Cell Biology Lecture #3

**Cell Biology is an interesting subject … So Enjoy** ^_^

The Nucleus

The main components of the cell:

1-Carbohydrates, lipids, proteins and so on

2-Water which is the main component of the cell 70% of the cell is formed by water.

3-Oxygen, carbon, nitrogen and so on

4-Small amounts of electrolytes (sodium, potassium, magnesium and calcium) These are forming ions (charged ions) They have very important function in the cell the charges are important for balancing the activity and transportation across the plasma membrane.

5-Nucleus, cytoplasm, plasma membrane and so on

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The Nucleus:

Is the central component of the cell and it is important component.

*It controls the activity of the cell.

*The structure of the nucleus:

1-Nuclear envelope

2-Chromatins-gentic material

3-Nuclear matrix

4-Nucleolus

**Nuclear envelope: is surrounded the contents and partially isolated the contents of the nucleus from cytoplasm around the nucleus.

**The nuclear envelope has two membranous layers and characterized by presence of pores

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(Openings in the envelope), these nuclear pores are important in the transportation of materials in the cytoplasm to the nucleus and from nucleus to the cytoplasm. There is traffic in both directions.

**Inside the nucleus there is chromatin (genetic materials) which is scattered inside.

**Between the chromatin there are homogenous substances which is called the matrix (nucleosomes)

Slide #2:

Here we can't see the nucleolus but we can see the chromatin scattered.

**Nucleolus: is a condensation of dark material consists of loops of DNA surrounded by RNA and large number of proteins and this is responsible for synthesis (formation) or ribosal RNA which will be transported to the cytoplasm then ribosal RNA with proteins will form ribosome.

**Ribosomes are essential in proteins synthesis.

**The outer layer of the membrane is usually in communication and continuation with the membrane of the endoplasmic reticulum (the membrane network in the cytoplasm).

**The inner membrane (which it is very similar to plasma membrane) is associated from inside by a special proteins which are called nuclear lamina (fibrous proteins).

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**The two layers of the nuclear envelope fuse together when come to surround the nuclear pores.

*Differences between granular and fibrous proteins:

Granular is more globular or spherical.

Fibrous like filaments

**The lamina layer of the network to the inside usually attached (or linking) to the chromatin of the nucleus (fixing the chromatin inside the nucleus)

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**Nuclear pores which are opening in the nuclear envelope in the site of the pores the two layers of the membrane fuse together .

**The nuclear pore regulates the passage of material (transportation of materials) from the nucleus to the cytoplasm.

**During cell division the nuclear envelope disappears and allows the chromatin (genetic material) to migrate half quantity to opposite part of the cell.

*After completion of the division of the cell, there will be assumptions (re-collecting the membrane and the envelope of the nucleus) so it is a dynamic envelope.

Slide #7:

Magnified illustration :

You can see the membranes, lamina, chromatin (thread-like structure and fixed to places by lamina)

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Also you can see the nuclear pore (it's not an empty space it's occupied by structures).

**The nuclear pore and its structures called nuclear pore complex.

**Nuclear pore complex formed by large number of proteins molecules (50 types of molecules) contributed to nuclear pore complex and these are forming columns or units which are about 8 units arranged as a cylinder (channel) around lumen so it's acting like a proteins channel and its diameter is variable (sometimes small, sometimes become wider).

**There are two rings in the pore one to the inside (to the nucleus) and on to the outside (to the cytoplasm).

**You can see from illustration in Slide #7 that the cytoplasmic ring has filaments (thread-like extension) into the cytoplasm. Also the nuclear ring has also fibers (extension) but these form a basket-like structure.

**The functions of the nuclear pore complex is to control active transportation from the cytoplasm to the nucleus (mostly proteins) and from the nucleus to the cytoplasm (mostly RNA).

**The nuclear pore complex transport large molecules by active transportation (e.g. RNA and proteins are large molecules).

**Small molecules and water could pass through smaller gaps in the complex.

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**Aquaporines: opening for the water and other small molecules and electrolytes.

