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TERMPAPER

Cell Biology (BTY-101)

PRESENTED TO:

Miss. Ruchi Bhardwaj

Kaur

B.tec

h-Biotechnology

RC78

02B53

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10809

630

ACKNOWLEDGEMENT

History of all great work is to witness that no great work was ever done without either the

active or passive support of a persons surrounding and ones close quarters. Thus it is not

hard to conclude how active assistance from seniors could positively impact the execution of

a project. I am highly thankful to our learned faculty Ms. RUCHI BHARDWAJ for her

active guidance throughout the project.

Last but not least, I would also want to extend my appreciation to those who could

not be mentioned here but have well played their role to inspire me behind the curtain.

SUKHJIT

KAUR

ABSTRACT

If a living being's body is a machine, then the brain is the processing unit or the control center

of this machine. We know that the smallest unit of life is the cell, which has to be a machine

as well. So, what is the processing unit of the cell? The control center of a cell is the nucleus,

also known as kernel of the cell. Let us see the nucleus function and structure, and understandthe working of nature's smallest processing unit.

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All the genetic contents of the cell are enclosed in the nucleus, organized as multiple long

chains of linear Deoxyribonucleic acid (DNA) molecules along with many kind of proteins,

like histones, which form chromosomes. Within these chromosomes, there aregenes enclosed,

which are nothing but the cell's nuclear genome. In brief, nucleus function can be stated as to

preserve the integrity of these genes and to preside over the activities in the cell by regulatinggene processing or functionality.

This termpaper involves the exploitation of various resources for the gatheration of full and

the latest information about the topic cell nucleus. Firstly all is being discussed about the

basic overview of nucleus and what is its use to the cell. Why cell nucleus is the important

and the necessary part or the component of the cell. Then the contents of the cell nucleus and

their functional roles in various processes. The cell nucleus is a characteristic of a eukaryotic

cell and this differentiates it from a prokaryotic cell separating it from cytoplasm thus leading

it being called a distinct organelle. Different portions of the nucleus are being shown with the

help of coloured photographs clearly showing the characteristics.

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INTRODUCTION

The nucleus is the heart of the cell. In this organelle almost all of the cells DNA is confined,

replicated and transcribed. It controls all metabolic as well as hereditary activities of the cell.

This is the unique feature that differentiates the prokaryotic cells from the eukaryotic cells

that in contrast have the nuclear region or the prokaryon or the nucleoid. This organelle is

central and commanding and is essential for the biosynthetic events that characterise cells

fraction and its type. It acts as a vault of genetic information encoding the past history and

future prospects being submerged in the cytoplasm. It is the seat of DNA replication, DNA

transcription, rNA processing. The nuclei were first discovered and named by RobertBrown in 1833 in plant cells. Nucleoli were discovered by M.J. Schleidon in 1838. The

term nucleolus was coined by Bowman in 1840. The term chromatin was coined by W.

Flemming for chromosomal meshwork.Strasburger introduced the terms nucloplasm and

cytoplasm.Barbara McClintock recognised and named nucleolar organisers in the

chromosomes in 1934. The role of nucleus in heredity was firmly established by grafting

experiments of Hammerling with Acetabularia in 1953. Ultrastructure of nuclear envelope,

pore complexes and nuclear lamina were worked out by kirschner, Schatten and

Thoman.

Nucleus is found in all the eukaryotic cells of plants and animals. The position and location

of nucleus in a cell is usually the characteristic of the cell type and is often variable. Usually

it remains oriented in the centre. But its position may change from tjme to time according to

the metabolic states of the cell. The shape of the nucleus normally remains related with the

shape of the cell but certain nuclei are irregular in shape. Generally occupies about 10% of

total cell volume. May vary in size from 3 micrometer to 25 micrometer in diameter

depending upon the cell type and contain diploid set of chromosomes. The haploid cells

contain small sized nuclei in comparison to the diploid cells.

ULTRASTRUCTURE

The nucleus is composed of the following structure:

Nuclear membrane or karyotheca or nuclear envelope

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The nuclear sap or nucleoplasm

The chromatin fibres

The nucleolus

Encloses the DNA and defines the nuclear compartment of interphase and prophase

nuclei. It is formed from two concentric unit membranes each 5-10 nanometer thick.

