Cells

[Ref. Junquira 15th, 62]

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.

Fibroblasts are the most common cells in connective tissue proper, produce and maintain most of the tissue’s extracellular components. Fibroblasts synthesize and secrete collagen (the most abundant protein of the body) and elastin, which both form large fibers, as well as the GAGs, proteoglycans, and multiadhesive glycoproteins that comprise the ground substance.

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

*** Normoblasts have nucleus, after this stage, they loss their nuclei to become reticulocytes, and the mature RBCs. 

Phagocytosis of ECM components and debris done by macrophage

Autosomal dominant disorders: 
Skeletal
1. Marfan syndrome
2. Osteogenesis imperfect
3. Achondroplasia
4. Ehlar Danlos Syndrome
Nervous
1. Huntington’s disease
2. Neurofibromatosis
3. Myotonic dystrophy
4. Tuberous sclerosis
Urinary
1. Polycystic kidney disease
Metabolic
1. Familial hypercholesterolemia
2. Acute intermittent porphyria
Gastrointestinal
1. Familial adenomatous polyposis coli
Haematopoeitic
1. Von Willebrand disease
2. Hereditary spherocytosis

The centromeres also move away from the centre, pulling along the remainder of the chromosome during Anaphase.

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.

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)

RBC has no mitochondria

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