Cell Biology for Clinical Pharmacy StudentsMD102Module I (Cell structure) Lecture # 5

Dr. Ahmed Sherif Attiaahmed.attia.e.learning@gmail.comhttps://sites.google.com/site/ahmedsattia/

Objectives

By the end of this lecture you should be familiar with:

• The structure of the nucleus and its components.

• Structure and types of nucleic acids.

The Nucleus• The nucleus is the largest cellular organelle in

mammalian cells (occupies about 10% of the total volume of the cell).

• Found in all eukaryotic cells (except erythrocytes & sieve tubes cells).

• It is surrounded by a nuclear membrane.

• The viscous liquid within it is called nucleoplasm, and is similar to the cytoplasm found outside the nucleus.

Functions of the nucleus

• It stores the cell's hereditary material, or DNA.

• It coordinates the cell's activities, which include metabolism, growth, protein synthesis, and reproduction (cell division).

The number of nuclei

• Although most cells have a single nucleus, some cell types have no nucleus, and others have many nuclei.

• This can be a normal process, as in the maturation of mammalian red blood cells, or an anomalous result of faulty cell division.

Anucleated cells

• Cells contain no nucleus and are therefore incapable of dividing to produce daughter cells.

• The best-known anucleated cell is the mammalian red blood cell which loses the nucleus during the process of differentiation.

• Anucleated cells can also arise from flawed cell division in which one daughter lacks a nucleus and the other is binucleate.

Polynucleated cells

• Contain multiple nuclei.

• Skeletal muscle cells, called myocytes, become polynucleated during development; the resulting arrangement of nuclei near the periphery of the cells allows maximal intracellular space for myofibrils.

• Multinucleated cells can accompany inflammation and are also implicated in tumor formation.

Components of the Nucleus

• Nuclear envelope

• Nuclear pores

• Nucleolus

• Chromosomes

The nuclear envelope

• Also known as the perinuclear envelope, nuclear membrane, nucleolemma.

• Double-layered membrane that encloses the contents of the nucleus during most of the cell's lifecycle.

• The space between the layers is called the perinuclear space and is connected with the rough endoplasmic reticulum.

The nuclear envelope• The outer membrane is continuous with

the rough endoplasmic reticulum.

• The inner membrane is erected upon the nuclear lamina, a network of intermediate filaments made of lamin.

• The envelope is perforated with tiny holes called nuclear pores.

Nuclear pore• The nuclear envelope is perforated with

thousands of pores.

• Is constructed from a number (30 in yeast; probably around 50 in vertebrates) different proteins called nucleoporins.

• The entire assembly forms an aqueous channel connecting the cytosol with the interior of the nucleus.

Nuclear pore• They regulate the passage of molecules

between the nucleus and cytoplasm.

• They permit some to pass through the membrane, but not others.

• Building blocks for building DNA and RNA are allowed into the nucleus as well as molecules that provide the energy for constructing genetic material.

Nuclear pore

Nucleolus

• It is a discrete densely-stained structure found in the nucleus.

• It is not surrounded by a membrane, and is sometimes called a suborganelle.

Nucleolus

• The nucleolus is made up of protein and RNA with very little DNA.

• It is the place where the ribosomes are assembled

• Nucleus may contain up to four nucleoli, but within each species the number of nucleoli is fixed.

• During cell division, the nucleolus disappears.

How DNA is packed inside the nucleus

• Human cell has nearly 6 feet of DNA

• They represent 46 individual molecules, one for each chromosome and each about 1.5 inches long.

• They are all packed in the microscopic cell nucleus.

• They must be tightly packed to fit within this space.

Chromatin • For DNA to function, it can't be crammed into the

nucleus like a ball of string.

• Instead, it is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin.

• Chromatin is seen only when the cell is just sitting around, When the cell is going to divide, the chromatin becomes very compact, condenses and appears as what is known as chromosomes .

