Cell Cycle and Cell Division Notes

Cell Cycle

• The cell cycle is an ordered series of events required for the faithful duplication of one eukaryotic cell into two genetically identical daughter cells.
• Precise replication of deoxyribonucleic acid (DNA) duplicates each chromosome.
• Duplicated chromosomes separate during karyokinesis (division of nucleus), followed by cytokinesis (division of cytoplasm).
• These transformations are accompanied by general cell growth, which provides enough material (membranes, organelles, cytosol, nucleoplasm) for the doubling of cell number.
• The cycle continues indefinitely in specialised cells called stem cells, found in the skin (Malpighian layer) or bone marrow, causing constant replenishment of discarded cells.
• Repetition of cell cycle may produce a clone of identical cells or lead to differentiation into distinctive cell types or the development of a complex organism.
• The DNA sequence of each cell's genome remains unchanged, but the resultant cellular forms and functions may vary.
• Cells enter interphase to prepare for the next cell division and grow to the same size as the mother cell. No change in chromosomes occurs in this stage.

Interphase

• Interphase prepares the cell for division.
• The cell grows to its maximum size, increasing the volume of the nucleus and nucleolus.
• The nucleus doubles the quantity of DNA, and the chromosomes become long and thread-like structures.
• Interphase is also called the resting phase because no change in chromosomes occurs in this phase.
• Interphase has three distinct phases:
  • G₁ phase (first growth phase)
  • S phase (synthesis phase)
  • G₂ phase (second growth phase)

G₁ Phase

• RNA and protein are synthesised, and the volume of cytoplasm increases.
• Mitochondria (in both plant and animal cells) and chloroplasts (only in plant cells) divide because they have their own DNA.
• It is a larger growth phase for the cells.
• In the late G₁ phase, cells either withdraw from the cell cycle and enter the R phase (resting phase) or start preparing themselves for the next S phase (synthesis phase).

S Phase

• The S phase is a DNA synthesis phase in which more DNA is synthesised and the chromosomes start to get duplicated.

G₂ Phase

• Before mitotic cell division, the cell enters a second, shorter growth phase called the G₂ phase.
• RNA and proteins necessary for cell division are synthesised, and the cell is ready to start for the next cell division.

Cell Division

• When a cell has attained its maximum size, it usually splits into half, forming two daughter cells. This process is called cell division.
• All structures in a parent cell are duplicated and distributed to each daughter cell.
• The daughter cells grow and divide.
• The process of cell division is controlled by the nucleus.
• In unicellular organisms, cell division is a means of multiplication of cells.
• In multicellular organisms, it brings about growth, replacement of old cells, repair of wear and tear tissues, and reproduction in organisms.
• Cell division also plays an important role in sexual and asexual modes of reproduction.

Cell Division for Growth

• All organisms begin their life with a single cell. This cell divides repeatedly and forms a cluster of cells.
• For different body functions, the cell cluster starts to shape out and form tissues and organs.
• Cell division helps in the growth of organisms.

Cell Division for Replacement

• Worn-out cells are replaced by new cells formed during cell division.
• Example: 20 million red blood cells are destroyed every minute in our body and are replaced by new cells formed in the bone marrow.

Cell Division for Repair

• The body can heal accidental injuries. When a cut occurs, the cells divide, cover up the gap, and join the broken ends of the cut.
• Cell division helps in the repair of the body.

Cell Division for Reproduction

• In the process of reproduction, cells divide to produce new organisms.
• Unicellular organisms reproduce asexually by dividing the parent cell into two similar independent cells by mitosis.
• In higher organisms, special cells in the maternal and paternal reproductive organs undergo meiosis to produce sperms and eggs, which receive only half of the chromosomes from their parent cells.

Steps Involved in Cell Division

• Replication of DNA
• Division of nucleus (karyokinesis)
• Division of cytoplasm (cytokinesis)
• The division of the nucleus is of great importance because it determines the distribution of chromosomes.
• Chromosomes are thread-like structures which appear only during cell division.
• They are responsible for the transmission of heredity traits or genes from parents to offspring.

