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Flashcards in The Nucleus Deck (91):

Function of the nucleus

1. Replicate dna
2. Synthesized and produce all types of rna
3. During interphase pore complexes in nuclear membrane regulate Macromolecular transfer between nucleus and cytoplasm


Why is protein synthesis restricted to the Cytoplasm

helps ensure that newly made rna molecules do not become involved in translation before processing is complete


Component of nucleus

1. Nuclear envelope
2. Chromatin
3. Nucleoli



Mass of DNA and its associated Proteins



Specialised regions of Chromatin


Characteristics of nuclear envelope

1. Selectively permeable membrane
2. 2 concentric membranes adulterated by perinuclear space and outer membrane Continuous with RER
4. Nuclear lamina associated inner membrane
5. Nuclear pore complex


Perinuclear space

Space between outer and inner nuclear membrane 30-50nm. Space is continuous with cytoplasmic network of RER


Nuclear lamina

Highly organized mesh work of proteins associated with the inner nuclear membrane.


Function of nuclear lamina

Stabilizes the nuclear envelope


Component of the nuclear lamina

Components are a class of intermediate filament protein called lamina. That bind to membrane proteins and associated with Chromatin in non dividing cells


Structure of nuclear protein

1. Bridge the inner and outer nuclear membrane
2. Made of Core proteins called nulceoporins
3. Display 8-fold symmetry around lumen


Function of pore complex

Regulate movement of Macromolecules.
Growing cells contain, 3000-4000 such channels providing passage for upto 1000 molecules per second


Macromolecules moved out of the nucleus

1. Ribosomal subunits
2. RNA associated with proteins


Macromolecules transferred into the nucleus

1. Ribosomal protein
2. Transcription factors
3. Chromatin protein
4. Enzymes


Mechanism for transportation of Macromolecules into and out of the nucleus

1.Protein complexes transported to cytoplasm have specific nuclear export sequences
2. Proteins to be imported have nuclear localization sequences
Both bind to specific transport proteins that interact with proteins of pore complex for transfer across across nuclear envelope. Energy derived from GTP with specific GTPase help provide directionality to the transfer.


Types of transport protein

Important, exportins, etc


Cells Chromatin divided into___ no. Of chromosomes

46 (23pairs)


After DNA replication but before cell division chromosomes contain identical Chromatin units called

Chromatids held together by complexes of cohesin protein at the centromere


Length of the DNA

2m long with 3.2 billion base pairs


What is a histone

Small basic protein


What is a histone

Small basic protein



It is a structural unit made of a core of 8 histone around which is wrapped about 150bp of DNA.
Gives a "beads on a string" appearance with 50-80bp of linker DNA separating each Nucleosome


H1 histone

Each Nucleosome has a large histone H1 associated with both the wrapped DNA and the surface of the core.


Why are Nucleosomes structurally dynamic

Because they can undergo modifications and rearrangement of the histones to allow temporary unwrapping of DNA and arrival of enzymes and other protein required for replication and gene Transcription.


Stages of DNA wrapping

1. 2m DNA wrap around histones forming Nucleosomes diameter of 11nm
2. Nucleosome undergo helical folding yield fiber of diameter 30nm
3. formation of transcriptionally active
DNA (euchromatin) into loops that are tethered to a central scaffold of proteins that include the
4. Formation of chromosomes


Function of condensins

Part of Central scaffold and promote compaction of Chromatin


two types of Chromatin

1. Heterochromatin
2. Euchromatin


appearance of Heterochromatin under an EM and light microscope

coarse, electron-dense material in the electron microscope and as intensely basophilic clumps in the light microscope.


appearance of Euchromatin under light and electron microscope

visible as finely dispersed granular material in the electron microscope and as lightly stained basophilic areas in the light microscope.


difference between Heterochromatin and Euchromatin

Heterochromatin Euchromatin
1.Tightly coiled 1.loosely packed
2. Darkly stained 2. Lightly stained
3. Little or no 3.Transcriptional
Transcription active


function of the nucleus

1.Cellular Regulations: houses genetic material, directs cell activities, regulates cell structure
2.production : produces Ribosomal subunits in the Nucleolus


The inner and outer nuclear membranes are bridged at

nuclear pore complexes


2 types of Heterochromatin

Constitutive Heterochromatin
Facultative Heterochromatin


Constitutive Heterochromatin

Contains mainly repetitive gene- poor DNA sequence


Chromosomal regions of Constitutive Heterochromatin

Centromere abs telomeres located near the middle and ends of the chromosome respectively


Facultative Heterochromatin

Contains regions of DNA with genes where Transcription is variable inactive by epigenetic mechanism.
Can undergo reversible transition from compact transcriptionally silent state to more open transcriptionally active confirmations


Indicator of cells metabolic and biosynthetic activity

Ratio of Heterochromatin to Euchromatin-Euchromatin predominates in active cells ex. Large neuron, while Heterochromatin is abundant in cells with little synthetic activity ex. Lymphocytes


Barr body

One of the 2 X chromosomes present in human females but not males. Made of Facultative Heterochromatin it remains tightly coiled while the other X chromosome is uncoiled.


