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*Self-practice*

10 th -Science Part 2

Topic 2: *Life Process in Living organisms Part-1*

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 *Today's exercise*

Read the unit "Mitosis" from your textbook. Page No. 18 and 19. 

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To understand the process of cell division, some basic concepts need to be clarified.  So the last two days I have been writing about it in detail.

 *Learn Chromosomes*

🔹 *What is the differences between DNA and chromosomes?*

◆ The nucleus is major organ of every cell. The nucleus contains the nucleic acids. This nucleic acid is DNA (deoxyribonucleic acid).  When viewed under an electron microscope, it becomes clear that these nucleic acids, which look like liquids, are long strands of DNA.

◆ As the cell division process begins, these threads begin to wind up like a spring, forming the X-shaped pair of chromosomes.  Midpoint of X-shaped chromosome is called as "centromere". Two 'chromosomes' are attached to this centromere.

🔹 *How many chromosomes are there in a human?*

In humans, a single cell contains a total of 46 chromosomes or 23 pairs of chromosomes.  Therefore, somatic cells are called diploid (2n) cells. However a single germ cell has only 23  chromosomes, they are called haploid (n) cells.

There is a misconception that our body has only 46 chromosomes.  There are 46 chromosomes in a single cell in our body.  This means that there are 46 chromosomes in each cell of our body.  Now you calculate the total number of chromosomes in the billions of cells in the body. Answer, would be 'Countless'. Isn't it?  But remember one thing, every cell has the same set of chromosomes. The sequence of genes changes only when there is a mutation.

🔹 *What is the number of chromosomes at the time of cell division?*

In cell division a duplicate cell is created from a single cell. So before division, the cell doubles the number of 46 chromosomes in its nucleus. That is, 46 (2n) chromosomes are doubled to 92 (4n) chromosomes. In a germ cell 23 (n) chromosomes were doubled to 46 (2n) chromosomes. (Here 'n' means one set of chromosomes i.e. 23 chromosomes)

🔹 *Why coiling of DNA takes place?*

Each chromosome within a cell has its own strand of DNA. So with 23 pairs of human chromosomes, every human cell should host 46 strands of DNA — each wrapped around hundreds of thousands of histones. This tight coiling helps the body to pack its long DNA  molecules into very tiny spaces.

DNA carries traits from one generation to the next.

🔹 *How many times does a cell divide in its lifetime?*

The lifespan of each cell is different. Different cells live from 4 days to 8 years. Cell division occurs at least 50 to 70 times during cell life. As it divides, the length of one arm in its chromosome decreases.  According the lenghth of telomeres chromosomes are classifieds in 4 groups.  1) Metacentric 2) sub-metacentric 3) Acrocentric and 4) Telocentric.

Detail information about types of chromosomes is given on page No.181 in class 9th textbook.

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 *Shrishail Mathapati,* Kolhapur

(9922242470)

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