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STEP 1 USMLE Exam Answered

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TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE" WHAT BINDS TO THE NUCLEOSOME AND TO LINKER DNA TO STABILIZE CHROMATIN FIBER? - ANSWER H1 TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRU... CTURE PHOSPHATE GIVES DNA WHAT CHARGE? - ANSWER NEGATIVE TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHAT GIVES HISTONES A POSITIVE CHARGE? - ANSWER LYSINE AND ARGININE TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHEN DOES DNA CONDENSE TO FORM CHROSOMES? - ANSWER IN MITOSIS TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHEN DOES DNA AND HISTONE SYNTHESIS OCCUR? - ANSWER DURING S PHASE TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE DOES MITCHONDRIA HAVE THEIR OWN DNA? - ANSWER YES TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHAT IS THE STRUCTURE OF MITOCHRONDRIA? - ANSWER CIRCULAR AND DOES NOT UTILIZE HISTONES TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHAT IS HIGHLY CONDENSED WITH BARR BODIES (INACTIVE X CHROMOSOMES), TRANSCRIPTIONALLY INACTIVE, INCREASES METHYLATION AND DECREASES ACETYLATION? - ANSWER HETEROCHROMATIN TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHAT IS LESS CONDENSED TRANSCRIPTIONALLY ACTIVE. HINT: EU = TRULY TRANSCRIBED" / EXPRESSED - ANSWER EUCHROMATIN TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE WHAT CHANGES THE EXPRESSION OF A DNA SEGMENT WITHOUT CHANGING THE SEQUENCE, AND IS INVOLVED IN AGING, CARCINOGENESIS, GENOMIC IMPRINTING, TRANSPOSABLE ELEMENT REPRESSION, AND INACTIVATION OF THE X CHROMOSOME? HINT: MAKES DNA MUTE - ANSWER DNA METHYLATION Removal of histones positive charge, relaxed DNA coiling and increases transcription - ANSWER DNA Acetylation Removal of acetyl group, tightens DNA coiling and decreases transcription - ANSWER Histone deacetylation NucleoSide - ANSWER Base + sugar NucleoTide - ANSWER Base + phosphate PURines (A.G)- 2 rings - ANSWER PURe As Gold PYrimidines (C,U,T) - 1 ring - ANSWER CUT the PY Deamination reaction Cytosine - Uracil - ANSWER 2 H bonds. Increase C-G content, increases melting temperature of DNA. C-G bonds are like "crazy glue" Deamination of adenine makes - ANSWER hypoxanthine Deamination of guanine forms - ANSWER Xanthine Deamination of 5-methylcytosine - ANSWER Thymine Uracil found in? - ANSWER RNA Thymine found in? - ANSWER DNA methylation of uracil makes - ANSWER Thymine Amino acids necessary for purine synthesis? - ANSWER Cats "purr" until the GAG Glycine Aspartate Glutamine *Pyrimidine synthesis* Leflunomide does what? - ANSWER inhibits dihydroorotate dehydrogenase *Pyrimidine synthesis* 5-fluorouracil (5-FU) and its prodrug Capecitabine does what? (Drug) - ANSWER Forms 5-F-dUMP, which inhibits thymidylate synthase (decreases dTMP) *Purine synthesis* 6-mercaptopurine (6-MP) and prodrug Azathioprine does what? (Drug) - ANSWER inhibit de novo purine synthesis *Purine synthesis* Mycophenolate and Ribavirin does what? (Drug) - ANSWER inhibit inosine monophosphate dehydrogenase *Purine and pyrimidine synthesis* Hydroxyurea (drug) does what? - ANSWER Inhibits ribonucleotide reductase *Purine and pyrimidine synthesis* Methotrexate (MTX), trimethoprim (TMP), pyrimethamine (drugs) does what? - ANSWER inhibits dihydrofolate reductase ( decreases deoxythymidine monophosphate (dTMP) in humans, bacteria, and protozoa. CPS1 - ANSWER Mitochondria (urea cycle) CPS2 - ANSWER Cytosol (cyTWOsol) Required for degradation of adenosine and deoxyadenosine. Decreases ADA and increases dATP. - ANSWER Adenosine deaminase deficiency Decreases reductase activity - ANSWER Adenosine deaminase deficiency