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3.1: Introduction to Cell Division

Growth in shape (width), size (height), and number (population), is characteristics of all living being including plants all are due to cell division. Growth in number means reproduction -a process by which living plants produce one of their own kind.

Rudolf Virchow 1855 first proposed that new cells arise from pre-existing cells by division-"Omnis cellula e cellula" or "All cells from cells." and are thus called daughter cells. Continuity of life depends on cell division. In unicellular plants reproduction takes place asexually by binary fission, or splitting into two cells. The two new daughter cells that are produced are identical to each other and to the parent cell.All multicellular plants start their life as a single cell-the zygote which is a product of the fusion (fertilisation) of usually two haploid gametes- a female and a male gamete. The zygote (diploid) divide and redivide to give rise multicellular plants have a huge variety of different types of cell, specialised for different functions.

  1. Haploid(n) - Plants/cells have only one set of chromosomes, denoted as N.
  2. Diploid(2n) -The union of two haploid gametes(male gametes N + female gametes N= zygote 2N) results in the diploid zygote, plants with two (di) sets of chromosomes are known as diploid and denoted as 2N. Ploidy is a term denoting the number of sets of chromosomes.
  3. Polyploid - Plants with more than two sets of chromosomes are termed polyploid.
  4. Homologous chromosomes - Chromosomes that carry the same genes are termed homologous chromosomes

Types of cell division

Three types of cell division occur in plants. These are:

  1. Amitosis or direct cell division (Robert Remak -1855)
  2. Mitosis or indirect cell division (Flemming -1882)
  3. Meiosis or reduction division (Farmer and Moore -1905)

Amitosis

Cell division in which there is first a simple cleavage of the nucleus without change in its structure (such as the formation of chromosomes), followed by the division of the cytoplasm; direct cell division; as opposed to mitosis. It is not the usual mode of division, and is occur mainly in highly specialized cells which are incapable of long-continued multiplication, in transitory structures, and in those in early stages of degeneration. This type of cell division is found mainly unicellular, prokaryotic plants e.g. cynobacteria, bacteria, yeast, internodal cells of Chara zeylanica and Chara contraria and endosperm cells of developing seed. etc.

Figure 3.1.1: Stages of Amitotic cell division

Amitosis occurs in two stages:-

  1. Nuclear division(Karyokinesis):- The nucleus elongates and a constriction appears in the centre(fig 1 B) resulting in a dumb-bell shaped structure. The constriction deepens and the nucleus gets divided into two(fig 1 C). There is no spindle formation and no formation of chromosomes.
  2. Cytoplasmic division(Cytokinesis):-The nuclear division is followed by cytoplasm division and the cell gets divided into two daughter cells(fig 1 D). The two daughter nuclei are always similar in shape and size. Thus amitosis is only a quantitative division.

Mitosis

Mitosis is the common method of nuclear division, followed by cytokinesis (cytoplasmic division). It usually occurs in vegetative or somatic cells therefore it is known as somatic division. It occurs in meristematic tissues - shoot, root tip. It results in the increase of size, shape and volume of plant parts and causes growth. The pattern of mitosis is fundamentally the same in all cells.In this division the mother cell produces two genetically identical daughter cells which resemble each other and also parent cell qualitatively and quantitatively.The separation of separate sister chromatids into two new cells with exactly the same number of chromosomes and half the amount of nuclear DNA is known as mitosis Therefore it is also called equational division. Mitosis is also known as indirect division because it is an elaborate process involving a series of important changes in nucleus as well as cytoplasm. In mitotic division not only the chromosomes are replicated but all necessary cytoplasm constituents and organelles are precisely divided between two daughter cells. In mitosis there is no change in chromosome number. Mitosis is observed in all types of cells -haploid, diploid or polyploid.If a parental cell has 1000 chromosomes, or even just 1 chromosome, the daughter cells have 1000 and 1 chromosomes, respectively after mitosis.

Some useful terms which are used:

  • Chromosome:- A gene is made up of DNA which codes for one or more polypeptides. A chromosome is made up of many genes. The DNA in the chromosome is wrapped around histone and non-histone proteins. Before DNA synthesis, there is only one double stranded helix of DNA in each chromosome.
  • Chromatid :- After DNA synthesis , there are two identical DNA helices connected by a structure called the centromere. Each DNA helix is called a chromatid.These chromatids are called sister chromatids.

