Lecture 10: The Nucleus

chromatin

Even in the dispersed form, the chromosomes have a unique staining pattern due to stainable fibers called ____.

nuclear pores

Number of nuclear pores: Some cells have a few, others have many. Generally, the more active the cell, the more ____ the cell will have, which reflects the more exchange taking place at that time.

heterochromatic S-period

The _____ regions are relatively inactive parts of chromosomes and also the latest to replicate during the ____ (replication of DNA) of the cell cycle.

heterochromatin, heterochromatin

The term ____ refers to the chromatin that remains in a condensed state during interphase. The ____ is usually seen associated with the inner nuclear membrane of the envelope.

nuclear envelope, chromosomes, nucleolus

The three principal structured components of the nucleus are the ____, the _____, and the ____.

imported into

The underlying molecular mechanism is best understood for proteins that are actively ____ from the cytoplasm ____ the nucleus.

histones equal

While all other components vary in proportionate amounts to DNA, ____ occur in ____ proportion by weight to DNA in the chromatin fiber.

Facultative heterochromatin

____ contains active genes, but may become condensed and genetically inactive in response to physiological and developmental conditions, and may revert to a euchromatic state at certain times.

lipids proteins

____, found principally as phospholipids, are structured components of the nuclear envelope. ____ of different kinds occur in all parts of the nucleus.

transcript, ribosomal, t- and m-

_____ RNAs migrate from their sites of synthesis along the chromosomal DNA. ____ RNA participates in ribosome subunit formation within the nucleolus of the nuclear compartment, while ____ RNAs are transported directly to the cytoplasm where they contribute to protein synthesis at ribosomes.

submetacentric acrocentric

_____ chromosomes have arms of somewhat unequal length while ____ have very unequal arm lengths.

Constitutive heterochromatin

______ is permanently condensed or turned off chromatin or genes, which many nuclei demonstrate that reflects evolution of the cell. At one time, this chromatin was active but through its evolutionary advancement, the chromatin is no longer needed by the cell, so it remains permanently turned off.

Membrane bound organelles

____distinguish eukaryotic cells from prokaryotic cells.

nucleolus physiological state

All nuclei have at least one ____, which is the ribosome factory of the cell. The size usually reflects the ____ of the cell.

chromosomes nucleolus

All or most of the DNA is confined to the ____ and RNA occurs primarily within the ____ but also is associated in varying amounts with the chromosomes.

exported

All the RNA molecules and partially assembled ribosomes needed for protein translation in the cytoplasm must be ____ from the nucleus to the cytoplasm.

imported

All the enzymes and other proteins required for chromosome replication and transcription of DNA in the nucleus must be ____ from the cytoplasm (site of translation).

nucleolus

At the start of mitosis as chromosome contraction begins, the ___ starts the disappear, because it is no longer present during metaphase and anaphase but reappears at the proper site of the NOR in the latter stages of mitosis.

centromere

Chromosome movement to the poles at anaphase will usually not take place unless a functional ____ is present.

differentiated

Chromosomes are ____ along their lengths into regions that perform unique functions.

telocentrics

Chromosomes with a terminal centromere have only one arm and are called ____.

enormous

Compared to prokaryotic circular DNA, eukaryotic linear DNA is ____ in size.

random ordered

Distribution of pores: In some cells, there may only be a ____ distribution of nuclear pores, but in some cells the nuclear pores are a very ____ pattern.

centromere primary constriction

Every chromosome has at least one ____ or site where spindle microtubules attach to the chromatid that is usually seen as a constricted place known as the ___ because of its appearance and its importance in relation to other constrictions, which have a different function.

nuclei

Every eukaryotic cell possesses one or more ____ during some stage of its existence.

octagonal

Nuclear pores of the NPC have a relatively uniform and highly ordered ____ structure made up of associated nonmembranous components of a distinctive type.

export out import

For the ____ of material out of the nucleus, the main difference is that transport ___ of the nucleus is used mainly for RNA molecules that are synthesized in the nucleus but function in the cytoplasm, whereas nuclear ____ is mainly for proteins that are synthesized in the cytoplasm but function in the nucleus.

important entryways for nucleocytoplasmic exchanges of molecules such as mono- and dissacharides and particles in both directions

Function of the nuclear pores:

H2A, H2B, H3, and H4 + linker H1

Histones are always found in equal proportion to DNA: two each of ___

syncytium

In animals, the multinucleate condition is referred to as being ____ ( striated muscle cells).