**The active transportation across the nuclear pore complex: there are proteins which are involved in the transportation of materials (especially proteins) from the cytoplasm to the nucleus.

**There are transports receptors of the nuclear pore complex recognize the protein by specific amino acid sequences.

**There are two types of proteins that play critical role in transportation of proteins:

1-Nuclear transport receptor Exportins and Importins

2-Small Protein called Ran small GTP-binding protein.

**The functions of Ran: has the ability of conjugating Guanosine triphosphate (GTP) which is energy rich nucleotide.

**Transportation of large molecules in the nuclear pore complex:

*Ran is attached to the GTP As a result GTP is going to hydrolyze (break the bond of phosphate) and release energy.

*The Energy is utilized by the molecule to conjugate to protein which is called importin.

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**Function of the importin leading the complex of molecules through the nuclear pore complex to the inside of the nucleus.

*The protein which is conjugated to the Ran and importin will pass through the nuclear pore complex that's sorted as a kind of carrier (carrier of proteins from the cytoplasm to the inside of the nucleus).

*Inside the nucleus: the protein that is transported will be released inside the nucleus and the GDP (Guanosine diphosphate) which is attached to Ran will be changed into GTP GDP changed into GTP inside the nucleus.

*GDP will lose energy so the importin with Ran and GTP will pass again to the cytoplasm (where GTP will be hydrolyzed into GDP again to transport other molecules).

**To Transport the material (especially RNA & Proteins) inside the nucleus to the cytoplasm There is similar complex which called Exportin inside the nucleus which conjugate to the protein and Ran transport RNA or proteins to the outside.

**GTP is going to hydrolyze releasing energy to lead the proteins and RNA into the cytoplasm.

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**So we have carrier for exportation (transport from the nucleus to the cytoplasm) Exportins

**And Carrier for importation (transport from the cytoplasm to the nucleus) Importins

** RNA transported in similar way to the protein which is conjugated to the complex.

**The transportation of RNA from inside the nucleus to the cytoplasm is very important for the function of the cell.

**The genetic material regulate their activity in the cell through RNA which is carrying the genetic information going to be coating on RNA and the RNA is going to leave the nucleus to the cytoplasm.

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*The genetic information which are stored in the DNA Chromatin.

**Chromatin: is the genetic material.

*DNA is a long molecule and is conjugated with protein in the nucleus (many kinds of proteins).

**The proteins that associate with DNA molecule are classified into:

1-Histones The major kind.

2-Non-histones Usually smaller.

**Histones have 5 types which are: H1, H2A, H2B, H3, and H4.

** The DNA associate with histones in specific arrangement.

**The Non-histone proteins are scattered, variable types (small and large) and also have functions in regulation the activity of DNA.

**DNA Deoxyribonucleic acid make up of units called nucleotide (chain of nucleotide united with other opposite chain).

**Each Nucleotide consists of three components:

1-Sugar (ribose in RNA, Deoxy ribose in DNA)

2-Phosphate group

3-nitrogenous base

**Arrangements of the components:

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Phosphate group conjugated to glucose of nucleotide and the glucose by itself associated to one side to the base.

**We have 4 kinds of bases classified into two major groups:

1-Purines contain Adenine & Guanine.

2-Pyramidines contain Thymine & Cytosine.

**The DNA with protein makes the chromatin (large masses inside the nucleus).

**There are two types of chromatin:

1-Heterochromatin (condensed chromatin): appear as a dark mass because it is coiled in itself thousands of times found in nuclei which are less active Mostly, this is seen when the cell is in stage of mitosis.

2-Euchromatin (extended chromatin): appear as a light mass because it is extended (not coiled) granule like scattered found in nuclei which are very active because it is extended (allow the transcription of the information of the DNA to form RNA) Active in protein synthesis.

**According to the chromatin we can classify Nuclei into:

1-Nuclei which are mostly heterochromatic (contain large amount of heterochromatin) appear darker less active.

