apoptosis can prevent tumor formation due to a balance between cell death rate and mitosis rate
- Necrosis results in inflammation of surrounding tissues, which can become chronic
Autolysis refers to the destruction of a cell by means of its own enzymes. It is also commonly known as self-digestion.
Phagocytosis is the process by which a cell engulfs a particle, forming an internal vesicle known as a phagosome.
Acute Neuronal Injury: The Role of Excitotoxic Programmed Cell Death Mechanisms.
The electron transport chain occurs at which location of a eukaryotic cell?
Mitochondrion
Cytosol
Nucleus
Ribosomes
Mitochondrion
The electron transport chain (ETC) and the Krebs cycle occur in the mitochondria. The Krebs cycle (TCA/citric acid cycle) takes place in the matrix of the mitochondrion. ETC occurs in the mitochondrial inner membrane. Glycolysis and gluconeogenesis occur in the cytosol of eukaryotic cells.
Which of the following is TRUE regarding type I diabetes?
Cells of the body do not respond to insulin
It is caused by defective insulin-producing cells
It is caused by defective insulin receptors
It is also known as adult-onset diabetes
It is caused by defective insulin-producing cell
Type I diabetes (proper term), juvenile onset diabetes or also called insulin-dependent diabetes is essentially a systemic condition caused by faulty beta insulin-producing cells of the pancreas. Due to the lack of insulin or smaller concentration of insulin produced, the body is incapable of regulating blood glucose, causing some tissues to receive too much glucose while insulin-dependent tissues do not receive enough. Type I diabetes is treated with insulin injections.
Type II diabetes (adult-onset, non-insulin-dependent diabetes mellitus) is caused by insulin resistance. Either the insulin cells are defective or the cells fail to use insulin appropriately resulting in elevated blood glucose levels. Type II is often manageable via diet and exercise or medication or both.
Fatty acid synthesis is activated during which of the following situations?
Increased levels of citrate, decreased levels of glucagon and insulin
Decreased levels of citrate, increased levels of glucagon and insulin
Increased levels of citrate and glucagon, decreased levels of insulin
Decreased levels citrate and insulin, increased levels of glucagon
Correct answer Increased levels of citrate and insulin, decreased levels of glucagon
Decreased levels of citrate and glucagon, increased levels of insulin
Increased levels of citrate and insulin, decreased levels of glucagon
Fatty acid synthesis is stimulated by insulin and inhibited by glucagon and epinephrine. The formation of fatty acids is catalyzed via the enzyme acetyl CoA reductase, which is activated by citrate. Acetyl CoA reductase is allosterically inhibited by palmitoyl-CoA. Fatty acid synthesis occurs in the cytosol and requires the use of ATP and NADPH, as this is a complex anabolic process.
A brief overview and important points of the pathway are as follows: Essentially, acetyl CoA combines with oxaloacetate (OAA) in the mitochondria to form citrate, which is then shuttled out into the cytosol by the citrate shuttle. Once in the cytosol, citrate is broken down again into its constituents, OAA and acetyl CoA. OAA is converted back into pyruvate to re-gain entry into the mitochondria. Acetyl CoA is converted to malonyl-CoA via acetyl CoA carboxylase (rate-limiting step and uses biotin as a cofactor). Malonyl CoA and the acyl carrier protein undergo several reactions to create fatty acid.
Which 3 of the following statements BEST represent the phenomena responsible for the dark appearance of the fovea during fluorescein angiography imaging? (Select 3)
Correct answer An increased density of xanthophyll in the foveal region
The retinal pigment epithelial cells in the foveal region are smaller and more densely packed
The absence of choroidal blood vessels in the center of the fovea
An increased density of carotenes in the foveal region
Correct answer The absence of retinal blood vessels in the center of the fovea
Correct answer The retinal pigment epithelial cells in the foveal region are larger and contain more melanin
n a normal fluorescein angiography image, the dark appearance of the fovea occurs as a result of three phenomena.
- The center of the fovea does not contain retinal blood vessels (foveal avascular zone)
- The choroid below the fovea does contain a network of blood vessels that fills with fluorescein in the early phase; however, there is blockage of this background choroidal fluorescence due to the increased density of xanthophyll in the foveal region
- Additionally, background choroidal fluorescence is blocked due to the retinal pigment epithelial (RPE) cells in the fovea, which contain more melanin and are larger than RPE cells elsewhere in the retina
Which of the following BEST describes the actions of the lacrimal system that occur when the eyes close during a blink?
Horizontal canaliculi are lengthened, puncta moves temporally, and lacrimal sac collapses
Correct answer Horizontal canaliculi are shortened, puncta moves medially, and lacrimal sac expands
Horizontal canaliculi are lengthened, puncta moves temporally, lacrimal sac expands
Horizontal canaliculi are shortened, puncta moves medially, and lacrimal sac collapses
As a person closes his eyes during a blink, the pre-tarsal orbicularis oculi compresses the vertical component of the canaliculi and shortens the horizontal canaliculi, which in turn causes the puncta to move medially. Simultaneously, the lacrimal portion of the orbicularis oculi also contracts, which results in expansion of the lacrimal sac. This action creates negative pressure, which draws the tears from the ocular surface through the canaliculi and into the sac.