The spherical inner nuclear membrane contains specific proteins that sct as binding

sites for the supporting binding sheath of intermediate filamesnts called nuclearlamina. Nuclear lamina has the contact with chromatin and nuclear RNAs. The inner

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nuclear membrane is surrounded by outer nuclear membrane which closely resembles

the membranes of the endoplasmic reticulum. It is also surrounded by the less

organised outer intermediate filaments. Outer surface is generally studded with

ribosomes engaged in the protein synthesis. The proteins made on these ribosomes are

transported into space between the inner and outer nuclear membrane called theperinuclear space. The perinuclear space is the 10 to 50 nm wide fluid filled

compartment which is continuous with the endoplasmic reticulum lumen and may

contain fibres, crystalline deposits, lipid droplets or dense material.

it is also called fibrous lamina, zonula nucleum limitans,

internal dense lamella, nuclear cortex and lamina densa.

Protein meshwork.

50 to 80 nm thick or 10 to 20 nm thick.

Lines the inside surface of inner nuclear membrane except the areas of

nucleopores.

Consists of a square lattice of intermediate filaments.

The lamins form dimers that have a rod like domain and two globular heads at

one end.

Very dynamic structure.

In mammalin cells undergoing mitosis, the transient phosphorylation of

several serine residues on the lamins cause the lamina to reversi

diassemble tetramers of hypophosphorylated lamin A and lamin c and

membrane associated lamin B.

During meiotic chromosome condensation, the nuclear lamina completely

disappears by the pachytene stage of prophase and reappears during diplotene

in oocytes.

May play a crucial role in the assembly of interphase nuclei.

In all eukaryotic forms, from yeasts to humans is perforated by nuclear pores

which have the following structure and function:

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- circular in surface view.

Diameter between 10nm and 100nm.

Annulus a diaphragm made of amorphous to fibrillar material extends across

each pore limting free transfer of material. It is observed in animal cells but

plant cells lack it.

Called nuclear pore complex because it has a complex structure.

Pore complex consists of two rings at its periphery with an inside diameter of

80nm, a large particle that forms the central plug and radial spokes that

extends from the plug to the rings.

Hole in the centre of the pore complex is an aqueous channel through which

water soluble molecules shuttle between the nucleus and the cytoplasm.

Pore complex perforates the nuclear envelope bringing the lipid bilayers of the

inner and outer nuclear membrane together around the margins of each pore.

In somatic cells are evenly or randomly distributed over the surface of nuclear

envelope.

In other cell types is not random but rather range from rows to clusters to

hexagonal.

Rate of transport through nuclear pore if the cell is synthesizing DNA, on an

average each pore needs to transport about 100 histone molecules per minute.

Ribosomal subunits are specifically transported through the nuclear pores by

active transport system. mRNA molecules complexed with proteins to form

ribonucloproteins are actively exported through the nucleus.

- there is considerable trafficking

across nuclear envelope during interphase.

Ions, nucleotides, structural, catalytic and regulatory proteins are

imported from the cytosol.

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- the nucleoplasm consists of many types of complex proteins. The

nucleoproteins can be categorised into the following two types:

(1) BASIC PROTEINS proteins which take up basic stain are known as

basic proteins. The most important proteins are the nucleoprotamines and the

nucleohistones.

(2) NON HISTONE OR ACIDIC PROTEINS the acidic proteins either

occur in the nucleoplasm or in the chromatin. The most abundant acidic

proteins of the euchromatin are the phosphoproteins.

- the nucleoplasm contains many enzymes which are

necessary for the synthesis of the DNA and the RNA.

Most of the nuclear enzymes are composed of the non-histone

proteins.

The most important nuclear enzymes are DNA polymerase, RNA

polymerase, NAD synthetase, nucleoside triphosphate, adenosine

diaminase, nucleoside phosphorylase, guanase, aldolase, enolase,3-phoshoglyceraldehyde dehydrogenase and pyruvate kinase.

The nucleoplasm also contains certain coenzymes and cofactors

such as ATP and acetyl CoA.

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- the nucleoplasm also contains several inorganic

compounds such as phosphorus, potassium, sodium, calcium and magnesium.the chromatin

comparatively contains large amount of these minerals than the nucleoplasm.

Thread like, coiled and much elongated structures which take readily the basic stains such as

the basic fuchsin. These are observed only in the interphase nucleus. During the cell division

these become thick ribbon like structures known as chromosomes. Chemically it consists of

DNA and proteins. RNA rarely accounts for more than 4% of the total chromatin present.

Most of the proteins present are the histone but non histone proteins are also present.

Histones are constituents of chromatin of all eukaryotes except fungi. The fibres of the

chromatin are twisted, finely anastomosed and uniformly distributed in the nucleoplasm. Two

types of chromatin material have been recognised heterochromatin and euchromatin.

- the darkly stained, condensed region of the chromatin.

