SSER Ltd Cell Division All eukaryotic multicellular organisms

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Cell Division All eukaryotic, multicellular organisms each develop from a single original cell, i. e. the fertilised egg (zygote) Repeated divisions of the zygote, by a process of cell division called mitosis, give rise to all the cells that make up a multicellular organism The fertilised egg divides repeatedly by mitosis to initially produce a ball of genetically identical cells - the early embryo. The function of mitosis is to produce new cells, each of which receives a set of chromosomes identical to those of the original cell that gave rise to them; genetically identical cells are the result of mitosis

Significance of Mitosis is involved in: DEVELOPMENT – the formation of a multicellular organism from a single fertilised egg GROWTH – involves an increase in size through the division of cells by mitosis CELL REPLACEMENT – many cells within a multicellular organism are constantly dying and being replaced by mitosis ASEXUAL REPRODUCTION – asexual reproduction in certain plants and lower eukaryotic animals involves the process of mitosis Budding in this species of Hydra is a form of asexual reproduction; new individuals are produced as outgrowths of the parent by the process of mitosis

The epithelial cells lining the intestine are continually being replaced by mitosis such that the entire lining is replaced every five days Epithelial cells

The dead layers of cells at the surface of the skin are constantly being lost and are replaced by the mitotic divisions of the cells in the layer beneath them Dead cells Constantly dividing cells replacing those lost at the surface

These nerve cells have lost the ability to undergo mitosis – once formed they do not divide

The Process of Mitosis maternal in origin paternal in origin centromere This diagram represents a parent cell containing a single pair of chromosomes – the pair of chromosomes are the same size and shape and are therefore described as being homologous chromosomes. One member of the pair was donated by the male at reproduction and the other member by the female. Each pair of homologous chromosomes thus has one member that is paternal in origin and another member that is maternal in origin. Humans possess 23 pairs of chromosomes in each body cell. Cells in which the chromosomes occur in homologous pairs are termed DIPLOID (two sets of chromosomes) Each chromosome replicates itself to form a pair of identical chromatids called sister chromatids that remain attached to one another at a region called the centromere The cell divides into two and each daughter cell receives one of each of the sister chromatids The daughter cells are genetically identical to the parent cell The function of mitosis is to construct an exact copy of each chromosome and then to distribute, through division of the parent cell, an identical set of chromosomes to each of the two daughter cells

The Process of Mitosis is a continuous process but for convenience of description is divided into FOUR MAIN STAGES: Prophase Please Metaphase Make Anaphase Another Telophase Two The following slides describe the process of mitosis in an animal cell containing two pairs of homologous chromosomes

Prior to mitosis the cell is in a stage called Interphase During interphase the DNA of the chromosomes replicates and new cell organelles are manufactured Although the chromosomes have duplicated, they are loosely coiled within the nucleus and visible only as granular material at this stage This granular chromosome material is known as chromatin Interphase Nucleolus Two pairs of homologous chromosomes – visible only as granular chromatin at this stage Nuclear membrane The centrioles replicate (animal cells only)

Transmission electron micrograph showing the granular chromatin of an interphase nucleus Interphase prepares the cell for division and the cell now enters the first phase of mitosis known as prophase During early prophase the chromosomes spiralise and condense – they become shorter and thicker and visible as clear threads

The nucleolus shrinks and the centrioles move to opposite poles of the cell Spindle fibres begin to form in the regions of the centrioles By late prophase, the chromosomes have condensed further and each chromosome can be seen to consist of two chromatids joined at the centromere Early Prophase

The nucleolus shrinks and the centrioles move to opposite poles of the cell Spindle fibres begin to form in the regions of the centrioles By late prophase, the chromosomes have condensed further and each chromosome can be seen to consist of two chromatids joined at the centromere Late Prophase Sister chromatids Centromere Each pair of sister chromatids is a pair of duplicated chromosomes resulting from the replication of DNA that occurred during interphase At the end of prophase the nuclear membrane begins to disintegrate and the cell enters the next phase known as metaphase During metaphase, the spindle fibres grow across the cell and the replicated chromosomes (pairs of chromatids) line up independently along the equator of the spindle attaching to the spindle at their centromeres

As the cell enters anaphase, the centromeres divide into two, separating the sister chromatids of each chromosome Metaphase

