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What is the difference between chromatin and chromosome?

Chromatin is a mass of genetic material composed of DNA and proteins.  Chromatin condense to form chromosomes during eukaryotic cell division.
During prophase of mitosis, chromatin fibers become coiled into chromosomes with each chromosome having two chromatids joined at a centromere.

What is the difference between chromosome and chromatid?

A chromatid is one of two identical halves of a replicated chromosome.  Following DNA replication, the chromosome consists of two identical structures called sister chromatids, which are joined at the centromere.

How does the gap 1 depend on the rate of division and cell differentiation, does it mean that the more the rate of replication and the less the cell replication the less gap 1?

When there is a rapid rate of cell division, Gap 1 becomes shorter.

I can't understand "G0 stage".

Following mitosis, the cell can enter the G0 (resting) phase, which is outside the cell cycle proper. In G0 the cell is not actively proliferating. Cells can spend long periods in G0 before they reenter the cell cycle in G1 phase. 
Some such as muscle cells and nerve cells remain there permanently; others, such as liver cells, can resume G1 phase in response to factors released during injury.

Is the meaning of differentiation the same meaning of division?

Differentiation does not mean division and not necessarily include division, it is change of the structure of the cell to be more mature (functioning).

Plz dr, why there is no S phase in between the first and second meiotic division?

No duplication of the DNA content so no S phase

I don't understand meiosis how bivalent arrangement occur at prophase1 I need a picture about it and difference between arrangement of chromosome to differentiate between meosis & mitosis?

I can't understand non disjunction clearly, could you please illustrate with a diagram.

Drs, it is not very clear for me "classification of chromosomes according to the position of the centromere".

What is kinetochore?

The site of attachment of spindle fibers on the chromosome during cell division, it is present on the centromere.

What is meant by homologous and non homologous chromosomes?

Homologous chromosomes - chromosomes of the same size and shape which carry the same type of genes, non-homologous chromosomes non-similar chromosomes e.g. :
Example - an organism is 2n = 4. 

Chromosomes 1 & 2 are homologous chromosomes

Chromosomes 3 & 4 are homologous chromosomes

Chromosomes 1 & 3 came from the mother

Chromosomes 2 & 4 came from the father


Are the two chromatids of the chromosome identical or not?

Yes, they are identical.

What is meant by non sister chromatids of homologous chromosomes?

Non sister means not from the same chromosome (one from mother and the other from the father).

Please, what's the difference between 46-s chromosome and 23-d chromosome? or they are the same?

Not the same
S chromosomes are formed of one chromatid (single), they are present in the stages: telophase and G1 phase.
D chromosomes are formed of 2 chromatids (double), they are present in S phase, G2 phase and mitotic stages from prophase to anaphase.

Doctor, what is the microtubular organizing center?

Microtubule-organizing centers (MTOC) is a term used to describe any cytoplasmic area rich in TUBULIN protein. MTOCs mediate the nucleation of the protein tubulin into microtubules, and help organize the microtubule cytoskeleton in the cell. Animal cells contain centrioles, which contain tubulin and other proteins organized into centrosomes. During mitosis, the centrioles divide and migrate to form two poles of the mitotic spindle, important for the segregation of chromosomes. Basal bodies are a type of MTOC able to organize microtubules into the parallel arrays at the center of cilia and flagellae.

Doctor, what's meant by anaphase lag?

Anaphase lag:  delayed movement during anaphase of one homologous chromosome in mitosis or of one chromatid in meiosis, so that the chromosome is not incorporated into the nucleus of one of the daughter cells; the result is one normal cell and one cell with monosomy (2n-1).

How are the blood cells and male germ cells examples of both continously renewing cells ( which don't divide ) and the stem cells?

Blood cells are examples that come from Pleuri/Multipotential stem cells which can give rise to more than one type of cells .. RBCs , Neutrophils , Basophils ... etc ..
But the Unipotential stem cells give rise to one type only! like the male germ cells .. give rise to Sperms .. It cannot give rise to Ova (as an example).
That's why Blood cells & Sperms are Continuously renewable cells.

Doctor I can't understand what really happens in diplotene stage of prophas I of meiotic division.

The first substage of prophase I:
Leptotene (Leptonema) : The term leptonema derives from Greek words meaning "thin threads". Chromosomes become visible as long slender threads bearing numerous bead-like nucleosomes (chromomeres). These are arranged in a linear fashion along their lengths. The nuclear envelope and the nucleolus are prominently visible. The thin chromosomes are scattered in the nucleus.

why are iso-chromosomes called "iso" although we have unequal fragments after division?

"Iso-chromosomes" means the 2 arms are identical in the resulting chromosomes.

Doctors, I can't understand "centric fusion".

Centric fusion in acrocentric chromosomes: fusion of the long arm of chromosome 21 and 14. A long chromosome is formed. The short arms are lost. The other name is "Robertsonian translocation". When the translocation is balanced, the person with it is called a Robertsonian translocation carrier. As carriers are healthy and have a normal lifespan, many never discover about their unusual chromosome rearrangement. In fact, the translocation can be passed down in families for many generations without anyone discovering.  An unbalanced Robertsonian translocation may come to light after a baby is born with a chromosome disorder.  The Robertsonian translocations between 14 and 21 may result in Trisomy 21 (Down) syndrome.  

The chromosomal aberrations are balanced or not: ring chromosome, centric fusion and duplication state?

Ring chromosome not balanced (lost parts).
Centric fusion balanced (the lost parts do not carry genetic information).
Duplication not balanced (contains extra piece).

Is the isochromosome unbalanced? plz explain why.

Isochromosome is not balanced because it contains double genetic information of one arm and lost genetic information for the other arm.

How can we call second mieotic division like mitotic despite the daughter cell "23 s " not identical to parent cell "23 d"?

They are similar but not identical, the mitotic division starts by a cell with 46 D chromosomes and ends by 2 daughter cells with 46 S chromosomes. The second meiotic division starts by cells with 23 D chromosomes and ends by daughter cells with 23 S chromosomes.

what does mitogenic agent mean?

Mitogenic agents: agents eg drugs irradiation etc that cause gene mutation

Please drs I cannot understand mosaic.

This is a post fertilization event. Some cells are normal (the above in the diagram) and others show chromosomal aberrations( the lower part) The symptoms are not severe because the individual has some normal cells.

Please drs: tetraploid results from failure of 1st cleavage of the fertilized ovum, I can't understand?

This is a post fertilization event.
First an ovum (1n) is fertilized by a sperm (1n) resulting into a zygote (fertilized ovum) (2n).
Then, the fertilized ovum fails to separate into 2 cells (failure of cleavage) with duplication of the chromosome number  resulting into a cell with 2x2 n= 4n.