The condensed portions of the nucleus are known as

chromocentres or karyosomes or false nucleoli

The heterochromatin occurs around the nucleolus and the

periphery.

It is supposed to be metabolically and genetically inertbecause it contains comparatively small amount of DNA and

large amount of the RNA.

- The light stained and different region of the chromatin is

known as the euchromatin. It contains comparatively large amount of DNA.

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Most cells contain in their nuclei one or more prominent spherical colloidal acidophilic

bodies called nuclei. Cells of bacteria and yeast lack nucleolus. Its size is found to be relatedwith the synthetic activity of the cell. Therefore, the cells with little or no synthetic activities

eg. Sperm cells, blastomeres, muscle cells etc. Are found to contain smaller or no nuclei

while the oocytes, neurons and secretory cells which synthesize the proteins or other

substances contain comparatively large sized nucleoli. The number of nucleoli in the cells

depends on the species and the number of the chromosomes. The position of the nucleolus in

the nucleus is eccentric. Nucleolus is often associated with the nucleolar organiser which

represents the secondary constriction of the nucleolar organising chromosomes and are 10 in

number in human beings.

ULTRASTRUCTURE AND FUNCTION OF

NUCLEUS

nucleolus are the sites where biogenesis of ribosomal subunits takes place.

In it three types of rNAs are transcribed namely 18S, 5.8S, 28S as parts of

much longer precursor molecule which undergoes processing, splicing by the

help of two types of proteins nucleolin and u3sn RNP.

The 5S rRNA is transcribed on the chromosome existing outside the

nucleolus and the 70S type of ribosomal proteins are synthesized in the

cytoplasm.

Nucleolus contains many more incomplete 60S ribosomal subunits than the

40S ribosomal subunits.

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The time lag in the migration of 60S and 40S ribosomal subunits prevents

functional ribosomes from gaining access to the incompletely processe3d

heterogeneous RNA molecule inside the nucleus.

the light stained and different region of the -----

Different stages of formation of ribosomes are completed in three distinct regions of the

nucleolus. Their initiation, production and maturation progresses from the centre

periphery. Following three regions have been identified in the nucleolus.

(1) FIBRILLAR CENTRE this pale staining part represents the innermost

region of nucleolus. The RNA genes of the nucleolar organiser of chromosomes

are located in this region. The transcription of these genes is also initiated in this

region.

(2) DENSE FIBRILLAR COMPOMNENT this region surrounds thefibrillar centre and RNA synthesis progresses in this region.

(3) CORTICAL GRANULAR COMPONENT outermost region of the

nucleus where processing and maturation of pre-ribosomal particles occur.

let us take a glance at all the functions of the nucleus:

Nucleus stores the genetic entropy necessary for reproduction,

growth and metabolism of not only the cell it controls, but also ofthe organism on the whole. It controls the transfer and replication ofhereditary molecules (DNA and RNA) between the parent cell andthe child cell. Nucleus ensures equal distribution and exact copyingof the genetic content during the process of cell replication. This isthe main nucleus function in animal cells. The nucleus sustains and controls the cell growth byorchestrating the synthesis of structural proteins in the cell. Nucleus is the place forDNA transcription in which messenger RNA(mRNA) are produced which synthesize protein. The nucleuscontains various types of proteins which can either directly controltranscription or are indirectly involved in regulating the process. The process of energy and nutrient metabolism in the cell isregulated by the nucleus, by directing the synthesis and functioningof enzymes, which is a type of protein. The selective diffusion of cell's regulatory and energymolecules through the pores in the nuclear membrane is presided bythe nucleus. Nucleus is responsible for the secretion ofribosomes. Theribosomes, in turn gets the credit for synthesizing all types ofproteins.

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TRANSCRIPTION SITES AND SFCs

One of the fundamental cell biological questions in nuclear architecture is : where are genestranscribed? The labelling of nascent RNAs has shown that RNA polymerase II (RNA pol II)transcription takes place in thousands of discrete dots distributed throughout the nucleus. Asexpected, RNA pol II components co-localize with these dots. However, RNA pol II alsolocalizes in numerous domains outside of transcriptionsites. These sites may be storage poolsfrom which factors are recruited. Using Fuorescent in situhybridization, several specificpre-mRNAs have been localized in extended spots or elongated tracks. The correspondingactive genes are positioned at the end of these tracks. Since splicing often