Causes lymphotoxicity - ANSWER Adenosine deaminase deficiency Major cause of SCID - ANSWER Adenosine deaminase deficiency SCID - ANSWER severe combined immunodeficiency disease Defective purine salvage due to absent HGPRT, which converts hypoxanthine to IMP and guanine to GMP. Results in excess uric acid production and de novo purine synthesis. - ANSWER Lesch-Nyhan syndrome Lesch-Nyhan syndrome: genetics? - ANSWER X-linked recessive. HGPRT gene: "H e's G ot P urine R ecovery T rouble" Lesch-Nyhan syndrome: findings? - ANSWER Retardation, self-mutilation, aggression, hyperuricemia, gout, choreoathetosis. *ORANGE "SAND" (sodium urate) in diaper***** Lesch-Nyhan syndrome: treatment? - ANSWER allopurinol or febuxostat (2nd line) What does HGPRT stand for? - ANSWER Hyperuricemia Gout Pissed off (aggression, self-mutilation) DysTonia What does unambiguous mean? - ANSWER Each codon specifies only 1 amino acid Degenerate/redundant - ANSWER Most amino acids are coded by multiple codons. Example of degenerate/ redundant genetic code? - ANSWER 'WOBBLE"- codons that differ in 3rd ("wobble") position may code for the same tRNA/ amino acid. Specific base pairing is usually required only in the first 2 nucleotide positions of mRNA codon. Degenerate/ redundant exceptions? - ANSWER Methionine (AUG) and tryptophan (UGG) encoded by only 1 codon Commaless, nonoverlapping - ANSWER read from a fixed starting point as a continuous sequence of bases Exceptions: some viruses Universal genetic codes - ANSWER Genetic code is conserved throughout evolution Exception in humans: Mitochondria DNA replication - ANSWER More complicated than prokaryotic replication and uses continuous and discontinuous (OKAZAKI fragments) synthesis. Occurs in a 5'-3' direction. replication fork - ANSWER A Y-shaped region on a replicating DNA molecule where new strands are growing. Helicase - ANSWER Unwinds DNA at replication fork *Deficient in Bloom syndrome (BLM gene mutation) Bloom Syndrome - ANSWER AR inheritance of Ch15 causing DNA fragility Feats: - short stature - telangiectatic erythema of the face (improves with age) - AbN facies incl microcephaly, prominent ears, malar hypoplasia, long arms, large hands/feet, acanthosis nigricans, Cafe au lait spots, syndactyly, polydactyly and clinodactyly - Normal IQ - primary hypogonadism Issue is INCREASED CANCER RISK, esp lymphoma, leukaemia, carcinomas and Wilm's tumour Deficient in BLM gene single stranded binding protein - ANSWER Prevents strands from reannealing Creates a single or double stranded break in the helix to add or remove supercoils - ANSWER DNA topoisomerase What inhibits topoisomerase (TOP 1) in eukaryotes? - ANSWER Irinotecan and topotecan What inhibits TOP II? - ANSWER Etoposide and teniposide What inhibits TOP II (DNA gyrase) and TOP IV in prokaryotes? - ANSWER Fluoroquinolones What makes RNA primer so DNA polymerase III can initiate replication - ANSWER Primase Prokaryotes only. Elongates leading strand by adding deoxynucleotides to the 3' end. Elongates lagging strand until it reaches primer of preceding fragment. - ANSWER DNA polymerase What has 5'-3' synthesis and proofreads with 3'-5' exonuclease - ANSWER DNA polymerase III True or false: Drugs blocking DNA replication have a modified 3' OH, preventing addition of the next nucleotide? - ANSWER True Prokaryotes only. Degrades RNA primer; replaces it with DNA - ANSWER DNA polymerase I same functions as DNA polymerase III, also excises RNA primer with 5'-3' exonuclease - ANSWER DNA polymerase I What catalyzes the formation of a phosphodiester bond within a strand of double-stranded DNA - ANSWER DNA ligase What joins Okazaki fragments? - ANSWER DNA ligase. "Ligase Links