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2-Nuclei which are mostly euchromatic (contain large amount of euchromatin) appear lighter very active.

**The unit of chromatin called nucleosomes.

**Chromatin basic components Nucleosomes :

(Revise Slide #17 & Slide #18)

Nucleosome consists of groups of histone (forming two rows of molecules) H2A, H2B, H3, H4 wrapped (surrounded) by DNA molecules twice (two taps around histones) the wrapping is fixed by the presence of a histone H1 to the outside.

**Nucleosome appears in microscope to have a diameter of 10 Nanometer.

**The chromatin will appear as a series of nucleosomes linked to each other by threads of DNA but still not the final structure.

**The chromatin thread usually coils several times on itself to form thicker and wider thread which it is about 30Nm this structure is usually functioning.

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**Sometimes chromatin coiled on itself hundreds and thousands times to form condensed protein which is not available for transcription.

**The threads of chromatin extended and coiled these are actually forming the chromosome.

**The chromosome can be condensed during division (mitosis) **During division chromosomes become shorter and highly condensed they take a specific shape so that specific shape of the chromosome can be seen in mitosis.

**During ordinary functions of the cell the chromatin thread which form the chromosome classify into:

1-extended

2-condensed it is not having the shape we know about chromosome.

***The shape we know of chromosome only appear during cell division (mitosis) .

**In animals There is variable number of chromosomes (independent and pairs).

**In human there are 46 chromosomes but they are arranged in pairs (each pair has similar function, structure … and so on).

**So we have 23 pairs of chromosomes.

**22 pairs called autosomal because they concerned with functions and characters of the body.

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**1 pair called sex pair which they are in female (XX) and in male (XY).

*Slide #21:

This is the appearance of the chromatin (independent threads) during the cell division … you can see its coiling and coiling until it reach the required shape of the chromosome during mitosis especially in metaphase.

*Slide #22:

Karyotyping: the study of the chromosomes

We can study the chromosomes by taking a cell at stage of metaphase of division and we cause the cell to rupture and stain the nuclear material we can see the chromosomes and identify their types and shapes.

*Slide #23:

Nowadays we have special techniques to show the bands on the chromosomes.

**Each Chromosome has a special region to be placed on and these regions appear in the map so we can identify if there is abnormality in the chromosome. That's called cytogenetic

**The Slide #23 illustrates the bands appearance (some are short and others are longer … and so on).

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**Each chromosome consists of two sisters of chromatids fused together at a region called centromere.

*Slide #26:

The association (fusion) of the two chromatids form a structure called kinetophore where we can find a protein called kinetochore.

**Kinetochore: protein around the chromosome in the centromere and it is important for attaching the rays of the mitotic spindle to the chromosome.

**The ends of chromosomes called telomeres.

**Telomeres are very important for the age of the chromosome (in each division become small, other division become small and shorter) so it represent the life of chromosome Express the age of the cells or of the body.

**The DNA on the chromosome two band … One opposite to other … they are linked by bases the bases of one band is linked to the bases of opposite band by special hydrogen bonds.

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sister chromatids

centromere

kinetochore

Replicated chromosome

short arm

long arm

**This double band molecule is twisted in spiral way that's why it is called double helix DNA.

**During function of the DNA one side is going to be copied in the formation of RNA this is called transcription.

**Function of the gene in DNA transcription of RNA from DNA.

**Genes: are segments of the DNA going to be transcribed to form certain polypeptide chain.

**Not all the DNA is going to form RNA.

**The DNA regions (sequences of bases) divide into:

1-Coded region

2-Non-coded DNA can't be used in protein synthesis

*Information transcribe into RNA.

**There are segments of gene transcribe RNA will be removed because they are not used in protein synthesis Called introns that will be removed by splicer in a process called splicing.

**While the segments that will be used in protein synthesis called exons.

**Alternative splicing: rearrangement of exons from this process there will have different genes.

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Done By

Amanda Yumurta

Supervised by

تبييض 2013لجنة

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