nuclear envelope

In general, the ____ is the most significant component in the control of flow between nucleus and cytoplasm and the pore complexes provide and adjunct system for controlling this exchange.

coencytic

In plants, the multinucleate condition is referred to as being ____ (Vaucheria).

imported NOR

It has been found that the proteins are manufactured in the cytoplasm and ___ by the nucleus to the nucleolus or ___ site.

heterochromatin euchromatic heterochromatin

It has been shown that ____ clearly alters to a ____ state before the mitotic division actually begins, but the nuclear chromatin all reverts to ____ during mitosis and meiosis and appears as distinguishable chromosomes.

actively

Large molecules and particles are ___ transported through nuclear pores by a selective process requiring active transport and energy.

mitochondria and chloroplast

Name the two other places that DNA can be found in the cell.

nuclear envelope

Nuclear materials are separated from the cytoplasm by a double-membrane system generally referred to as the ____.

constitutive heterochromatin

Primitive cells usually don't contain ____ while advanced cells may have quite a lot.

nuclear localization signals (NLS)

Proteins actively imported from the cytoplasm into the nucleus usually posses one or more ____, which are amino acid sequences that enable the protein to be recognized by the nuclear pore complex.

nuclear export signals (NES) exportins importins

Proteins called ____ assist the export of RNAs after binding to the RNAs destined for export. These sequences are recognized by nuclear transport proteins called _____ that bind to the molecules containing NES seuqences and mediate their transport out through the nuclear pores by mechanism that resembles the way in which ____ transport or import cytoplasmic molecules into the nucleus.

8 central granule

Rings of the annulus are not homogenous; they have an eightfold radial symmetry caused by the presence of ____ symmetrically distributed granular subunits of a nonmembrane nature. Each ring or rim has 8 spokes extending into the lumen of the pore connecting to wheel's hub, which is the ____.

heterochromatin

Segments of chromatin so highly compacted that they show up as dark sports in micrographs are called ____,

packaging, 30-nm

Since the 30-nm fiber does not form in chromatin preparations whose histone H1 molecules have been removed, suggest that histone H1 facilitates the ____ of nucleosomes into the ____ fiber.

matrix

Specific locations of nuclear pores on a nucleus imply that there are chromosomes attached to the nuclear ____ along the inner membrane in that area of the nucleus.

active transcribed

The 300nm in diameter looped portions of chromatin often contain ___ regions of DNA (DNA that is being ____).

secondary constriction

The NOR site is detectable as a constriction (also known as the ____ of that chromosome) on one arm of the chromosome, usually near the free end of the chromosome arm.

chromatin fiber

The ____ can be considered as the basic structural unit of the chromosome in eukaryotes.

nuclear lamina, intermediate filaments, lamins

The ____ is a thin but dense meshwork (fibrous structure) that lines the inner surface of the inner nuclear membrane and helps support the nuclear envelope. It is abut 10-40 nm thick and is constructed from ____ made of proteins called ____.

nuclear matrix

The ____ may be involved in anchoring chromatin fibers at locations where DNA or RNA is being synthesized, thereby organizing the DNA for orderly replication and transcription and perhaps even providing tracks that guide and propel newly formed mRNA to the nuclear pores for transport to the cytoplasm.

inner membrane nuclear matrix heterochromatin

The ____ of the envelope contains no attached ribosomes but is closely associated with the ____, which helps maintain the shape of the nucleus and provide and organizing skeleton for the condensed chromatin fibers (called ____).

H1

The ____ proteins are thought to aid in the coiling of the 10nm fiber, with nucleosomes, to coil into the 30 nm size.

9-nm

The pore channel itself is only ____ wide, allowing for only small particles, while larger particles are excluded entirely.

centromere region, centromere, centromere plate (a type of MTOC)

The area of chromosome with the centromere is called the ____ and the visible structures in this region is the ____. In animals it is often a 3-layered disc-like structure. The outer layer is called the ____; the inner dense layer is formed from compacted chromatin fiber and the middle layer is transparent.

transporter

The central granule is usually called the ____ because it is thought to move macromolecules across the nuclear envelope. Proteins extending from the rim into the perinuclear spaces are thought to help anchor the pore complex to the envelope.

linear, histone proteins, nonhistone proteins, DNA- & RNA-polymerases

The chromatin fiber consists of a continuous ____ DNA duplex strand with the association of basic ____, acidic or neutral _____, various amounts of RNA, and enzymes such as _____.

discrete

The chromatin threads corresponding to each individual chromosome are not randomly distributed but each chromosome has its own ____ location, referred to as "chromosome territories."