X chromosome is males

Remains largely Euchromatic


Chromosomal territories

When sequences on each individual chromosomes are labeled differently with florescent probes, each structure occupies a discrete Chromosomal territory


Why are memers on each Chromosomal pair called homologous

Although they come from different parents they contain alleles of the same gene


How is microscopic analysis of chromosomes done

Begins with cultured cells arrested in mitotic metaphase by colchicine or other compounds that disrupt microtubules. After processing and staining the cell the condensed chromosomes are photographed by a light microscopy and rearranged digitally to produce a karyotype in such Stained chromosomes can be analyzed


Medical application of Barr bodies

1.Permit gender to be determined microscopically in patients whose external sex Organs do not permit determination ex. In hermaphroditism and pseudohermaphroditism
2. Reveals other anomalies involving the sex chromosome ex. Klinefelters syndrome


Medical application of karyotyping

1. Prenatal diagnosis - detect certain genetic anomalies
2. In adults - missing or extra chromosomes and Chromosomal deletions or translocations easily seen


Characteristic of a Nucleolus

1. Spherical
2. Highly basophilic
3. In nuclei of cells actively engaged in protein synthesis
4. rRna production and ribosome assembly
5. Contains fibrillar and granular subregions


Intense basophilia of nucleoli is due to

Densely concenrated rRNA that is transcribed processed and assembled into Ribosomal subunits


What is neoplastic profileration

Cells become transformed to grow at a higher rate and in an uncoordinated manner


Cause of neoplastic profileration

Damage to the DNA of proto-oncogenes and failure of cells to be eliminated.



Mutation in the gene coding for particular lamin. Nuclear envelope is abnormal.


Progeria disorder

Mutation in the gene coding for lamin A. Causes Premature aging


Cell cycle

Regular sequence of events that produce new cells


Phases of cell cycle

1. Mitosis
2. G1
3. S
4. G2


G1 phase

1.Period between mitosis and beginning of DNA replication. Longest and most variables
2. Active RNA and protein synthesis
3. Cells volume, reduced by half during mitosis, return to its pervious size


S phase

1.Period of DNA synthesis- DNA replication
2. Histone synthesis
3. Centrosome duplication


G2 phase

1. Gap between DNA Duplication and the next mitosis
2. Proteins required accumulated


G0 phase

New postmitotic cells specialize and differentiate, cell activities may be temporarily or permanently suspended.
Some differentiated cells like liver cells renew cycling under certain conditions, while others like most muscle and nerve cells are terminally differentiated.


How is cycling activated in the postmitotic G0 cells

Activated by protein signals from the extracellular environment called mitogens or growth factors which attach to cell surface receptors and trigger a cascade of kinase signaling in the cells. Then maintained at the restriction point at G1/S boundary until sufficient nutrients and enzymes required for DNA synthesis have accumulated, once all is ready S phase begins


Checkpoints of cell cycle

Entry and progression through other phases of the cycle are monitored at these points, where certain condition must be met before the cell continues cycling.


Cell cycling is regulated by

A family is cytoplasmic proteins called cyclin


How do Cyclin proteins regulate cells cycle

Different Cyclin for different cell cycle phase each activate one or more specific Cyclin-dependent kinase (CDKs). Activated CDKs then phosphorylates specific proteins including enzymes, Transcription factors for specific genes and cytoskeletal subunits, triggering the activities that characterize the next phase of the cycle. Once each successive set of activities is completed the Cyclin controlling that phase is removed by proteasomes and a new Cyclin that promotes the activities of for the next phase takes over.


Commercially produced mitogens growth factors

Analogs of granulocyte colony-stimulating factor (G-CSF)


Three important checkpoints

1.The start or restriction checkpoint just before the start of S- G1/S
2. The G2/M checkpoint that ensures that DNA replication is complete
3.The metaphase spindle checkpoint that ensures that all chromosomes will be segregated- metaphase/anaphase checkpoint


DNA changes (mutations) resulting from damage detected by

tumor suppressor proteins such as p53. many forms of human cancer, the gene for the key tumor suppressor p53 is itself mutated, thus reducing the ability to eliminate cells with damaged DNA and facilitating proliferation of cells with new genetic defect.



is a type of cancer occurring in the eyes, usually in young children.