cotranscriptionally, these transcription sites may also represent sites of pre-mRNA splicing [.However, the majority of pre-mRNA splicing factors are not localized at active transcriptionsites, but are enriched in distinct domains, termed speckles or SFCs.Using electrmicroscopy, SFCs correspond to perichromatin fibrils and interchromatin granuleclusters.Perichromatin fibbrils are fibres of variable diameter with particles of irregular sizeassociated with them and they are thought to represent nascent transcripts.Interchromatingranule clusters are composed of dense granules of 2025 nm in diameter connected byfibres. SFCs are not primary sites of pre-mRNA splicing. Most active genes are found at theperiphery of SFCs, and only rarely within the compartment. The function of SFCs appears tobe the storage assembly of spliceosomal components. Upon expression of introncontaininggenes or viral infection, splicing factors are redistributed from SFCs to the new transcriptionsites . While these observations suggest that splicing factors generally move towards a gene,it is likely that pre-mRNA also moves toward SFCs.Recruitment of splicing factors fromSFCs to sites of transcription is believed to involve a cycle of phosphorylation anddephosphorylation. Furthermore, the accumulation of splicing factors at sites of transcriptionis dependent on the C-terminal domain of the largest subunit of RNA pol II. Theseobservations are consistent with the emerging view that all RNA-processing machineries arephysically linked to the transcription machinery.

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Initially described as ``nucleolar accessory bodies'' by Santiago Ramon y Cajal at the start ofthe last century, these cytologically distinct bodies were renamed coiled bodies due to theircharacteristic.

appearance as a tangle of coiled fibrillar strands of 4060 nm indiameter.

Recently, in honour of its initial discoverer, the coiled body wasrenamed Cajal bodies.

CBs are small spherical structures, which are typically present as 15copies per nucleus, ranging in size from 0.11.0.

CBs contain a large number of components, including thespliceosomal snRNPs, U3, U7, U8 and U14 snoRNAs, basaltranscription factors TFIIF and TFIIH, cleavage-stimulation factor andcleavage and polyadenylation specificity factor, and nucleolarcomponents fibrillarin, Nopp140, and protein B23.

The function of the CB is unknown. CBs are generally found within the nucleoplasm, but they have been

detected within the nucleolus in human breast carcinoma cells, brownadipocytes and hepatocytes of hibernating dormice.

Several observations indicate that CBs might pass through thenucleolus.

Recently, it has been shown that CBs are highly mobile structuresmoving throughout the nucleoplasm.

In all cases they appear to have the ability to move to and from thenucleolar periphery and within the nucleolus.

At least two size classes of CBs have been characterized in living cells,the larger (termed CB) and smaller (termed mini-CB). Each appears toarise through joining and separation events

Formation of CBs inside the nucleolus can be induced by a singlepointsubstitution (serine to aspartate at position 202) in transientlyexpressed p80-coilin protein. In addition, Nopp140, which interactswith p80-coilin in vitro, is present in both CBs andthe nucleolus, andaffects both structures when non-functional mutants are transientlyexpressed.

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After RNA molecules (mRNA, tRNA and rRNA) are produced in the nucleus, they mustbe exported to the cytoplasm for protein synthesis. On the other hand, many proteinsoperating in the nucleus must be imported from the cytoplasm. The traffic through thenuclear envelope is mediated by a protein family which can be divided into exportins andimportins. Binding of a molecule (a "cargo") to exportins facilitates its export to thecytoplasm. Importins facilitate import into the nucleus.

The function of exportins and importins is regulated by a G protein called "Ran". Thereare two types of G proteins: heterotrimeric G proteins and monomeric G proteins (or smallG proteins). The latter includesRas, Rho, Rab, Arf, and Ran. Like other G proteins, Ran

can switch between GTP-bound and GDP-bound states. Transition from the GTP-bound tothe GDP-bound state is catalyzed by a GTPase-activating protein (GAP) which induceshydrolysis of the bound GTP. The reverse transition is catalyzed by guanine nucleotideexchange factor (GEF) which induces exchange between the bound GDP and the cellularGTP. The GEF of Ran (denoted by RanGEF) is located predominantly in the nucleus whileRanGAP is located almost exclusively in the cytoplasm. Therefore, in the nucleus Ranwill be mainly in the GTP-bound state due to the action of RanGEF while cytoplasmic Ranwill be mainly loaded with GDP.This asymmetric distribution has led to the followingmodel for the function of exportins and importins.

It is thought that binding between an exportin or importin and its cargo depends on theirinteraction with Ran: RanGTP enhances binding between an exportin and its cargo butstimulates release of importin's cargo; RanGDT has the opposite effect, namely, itstimulates the release of exportin's cargo, but enhances the binding between an importinand its cargo. Therefore, the exportin and its cargo may move together with RanGTPinside the nucleus, but the cargo will be released as soon as the complex moves into thecytoplasm (through nuclear pores), since RanGTP will be converted to RanGDP in thecytoplasm. By contrast, the importin and its cargo may move together with RanGDP in thecytoplasm, but the cargo will be released in the nucleus since RanGDP will be converted toRanGTP in the nucleus.

