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CSIR NET Life Science Notes – UNIT 2D Notes – Cell Division & Cell Cycle

CSIR NET UNIT 2 Part D – Cell division and cell cycle is a very important topic with respect to the CSIR NET Life science exam. One must never skip this topic as it’s not only important from the CSIR NET exam point of view but also clarity in concepts in this subject helps a lot in future research career too.

Reference Books:

  • Cell & Molecular Biology: Concepts & experiments By Gerald Karp
  • Molecular Biology of the cell By Alberts
  • Molecular Cell Biology By Baltimore & lodish

CSIR NET Life Science Notes – UNIT 2 D

Cell Cycle

  • The process of Generating Identical cells
  • To produce a pair of genetically identical cells, DNA must be perfectly replicated and the resulting chromosomes must be segregated into two separate cells.
  • Most cells must also double their mass and duplicate their cytoplasmic organelles. To accomplish these goals, a cell must go through the Cell Cycle.
  • Cell cycle – sequence of events by which cell duplicates
  • Two main phases: M phase – mitosis, Interphase – time by which cell prepares for division by cell growth and DNA replication
  • Interphase is divided into
  • G1 –Gap 1- the period between the end of M and the start of DNA replication
  • S phase- synthesis period
  • G2- Gap 2- after DNA replication to initiation of M
Fig: Phases of Cell Cycle

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  • G1 – Cells undergo majority of growth
  • S – Each chromosome replicates (synthesizes) to produce sister chromatids
    Attached at centromere
    Contains attachment site (kinetochore)
  • G2 – Assemble machinery for division such as centrioles

Length of each cycle

  • 95% cell cycle – interphase
  • M takes about 1h
  • G1- 11h; S- 8h; G2- 4h M-1h
  • S phase: DNA doubles
  • No increase in the number of chromosomes

G1 Phase

  • G1 is the period of time between the last mitotic division and the beginning of the synthesis of new DNA
  • During G1, the cell surveys the environment and makes the decision to enter the cell cycle
  • Proteins that promote cell division will be activated and force the cell towards the cell cycle

S phase

  • The S phase, short for the synthesis phase, is a period in the cell cycle during interphase, between the G1 phase and the G2 phase.
  • Following G1, the cell enters the S stage, when DNA synthesis or replication occurs.
  • At the beginning of the S stage, each chromosome is composed of one coiled DNA double helix molecule, which is called a chromatid.
    The enzyme DNA helicase splits the DNA double helix down the hydrogen bonds (the middle bonds). DNA polymerase follows, attaching a complementary base pair to the DNA strand, making two new semi-conservative strands.
  • At the end of this stage, each chromosome has two identical DNA double helix molecules
  • During the S phase, the centrosome is also duplicated.
  • The end result is the existence of duplicated genetic material in the cell, which will eventually be divided into two.

G2 Phase

  • The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs
  • A phase within the interphase of the cell division cycle that prepares cells for mitosis
  • Since the formation of new DNA 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.

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  1. Iam pursuing 12 th standard ISC n preparing for NEET. Planning to appear for bio Olympiad . Please share this material

    Thank you


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