Mitotic Cell Cycle - Session 1

We know that every living organism has DNA. DNA stands for deoxyribonucleic acid. It contains the genetic information. It is like a blueprint or instruction manual for creating and maintaining an organism. It contains all the information needed to determine our traits, such as eye color and height. Do you know how traits are transferred from parents to child? It happens through DNA. In our cells, DNA is organized into structures called chromosomes.

Chromosomes are structures that look like threads. They are found inside the nucleus of our cells. They act as packages that hold and protect our DNA. Now, let us zoom in on chromosomes and talk about chromatids. When a cell is getting ready to divide, the chromosomes undergo a process called replication. During replication, each chromosome makes an identical copy of itself. These copies are called chromatids.

So, now we have a pair of chromatids for each chromosome. The chromatids are exact copies of each other because they contain the same genetic information. Chromatids are held together by a special region called centromere. Think of it as a glue that keeps the chromatids attached at a specific spot on the chromosome.

Now that we understand chromatids and centromeres. Let us shift our focus to the structure of chromosomes at a molecular level. Chromosomes are made up of DNA, but there's more to it. DNA is wrapped around proteins called histones. Histones are like spools around which the DNA is tightly wound. They provide support and help organize the long DNA molecule into a more compact and manageable form. The DNA histone complex is called chromatin. It forms the basic unit of chromosomes.

Within the chromatin, there are smaller units called nucleosomes. Nucleosomes are the basic structural units of chromatin. Nucleosomes consist of DNA wrapped around a cluster of histone proteins. They look like beads on a string, where the string represents the DNA and the beads represent the histones. These nucleosomes take up a vital role in the packaging of DNA within chromosomes. They help protect the DNA and ensure that it is properly organized and compacted.

Let us explore the process by which cells divide. This process is called the cell cycle. It consists of several distinct stages. These stages are interphase, mitosis and cytokinesis. We'll start with interphase, which is a crucial part of the cell cycle. Interphase is the longest phase of the cell cycle. It's like a preparation phase where the cell gets ready for division. There are three stages in interphase. These are G1 phase, S phase and G2 phase.

The first stage of interphase is called the G1 phase. During this phase, the cell grows in size and carries out its normal functions. The cell also checks its internal environment and external signals to ensure it is ready for DNA replication. Think of the G1 phase as the growth and preparation stage. In this phase cell accumulates the necessary resources and energy to proceed with the cell cycle.

The next stage of interphase is the S phase. S phase is also known as the synthesis phase. During this phase, the cell undergoes DNA replication. The DNA molecules unwind and separate. New exact copies of the original DNA are created in S phase. Imagine the S phase as a DNA replication factory. The cell copies its genetic information to ensure that each daughter cell will receive a complete set of chromosomes.

After the S phase, the cell enters the G2 phase. During this stage, the cell continues to grow. It synthesizes proteins and prepares for cell division. It checks for any errors in the replicated DNA and repairs them. The cell also produces additional organelles and molecules needed for the upcoming division. Think of the G2 phase as the final preparations before the cell moves on to mitosis. It ensures that the cell is equipped with all the necessary components for successful division.

After the G2 phase, cell undergoes mitosis. Mitosis is the phase of the cell cycle where the nucleus divides. This ensures that each daughter cell receives an identical set of chromosomes. Mitosis consists of several distinct stages. These stages are prophase, metaphase, anaphase, and telophase.

During prophase, the chromatin condenses and becomes tightly coiled. The chromosomes become visible under a microscope as distinct structures. The nuclear membrane starts to break down. The centrosomes begin to move to opposite poles of the cell.

In metaphase, the condensed chromosomes line up along the middle of the cell. These chromosomes are aligned at the equatorial plane of the cell. This results in the formation of a single plane. This plane is called the metaphase plate. This process ensures that each daughter cell will receive an equal and complete set of chromosomes.

The next stage of mitosis is anaphase. During anaphase the centromeres split. This allows the sister chromatids to separate and move towards opposite ends of the cell. The spindle fibers, which are protein structures, assist in pulling the chromatids apart.

During telophase, the separated chromatids reach the opposite ends of the cell. The nuclear membranes begin to reform around each set of chromosomes. The chromosomes start to uncoil and return to their chromatin form. The spindle fibers disassemble. The cell now prepares for cytokinesis.

Cytokinesis is the process by which the cytoplasm divides. This division of cytoplasm creates two separate daughter cells. This process follows immediately after mitosis. Cytokinesis completes the cell division process. After the cytokinesis, cell undergoes interphase again and whole cell cycle repeats. In animal cells, cytokinesis occurs through a process called cleavage. A contractile ring forms around the center of the cell. This contractile ring consists of actin filaments and myosin proteins. As the contractile ring contracts, it squeezes the cytoplasm of cell. This creates a furrow that deepens until it eventually pinches the cell into two daughter cells.

Cytokinesis in plant cells differs from animal cells due to the presence of a rigid cell wall. During late telophase, vesicles containing cell wall material gather at the middle of the cell. These vesicles form a structure called the cell plate. The cell plate gradually enlarges and fuses with the existing cell wall. This results in division of the cell into two daughter cells.