cell bio review 1,000,000 david pearce. 3 prophase i leptotene – chromatin begin to condense, 2...
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Cell Bio Review 1,000,000
David Pearce
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Prophase I
• Leptotene– chromatin begin to condense, 2 sister chromatids so close, cannot be
distinguished• Zygotene
– synapsis– synaptonemal complex
• Pachytene– bivalent – tetrad – Recombination
• Diplotene– Synaptonemal complex disappears– Bivalents begin to separate but held together at centromeres and ~ 50 points of
crossing over – chiasmata• Diakinesis – stage of max condensation
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Homologous chromosomes exchange segments during crossing over
• Crossing over occurs during Pachytene
During meiosis in males, the X and Y chromosomes form a bivalent and exchange information in the pseudoautosomal regions
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RNA Polymerase produces a complementary RNA to the template strand
• The template strand of DNA serves as a template for RNA formation during transcription– Sometimes called the Antisense strand
• The coding strand is not directly involved in transcription.– Sometimes called the Sense strand– The coding strand sequence is extremely similar to the single-stranded RNA
molecule • Coding strand has T• RNA sequence has U instead of T
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Prokaryotic Termination of Transcription
– Rho dependent• Rho factor, an ATP-
dependent unwinding factor binds to specific termination sequence and unwinds RNA from DNA template
– Rho independent• Formation of a GC-rich
hairpin loop forms and pulls RNA away from DNA
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Rifampin and Actinomycin:Two Antibiotics that Inhibit Transcription
• Rifampin specifically inhibits initiation of RNA synthesis in BACTERIA
• Interferes with formation of 1st few phosphodiester bonds in RNA chain– semisynthetic derivative of rifamycins, derived from
strain of Streptomyces.– Used to treat TB (tuberculosis)
• Actinomycin D binds tightly to dsDNA, prevents it from being effective template for RNA synthesis in both pro and eukaroytes – Polypeptide antibiotic from different strain of
Streptomyces, – Used infrequently in treatment of various malignant
neoplasms: Wilm’s tumour, sarcomas.– Adverse effects: bone marrow depression, GI toxicity; it
is extremely irritating, produces severe tissue damage (toxic reactions frequent and severe)
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Eukaryotic RNA Polymerases
• RNA Polymerase I, II, and III carry out transcription in the eukaryotic nucleus– RNA Polymerase I: Precursor for 28S, 18S and 5.8S rRNAs– RNA Polymerase II: Pre-mRNA, snRNA and microRNAs– RNA Polymerase III: Pre-tRNA, 5S rRNA
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Steps in RNA Pol II TF binding• TFIID binds to TATA box
• Can bind to DPE sequence in case of DPE-driven promoters
• RNA Polymerase II recruited to promoter with TFIIF attached
• Pol II Large Has a C terminal repeat on C-terminal domain• Phosphorylation State of CTD important for Active
• TFIIH has both helicase and protein kinase activity
• Pol II phosphorylated by TFIIH at CTD
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Ribosomal RNA• rRNA participates in protein
synthesis as part of ribosome (RNA + protein complexes)
• Prokaryotes: 70S ribosome– Small subunit = 30S
• Small subunit composed of 16S rRNA and 21 proteins
– Large subunit = 50S • Large subunit composed of 23S + 5S rRNA
subunits, and 32 proteins
• Eukaryotes: 80S ribosome – Small subunit = 40S
• Small subunit composed of 18S rRNA and ~30 proteins
– Large subunit = 60S• Large subunit composed of 28S, 5.8S and 5S
rRNA and ~ 50 proteins
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Splicing• Exons: sequences destined to appear in final mRNA• Introns: sequences within the primary transcript that do not appear
in mature, functional RNA• Splice sites 5’-GU with 3’ AG
– Splice Donor: GU at the 5’ boundary of the intron– Splice Acceptor: AG at the 3’ boundary of the intron– Branch Point: A
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