Subcellular Structures

Post-translational Modification of Protein Occurs in:
1. Golgi apparatus
2. Ribosome
3. RER

Organelles Involved in Protein Synthesis:
1. Ribosome
2. RER
3. Mitochondria

Microvilli is concerned with absorption.

Mitochondria are membrane enclosed organelles with arrays of enzymes specialized for aerobic respiration and production of ATPs. Hence it is called the power house of cell.

- Size: 0.5-1 um in diameter & up to 10 um in length

- Shape: Elongated/rod shaped

- Number: The number of mitochondria is related to the cell's energy needs: cells with a high- energy metabolism (eg, cardiac muscle, cells of some kidney tubules, hepatocytes etc.) have abundant mitochondria, whereas cells with a low-energy metabolism have few mitochondria.

- Mitochondria are absent in RBCs and terminal keratinocytes of skin.

- Mitochondrial matrix contains a small circular chromosome of DNA, ribosomes, mRNA, and tRNA, all with similarities to the corresponding bacterial components.

- Mitochondrial DNA is a small circular double helix DNA that contains 37 genes.
- Mitochondrial DNA is inherited from mother (ovum), as cytoplasm of sperm do not contribute to zygote.
- Due to mitochondrial DNA, mitochondria are self replicating.

- Mitochondria are highly plastic, rapidly changing shape, fusing with one another and dividing, and are moved through the cytoplasm along microtubules.

Barr body:
- Tightly coiled clump of chromatin mass (heterochromatin), genetically inactive
- It represents inactivated X chromosome.**
- During cell division Barr body disappears
- Number of Barr body is always 1 less than the number of X chromosome
- Female has 1 Barr body, male has no Barr body normally.
- Klinefilter syndrome : 47,XXY ; 1 Barr body

The genetic code is a series of codons by which DNA directs transcription of RNA and its translation into proteins

Properties of genetic code:
 Specificity
 Universality
 Redundant (degenerate)
 Non-overlapping
 Comma less

1. Euchromatin:
- It is a partially condensed and less coiled 
- It is more active and lightly stained
- It is expressed during interphase
- Prominent in metabolically active cells
 
2. Heterochromatin:
- It is a condensed and more coiled 
- It is inactive and darkly stained (intensely basophilic in light microscope)
- It does not express during interphase
- Example: Sex chromatin (Barr body), chromatin in circulating lymphocyte

Lysosomal enzymes:
The most common lysosomal enzymes are acid hydrolases such as
 Proteases
 Nucleases
 Phosphatase
 Lipase
 Phospholipases
 Sulfatases
 ß-glucuronidase

Enzymes of peroxisomes: Catalase, D-and L-amino oxidases, urate oxidase, hydroxyacid oxidase

Peroxisomes are spherical organelles enclosed by a single membrane and named for their enzymes producing and degrading hydrogen peroxide, H₂O₂

Enzymes of peroxisomes: Catalase, D-and L-amino oxidases, urate oxidase, hydroxyacid oxidase

Functions of peroxisome:
1. Peroxisomes contain oxidative enzymes that are required for amino acid oxidation and ß-oxidation of fatty acids.
2. Inracellular hydrogen peroxide H₂O₂ is eliminated by the enzyme catalase which is present in the peroxisomes.
3. Catalase also degrades several toxic molecules and drugs, particularly in liver and kidney peroxisomes

Functions of Golgi apparatus:
1. Post-translational modification of proteins:
Freshly synthesized proteins are transferred from RER to the Golgi apparatus. These proteins are modified by the Golgi apparatus. This is done by glycosylation, sulfation, phosphorylation, and limited proteolysis of proteins.
2. Packaging Enzymes for lysosomes and proteins for secretion
3. Storing of all materials for lysosomes, secretion, and incorporation into the plasma membrane.
4. Lysosome may be produced in the Golgi complex.
5. It is necessary for the synthesis of acrosome in the spermatozoa.

Nucleolus:
Nucleolus is a highly basophilic spherical mass of heterochromatin in cells actively engaged in protein synthesis. The intense basophilia of nucleoli is due not to heterochromatin but to the presence of densely concentrated ribosomal RNA (rRNA).

Function of nucleolus: Ribosomal RNA (rRNA) synthesis

Smooth endoplasmic reticulum is abundant in cells involved in lipid synthesis-
• Liver cells
• Cells of adrenal cortex
• Leydig cells of testis
• SER also appears in a specialized form, called sarcoplasmic reticulum, found in the striated muscle cells.

Mitochondria have two membranes: a porous outer membrane encloses the intermembrane space and an inner membrane with many folds (cristae) enclosing a gel-like matrix.

