Nucleus Flashcards
Nucleus
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Nuclear Staining Varies Greatly Between Cell Types:
Some Nuclei Stain Pale and Have Uncondensed DNA -
Some Nuclei Stain Dark and Have Condensed DNA-
Euchromatin (dispersed and transcriptionally active DNA)
Heterochromatin (condensed and transcriptionally inactive DNA)
Cells with abundant heterochromatin are transcribing few genes
Why Do Nuclei Vary So Much?
- Variations in nuclear function and framework
- Variations in gene transcription
- Complex of proteins + rRNAs that forms around the genes for ribosomal RNA
- Site of assembly of ribosomal subunits
Nucleolus
Cells with prominent or multiple nucleoli produce larger amounts or ribosomes
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DNA and Associated Proteins
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Chromatin
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DNA in Chromatin percentages
- About 1% of DNA in chromatin actually codes for proteins (20,000 genes)
- About 3% of DNA is “satellite” DNA (14-500 base sequences repeated 1000’s of times)
- About 45% of DNA is formed from “moderately repeated” sequences of several thousand bases (transposons)
Allows Specific Labeling of All Chromosomes
In Situ Hybridization of Chromosome-Specific Repeated Sequences
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- Performed on squashed cells that are halted in metaphase by microtubule-destabilizing drugs
- Used to assess chromosomal number, banding patterns, and possible chromosomal breakages
Karyotype Analysis
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Occupy discrete territories within the nucleus
Interphase Chromosomes
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In the Cell Cycle, 2 things can happen to chromosomal DNA:
Duplication and Separation
DNA Synthesis is Initiated When
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a Kinase (Cyclin Dependent Kinase) Phosphorylates Rb
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Cyclins and Cyclin-dependent Kinases
- 11 Cd kinases are known, but only 4 seem important for the cell cycle
- All 4 Cdk’s (Cdk1,2,4 & 6) can inhibit Rb and initiate DNA synthesis
- Cdk2 phosphorylates Lamins and causes the disappearance of the nuclear envelope during mitosis
- All Cdk’s must bind a protein, cyclin, to be active
Mitosis
- Prophase (condensation)
- Prometaphase (nuclear envelope breaks down)
- Metaphase (chromosomes line up & attach to spindle)
- Anaphase (chromosomes are pulled apart)
- Telophase
Prophase
- All chromosomes become greatly condensed
- Chromosome 22 shrinks in length from 1.5 centimeters to about 2 micrometers (10,000 fold decrease in length)
- Centrioles are duplicated and the mitotic spindle of microtubules is assembled
Metaphase
- Nuclear envelope breaks down, permitting attachment of microtubules to the kinetochore (a complex of proteins attached to each chromosome at the centromere)
- Chromosomes line up in the middle of the mitotic spindle
During Metaphase, ________ is released from sister chromatids except at the ________
cohesin; kinetochore