Cell Bio Review 1,000,000

David Pearce

3

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

4

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

6

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

7

8

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

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)

10

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

11

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

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

13

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