Basic Terms Used in Cell Division

• Gene: Unit of heredity which is composed of DNA that determines a particular character.
• Genome: The full one set of chromosomes.
• Diploid: Cells contain two versions of every gene or double the number of chromosomes.
• Haploid: Cells contain one version of every gene or half the number of chromosomes.
• Chromosome: A thread-like structure present in the nucleus which serves as a carrier of genes.
• Homologous chromosomes: Similar but non-identical chromosomes consisting of same genes at the same position.
• Chromatid: During cell division, the chromosomes condense and appear as a double structure. Each unseparated chromosome within such a pair is called a chromatid.
• Sister chromatids: When two chromatids are genetically identical, they are called sister chromatids.
• Bivalent: A pair of homologous chromosomes.
• Locus: The location of a gene on a chromosome.

Types of Cell Division

• Mitosis is responsible for growth and development in an organism.
• Meiosis is responsible for the production of gametes (sperm and ovum).
• In each type of cell division, karyokinesis occurs before cytokinesis.

Karyokinesis and Cytokinesis

• All the nuclear changes occurring in a cell during cell division are collectively known as karyokinesis.
• It is followed by cytokinesis.

Mitosis Cell Division

• Mitosis is a kind of cell division which leads to growth and repair of the body.
• The two daughter cells formed as a result of mitosis are similar to the parent cells in all respects.
• Mitosis is also called an equational cell division.
• In animals, mitosis occurs in somatic cells, and in plants, it takes place in the meristematic tissues and during the growth of leaves, flowers, and fruits.
• Mitosis is an elaborate process which involves karyokinesis and cytokinesis, followed by the separation of the daughter cells.

Phases of Mitosis

• Mitosis involves a series of changes in the nucleus and the cytoplasm.
• Karyokinesis of mitosis is a continuous process which includes four phases:
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

Prophase

• In early prophase, the chromosomes become short and thick and are clearly visible inside the nucleus.
• Each chromosome gets duplicated to form two sister chromatids which are attached to each other at a point called centromere.
• The centrosome with two centrioles splits into two, which move apart to the opposite poles.
• Each centriole is surrounded by radiating rays called asters, which forms the spindle fibres for the attachment of chromatids.
• The nucleolus and nuclear membrane disappear, and the duplicated chromosomes start moving towards the equator of the cell.

Metaphase

• Metaphase is a short and simple phase of mitotic cell division.
• The paired chromosomes start moving towards the centre of the equator and get arranged in the form of an equatorial plate.
• Each chromosome gets attached to a spindle fibre by its centromere.

Anaphase

• At the beginning of anaphase, centromeres divide into half and together with their attached chromatids are drawn apart towards the opposite poles.
• They are drawn by the contraction of spindle fibres.
• In late anaphase, a furrow starts to develop in the middle of the animal cell.

Telophase

• Two sets of daughter chromosomes reach at the opposite ends and gradually thin out in the form of chromatin fibres.
• These chromatin fibres get condensed to form two identical nuclei.
• A nuclear membrane is formed around each nucleus, and the nucleolus reappears.
• At the late telophase, a furrow appears in the middle of the cell membrane of the animal cell, which deepens and finally splits the cytoplasm into two, thus forming two new daughter cells.
• In the plant cell, instead of a furrow, a cell plate is laid down in the cytoplasm at the equatorial plane and divides the parent cell into two daughter cells.

Mitosis in Plant and Animal Cells

• Mitosis occurs at the growing tips (in the cells of meristematic tissues) and side portions of the plant body in plant cells.
• Mitosis occurs in the cells throughout the body for proportionate growth in animal cells.
• Asters are not formed because centrioles are absent in the plant cell.
• Asters are formed because centrioles are present in the animal cell.
• In cytokinesis, a cell plate is formed which divides the parent cell into two daughter cells in plant cells.
• Cytokinesis occurs by the formation of a furrow in the cytoplasm of the cell which divides the parent cell into two daughter cells in animal cells.