Microtubules are non branching hollow tubules made up of tubulin proteins (α and b tubulin) with a diameter of 25 nm

Locations:
• Cilia
• Flagella
• Centrioles
• Mitotic spindle
• Elongating cell processes and
• Growing axons

Functions:
1. Movement of cilia, flagella (tail of sperm)
2. Intracellular transport of vesicles
3. In cell division, formation of mitotic spindle
4. Maintenance of cell shape

[Ref. Junquira 15th, 43]

Explanation-
Post translational modification occurs in-ribosome, RER and Golgi complex.

Ribosome lies outside nucleus in the cytoplasm.

Functions of mitochondria:
1. Production of ATP by aerobic respiration
2. Mitochondrial DNA takes part in RNA & protein synthesis
3. Role in cellular catabolism by apoptosis (programmed cell death)
4. Mitochondria contains enzymes that take part in-
   a. Respiratory chain or electron transport
   b. Oxidative phosphorylation
   c. Krebs/citric acid cycle
   d. β-oxidation of fatty acid
   e. Ketogenesis

Inner mitochondrial membrane shows long infoldings called cristae (for increasing surface area).
The lipid bilayer of the inner membrane contains unusual phospholipids and is highly impermeable to ions.
Inner mitochondrial membrane is a site for oxidative phosphorylation, respiratory (electron transport) chain, and ATP synthesis.

[Ref. Junquiera 15th, 39]

Reference: Junqueira’s Basic Histology; 14th Edition; Page-370

Functions of SER:
1. Synthesis of lipid: SER is the main site for synthesis of lipid
2. Carbohydrate metabolism
3. Synthesis of phospholipids
4. Detoxification of potentially harmful exogenous molecules such as alcohol & barbiturates in liver
5. Synthesis of steroid hormones
6. Synthesis & storage of cholesterol
7. Participate in contraction process in muscle cells (sarcoplasmic reticulum)

Functions of RER:
1. Protein synthesis: Site for translation (mRNA -> protein)
· Membrane associated proteins
· Proteins of membranous organelles
. Proteins to be secreted by exocytosis
2. Initial glycosylation of glycoprotein
3. Post-translational modification of protein
4. Assembly of multichain proteins
5. Checkpoint: RER destroys defective proteins

Smooth ER is the site of Lipid synthesis

* The basic structural unit of chromatin is the nucleosome.
Nucleosomes are composed of  DNA wrapped around a set of eight histone proteins. 

* Gene, the functional unit of DNA, is the unit of inheritance.

* The nucleotides are the basic structural units of the DNA.
• Nitrogen base + sugar = Nucleoside
• Nucleoside + phosphate group = Nucleotide
• Nucleotide polymer = Nucleic acid (DNA/RNA)

Chromatid is formed during Prophase

Cytoplasmic organelles Involved in Protein Synthesis (Translation):
1. Ribosome
2. RER
3. Mitochondria

Post-translational Modification of Protein Occurs in:
1. Golgi apparatus
2. Ribosome
3. RER

A. Prophase I: It lasts for a long time and is subdivided into the following 5 phases:

1. Leptotene : Individual chromosomes, composed of two chromatids joined at the centromere, are visible as long strands in the nucleus.

2. Zygotene: Homologous chromosomes are arranged in pairs lengthwise. This process is called synapsis or conjugation.

3. Pachytene: The two chromosomes together constitute a bivalent. A bivalent contains four chromatids in it, forming a tetrad. The two central chromatids become coiled over each other, so that they cross at a number of points (chiasmata). This is called crossing over. 

4. Diplotene: Chromosomes continue to condense and then begin to separate, revealing chiasmata.

5. Diakinesis: The homologous centromeres pull apart and the chromosomes are separated. Meanwhile the nucleolus disappears, as does the nuclear membrane, freeing the chromosomes into the cytoplasm.

Posttranslational modifications of proteins take place in Golgi complex

Plasma Membrane: 

Thickness: 7.5 to 10nm 

Composition: It is composed of 
a.Protein -55% 
b. Lipids: 42% 
  ✓ Phospholipids -25% 
  ✓ Cholesterol - 13% 
  ✓ Other lipids- 4% 
c. Oligosaccharides -3%

Protein synthesis occurs in-ribosome, RER and mitochondria.

Autophagy is the function of lysosome.

Lysosomes can digest material into the cell from its environment by phagocytosis. Hence they are known as the suicidal bags of the cell.

Functions of centriole/centrosome:
1. Centrosome initiate formation of microtubules.
2. Centrosome forms mitotic spindle.
3. Centrosome provides basal bodies for cilia and flagella.
4. Centrioles self-replicate just before cell division.