Chapter 2: STRUCTURE OF CHROMOSOMES, CELL CYCLE AND CELL DIVISION
Chromosomes (chroma: coloured; soma: body) are highly coiled and condensed chromatin fibres.
Each chromosome contains one long DNA molecule (40%) associated with histone proteins (60%). This complex of protein and DNA is called Chromatin
Chromosomes were discovered in animals by a German scientist, Walther Fleming in 1882, when he observed rapidly dividing threads in the cells of the larvae of salamander (amphibian). He called this division Mitosis (literally meaning “thread”)
Molecular structure of DNA molecule was first studied by Rosalind Franklin in 1953.
The double-stranded helical structure of DNA molecule was discovered by Watson and Crick in 1953, for which they were awarded the Noble prize in 1962.
A single DNA molecule is very large, and hence it is described as s Macromolecule. Each single DNA strand is composed of repeating nucleotides which are made of three components, phosphate, sugar (pentose) arranged lengthwise and the nitrogenous bases –Adenine (A), Guanine(G), Cytosine(C), and Thymine(T) attached to sugar inwards which extend to join (by a hydrogen bond) the complementary nitrogenous base from the other strand, forming a ladder-like arrangement with nitrogenous bases forming the “rungs“ of the ladder.
Adenine pairs with Thymine with two hydrogen bonds, Guanine pairs with Cytosine with three hydrogen bonds.
Histones are the proteins that help in the coiling and packaging of DNA into structural units called Nucleosomes. The DNA strand winds around a core of eight histone proteins (histone octamer). Each such complex is called Nucleosome
Each chromosome has 2 sister chromatids joined at a point called the centromere, which appears as a small constricted region. Centromere also serves to attach to the spindle fibre during cell division. Sister chromatids are separated at the centromere when the spindle fibres contract.
Genes are specific sequences of nucleotides on a chromosome that encode particular proteins in the form of some particular feature of the body. They are units of heredity which are transferred from parents to offspring.
New cells need to be produced for-growth, replacement, repair (by mitosis) and reproduction (by meiosis).
The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells. It has 2 phases-a non-dividing phase called the Interphase, and a dividing phase called the M-phase or Mitosis. In animal cells, mitosis occurs in most tissues throughout the body (for growth and replacement) whereas, in plant cells, mitosis occurs mainly at the growing tips (for lengthening) and sides (for the increase in girth).
During Interphase (Resting phase-no visible change in chromosome), the two daughter cells with a full-sized nucleus but relatively less cytoplasm, prepare for the next cell division and grow to the same size as their mother cell. It has 3 phases-first growth phase (G1), synthesis phase (S) and second growth phase (G2).
G1 phase – RNA and protein synthesized volume of cytoplasm increases. After late G1 phase, the cell either withdraws from the cell cycle and enters the Resting phase (R) OR enter next Synthesis phase(S).
S phase- more DNA is synthesized, the chromosomes are duplicated (double helix opens at one end, making the two strands free to which the new strands begin to form and the process continues in a sequence for the whole length of the DNA)
G2 phase (shorter)- more RNA and proteins synthesized necessary for cell division.
There are two types of Cell Division- 1.Mitosis: cell division leading to the production of diploid cells (2n)for growth and development, 2- Meiosis—cell division leading to the production of haploid cells (n) or gamete(sperms or egg)
Mitosis- same normal chromosome number is maintained at each cell division. Phases-Karyokinesis (division of the nucleus) and Cytokinesis (division of the cytoplasm)
4 phases of Karyokinesis—Prophase, Metaphase, Anaphase, Telophase
Prophase –chromosomes become distinct; chromosomes are already duplicated as paired chromatids; sister chromatids attach at centromere; centrosome (in animal cells) splits into two along with simultaneous duplication of the centrioles contained in it, and the daughter centrioles move to the poles. Each centriole get surrounded by radiating rays called aster (aster are not formed in plant cells); spindle fibres appear between daughter centrioles forming the achromatic spindle; nuclear membrane and nucleolus disappear; duplicated chromosomes start moving towards the equator of the cell.
Metaphase – each chromosome gets attached top its spindle by centromere; duplicated chromosomes arrange on the equatorial plane.
Anaphase – centromere attaching the two chromatids divide; the two sister chromatids of each chromosome separate and are drawn apart towards opposite poles pulled by shortening of spindle fibres; a furrow starts in the cell membrane at the middle in an animal cell.
Telophase – two sets of daughter chromosomes reach the poles; spindle fibres disappear; chromatin thin out in the form of chromatin fibres; the nuclear membrane is formed; nucleoli reappear; cleavage furrow starts deepening in the animal cell and a cell plate is laid down in the cytoplasm at the equatorial plane in a plant cell.
Cytokinesis – in animal cells, cleavage furrow deepens totally and separates the two daughter cells whereas, in plant cells, cell plate grows from the centre to the periphery, resulting in two cells.
Significance of Mitosis- growth, repair, replacement, asexual reproduction in unicellular organisms, maintain the same chromosome number in daughter cells.
Mitochondria and Chloroplasts have their own DNA and ribosomes which help in the production of particular proteins. They divide by mitosis.
Meiosis (meion= to lessen, referring to the reduction in the number of chromosomes) takes place in the reproductive organs (testis and ovary) in humans and to produce sex cell or gametes (sperms and ova respectively). In flowering plants, it takes places in anthers and the ovule to produce pollen grains and female gametophyte. The number of chromosomes is halved in the sex cells. Only one member from each pair is passed on to each daughter cell. This is the haploid (n) number of chromosomes.
Meiosis involves two divisions-first reduction division and second mitotic division. In the first division, the chromosomes arranged in homologous pairs (2n) and then they separate to reduce the number to half (n) in the resulting cells.
G2 phase is absent in meiosis. Prophase of meiosis 1 corresponds most closely to the G2 phase of the mitotic cell cycle.
Significance- firstly, the chromosome number is halved. Secondly, there is mixing up of genes between maternal and paternal chromosomes during meiotic division which provides for innumerable variations in the progeny.
The exchange of chromatid /genetic material between the two members of homologous pair of chromosomes is called Crossing over and the point of crossing over is called Chiasma (plural: chiasmata) which is the X-shaped structure formed due to crossing over between two non-sister chromatids of the paired homologous chromosomes.