This electron micrograph shows a metaphase chromosome about to enter anaphase Centromere Sister chromatids As the cell enters anaphase, the divided centromeres repel one another and the chromatids begin to move apart

Spindle activity pulls the chromatids apart and the separated chromatids (know called chromosomes) move to opposite poles of the cell Early Anaphase

Spindle activity pulls the chromatids apart and the separated chromatids (know called chromosomes) move to opposite poles of the cell Anaphase

The chromosomes begin to uncoil and a nuclear membrane begins to form around each set of chromosomes Telophase Cytokinesis (division of the cytoplasm) now begins as the cell membrane begins to constrict towards the centre of the cell The spindle fibres begin to disintegrate A single chromatid from each chromosome has reached the poles of the spindle - the chromatids are now described as chromosomes

Cytokinesis As the membrane continues to constrict, the cytoplasm becomes completely divided forming two genetically identical daughter cells Each cell now possesses an exact copy of each chromosome that was present in the nucleus of the original cell In plant cells, constriction of the membrane at cytokinesis cannot occur due to the presence of the cell wall In plant cells, a cell membrane forms in the middle of the dividing cell and this membrane secretes cell wall material on each side to form new cell walls for the daughter cells

Summary of Mitosis Interphase – cell prepares Early Prophase – chromosomes Late Prophase – spiralise and condense and for division; DNA replicates, chromosomes spiralise and become visible as threads; new organelles are condense further and manufactured and cell grows; centrioles move to opposite can now be seen to consist poles of the cell and spindle chromosomes present as of two sister chromatids fibres begin to form granular material (chromatin) joined at the centromere Metaphase – Early Anaphase – nuclear membrane as the cell has disintegrated and enters anaphase, spindle fibres have the centromeres grown across the cell; divide into chromosomes line up two separating independently along the sister the equator of the chromatids of spindle attaching to each chromosome the fibres via their centromeres

Summary of Mitosis Anaphase – spindle activity pulls the chromatids apart and the separated chromatids move to opposite poles of the cell Cytokinesis – as the membrane continues to constrict, the cytoplasm becomes divided forming two genetically identical daughter cells. Each cell now possesses an exact copy of each chromosome that was present in the nucleus of the original cell Telophase – the chromatids are now described as chromosomes and they begin to uncoil. The spindle fibres disintegrate and the cell begins to constrict along its central axis. A nuclear membrane begins to form around each set of chromosomes Daughter cells

A D B These photographs are taken from prepared slides of onion root tip cells that were undergoing mitosis: Identify the photograph showing Interphase and the photographs showing the four stages of mitosis – Prophase, Metaphase, Anaphase and Telophase C E

Answers This cell is in Interphase The chromosomes are not visible as threads but appear as chromatin (granular material) in the nucleoplasm The nucleolus is clearly visible B During this stage the cell prepares for mitosis – DNA replicates and new organelles are manufactured

This cell is in Prophase The chromosomes have spiralised and condensed – they are shorter and thicker and visible as clear threads At this stage the chromosomes can be seen to have replicated with each chromosome now consisting of two chromatids The nucleolus has shrunk in size E Spindle fibres are beginning to form close to the nucleus and the nuclear membrane disintegrates

This cell is in Metaphase Spindle fibres have grown across the cell Replicated chromosomes attach to the spindle fibres by their centromeres A Each replicated chromosome lines up independently along the equator of the cell

This cell is in Anaphase The centromeres of each chromosome replicate and the chromatids repel one another D The spindle fibres contract and pull the separated chromatids to opposite poles of the cell

This cell is in Telophase A single chromatid from each chromosome has reached the poles of the spindle – the chromatids are now described as chromosomes The chromosomes begin to uncoil and appear as chromatin once again; a nuclear membrane forms around each set of chromosomes C Cytokinesis (division of the cytoplasm) follows telophase In animal cells this involves constriction of the cell membrane along its central axis and division of the cell into daughter cells In the plant cell shown in the photograph, this involves the formation of a cell membrane in the middle of the cell followed by secretion of cell wall material on either side of this membrane

The Cell Cycle The stages through which a cell passes from one cell division to the next constitute the cell cycle The cell cycle is divided into two major phases: the M phase or mitotic phase and the interphase

