USMLE Biochemistry Videos & Books

2022-06-17 08:29:01 A research scientist purifies DNA polymerase Ill from Escherichia coli extract. When the enzyme is incubated in a mixture containing DNA templates, RNA primer oligonucleotides, and tagged deoxynucleotides, she finds that the enzyme possesses 3' to 5' exonuclease activity. ओपन / कमेंट

2022-06-16 20:29:01 Educational objective: Deletion or addition of a number of bases that is not divisible by 3 in the coding region of a gene will cause a frameshift mutation. Frameshift mutations alter the reading frame of the genetic code, resulting in the formation of nonfunctional proteins. ओपन / कमेंट

2022-06-16 08:28:01 ओपन / कमेंट

2022-06-16 08:27:02 A 3-year-old boy is being evaluated for recurrent respiratory infections. The patient's family immigrated to the United States 5 months after his birth. Since then, the boy has experienced multiple episodes of pneumonia and bronchitis, and has developed a persistent cough and failure to thrive. His older brother has no medical issues. A genetic test is performed and reveals a mutation in an exon of a gene that codes for a transmembrane chloride channel. The abnormal mRNA is isolated from cultured epithelial cells, and its complementary DNA is synthesized. Amplified cDNA samples from both the patient and his healthy sibling are analyzed using gel electrophoresis and compared to DNA fragments of known size to determine base pair length. The results are shown below. ओपन / कमेंट

2022-06-15 20:27:01 Educational objective: The TATA box is a promoter region that binds transcription factors and RNA polymerase II during the initiation of transcription. It is located approximately 25 bases upstream from the beginning of the coding region. ओपन / कमेंट

2022-06-15 08:27:01 ओपन / कमेंट

2022-06-15 08:26:01 ओपन / कमेंट

2022-06-14 20:26:01 Educational objective: Telomeres help maintain chromosomal integrity and are preserved in rapidly dividing cell lines (eg, epithelial cells, lymphocytes, hematopoietic stem cells) by telomerase. Disorders involving telomerase function (eg, dyskeratosis congenita) result in premature death of cells with high turnover, characteristically causing mucocutaneous changes (eg, oral leukoplakia, dystrophic nails), bone marrow failure, and pulmonary fibrosis. ओपन / कमेंट

2022-06-14 08:25:17 ओपन / कमेंट

2022-06-14 08:25:16 An 18-year-old man comes to the office due to abnormal skin pigmentation and premature hair graying. His nails are also thin and break easily, and he has white patches on his tongue. The patient's father had similar features at a young age and died of progressive pulmonary fibrosis. Physical examination shows diffuse gray hair, areas of reticular hyperpigmentation on the torso, dystrophic nails, and oral leukoplakia. Genetic testing reveals a loss-of-function mutation affecting the telomerase reverse transcriptase gene. ओपन / कमेंट