DNA" Telomerase - ANSWER Eukaryotes only. A reverse transcriptase (RNA dependent DNA polymerase) that adds DNA (TTAGGG) to 3' ends of chromosomes to avoid loss of genetic material with every duplication. What is dysregulated in cancer cells, allowing unlimited replication. - ANSWER Telomerase "Telomerase TAGs for Greatness and Glory Transition mutation - ANSWER purine to purine (A to G) or pyrimidine to pyrimidine (C to G) Transversion mutation - ANSWER A point mutation in which a pyrimidine is substituted for a purine (C to G), or vice versa (A to T). silent mutation - ANSWER Nucleotide substitution codes for same (synonymous) amino acid; often base in 3rd position of codon (tRNA wobble) Missense mutation - ANSWER A base-pair substitution that results in a codon that codes for a different amino acid. Sickle cell disease is an example of what kind of mutation? - ANSWER Missense mutation What nucleotide substitution results in early STOP codon. Usually results in nonfunctional protein. - ANSWER Nonsense mutation What are the STOP codons? - ANSWER UAA, UAG, UGA frameshift mutation - ANSWER mutation that shifts the "reading" frame of the genetic message by inserting or deleting a nucleotide. Deletion or insertion of a number of nucleotides not divisible by 3 What kind of mutations are Duchenne muscular dystrophy and Tay-Sachs disease? - ANSWER Frameshift mutation splice site mutation - ANSWER mutation at a point where coding and non-coding regions meet in a section of DNA. Retained intron in mRNA, protein with impaired or altered function. what are examples of splice site mutation? - ANSWER rare causes of cancers, dementia, epilepsy, some types of B-thalassemia, gaucher disease, marfan syndrome Preferred metabolic substrate of E. Coli? - ANSWER Glucose What happens when the glucose is absent and lactose is available? - ANSWER The lac operon is activated to switch to lactose metabolism. lac operon - ANSWER a gene system whose operator gene and three structural genes control lactose metabolism in E. coli What happens with the lac operon with low glucose? - ANSWER 1) increased adenylate, 2) increased cyclase activity, 3) increased generation of cAMP from ATP, 4) activation of catabolite activator protein (CAP) and 5) increased transcription What happens with lac operon with high lactose? - ANSWER 1) unbinds repressor protein from repressor/ operator site and 2) increases transcription DNA repair: single strand nucleotide excision repair - ANSWER Specific endonucleases release the oligonucleotides containing damaged bases; DAN polymerase and ligase fill and reseal the gap. What repairs bulky helix-distorting lesions? - ANSWER Nucleotide excision repair what phase does nucleotide excision repair take place? - ANSWER G1 phase what is defective in xeroderma pigmentosum ? - ANSWER nucleotide excision repair Xeroderma pigmentosum - ANSWER Mutated single strand nucleotide excision repair gene, which prevents repair of thymidine dimers.; Dry skin w/ melanoma and other cancers ("children of the night"). Also extreme light sensitivity base excision repair - ANSWER Base specific glycosylases remove altered base and creates AP site (apurinic/ apyrimidinic). One or more nucleotides are removed by AP endonucleases that cleaves the 5' end, AP-Lyase cleaves the 3' end, and DNA polymerase-B fills gap and ligase seals it. what is important in repair of spontaneous/ toxic deamination? - ANSWER Base excision repair What does GEL PLease stand for with base excision repair? - ANSWER Glycosylase Endonuclease Lyase Polymerase-B Ligase mismatch repair - ANSWER Mismatched nucleotides in newly synthesized [Show More]

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