STEP 1 USMLE Exam Answered

TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE" WHAT BINDS TO THE NUCLEOSOME AND TO LINKER DNA TO STABILIZE CHROMATIN FIBER? - ANSWER H1 TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE PHOSPHATE G ...

TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE" WHAT BINDS TO THE NUCLEOSOME AND TO LINKER DNA TO STABILIZE CHROMATIN FIBER? - ANSWER H1 TOPIC: BIOCHEMISTRY - MOLECULAR / "CHROMATIN STRUCTURE 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: