স্পেশাল বিসিএস (স্বাস্থ্য) - লং কোর্স

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%

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

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.

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.

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

***Microvilli has core of microfilaments.

Microtubules:
Microtubules are non-branching hollow tubules made up of tubulin proteins (∝ and ß 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

Most of the cell contain single nucleus except
– RBCs and platelets do not have nuclei
– Striated muscle cells are multinucleated
– Few hepatocytes and transitional epithelial cells are binucleated

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

Events of cell cycle:
1. Mitosis (M phase)
2. Interphase: The long period between two mitosis division (the G1, S, and G2 phases) is  called interphase.
• G1 phase (pre-synthetic phase)
• S phase
• G­­­2 phase (post-synthetic phase)

G1 Phase:
- The period between mitosis and the beginning of DNA replication.
- The G1 phase, usually the longest and most variable part of the cycle.
- It is a period of active RNA and protein synthesis (RNA & regulatory proteins essential for DNA replication)
- Also in G1, the cell volume, reduced by half during mitosis, returns to its previous size.

S Phase:
- The S phase is characterized by DNA replication, histone synthesis, and the beginning of centrosome duplication.
- The cell now contains twice the normal amount of its DNA, that is - the (2n) amount of DNA is doubled (4n) in preparation for cell division.

G2 Phase:
- In the relatively short G2 phase, RNA and proteins essential for cell division are synthesized.

Stages of Mitosis:

1. Prophase: 
- The nucleolus disappears.
- The chromatins become coiled. They condense, shorten, and thicken into discrete chromosomes.
- The microtubules of the mitotic spindle appear between the two centrosomes.

2. Metaphase:
- Chromosomes migrate to the equatorial plane of the cell due to the activity of microtubules.
- Each chromosome divides longitudinally to form two chromosomes called sister chromatids.
- The chromatids attach to the microtubules of the mitotic spindle at centromere

3. Anaphase:
- The sister chromatids of each chromosome separate from each other and migrate toward the opposite poles of the cell.
- The centromeres also move away from the centre, pulling along the remainder of the chromosome.

4. Telophase:
- Nucleoli reappear in the daughter cells.
- The chromosomes become less distinct and a new nuclear envelope appears around each daughter chromosomes.
- A constriction develops at the equatorial plane of the parent cell and progresses until the cytoplasm and its organelles are divided in two.
- Formation of a new plasma membrane between the two portions of the dividing cell completes the two new cells, termed daughter cells.

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.

The first meiotic division reduces the chromosome number (Reductional division), but the DNA amount is unchanged (2n)

The second meiotic division reduces the DNA amount (n), but the chromosome number is unchanged (Equatorial division)

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

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

Autosomal Dominant Disorders
• Disorders of autosomal dominant inheritance are manifested in the heterozygous state, so at least one parent in an index case usually is affected. 
• Both males and females can be affected 
• Both sexes can transmit the condition.
• When an affected person marries an unaffected one, each child has one chance in two of having the disease (50%).
• New mutation can occur
• Variable expressivity
• The age of onset is delayed
• Vertical transmission (consecutive generations are affected)
• No carrier state
• Father-son transmission possible
• Condition usually less severe than autosomal recessive
• Usually associated with structural abnormalities

Autosomal Recessive Disorders 
• They make up the largest group of Mendelian disorders. 
• Manifested in the homozygous state. They occur when both of the alleles at a given gene locus are mutants.
• The trait does not usually affect the parents
• Both the parents are carriers
• Siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for each birth); 
• May be result of a consanguineous marriage. 
• The expression of the defect tends to be more uniform than in autosomal dominant disorders.
• Complete penetrance is common
• Onset is frequently early in life. 
• In many cases, enzymes are affected by the mutation (Metabolic abnormalities).

Hemophilia- X-linked recessive
Myotonic dystrophy, Osteogenesis imperfecta- Autosomal dominant

Examples of autosomal recessive disorders: 

Metabolic:
1. Homocystinuria/Alpha-1 antitrypsin deficiency
2. Cystic fibrosis
3. Phenylketonuria
4. Alkaptonuria
5. Wilson’s disease
6. Galactosemia
7. Lysosomal storage disease
8. Glycogen storage disease

Haematopoietic:
1. Thalassaemia
2. Sickle cell anaemia

Endocrine:
1. Congenital adrenal hyperplasia

Nervous: 
1. Neurogenic muscular atrophy
2. Friedrich’s ataxia
3. Spinal muscular atrophy

Skeletal: 
1. Some variants of Ehlers-Danlos syndrome

*** There is no male to male transmission in X-linked disorders, because in male the X chromosome comes from the mother. 

Criteria of X-linked recessive disorders:
1. Affected cases are usually males carrying the gene.
2. Homozygous females can be affected, but this is rare. 
3. Heterozygous females usually do not express the full phenotypic changes because of the paired normal allele.
4. Only females are carriers.
5. If a female has the allele (female carrier)- with a normal male, off-springs will be-
    - 50% daughters carrier, 50% normal
    - 50% sons affected, 50% normal
6. If a male has the allele (affected male)- with a normal male the off-springs will be-
    - All daughters carrier
    - All sons normal (no male to male transmission)
7. Affected cases have affected brothers and maternal uncles.
8. Since females have two copies of X-chromosomes and males have only one, X-linked recessive disease are much more common among males than in females.

45XO- Turner's syndrome
47 XXY- Klinefelter's syndrome
47 XX/XY+18- Edward's syndrome (Trisomy 18)
47 XX/XY+13- Patau's syndrome (Trisomy 13)

Down's syndrome:
Also known as Trisomy 21 is the most common numerical chromosomal abnormality.

Cause :
- Non disjunction (94%)- 47 XX/XY+21 (Trisomy 21)
- Mosaicism (2%)
- Transloation (4%)

***Wall of splenic sinusoids does not have pericytes or smooth muscle cells. Elongated endothelial cells called stave cells line the splenic sinusoids.

Features of epithelium:
- Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of extracellular matrix (ECM).
- The forms and dimensions of epithelial cells range from high columnar to cuboidal to low squamous cells. Epithelial cell nuclei vary in shape corresponding roughly to the cell shape.
- Epithelium is an avascular structure. It receives nutrition by diffusion.
- The epithelia have got nerve supply
- Epithelial cells rest on basement membrane. 
- Epithelial cells show various types of junctions with adjacent cells and basement membrane.
- Epithelial cells isolate deeper structures from surface environment.
- Epithelial cells show surface modifications as per functional need. 
- Epithelial cells undergo mitosis and can regenerate damaged portion.
- Epithelium is derived from all three germ layers.
- Epithelial cells generally show polarity. 

Classification of glands based on nature of secretion:
1. Mucous glands- Secrete thick mucoid secretion called mucin.
Example: Goblet cells, sublingual salivary glands

2. Serous glands- Secrete a protein rich watery secretion.
Example: Exocrine pancreas, parotid glands, lacrimal gland, von Ebnar's gland in tongue.

3. Mixed glands- Which contain both mucous and serous cells and produce sero-mucous
secretion. Example: Submandibular salivary gland.

1. Intramembranous ossification:
Involves direct conversion of mesenchymal tissue to bone. Bones produced by this process are called membranous (flat) bones. 
Examples: Bones of cranial vault, facial bones.

2. Endochondral (cartilaginous) ossification
Involves conversion of mesenchyme to hyaline cartilage that later gets replaced by bone.
Examples: All long bones (except clavicle), base of skull, vertebrae & ribs are formed by cartilaginous ossification. 

Both "ক) Atlantoaxial joint" and "খ) Superior radioulnar joint" are correct answer.
The question has been canceled due to double answer options.

Function of thymus:
1.Production, differentiation and maturation and maturation of T-lymphocytes
2.Secretion of thymopoietin by epithelioreticular cells. Thymopoietin stimulates T cell production.
3.Thymus is essential for development of immunity in early life. 
4. Another main function of the thymus is induction of central tolerance, which along with regulatory T cells prevents autoimmunity

Paracortex: 
The region between the cortex and medulla, the paracor­tex or deep cortex does not have precise boundaries but can be distinguished from the outer cortex by its lack of nodules (B-lymphocytes). Paracortex mainly consists of T cells; hence, it is also called thymus-dependent cortex.

Myofibrils are the structural and functional subunits of muscle fiber. Myofibrils are arranged longitudinally in the muscle fiber and runs in the entire length of muscle fiber.

Sarcomere is the structural & functional unit of myofibrils.