Cell cycle

Cell cycle: The cell cycle is an ordered set of events, which occurs between the formation of a cell and its division into two daughter cells.

The cell cycle is composed of 4 distinct phases: G1 phase, S phase, G2 phase, and M phase and C phase. G1, S, and G2 phases (the first three phases) together constitute the interphase and the M stage stands for mitosis and C phase for cytokinesis.In the simplest sense, a cell duplicates its contents and then divides in two.The cycle of duplication and division is known as the cell cycle.

Interphase + Nuclear division(mitosis) + Cytokinesis = Cell cycle

  1. Interphase: During interphase, the cell is growing and preparing for mitosis (M phase) by accumulating nutrients and replicating DNA.Interphase is the longest phase in cell cycle.Though this phase is sometimes called resting stage, but it is in fact the most active phase of the cell cycle.

 

Figure 3.1.2: Interphase

  1. G1 phase: G1 stage separates the end of mitosis and the start of the S phase. The timing of the cell cycle and the relative lengths of the various stages depends on the specific type of cell and on the local conditions. Cell cycle ranges 8 hours to 100 days or more. Differences in cell cycle times are mainly due to the variations in the length of G1 phase. Cells in a rapidly developing tissue have thus a short G-1 phase. Slowly dividing cells stay in G1 phase for days or more or in many organisms takes up most of the cell's life.

Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They acquire ATP and increase in size during the G1 phase. In this phase cytoplasmic growth occurs and the cell is preparing its enzymatic machinery to be ready for the next stage S phase(synthesis). The daughter cells become as large as the mother cell, the chromosomes are thread-like and invisible(dispersed state), no change in DNA amount or chromatin. Each chromosome contains only a single molecule of DNA is called an unduplicated or unreplicated chromosome. In G1 (first gap) intensive formation of biochemicals and cellular synthesis, production of mitochondria, plastids, endoplasmic reticulum, lysosomes, golgi apparatus, vacuoles takes place. Nucleolus produces rRNA, Synthesis of rRNA, mRNA and ribosomes. Structural and function (enzymes) proteins, amino acids for histone formation, nucleotides and energy rich substance ATP - synthesized. Cell metabolic rate high, controlled by enzymes.

During G1 phase, a cell may follow one of the three options:

  • cell has reached the restriction point (R point). After a short rest it continue on the cycle and divide.
  • The cell permanently stop division and enter G0 or quiescent stage.
  • The cell cycle have been arrested at a specific point of G1 phase. The cell in the arrested condition is said to be in the G0 state. The cell in the G0 may be considered to be withdrawn from the cell cycle.When conditions change and growth is resumed the cell re-enters the G1.
  1. S (synthetic phase) Synthesis of DNA and histones phase:- It is bordered by both of the G1 and G2gap phases.During the S phase, new DNA is synthesized by the cell resulting in each chromosome with two molecules of DNA.The DNA content of the nucleus is doubled and proteins are synthesized. During this stage every double-helical DNA molecule is duplicated, making two strands of DNA that are exactly identical( the DNA breaks apart at different points along the strands. New single strands join the two halves of original strands). Two new DNA strands are formed ,which are attached together by specific proteins, at a short sequence of DNA (which is found on each double helix) and called a centromere. The two DNA copies that result from S phase are not visible through a light microscope because they have not yet condensed to form chromosomes (i.e., they remain chromatin). Each chromosome has become two chromatids. Once duplication is complete, histones proteins synthesized which cover each DNA strand (chromatin synthesis). The number of chromosomes remain same as were present in the newly formed cell(1n or 2n) but each chromosome is changed from single stranded form to two stranded form.
  2. G2 phase:-Since the formation of new DNA(S phase) is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case mitosis).During the pre-mitotic gap phase (G2), synthesis of RNA and protein continues, but DNA synthesis stops.The mitotic spindle proteins are formed. The mitotic spindle is structure that is involved with the movement of chromosomes during mitosis. The multiplication of the chloroplasts and mitochondria (by binary fission!) and production of materials needed for mitosis (a nuclear! event) takes place. During G2 phase chromatin begins to condense into the relatively compact structures called chromosomes (which, as a result of condensation, become visible through a light microscope). These chromosomes remain attached through their centromeres. Preparation for segregation: Preparation is made for chromosomal segregation, though actual segregation is not yet initiated (e.g., the nuclear membrane remains intact). The durations of the S phase, the G2 phase and mitosis is generally constant in most cell types.

Figure 3.1.3: Phases of cell cycle