Protein transport

A protein destined for the nucleus and/or cytoplasm contains a specific sequence whichcan be recognized directly by importin/exportin or through an adaptor protein. Forexample, importin b is a well characterized importin. It cannot recognize the specificsequence but can be assisted by importin a which is an adaptor. By contrast, the importinfor the heterogeneous nuclear ribonucleoprotein (hnRNP) can recognize directly the

specific sequence in hnRNP. This importin is named "transportin".

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(a) The two states of Ran: GTP-bound and GDP-bound. (b) General function of importinsand exportins.

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Latest research suggests cell nucleus plays a rolein ageing

Scientific research is increasingly pointing to the fact that the cell nucleus plays an

important part in the ageing process - a discovery that could revolutionise anti-

ageing treatments.

The study, which was conducted by scientists at the National Cancer Institute (NCI) in theUS, indicates that mutations in the nucleus structural protein lamin A can cause premature ageingsyndrome.

However the research team have discovered that reversing inhibition of a splice contained inthe lamin A protein can reverse the onset of mutations that often lead to signs of ageing,suggesting that lamin A is implicated in physiological ageing.

Further more the study also found that the cells of people aged over 80 tended to havespecific problems that the nucleus of young children do not have, leading to obvious signs ofageing such as graying hair and wrinkling.

What the scientists now intend to do is work on establishing the idea that the normal ageingprocess is at least partly concerned with the decay of the nucleus. The biggest challenge willalso be discovering a means of blocking out this process.

"If this really has a physiological role in normal elderly people then it's a huge deal," David

Sinclair, who heads up research into ageing at Harvard Medical School in the US, toldNature.com.

Cosmetic and personal care companies are currently making huge investments into theresearch and development ofanti-ageing products. This falls in line with the demands of anageing and increasingly affluent population that wants to maintain youthful and healthylooks.

Eco-Frost Brings a New Dimension to Your Packaging

Technigraph's new, environmentally-friendly spray frost sets your packaging apart. Clearfrosts, tints and unique effects combine with direct screen printing for a positive impact onplastic or glass packaging...This is now fuelling a multi-billion dollar industry that hastriggered one of the fastest growing segments in the personal care market. All this means thatany solution that shows promise in fighting signs of ageing is bound to make ears prick up.

Currently nanotechnology is thought to hold great potential in the fight against wrinkles, as itis believed that creating active anti-ageing ingredients in nano molecular form can increaseefficacy.

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However, this most recent research into cell nucleus protein could help further focus thedevelopment of technologies such nanotechnology-based active ingredients to improveefficacy still further.

Researchers Tom Misteli and Paola Scaffidi from the NCI says that healthy cells alwaysmake a trace amount of an aberrant form of lamin A, but that younger cells can sense andeliminate it. A proliferation of aberrant cells, found in older humans, causes the nucleus towrinkle up, leading to physiological signs of ageing.

The big challenge the research team and other scientists now face is to try and find a meansof blocking this proliferation.

According to Misteli pinpointing a solution to this means, "You can take these old cells andmake them young again."

There are already drugs on the market that effectively enable this process to be carried outand animal experiments are currently being carried out to determine effectiveness.

CONCLUSION

It is now well established that the nucleus contains numerous distinct compartments. Anongoing challenge is to establish the functions of these compartments. A critical step in thisendeavour will be the biochemical purification and characterization of nuclear compartments

and, perhaps more importantly, the invitroreconstitution of compartments and theirfunctions. Recent observations suggest that the nucleus is a stable, yet highly dynamic,structure and that the dynamic properties are crucial for its accurate functioning. Whereasmost studies to date have analysed the dynamic properties of large mixed populations ofproteins, important insights will be gained in the future from the study of proteins at specificloci within the nucleus, the visualization of single-molecule dynamics using high-speedmicroscopy, and from the analysis of protein dynamics within larger complexes. Although wehave recently learnt much about protein dynamics, a complete picture of how genomesfunctioncan only be obtained by uncovering the organizational principles and microsocopic dynamics of chromosomes and chromatin. Regardless of the pace of futureprogress, the ability to visualize dynamic properties of chromatin, RNA and proteins in living

cells has already given a more complete picture of how nuclear processes are organized, andhas added an additionaldimension in the study of nuclear processes and gene expression.

References:www.cellbio.com

www.cellnucleus.com

www.cellresearch.com

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T

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