Nucleus Flashcards
Nucleus
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
DNA and Associated Proteins
Chromatin
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
- 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
Occupy discrete territories within the nucleus
Interphase Chromosomes
In the Cell Cycle, 2 things can happen to chromosomal DNA:
Duplication and Separation
DNA Synthesis is Initiated When
a Kinase (Cyclin Dependent Kinase) Phosphorylates Rb
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
Anatomy of Kinetochore
Anaphase
- The so-called Anaphase Promoting Complex (APC), attached to the kinetochore, degrades cohesin
- Chromatids move apart
- APC also degrades cyclins, so that cyclin-dependent kinases stop functioning and mitosis completes itself
As chromosomes move apart along kinetochore microtubules, so called _______ repel each other and push the two spindles apart
polar microtubules
Mitosis vs. Meiosis
Unique features of 1st Meiotic Division:
- Pairing of homologous chromosomes
- Crossing over during 1st divison
- 2nd Division
- Haploid cells
How do homologous chromosomes pair so precisely?
Chromosome tips attach to nuclear envelope during prophase 1 so that homologous pairs are aligned perfectly
How do homologous chromosomes “recognize” each other?
- Uncharacterized proteins prevent improper pairing?
- Or Epigenetic Marks?
What holds homologous chromosomes together?
Formation of Synaptonemal Complex (SCP1 protein)
Proteins of Synaptonemal Complex
- Synaptonemal Complex Protein 1 Binds adjacent DNA strands to each other
- RAD51 recombinase protein causes cutting and splicing of DNA during Crossing Over
- (Also found in lymphoid organs)
Protein that is localized between paired chromosomes
SCP1
What drives the switch from mitosis to meiosis?
Retinoic acid appears to be a key player in this process
Gene Transcription Varies Between Cell Types
Control of Gene Transcription
- Each gene is marked by a specific sequence called the TATA box
- The TATA box binds proteins needed for DNA transcription
- All of these TATA binding proteins are only weakly active
- To increase their activity, a Transcription Regulating Factor is required
- 2000 different TRF’s are known; each cell produces only its own subset, leading to a unique pattern of DNA transcription for each cell
Control Nanog gene to maintain pluripotency of stem cells
OCT4 & SOX2 Transcription Regulating Factors
Transcription Factors Drive Cell Differentiation
X-Chromosome Inactivation
- In each female cell, one X-chromosome is randomly inactivated and condensed
- In many cells, the X-chromosome becomes attached to the nucleolus
In some cells (white blood cells) the X-chromosome appears as a dark-staining Barr Body within the nucleus
Lamin Proteins
Lamin Proteins
- Intranuclear varieties of intermediate filaments
- Form the so-called nuclear lamina
- Lamin A, B, and C subtypes are known
- Interact with DNA, Transcription Regulating Factors (Rb protein), and the inner nuclear envelope to regulate nuclear shape and function
Inherited abnormalities in Lamin A protein cause a syndrome, _______, with symptoms of ________
Progeria; accelerated aging
Symptoms include: weak bones, facial abnormalities, and distorted nuclear shapes;
Disease is very rare
Abnormal Lamin A Protein Becomes Inapproperiately ________
Farnesylated
Abnormally farneslated Lamin-A interacts abnormally with a major depressor of transcription called the ___________
retinoblastoma (Rb) Protein
Drugs that block the farnesylation of lamin-A relieve many of the symptoms of progeria in mice
A Typical Cell Nucleus Possesses _________Nuclear Pores
3000-4000
Pores are composed of
Nucleoporin proteins
Nucleoporin Proteins
- 456 individual protein molecules are found in each pore; there are about 30 varieties of nucleoporins
- Function to bind and import proteins into the nucleus. Examples:
- DNA, RNA polymerases
- Transcription factors, histones
- Nuclear proteins contain a sequence of basic amino acids termed a Nuclear Localization Sequence
Nucleoporin Proteins Function:
- Function to transport out of the nucleus proteins that bind to mRNA molecules
- Function to export ribosomal subunits, which are assembled within the nucleus
