USMLE Biochemistry Videos & Books

2022-06-13 20:24:01 Educational objective: DNA replication occurs in the 5'--+3' direction on both strands. In contrast to the continuous synthesis of the leading strand, lagging strand synthesis occurs discontinuously and is composed of short stretches of RNA primer plus newly synthesized DNA segments (Okazaki fragments). As a result, lagging strand synthesis requires the repetitive action of DNA primase and DNA ligase. ओपन / कमेंट

2022-06-13 08:24:01 ओपन / कमेंट

2022-06-13 08:23:01 A 65-year-old woman with chronic obstructive pulmonary disease and type II diabetes mellitus comes to the emergency department due to profound fevers and malaise. After initial evaluation, she is hospitalized for septicemia. Blood cultures plated on lactose-containing media grow rapidly dividing gram-negative bacteria. Replication of these microbial cells requires synthesis of two daughter strands of DNA using the parent strands as templates. ओपन / कमेंट

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2022-06-12 20:23:01 Educational objective: Nucleosomes are composed of DNA wrapped around a core of 8 histone proteins (2 molecules each of H2A, H2B, H3, and H4). H1 histone is located outside of this histone core and helps package nucleosomes into more compact structures by binding and linking the DNA between adjacent nucleosomes. ओपन / कमेंट

2022-06-12 08:22:01 ओपन / कमेंट

2022-06-12 08:21:01 Electron microscopy of a eukaryotic cell during interphase of the cell cycle shows 10-nm thick chromatin fibers with a "beads on a string" appearance. These chromatin fibers are extracted and treated with an endonuclease, which preferentially cleaves the "string" portions of the chromatin. Further evaluation of the "beads" reveals that they are composed of DNA wrapped around a core of proteins. ओपन / कमेंट

2022-06-11 20:21:01 Educational objective: Alternative splicing is a process by which a single gene can code for various unique proteins by selectively including or excluding different DNA coding regions (exons) into mature mRNA. ओपन / कमेंट

2022-06-11 08:21:01 ओपन / कमेंट

2022-06-11 08:20:12 A researcher is studying the Fas receptor (FasR), a protein widely expressed on cell surfaces. The signaling cascade of programmed cell death is initiated when FasR binds to its ligand (Fasl), which is expressed on cytotoxic T cells. In an experiment, cancer cells that escaped elimination by the immune system were found to contain soluble Fas proteins that did not promote apoptosis. The soluble Fas proteins were shorter and lacked the transmembrane domain. DNA analysis of these cells revealed no FAS gene mutations. ओपन / कमेंट