Nucleus

  • It controls all the cellular activities of cell.
  • It consists of the following.
    a. Nuclear Membrane- It has perforations called pores.
    b. Nucleoplasm
    c. Nucleolus
  • Nucleolus is the site of ribosomal RNA formation.
  • The network of nucleoprotein fibres are called chromatin.
  • Chromatin contains of DNA, histones(basic proteins), non-hiostone proteins, and RNA
  • Chromatin threads condense and organize to form chromosomes.
  • The primary constriction in the chromosome is called centromere.
Structure of DNA
  • It has a double-helix structure, similar to a ladder.
  • It is made up of nucleotides.
  • nucleotides are made of sugar, phosphate groups and nitrogenous bases (adenine, guanine, cytosine, and thymine)
Components of DNA
  • Sugar
  • Phosphate groups
  • Nitrogen base
Genes
  • It is a unit of DNA
  • Located on Chromosomes.
  • Controls the development of one or more traits.
  • It is the basis of inheritance.
Cell Cycle
  • The cell cycle is divided into two basic phases: Interphase and M phase
Interphase
  • Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and DNA replication in an orderly manner.
  • It is divided into three phases:
    a. G1 phase- It involves growth of cell and preparation of DNA replication.
    b. S pahse- It involves DNA replication. The amount of DNA doubles, but the chromosomes number remains the same.
    c. G2 phase- It involves protein synthesis and further growth of cell, which it for division.
M phase
  • It represent the phase where the cell actually divides. It starts with nuclear division and ends with the division of cytoplasm.
Significance of Cell Division.
  • It is the mean of asexual reproduction in unicellular organisms.
  • it is essential for the growth of a single-celled Zygote into a whole new multicellular organism.
  • It helps in the repair of injuries and worn out tissues.
  • it replaces dead cells of the body and thus is essential for the growth of organism.
  • In sexual reproduction, meiosis occurs. This type of cell division not only results in the production of gametes, but also brings a new combination of genes, thus results in variation among population. this also leads to the evolution of species.
Types of Cell division
    1. Mitosis
    2. Meiosis

Mitosis
  • Also called equational division as the number of chromosomes in parent cell and progeny remain the same.
  • Mitosis (M phase) is divided into 4 stages (PMAT: Prime Minister At Tulshipara)
    1. Prophase (P)
    2. Metaphase. (M)
    3. Anaphase (A)
    4. Telophase (T)
  • PROPHASE
    -It involves the initiation and condensation of chromosomes.
    -Nucleolus and nuclear membrane disappear.
  • METAPHASE
    - Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at the equatorial plate.
  • ANAPHASE
    - Centromere splits and chromosomes move apart towards two opposite poles due to the shortening of spindle fibres.
  • TELOPHASE
    -Chromosomes finally reach their respective poles.
    - Nuclear envelope assembles around each chromosome cluster.
    -Nucleolus and other organelles reform.


Meiosis and Its Significance
Meiosis
      • It is a type of cell division that produces gametes.
      • It occurs in the reproductive organs of various organisms. In humans, it occurs in testis and ovary to produce sperm and ova, respectively, whereas in plants, it occurs in anthers and ovules to produce pollen grains and egg, respectively.

Important characteristic:
    • In normal body cells, or somatic cells, the chromosomes are present in the form of pairs; each set of a pair comes from each parent. Such a cell is said to be diploid,
represented as 2n.
    • During meiosis, the number of chromosomes is halved, so that the daughter cells thus
formed contain single set of the chromosome pair. The cells thus formed (or gametes) are said to be haploid, represented as n.
    • This reductional nature of meiosis is important to maintain the genetic makeup of
an organism undergoing sexual reproduction. In sexual reproduction, the haploid male and female gametes produced through meiosis get fused together during fertilisation. As a result, the chromosome number in the zygote remains equal to that of the parents,
and thus the diploid nature is restored.

Significance of meiosis:-
    • Chromosome number is halved.
    • It helps in mixing up of genes. This occurs in two ways:







Visual representation of Mitosis









For more notes click here: Genetics Notes