Your net retinoscopy findings are : pl -1.50 x 180. Spherical refinement results in no change. However, after performing Jackson crossed cylinder your patient accepts an additional -1.00 D at axis 180. How should the spherical portion of the findings be adjusted?
Decreased by -0.25 D
Correct answer Increased by +0.50 D
Increased by +0.25 D
Decreased by -0.50 D
The spherical component should be increased by +0.50 D. For every 0.50 D change in astigmatism, the spherical component should be altered by 0.25 D in the opposite direction. Therefore, if the astigmatism is increased by -0.50 D then +0.25 D should be added to the spherical component. This ensures that the circle of least confusion remains on the retina.
A patient with against-the-rule astigmatism in the right eye will exhibit which of the following bowtie configurations on corneal topography imaging?
Vertical bowtie
Bowtie slanted right
Correct answer Horizontal bowtie
Bowtie slanted left
The orientation of the bowtie on corneal topography is aligned with the location of the steep keratometric meridian (90 degrees away from the axis). Therefore, in against-the-rule astigmatism (axis near 90 degrees), the bowtie will be oriented horizontally. With-the-rule astigmatism (axis near 180 degrees) will reveal a vertically aligned bowtie pattern on topography. Additionally, patients with oblique astigmatism will have the bowtie slanted either left or right. For example, if a patient reveals astigmatism with an axis of 45 degrees, topography will show a bowtie pattern that is slanted to the right by 45 degrees (aligned with the 135 degree meridian).
Which of the following types of ultraviolet (UV) light exposure possesses the GREATEST link with sunburn?
UV-C
All types of UV light have an equal affinity for causing sunburn
Correct answer UV-B
UV-A
UV-A is the longest of the three types of UV light, ranging between 315-380 nm. UV-A is considered the least damaging of the types of UV light and penetrates into the deeper layers of the skin. UV-A radiation is much more abundant than any of the other types of UV light that reaches the surface of the earth. UV-B light ranges between 280 and 315 nm, and over-exposure to this type of UV light can lead to sunburn. UV-B exposure has a higher correlation with skin cancer and cataract formation than UV-A. UV-C light ranges from 100 to 280 nm, and because it has a shorter wavelength, it is also very high-energy; therefore, prolonged exposure can lead to solar keratitis. The majority of UV-C light is absorbed by the ozone layer.
The cornea absorbs the majority of UV-C when the eye is exposed to this very short wavelength of light. The crystalline lens absorbs the majority of UV-A and UV-B, protecting the retina from potential UV damage. However, over time, the lens may become impaired and result in the formation of a cataract.
A pregnant woman presents with a complaint of abnormal swelling of the face and hands and sudden excessive weight gain. She is sent for urine testing, which is positive for protein. This patient is most likely experiencing which of the following complications?
Anemia
Placental abruption
Correct answer Pre-eclampsia
Premature rupture of fetal membranes
Eclampsia
- Pre-eclampsia is a combination of several classic factors that occur during pregnancy, including high blood pressure, fluid retention, and high levels of protein in the urine. This condition typically occurs in women after their 20th week of pregnancy and usually affects about 1 in 20 women. Some women may be asymptomatic; however, most patients experience symptoms of a reduction in the amount of urine produced, swelling of the face, hands, and feet, and/or sudden or excessive weight gain over a period of a couple of days. In addition, blood pressure is commonly elevated to levels above 140/90.
Pre-eclampsia is a dangerous condition, and if not properly treated, it can progress to eclampsia, which is a potentially fatal condition that involves seizures and comas. It is unclear as to the cause of pre-eclampsia, but certain women have been shown to posses a higher risk (first pregnancy, women over 40, African-American women, women who have already suffered from pre-eclampsia, and women who have a history of high blood pressure, diabetes, or kidney disease). Treatment typically involves bed rest, drinking a lot of water, and close monitoring of blood pressure. In some cases, inducing labor early may be necessary.
Next Question
End Practice
A patient with a visual acuity of 20/30 will likely display a high spatial frequency cut-off of how many cycles per degree (cpd)?
50 cpd
40 cpd
Correct answer 20 cpd
30 cpd
In order to convert from Snellen acuity to cycles per degree, simply divide 600 by the Snellen denominator. For the above example: 600/30 = 20 cycles per degree.
The anterior knee of von Willebrand is composed of optic nerve fibers carrying information from which of the following quadrants of the retina?
Inferior-temporal
Superior-temporal
Superior-nasal
Correct answer Inferior-nasal
The optic chiasm is formed by the union of the right and left optic nerves. It is in this area that the fibers originating from the nasal retina (temporal visual field) decussate to join the uncrossed temporal fibers (nasal visual field) and course posteriorly as the optic tracts.
The inferior nasal fibers remain low as they traverse the optic chiasm and decussate more anteriorly. Because of their location, these fibers are more vulnerable to damage from expanding pituitary lesions, leading to involvement of the superior temporal visual fields early in the disease. It is also important to note that these inferior-nasal fibers actually loop forward into the contralateral optic nerve before coursing posteriorly in the optic tract. This region is known as the anterior knee of von Willebrand.
The superior nasal fibers remain high as they traverse the optic chiasm and decussate more posteriorly. Therefore, these fibers are typically first involved in cases of lesions growing from above the optic chiasm (craniopharyngiomas), affecting the inferior temporal visual fields early in the disease progression.