nucleoplasm

The chromosomes and nucleolus are bathed in the ____ (nuclear juice), which is an amorphous suspension of proteins, particles, fibers, and various organic and inorganic materials.

cell cycle

The conversion of heterochromatin to the euchromatin confirmation is evidence that parts of the chromatin fiber may undergo different degrees of condensation at different times in the ____ or life of the cell.

nucleosomes 10nm 30nm 300nm

The formation of ____ is only the first step in the packaging of nuclear DNA. Isolated chromatin fibers exhibiting "beads-on-a-string" appearance measure about ____ in diameter. The chromatin of intact cells often forms a slightly thicker fiber about _____ in thickness. The next level of chromatin-packing is the folding of the 30-nm fibers into looped domains measuring ____ in diameter.

rRNA synthesis

The function of the nucleolus is the site of ____ and subsequent ribosome assembly and is rich in ribonucleoproteins since the granular and fibrillar regions are reduced following RNase or proteolytic enzymes, and little free RNA is present.

perinuclear space nuclear pore complex (NPC)

The inner membrane space or _____ is not continuous; it is interrupted by pores or openings formed by fusion of the inner and outer membranes, forming a channel that is lined with the intricate protein structure called the ____, measuring about 120 nm.

ribosomes r-RNA

The larger the nucleolus, the more ____ being produced (function: ____ production), and therefore the more protein being produced by the cell in the cytoplasm.

importin

The leading amino acids forming the NLS is recognized by a special type of receptor protein called an ____, which binds to the NLS and mediates the movement of the NLS-containing protein to the nuclear pore and into the nucleus. Recognition sequences directing the proteins to the nucleus.

nucleus

The mammalian red blood cell typically loses its ____ at maturity.

nucleolar organizing chromosomes genome

The maximum number of nucleoli in a typical cell indicates the number of ____. There is a minimum of one such chromosome per ____ (haploid chromosome compliment of a cell).

euchromatin

The portion of chromatin which becomes extended during interphase is called ____.

nucleosomes, eight, octomer, 10nm, 200, linker

The presence of particles called ____ have been described which involves treatment of the DNA fibers with dilute salt solutions, which reveals a periodic structure to the DNA fiber. These have been interpreted as ___ histone proteins wrapped by the DNA fiber forming an ____. Digestion of the ____ in diameter DNA fiber by endonucleases causes DNA to fragment into lengths ____ base pairs long. Examination of these segments reveals a length of DNA - 140 base pairs long and wrapped around the 8 histone proteins and connected by a ___ of base pairs 60 pairs long.

histone

The proteins with the most important role in chromosome structure are ____ proteins.

euchromatin

The more loosely compacted segments of chromatin, diffuse form of chromatin, are called _____.

1. structural support for the nuclear envelope 2. attachment sites for chromatin

The nuclear lamina provides: (2)

nucleolar organizing region (NOR)

The nucleolus forms at a specific site called the ____ on that particular chromosome that carries multiple copies of rRNA genes in repeat sequences (also known as rDNA).

active inactive

The nucleolus is largest in the most ____ cells and shrinks in ____ cells.

Nuclear envelope perinuclear space outer and inner membrane

The nucleus is surrounded by two membranes, called the ____, with a space or lumen, called the ____, between the ____ and _____ system. The nuclear membranes virtually assure exclusion of cytoplasmic structures from the nuclear compartment.

replication, transcription, messenger, transfer, and ribosomal

The nucleus is the site of gene ____ (the process of making new DNA) and ____ (the synthesis of the three types of RNA - _____, _____, and ____), since the chromosomes, the major concentration of cellular DNA, is located within the nucleus.

outer membrane polysomes

The outer face of the ____ has associated ribosomes (which resemble ribosome groups reading a specific m-RNA or ____ found elsewhere in the cytoplasm or on the RER), Golgi apparatus, and occasionally forms rough ER profiles.

annulus

The perimeter of the pore is accentuated on its upper and lower surfaces by a ring-like system called an _____, resembling a wheel laid on its side with two parallel octagonal shaped rings within the nuclear envelope.

facultative and constitutive

There are two types of heterochomatin:

membrane

There is no ____ around the nucleolus and two or more may fuse to form a single body in some kinds of cells.

centromere secondary constriction nucleolar organizing region (NOR)

Two particular differentiated regions of the chromosomes are the _____ (or kinetochore) and the ____ (which is associated with the ____)

metacentric

When the centromere is median in location, the chromosome is termed ____.