Significance of Mitosis

• Genetic stability: Mitosis results in two daughter cells which are similar to the parent cell in all respects. The daughter cells have the same amount of DNA, which is identical to the parent DNA. The two daughter cells are genetically identical to each other.
• Growth: The mitotic division of somatic cells results in the growth of the organisms. After fertilisation, an egg develops into an embryo, and finally a full grown body is formed due to repeated mitotic cell division.
• Asexual reproduction: In asexual reproduction, the parent cell divides mitotically to produce two identical daughter cells.
• Repair: Mitosis forms new cells and replaces old worn-out cells. A wound or cut can be heeled due to mitotic cell division.
• Replacement: The mitotic cell division replaces the old and worn out cells with new ones.

Meiosis Cell Division

• Meiosis is a reductional cell division in which the number of chromosomes is reduced to half in the daughter cells.
• It is an essential cell division for producing sex cells or gametes.
• The meiosis cell division occurs in the reproductive organs.
• In humans, it takes place in the testes of males and the ovaries of females. The testes produce sperms and ovaries produce ovum.
• In flowering plants, it takes place in the anther (male reproductive part) to produce pollen grains and in the ovary (female reproductive part) to produce ovules.
• The most significant aspect of meiosis is that in meiosis both sperm and egg cells receive only half the number of chromosomes from their parent cells.
• During fertilisation, the haploid (n) male and female gametes (sperm and ovum) fuse together and form a diploid (2n) zygote.
  Note: Meiosis is a type of reductional division, which is completed in two stages called meiosis I and meiosis II. Meiosis I is the reductional division whereas meiosis II is a simple mitotic cell division.

Significance of Meiosis

• It reduces the number of chromosomes to half in the gametes: For example, humans have 46 chromosomes in each of their body cells. But the human egg and sperm cells have only 23 chromosomes each.
• Meiosis helps to restore the diploid number of chromosomes present in body cells: The gametes (male and female sex cells) contain haploid number (n) of chromosomes. During fertilisation, they fuse to form a zygote with diploid (2n) number of chromosomes. In this way, the normal diploid number of chromosomes is restored in the body cells.
• It mixes the maternal and paternal chromosomes: The first stage of meiosis is a reductional division. In this phase, the parental chromosomes are separated from their homologous pairs and often undergo crossing over which results in genetic recombination.
• It plays an important role in evolution: As new combinations of genetic materials are formed it produces variation.

Differences between Mitosis and Meiosis

Homologous Chromosomes and Crossing Over

• During meiosis, chromosomes are usually found in a pair in which one chromosome is of paternal origin and the other is of maternal origin. Chromosomes of such pair are called homologous chromosomes.
• During the first phase of meiosis, crossing over takes place between non-sister chromatids.
• During crossing over, homologous chromosomes align lengthwise with each other and exchange genetic material between them.
• The pairing of homologous chromosomes is known as synapsis.
• The point at which these chromosome exchange genetic material between two chromatids is known as chiasma.
• Crossing over helps bring about precise shuffling of genes during gamete formation.
• The fusion of these gametes gives rise to individuals that are genetically distinct from their parents.
• Exchange of genes during meiosis causes variation in chromosomal make-up.
• This is the reason why the children of same parents, howsoever similar, are different from each other in certain respects.
• Variation produces new species over time and contributes in evolution.
• Crossing over occurs in meiosis is defined as the exchange of genetic material between non-sister chromatids.
• The new combinations of genes which are not found in either parent can lead to variations.
• These variations greatly contribute to evolution.