Rb gene

codes for a key protein active at the G1
restriction point that blocks cell cycle
progression until a mitogenic stimulus arrives. A kinase activated by a growth factor receptor
phosphorylates the Rb protein, causing it to release the E2F transcription factor. This factor then activates genes needed for DNA replication.


What causes cell cycle to halt

By adverse conditions such as inadequate nutrition, inappropriate cellular microenvironment, or DNA damage


Function of G-CSF

Stimulates neutrophil production in immunocompromised patients and erythropoietin, which can stimulate red blood cell formation in patients with anemia


When there is DNA damage where is the cell cycle stopped

It can be arrested at G1 restriction point, S or G2 checkpoint


What happens if dna damage encountered at check point cannot be corrected while cycle is halted

Protein enclosed by tumor suppressor gene such as p53 are activated and cells activity get redirected towards apoptosis



Genes coding for proteins that control cell profileration and Differentiation. They encode almost any protein involved in the control of mitotic activity



Change in structure or expression of a proto-oncogene convert them into oncogenes that cause uncontrolled cell growth and potential for cancer


Morphologically distinct regions within the Nucleolus

1. Light staining fibrillar centers - contain DNA with the rRNA genes
2. Darker fibrillar material - accumulating rRNA transcripts
3. Granular Material- larger and smaller Ribosomal subunits being assembled


Function of E2F Transcription factors

Activates genes needed for DNA replication



Present cell divides and each of the 2 daughter cells receive a Chromosomal set identical to that of the parents cell


Long period began mitosis

Interphase - G1, S and G2 phases



2. Nucleolus disappears
3. Chromosome with sister chromatids are condensed
4. Duplicated centrioles separate to opposite poles and organize microtubules of mitotic spindle
5.lamin and nuclear membrane phosphorylated causing them to disassemble



1. Chromosome condensed further
2. Kinetochores attach to the mitotic spindle
3. Cell is more Spherical
4. Chromosome moved into alignment at the equatorial plate



sister chromatids separate move toward opposite poles



1. 2 sets of chromosomes are at the poles
2. Chromosomes decondense
3. Spindle depolymerizes
4. Nuclear envelope reassembles around each daughter chromosome
5. Contractile ring of actin filaments develops in cortical cytoplasm at the cells equator to produce cytokinesis


Duration of cell cycle

Varies from 24-36 hours. G1 depends on many factors usually the longest memoir variable 25h, S depends on genomic size, G2 and mitosis together last only 2-3hs


capacity for mitosis within a tissue, either by differentiated cells or by reserve
cells, largely determines that tissue’s potential to



Kinetochores microtubules

Polar microtubules which interdigitate near the equatorial plate


Astral microtubules

Anchoring the spindle to cell membrane


Stem cells

1.Undifferentiated cells whose cycling serves to renew the differentiated cells of tissue.
2.Divide infrequently
3.Division is asymmetrical - one daughter cell remain as a stem cell, the other leads to Differentiation


Where are stem cells found

In specific locations or niches where the microenvironment helps maintain their uniquely undifferentiated properties


Progenitor cells

Aka transit amplifying cells are in transit asking the path from the stem cell niche to a differentiated state, and amplifying by mitosis the number of new cells available for the differentiated tissue


Meiosis occurs in

Cells that will form spiel and egg cells


2 key features of Meiosis

1. Synapsis and crossovers
2. Produces haploid cells


Events in Meiosis

1. Completes S phase, replicated it's DNA, chromosome contains 2identical DNA molecule called sister chromatids
2. Prophase I partially condensed Chromatin of homologous chromosome undergo Synapsis and refered to as tetrads
3.crossing over occurs
4. Chromosome fully condense undergo metaphase, anaphase, and telophase separating the homologous chromosomes
5. New cells divide again rapidly without S phase
6. meiosis II separation of chromatids form new haploid set of chromosomes


Difference between mitosis and Meiosis

1.Mitosis produces 2 diploid cells, Meiosis involves 2 cell division and produce 4 haploid cells
2. In Meiosis every haploid cell is genetically unique, mitosis yield 2 cells that are the same genetical


What cause trisomy 21

Chromosome 21 is a very small chromosome and most likely overlooked at the metaphase /anaphase checkpoint. Failure of the homologous chromosomes to divide causes trisomy 21