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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Cell reproduction is the process by which cells divide to produce new cells. This process is essential for Growth, development, and repair of Tissues. There are two types of cell reproduction: mitosis and meiosis.
Mitosis is the process by which a cell divides into two identical daughter cells. This process is essential for growth and repair of tissues. Mitosis occurs in four stages: prophase, metaphase, anaphase, and telophase.
Prophase is the first stage of mitosis. During prophase, the chromosomes condense and become visible. The nuclear envelope breaks down, and the spindle fibers form.
Metaphase is the second stage of mitosis. During metaphase, the chromosomes line up in the center of the cell. The spindle fibers attach to the chromosomes.
Anaphase is the third stage of mitosis. During anaphase, the centromeres of the chromosomes split, and the sister chromatids are pulled to opposite poles of the cell.
Telophase is the fourth and final stage of mitosis. During telophase, the chromosomes decondense and become invisible. The nuclear envelope reforms, and the spindle fibers disappear. The two daughter cells are now complete.
Meiosis is the process by which a cell divides into four daughter cells, each with half the number of chromosomes as the parent cell. This process is essential for sexual reproduction. Meiosis occurs in two stages: meiosis I and meiosis II.
Meiosis I is the first stage of meiosis. During meiosis I, the homologous chromosomes pair up and cross over. The homologous chromosomes then separate, and each daughter cell receives one of each homologous chromosome.
Meiosis II is the second stage of meiosis. During meiosis II, the sister chromatids of each chromosome separate, and each daughter cell receives one of each sister chromatid.
The products of meiosis are four haploid cells, each with half the number of chromosomes as the parent cell. These haploid cells can then fuse with haploid cells from another organism to form a diploid cell.
Cell reproduction is a complex process that is essential for life. Mitosis and meiosis are the two types of cell reproduction, and they play different roles in the life of an organism. Mitosis is responsible for growth and repair of tissues, while meiosis is responsible for sexual reproduction.
Cell reproduction is the process by which cells divide to produce new cells. It is essential for growth, development, and repair of the body. There are two types of cell reproduction: mitosis and meiosis.
Mitosis is the process of cell division that produces two identical daughter cells. It is the most common type of cell division and occurs in all eukaryotic cells. The process of mitosis can be divided into four stages: prophase, metaphase, anaphase, and telophase.
Prophase is the first stage of mitosis. During prophase, the chromosomes condense and become visible. The nuclear envelope breaks down and the spindle fibers form.
Metaphase is the second stage of mitosis. During metaphase, the chromosomes line up at the center of the cell. The spindle fibers attach to the centromeres of the chromosomes.
Anaphase is the third stage of mitosis. During anaphase, the centromeres of the chromosomes split and the sister chromatids are pulled to opposite poles of the cell.
Telophase is the fourth and final stage of mitosis. During telophase, the chromosomes decondense and become invisible. The nuclear envelope reforms and the spindle fibers disappear. The two daughter cells are now complete.
Meiosis is the process of cell division that produces four daughter cells with half the number of chromosomes as the parent cell. It is essential for sexual reproduction. The process of meiosis can be divided into two stages: meiosis I and meiosis II.
Meiosis I is the first stage of meiosis. During meiosis I, the homologous chromosomes pair up and exchange genetic material. The homologous chromosomes then separate and go to opposite poles of the cell.
Meiosis II is the second stage of meiosis. During meiosis II, the sister chromatids of each chromosome separate and go to opposite poles of the cell. This results in four daughter cells with half the number of chromosomes as the parent cell.
Frequently asked questions
What is the difference between mitosis and meiosis?
Mitosis is the process of cell division that produces two identical daughter cells. Meiosis is the process of cell division that produces four daughter cells with half the number of chromosomes as the parent cell.
What are the stages of mitosis?
The stages of mitosis are prophase, metaphase, anaphase, and telophase.
What are the stages of meiosis?
The stages of meiosis are meiosis I and meiosis II.
What is the purpose of cell reproduction?
The purpose of cell reproduction is to produce new cells for growth, development, and repair of the body.
What are the benefits of cell reproduction?
