Cell Cycle

Interphase - preparation for division

• The cell synthesises new cell components such as organelles and membranes, and new DNA.
• The formation of new cellular proteins occurs throughout interphase, whereas DNA synthesis occurs during the S phase.
• The length of interphase differs depending on the role of the cell.

Developing Embryos

• In the developing human embryo, there is no interphase for the first few divisions - the zygote already contains the materials needed to form the first 16 or so cells.
• In these first few divisions, the embryo divides without growing in size, producing smaller cells with each cell cycle.
• This makes the embryonic cell cycle much faster than those of other body cells.

Duration of Interphase

• The S and G2 phases of most cells remain relatively constant in duration.
• The length of the G1 phase is more variable; some cells can take weeks, months or even years to complete this phase.
• e.g. liver cells may divide only once every one or two years due to an extended G1 phase.
• In seeds, the cells of the embryo usually remain in G1 phase until germination occurs.
• Some cells such as nerve and muscle cells never divide again, remaining permanently in a non-dividing state.
• In plants, cell division is localised in small groups of cells called meristems, with most other cells not dividing.

Nucleus during Interphase

• The interphase nucleus is a fairly uniform, featureless structure with one or two darker-staining regions called nucleoli (singular nucleolus).
• Ribosomes are formed in the nucleoli, and they give nucleoli their dark appearance in electron micrographs.
• Ribosomes are made of protein and rRNA (ribosomal RNA).
• The rest of the nucleus contains the chromosomes, which are made up of DNA associated with proteins.

Chromosomes during Interphase

• During interphase, the individual chromosomes are unravelled. This allows access to the genetic material enabling new proteins to be synthesised.
• In preparation for cell division, the cell synthesises additional cytoplasmic proteins and organelles.
• The cell must also produce copies of DNA for the 2 new cells. It is vital that this DNA is identical in both structure and quantity to the DNA in the original cell - achieved by DNA replication.

Cell Division

• By the end of interphase, the cell contains enough cytoplasm, organelles and DNA to form two new cells. The DNA and the contents of the cytoplasm must be separated so that each new cell can function independently. The DNA is separated in nuclear division (mitosis). Cytoplasmic division follows this.
• Cell division is a continuous process.

Importance of Mitosis

• Mitosis ensures genetic consistency, with daughter cells genetically identical to each other and to the parent cell, each containing exactly the same number and type of chromosomes as their parent cell. This is achieved by:
• DNA replication prior to nuclear division
• The arrangement of the chromosomes on the spindle and the separation of chromatids to the poles
• This genetic consistency is important in growth and repair, and also in asexual reproduction.

Growth and Repair

• Mitosis occurs in the growth of any organism as it develops from a single cell into a multicellular organism.
• It ensures that a multicellular organism has genetic consistency; all the cells in the body have the same genetic information.
• Some organisms can regenerate lost or damaged parts of their bodies using mitosis.
• Mitosis also allows old and damaged cells to be replaced with identical new copies.

Asexual reproduction

• Asexual Production: reproducing without producing gametes - growing copies of themselves through mitosis, producing offspring that are genetically identical to each other and to their parent.
• E.g. binary fission in bacteria; first the bacteria cell grows and then divides into 2 new cells.
• Hydra reproduce asexually by budding; an outgrowth from the parent detaches to form a new individual. The single-celled fungus yeast also reproduces this way.
• Asexual reproduction is common in plants too, where it is sometimes known as vegetative reproduction - growth of new plants from tubers and bulbs.
• Some organisms reproduce asexually at one stage in their life cycle and sexually at another.