Chromosomes

• Chromosomes are highly condensed, coiled and thread-like chromatin fibres through which the heredity traits are transmitted from one generation to the next.
• They are only visible through the microscope during cell division.
• Number of chromosomes is constant for each species.
• In a normal body cell, chromosomes are diploid (2n). In the sex cells (sperm and ovum), they are haploid (n).
• Each sex cell contains only one member of a chromosome pair.
• Humans each body cell contains 46 or 23 pairs of chromosomes (22 pairs called autosomes and one pair of sex chromosomes); each sex cell has 23 chromosomes only.

Discovery of Chromosomes

• Chromosomes (Gk. chrome meaning colour and soma meaning body) were first discovered by Walther Flemming in 1882.
• He studied the cells of salamander larva and noticed minute threads in the rapidly dividing cells and coined the name mitos, which means a thread.

Structure of Chromosomes

• In the interphase, chromatin fibres get thickened and shortened and move into the first stage (prophase) of cell division and form chromosomes.
• In mitotic cell division, each chromosome forms its own replica and duplicates itself with the synthesis of a new DNA substance, now termed as chromatids.
• Each chromosome consists of two chromatids held together at centromere (a small constricted region).
• During cell division, chromatids become long, thread-like structures called chromatin fibres.
• Each chromatin fibre is made up of 40% DNA and 60% histone proteins.
• In a chromosome, the chromatin fibre is coiled and super-coiled like the cable of a typical telephone cord.
• The histones and the DNA form a complex in which the DNA strand winds around a core of eight histone molecules, known as a nucleosome.

Gene Structure of DNA

• DNA stands for deoxyribonucleic acid.
• It is the hereditary material which carries hereditary information from one generation to the next in the form of genes.
• A large, single molecule of DNA is composed of two complementary strands of nucleotides that are wound around each other.
• Each nucleotide is made up of three components, a phosphate group, a pentose sugar and a nitrogenous base.
• In a nucleotide, the sugar molecule is arranged lengthwise, and the nitrogenous base is attached to the sugar inwards and extends to join the complementary nitrogenous base from the other strand by a hydrogen bond.
• The two strands together make a spiral, ladder-like arrangement called the double helix.
• There are four bases involved in the double-helix model of the DNA: adenine (A), cytosine (C), guanine (G), and thymine (T).
• Adenine is paired with thymine with two hydrogen bonds, while guanine is paired with cytosine with three hydrogen bonds.

Formation of New DNA Strand or DNA Replication

• The new DNA strands start forming during the mitotic cell division in its interphase stage.
• The chromosomes form their own replica and duplicate themselves to maintain their number, at the end of cell division in the daughter cells.
• The parent DNA double helix opens at one end and forms two free strands (like a letter 'Y'), where each strand acts as a template for the synthesis of the new one, complementary to itself.
• The process continues until all the nucleotides on the templates have joined with appropriate free nucleotides, and the two identical free DNA molecules are formed.
• Chromosomes are coiled and highly condensed chromatin fibres.
• Genes are the specific sequences of nucleotides encoding particular proteins which express in the form of some specific feature in the body.
• In a single human chromosome, millions of nucleosomes are present.

Contribution of Scientists Towards the Discovery of DNA

• Frederich Meisher (1869) isolated DNA from the nucleus of the pus cells.
• Rosalind Franklin (1951) gave the name nuclein to nucleic acid because of its acidic nature.
• J. D. Watson and F. H. C. Crick (1953) gave the three-dimensional double helical model of DNA, for which they were awarded Nobel Prize in 1962.
• Chromosomes are the carrier of hereditary units called genes, which define the characteristics or traits from parents to offspring at the time of cell division.
• The genes are composed of DNA (deoxyribonucleic acid) and proteins called histones.
• A gene (DNA) codes for the specific protein which controls the expression of a particular characteristic in an individual.
• Humans have 20,000 to 25,000 different types of genes, found on the chromosomes, which occupy a specific position on it.
• The chromosomes can exist in different forms called alleles, which determine the aspect of the characteristic shown (e.g. tallness or shortness for the characteristic of height, brown or black for the colour of eyes, etc.).