Cell reproduction – Cell cycle Mitosis and Meiosis

CELL REPRODUCTION

Cell reproduction is the process by which cells divide to form new cells.  Each time a cell divides, it makes a copy of all of its chromosomes, which are tightly coiled strands of DNA, the genetic material that holds the instructions for all life, and sends an identical copy to the new cell that is created.

Cell cycle

The cell cycle or cell-division cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells.The cell cycle has 5 phases:

G 1 Phase: During the G1 Phase, the cell grows and stores up energy that it will use during cell division.  Nutrients are taken in and all the usual cell processes take place.  Once cells are fully grown, they proceed on to the S Phase.

S Phase: During the S Phase, the DNA in the cell’s nucleus is copied.  This means that the cell then attains two copies of all the necessary DNA for normal cell activity, leaving a full set to be transferred into the new cell that will be created after the cell divides.

G 2 Phase: During this phase, the cell prepares for cell division.  This phase represents a time gap between the time when the cell copies its DNA and when it divides.

M Phase: During this phase, cell division takes place through Mitosis.

Cytokinesis: During Cytokinesis, the cytoplasm in the cell divides and the cell’s membrane pinches inward and the cell begins to divide.  Also, when plant cells divide, a cell plate forms between the two new cells to divide them.  After this step, the new cell and sometimes the original cell also restart the cell cycle by beginning G1 Phase again.  However, sometimes cells enter G0 phase, which is a phase where cells exit the cell cycle after they are fully grown and continue to serve their purpose in an organism.

 

MITOSIS

Mitosis, a process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells. Strictly applied, the term mitosis is used to describe the duplication and distribution of chromosomes, the structures that carry the genetic information.

Phases of mitosis

Interphase

Before a cell can reproduce, it has to perform a variety of activities to get ready. The stage of the cell cycle when a cell is preparing itself to duplicate is called interphase. Since so many things are happening in the cell at this time, most of the cell’s life is spent in this stage. While preparing to reproduce, the cell makes more cytoplasm (the gel-like substance found inside the cell membrane that bathes the organelles) and increases its supply of proteins. When it’s ready, it goes through three sub-phases of interphase: G1, S, and G2.

Preprophase

This phase takes place in plant cells only. The preprophase band is a microtubule array found in plant cells that are about to undergo cell division and enter the preprophase stage of the plant cell cycle. Besides the phragmosome, it is the first microscopically visible sign that a plant cell is about to enter mitosis.

Just before mitosis starts, the preprophase band forms as a dense band of microtubules around the phragmosome and the future division plane just below the plasma membrane. It encircles the nucleus at the equatorial plane of the future mitotic spindle when dividing cells enter the G2 phase of the cell cycle after DNA replication is complete. The preprophase band consists mainly of microtubules and microfilaments (actin) and is generally 2-3 µm wide. When stained with fluorescent markers, it can be seen as two bright spots close to the cell wall on either side of the nucleus.

Prophase

The process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. During prophase, the complex of DNA and proteins contained in the nucleus, known as chromatin, condenses. The chromatin coils and becomes increasingly compact, resulting in the formation of visible chromosomes. Chromosomes are made of a single piece of DNA that is highly organized. The replicated chromosomes have an X shape and are called sister chromatids. The sister chromatids are pairs of identical copies of DNA joined at a point called the centromere. Then, a structure called the mitotic spindle begins to form. The mitotic spindle is made of long proteins called microtubules that begin forming at opposite ends of the cell. The spindle will be responsible for separating the sister chromatids into two cells.

Premetaphase

In this phase, the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. During prometaphase, the physical barrier that encloses the nucleus, called the nuclear envelope, breaks down. The breakdown of the nuclear envelope frees the sister chromatids from the nucleus, which is necessary for separating the nuclear material into two cells. Another important event during prometaphase is the development of a protein formation called a kinetochore around the centromere, the central point joining the sister chromatids. Long protein filaments called kinetochore microtubules extend from poles on either end of the cell and attach to the kinetochores.

Metaphase

In this nuclear envelope has entirely vanished and the chromosomes have condensed , which means that they have become tightly coiled and are now clearly visible even under an ordinary light microscope. In addition, the microtubules of the spindle apparatus have attached to the centromeres at their kinetochores. The centrosomes are now at opposite ends (“poles”) of the cells.

Anaphase

During anaphase A, the cohesins that bind sister chromatids together are cleaved, forming two identical daughter chromosomes. Shortening of the kinetochore microtubules pulls the newly formed daughter chromosomes to opposite ends of the cell. During anaphase B, polar microtubules push against each other, causing the cell to elongate. In late anaphase, chromosomes also reach their overall maximal condensation level, to help chromosome segregation and the re-formation of the nucleus. In most animal cells, anaphase A precedes anaphase B, but some vertebrate egg cells demonstrate the opposite order of events.

Telophase

Telophase is the fifth and final phase of mitosis, the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. Telophase begins once the replicated, paired chromosomes have been separated and pulled to opposite sides, or poles, of the cell. During telophase, a nuclear membrane forms around each set of chromosomes to separate the nuclear DNA from the cytoplasm. The chromosomes begin to uncoil, which makes them diffuse and less compact. Along with telophase, the cell undergoes a process called cytokinesis that divides the cytoplasm of the parental cell into two daughter cells.

Cytokinesis

Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. It occurs concurrently with two types of nuclear division called mitosis and meiosis, which occur in animal cells. Mitosis and each of the two meiotic divisions result in two separate nuclei contained within a single cell. Cytokinesis performs an essential process to separate the cell in half and ensure that one nucleus ends up in each daughter cell. Cytokinesis starts during the nuclear division phase called anaphase and continues through telophase. A ring of protein filaments called the contractile ring forms around the equator of the cell just beneath the plasma membrane. The contractile ring shrinks at the equator of the cell, pinching the plasma membrane inward, and forming what is called a cleavage furrow. Eventually, the contractile ring shrinks to the point that there are two separate cells each bound by its own plasma membrane.

MEIOSIS

Meiosis The form of cell division that creates gametes, or sex cells (eggs or sperm) is called meiosis. It is a special form of reproduction that results in four next-generation cells, rather than just two, from each cell.

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange DNA by homologous recombination leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and two small polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal DNA, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act.

Phases of meiosis

There are two main phases of meiosis: meiosis 1 and meiosis 2

Meiosis 1 has following phases:

Prophase 1

Prophase I is typically the longest phase of meiosis. During prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). This often results in chromosomal crossover. This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma). Prophase I has historically been divided into a series of substages which are named according to the appearance of chromosomes. Prophase 1 involves Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Metaphase 1

  • The centrioles are at opposite poles of the cell.
  • The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate.
  • Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).

Anaphase I

  • Anaphase 1 begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle.
  • Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.

Telophase I

The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I. Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.

Meiosis 2

Prophase 2

  • While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II.
  • The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes.
  • The nuclear envelope breaks down, and the spindle apparatus forms.

 

Metaphase 2

  • Each of the daughter cells completes the formation of a spindle apparatus.
  • Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate.
  • For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole.

Anaphase 2

The centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes in their own right.

Telophase 2

  • A nuclear envelope forms around each set of chromosomes.
  • Cytokinesis takes place, producing four daughter cells (gametes, in animals), each with a haploid set of chromosomes.
  • Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental chromosomes.
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