Components of the Chromatin• Chromatin is DNA plus the proteins (and

RNA)

• There two types of proteins:

• Histones: They are the chief protein components of chromatin.

• Non histones: They are the minor protein components of the chromatin.

Histones

• Basic proteins connected with the DNA.• They act as spools around which DNA winds

and they play a role in gene regulation.• Histones are found in the nuclei of eukaryotic

cells.• 6 CLASS: H1, H2A, H2B, H3, H4, H5.• Two each of the class H2A, H2B, H3 and H4

assemble to form one octameric nucleosome core particle by wrapping 146 base pairs of DNA around the protein spool.

Non-histone proteins

ENZYMES: Involved in synthesis and modification of nucleic acids and involved in modification of nuclear proteins.

REGULATORY PROTEINS:Involved in regulation of expression of genetic information.

STRUCTURAL PROTEINS: Involved with interaction with chromatin and DNA. Structural component of the chromatin.

Types of chromatin

1- Euchromatin

• It is a lightly packed form of chromatin that is rich in gene concentration.

• It is often (but not always) under active transcription.

• It is the only form of chromatin present in prokaryotes.

Types of chromatin

• 2- Heterochromatin

• It is a tightly packed form of DNA.

• It is usually localized to the periphery of the nucleus.

• It is found only in eukaryotes.

• Its functions involves gene regulation and protection of the integrity of chromosomes.

Types of Heterochromatin

1- Constitutive heterochromatin• DNA contained within this region will be poorly

expressed. • It usually contains repetitive sequences.• All cells of a given species will package the

same regions of DNA.

2- Facultative heterochromatin • Contains DNA that is sometimes expressed.• The regions of DNA packaged in facultative

heterochromatin will not be consistent within the cell types of a species.

Nucleic acids structure

• Polymers of ribonucleotide units

• Each monomer (unit):

Nitrogenous base

Sugar

Phosphate group

NUCLEIC ACIDS

DNA

DEOXYRIBONUCLEIC ACID

Sugar: deoxyribose

BASES: A, T, G, C

Double stranded

RNA

RIBONUCLEIC ACID

Sugar : ribose

BASES: A, U, G, C

Single stranded

Chemical structures of the principal bases in nucleic acids

DNA

• There are 4 different bases in a DNA molecule:

a) adenine (a purine)

b) cytosine (a pyrimidine)

c) guanine (a purine)

d) thymine (a pyrimidine)

• The number of purine bases equals the number of pyrimidine bases.

• The number of adenine bases equals the number of thymine bases.

• The number of guanine bases equals the number of cytosine bases.

DNA Composition

• The two strands are held together through hydrogen bonds:

• A and T are by double bonds.

• G and C are held by triple bonds.

DNA chain has polarityOne end of the chain has a free 5’ -OH group the other end has a 3’ -OH group in the sugar molecule.

DNA sequence

• The base sequence is written and read in the 5’ -to-3’ direction.

RNARibonucleic Acid• Polymer of nucleotides• Nucleotide is made of three parts:

1. 5-C sugar: ribose sugar -oxygen at C-2

2. Phosphate

3. Nitrogen base• 4 types of bases: Adenine (A), Guanine (G),

Cytosine (C), Uracil (U).

Types of RNA

1. Messenger RNA (mRNA):

• It is synthesized by the transcription of DNA.• It is selectively transported from the nucleus to the

cytoplasm.

2. Ribosomal RNA (rRNA)

• It is a structural component of the ribosome.

Types of RNA

3. Transfer RNA (tRNA):

• It is used in synthesis of protein as carriers of amino acid.

• There at least 20 different types of tRNA.

From DNA to Protein

• Transcription : the synthesis of RNA under the direction of DNA.

• Translation : the actual synthesis of a protein, which occurs under the direction of mRNA.

Summary

• The nucleus and its functions

• Components of the nucleus:

- Nuclear envelope

- Nuclear pores

- Nucleolus

- Chromosomes (Chromatin)

• Structure and types of nucleic acids.