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ALevel

Biology-9700

Replication And Division Of Nuclei And Cells

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Replication And Division Of Nuclei And Cells

Replication And Division Of Nuclei And Cells


Chromosomes

Chromosomes consist of DNA molecules which are coiled upon histone proteins. Eight histone proteins coil together to form a structure called a nucleosome. This is then coiled upon itself.

When the cell is not diving DNA is in uncoiled form. Thus, it appears as very thin threads present in the cytoplasm. This is called the chromatin network.

During cell division, this coils around centromere to form chromosomes. Chromosomes have two arms one long arm is called p and a short arm is called q.

Chromosomes can be classified according to the position of the centromere.

The portion of the chromosome from centromere to its end is called the arm of the chromosome.

The identical strands of DNA are called a chromatid. Before cell division, each chromosome has only one chromatid. After DNA replication each chromosome will have two chromatids. Thus, during cell division, the number of chromosomes remains the same while the amount of DNA is doubled.

Telomere

Each chromosome consists of a sequence of repeating nucleotides at its end called the telomere.

During every cycle of DNA replication, the end of chromosomes is shortened. To protect the DNA from cell damage telomere are present at the end of chromosomes like a cap.

Thus, after cell division telomeres are shortened instead of chromosomes.

Cell Cycle

Each cell cycle consists of two phases.

1. Interphase
2. Mitosis

Interphase

Interphase is the nondividing phase of the cell cycle. During this phase, the cell grows and prepares itself for the next division.

Interphase has three phases.

1) G1 Phase

During this phase, cell synthesizes RNA and proteins that are required for cell division.

2) S Phase

S phase also called the synthesis phase. In this phase, the cell replicated its DNA. Before replication cell has one copy of DNA after replication it has two copies.

3) G2 Phase

The cell continues to grow in this phase. The organelles are replicated.

Mitosis

Mitosis is the dividing phase of the cell cycle. In this, the nucleus is divided into two daughter nuclei called karyokinesis.

After mitosis, the cell performs cytokinesis to divide the cytoplasm.

Phases

Mitosis is further divided into further phases.

Prophase


During this phase, the nuclear membrane starts disappearing.

The centrioles replicate and move towards the opposite pole.

Chromosomes Start Coiling And Become Visible.

Centriole starts forming spindles.
 

Metaphase


Chromosomes are arranged at the equatorial plane right angle to the fibers formed by centriole.

Chromosomes form kinetochore at centromere for attachment of spindle fibers.

Anaphase


Spindle fibers start shortening thus pulling each chromosome towards the opposite pole.

Chromosomes breaks at the centromere and each half are moved towards the opposite side.

Telophase


Chromosomes reach the opposite pole.

Spindle fibers start disappearing

The nuclear membrane starts forming around each set of chromosomes.

Hence Two Daughter Nuclei Are Formed.

Cytokinesis

After the division of the nuclei, cytoplasm starts diving. This process is called cytokinesis.

Significance Of Mitosis


1. Formation Of Identical Daughter Cells

Through mitosis daughter cells can be formed which are an exact copy of the parent cells.

2. Healing Of Wounds

New cells are required for tissue repair after injury. This task is fulfilled by mitosis.

3. Growth

Mitosis helps in the production of new cells which are required for increasing the size of the organism.

4. Asexual Reproduction

Mitosis is the basis of asexual reproduction. New daughter cells are formed by a single parent.

Mitosis And Uncontrolled Cell Division


There are various genes in the cell that act as checkpoints in the cell cycle. They control when a cell will undergo division and when it will stop by responding to various stimuli.

Mutations in these genes result in uncontrolled mitosis. The cell will keep on dividing. This will result in the formation of a large number of abnormal cells. This is called a tumor. Some tumor cells might break and enter the bloodstream. Now the tumor is said to be metastasized. These abnormal cells will deposit in other organs and form tumors there.

Stem Cells

Stem cells are pluripotent cells. These cells are undifferentiated and can divide and specialize to form cells of any lineage.

For example:
A stem cell can specialize to form a red blood cell or it can be specialized to form liver cells. This depends on the group of genes that are activated during their maturation. In order to become RBC, only genes coding hemoglobin will be activated while others will be turned off.

Significance Of Stem Cell In Tissue Repair

There are two types of stem cells.

1) Embryonic stem cell
2) Adult stem cell

These cells can be used for repair of various tissues due to their ability to form cells of any lineage. They divide by mitosis either to form a new stem cell or a cell that goes on to mature into specialized cell type.

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