The benefits of cell reproduction include:
- Growth: Cell reproduction allows the body to grow and develop.
- Repair: Cell reproduction allows the body to repair damaged cells.
- Replacement: Cell reproduction allows the body to replace old or dead cells.
What are the risks of cell reproduction?
The risks of cell reproduction include:
- Cancer: Cancer is a disease that occurs when cells divide uncontrollably.
- Genetic disorders: Genetic disorders can occur when there is a mistake in the DNA during cell division.
- Infection: Infection can occur when cells are damaged or destroyed.
How can I protect myself from the risks of cell reproduction?
You can protect yourself from the risks of cell reproduction by:
- Eating a healthy diet.
- Exercising regularly.
- Getting enough sleep.
- Avoiding smoking and excessive alcohol consumption.
- Getting regular check-ups with your doctor.
Which of the following is not a stage of the cell cycle?
(A) Interphase
(B) Mitosis
(C) Meiosis
(D) CytokinesisWhich of the following is the correct order of the stages of mitosis?
(A) Prophase, metaphase, anaphase, telophase
(B) Prophase, metaphase, telophase, anaphase
(C) Anaphase, metaphase, prophase, telophase
(D) Telophase, anaphase, metaphase, prophaseWhich of the following is not a characteristic of mitosis?
(A) It is a type of cell division that results in two daughter cells that are genetically identical to the parent cell.
(B) It is a type of cell division that occurs in somatic cells.
(C) It is a type of cell division that occurs in gametes.
(D) It is a type of cell division that results in four daughter cells.Which of the following is not a characteristic of meiosis?
(A) It is a type of cell division that results in four daughter cells.
(B) It is a type of cell division that occurs in gametes.
(C) It is a type of cell division that results in daughter cells that are genetically different from the parent cell.
(D) It is a type of cell division that occurs in somatic cells.Which of the following is not a difference between mitosis and meiosis?
(A) Mitosis results in two daughter cells, while meiosis results in four daughter cells.
(B) Mitosis is a type of cell division that occurs in somatic cells, while meiosis is a type of cell division that occurs in gametes.
(C) Mitosis results in daughter cells that are genetically identical to the parent cell, while meiosis results in daughter cells that are genetically different from the parent cell.
(D) Mitosis is a type of cell division that does not involve crossing over, while meiosis is a type of cell division that involves crossing over.Which of the following is the correct order of the stages of meiosis?
(A) Prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II
(B) Prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, telophase II, anaphase II
(C) Prophase II, metaphase II, anaphase II, telophase II, prophase I, metaphase I, anaphase I, telophase I
(D) Prophase II, metaphase II, anaphase II, telophase II, anaphase I, metaphase I, prophase I, telophase IWhich of the following is not a characteristic of prophase I of meiosis?
(A) The nuclear envelope breaks down.
(B) The chromosomes condense.
(C) The homologous chromosomes pair up.
(D) The centromeres divide.Which of the following is not a characteristic of metaphase I of meiosis?
(A) The homologous chromosomes line up at the metaphase plate.
(B) The centromeres attach to the spindle fibers.
(C) The chromosomes are pulled apart by the spindle fibers.
(D) The nuclear envelope reforms.Which of the following is not a characteristic of anaphase I of meiosis?
(A) The homologous chromosomes are pulled apart by the spindle fibers.
(B) The centromeres divide.
(C) The chromosomes move to opposite poles of the cell.
(D) The nuclear envelope reforms.Which of the following is not a characteristic of telophase I of meiosis?
(A) The chromosomes decondense.
(B) The nuclear envelope reforms.
(C) The spindle fibers disappear.
(D) The chromosomes move to opposite poles of the cell.Which of the following is not a characteristic of prophase II of meiosis?
(A) The nuclear envelope breaks down.
(B) The chromosomes condense.
(C) The centromeres divide.
(D) The homologous chromosomes pair up.Which of the following is not a characteristic of metaphase II of meiosis?
(A) The chromosomes line up at the metaphase plate.
(B) The centromeres attach to the spindle fibers.
(C) The chromosomes are pulled apart by the spindle fibers.
(D) The nuclear envelope reforms.Which of the following is not a characteristic of ana