2.5 Cell Division (Mitosis)

Topic 2 Cells

Cell Division2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

2.5.2 State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.

2.5.3 State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

Cell Division2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).

2.5.5 Explain how mitosis produces two genetically identical nuclei.

2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

The Continuity of Life

In 1855, Rudolf Virchow claimed:“Where a cell exists, there must have been a preexisting cell”

He summarised with the Axiom:“Omnis cellula e cellula” meaning “All cells from cells”

The continuity of life is based on the reproduction of cells, or Cell Division.

The Cell Cycle

Cells go through repetitive stages of growth and division.This is called the Cell Cycle.The cell cycle consists of a number of stages:– Interphase– Mitosis– Cytokinesis

The Cell Cycle

Ref: Biology, Campbell

Events occurring in the Cell CycleStage of cell cyclePhase Major events

Interphase G1 Period of high metabolic activity resulting in cell growth.

New proteins synthesised.Cell organelles are produced.

S DNA Replication occurs.G2 Mitochondria and chloroplast divide.

Centrioles (if present) divide.Mitosis Prophase Chromosomes form/coil up.

Metaphase Chromosomes line up along equator of cell.

Anaphase Chromosomes move to each end of cell.Telophase New nuclear envelope forms.

Cytokinesis Division of cytoplasm and cell organelles. into two approximately equal halves.

Mitosis

Mitosis is used in eukaryotes whenever genetically identical cells are needed:– during growth;

• In areas where organisms are getting bigger.

– tissue repair;• In areas of an organism that have been damaged and need to be repaired.

– asexual reproduction;• Some organisms reproduce without sex.

The Products of Mitosis

Mitosis produces:– Two daughter cells with genetically identical nuclei.

ie: each cell has the same number and type of chromosomes as the parent.The two cells are considered genetically identical to each other and the parent cell.

Mitosis

Mitosis occurs in organisms where there is a need for growth or repair.When Eukaryotic organisms reproduce Asexually.Mitosis is a part of the cell cycle.Mitosis is usually divided into 4 phases:– Prophase– Metaphase– Anaphase– Telophase

Prophase (before phase)

In the nucleus, chromatin strands condense and become clearly visible as thick chromosomes.Each chromosome is composed of two identical strands called Chromatids (sister chromatids), which are joined at the Centromere.Late in the phase, the nuclear membrane and nucleolus begin to disappear.In animal cells, the centrioles move to opposite sides of the cell (spindle poles). Spindle fibres begin to form.

Metaphase (between phase)

By this phase, the nuclear membrane has totally broken down.The nucleolus has disappearedThe chromosomes, previously randomly arranged within the cell, become aligned at the equator of the cell, half way between the two spindle poles (centrioles in animal cells).Spindle fibres stretch from the chromosomes (attached to the centromere) to the spindle poles.

Anaphase (back phase)

At the beginning of this phase, each centromere divides, releasing the attachment of sister chromatids.Each chromatid is pulled along the spindle fibres to opposite spindle poles, lead by its centromere.Once near the opposite ends of the cell, Cytokinesis begins.

Telophase (end phase)

During this phase, the spindle fibres break down and each chromosome unwinds as the chromatin that forms it again becomes extended and thin.The nuclear membrane and nucleolus reform in each daughter cell.Cytokinesis nears completion.

Cytokinesis

This is the process of cytoplasmic division and enclosure of the cytoplasm of the two new daughter cells.In animals, cytokinesis occurs by the pinching of the cytoplasm (a cleavage furrow), pinching the cell into two.In plant cells, a cell plate forms between the 2 new daughter nuclei, dividing the cell into two.Cytokinesis may begin as early as Anaphase.

Cancer

A Cancer is an abnormal growth of cells which invades other tissue and may move to other parts of the body.A cancer begins with just one abnormal (mutated) cell.The cell divides by mitosis to form a lump or tumour.They can occur in any organ or tissue.

Ref: Biology Key Ideas

IBO guide:2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

2.5.2 State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.

2.5.3 State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

IBO guide:2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).Include supercoiling of chromosomes, attachment of spindle microtubules to centromeres, splitting of centromeres, movement of sister chromosomes to opposite poles, and breakage and re-formation of nuclear membranes.Textbooks vary in the use of the terms chromosome and chromatid. In this course, the two DNA molecules formed by DNA replication are considered to be sister chromatids until the splitting of the centromere at the start of anaphase; after this, they are individual chromosomes. The term kinetochore is not expected..

IBO guide:Aim 7: Students could determine mitotic index and fraction of cells in each phase of mitosis. Individual groups could paste data into a database. Pie charts could be constructed with a graphing computer program. If a graphing computer program is used in DCP for internal assessment, it should be according to the IA and ICT clarifications.

2.5.5 Explain how mitosis produces two genetically identical nuclei.

2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.