The Cell Cycle INTERPHASE S G 1 G 2 Interphase is divided into G 1, S and G 2 phases MITOSIS (M PHASE)

The Cell Cycle The M phase occupies only a small portion of the cycle, lasting from about 30 minutes to one hour Interphase occupies the largest portion of the cycle S G 1 M G 2 The M phase consists of the process of mitosis, when the chromosomes separate, together with cytokinesis, when the entire cell is physically divided into two daughter cells

The Cell Cycle G 1 - First Growth Phase The cell grows and new organelles and proteins are manufactured M S DNA replication takes place G 2 - Second Growth Phase The cell grows and prepares for mitosis During interphase, preparations for mitosis take place

Variations in the Cell Cycle G 1 is the most variable of all the phases The rapidly dividing epithelial cells lining the human intestine remain in the G 1 phase for only about 2 hours S G 1 M Some cells, such as nerve and muscle cells never divide; they may be considered to be permanently in the G 1 phase and never enter the S phase G 2 Slowly dividing liver cells may take many months to move through G 1 to the S phase

Cancer: The Cell Cycle Out of Control The multiplication of cells is a closely regulated process Cell division is under genetic control, and it is known that there are specific genes which code for proteins that ‘switch on’ and ‘switch off’ the process Cancer is a disease that results from uncontrolled cell divisions Normal cells become transformed into cancer cells when the genes that control cell division mutate and become ONCOGENES Environmental cancer-causing agents, known as carcinogens, play a part in causing the alteration of DNA structure that leads to oncogene formation Known carcinogens include ultraviolet radiation, cigarette smoke and X-rays

Scanning Electron Micrograph of Dividing Cancer Cells

Cancer: The Cell Cycle Out of Control When a normal body cell mutates it may divide to produce a clone of cells that form a tumour mutation normal body cell mitosis mutated body cell tumour Many such tumours are found to be BENIGN and do not spread from their site of origin – they may nevertheless compress and damage adjacent tissues Malignant, cancerous tumours may spread from their site of origin These tumours develop their own blood and lymph supply which can transport malignant cells from the tumour to other sites in the body malignant cancer cells carried to other body sites malignant tumour these cells invade other body regions to form secondary cancers This is called metastasis secondary tumour

Observing Mitosis in Plant Tissues Plant roots grow by mitotic division of the cells at the root tip Onion root tips are an ideal source of material for observing the stages of mitosis • A scalpel is used to cut about 4 mm from the tip of the growing onion root • Acetic acid is added to the tip on a watch glass and warmed gently for about 5 minutes; the acid helps to macerate the cells • The root tip is transferred to a slide where two or three drops of aceto-orcein stain are added; this stain is taken up by the chromosomes and makes them more visible as they stain red • The tip is gently broken up with a mounted needle and the cells are spread across the slide • A coverslip is placed over the root preparation and gently squashed • The slide is examined for stages of mitosis using an optical microscope

Photomicrograph showing cells from an onion root tip Note that many of the cells are in interphase

The Timing of the Cell Cycle The different phases of the cell cycle last for different periods of time An onion root slide was examined for the different stages of the cell cycle and the number of cells in each phase was recorded The percentage of cells in each stage was then calculated The percentage of cells in each stage is used as a measure of the percentage of time that the cells spend in each phase of the cell cycle Interphase is the longest stage of the cell cycle with anaphase being the shortest Stage of Cell Cycle Number of Cells Cell % (approx) Interphase 138 66 Prophase 56 27 Metaphase 6 3 Anaphase 2 1 Telophase 7 3 209 100 Total

An Introduction to Meiosis is another form of cell division that is associated with reproduction in many organisms In humans, meiosis is responsible for the formation of the reproductive cells or gametes In humans, these are the egg and sperm cells Whereas most body cells have a complement of 23 pairs of chromosomes, human gametes possess only 23 single chromosomes. A gamete’s complement of 23 single chromosomes is constituted by one chromosome taken from each of the 23 pairs of chromosomes Within the human ovaries and testes, gametes are produced by meiosis and this process halves the chromosome number Human body cells are DIPLOID as they possess two sets of chromosomes (23 pairs) Human gametes are described as being HAPLOID as they possess only one set of chromosomes (23 chromosomes) SPERM If the gametes were diploid then the number of CELL chromosomes would double at every generation after fertilisation EGG CELL

MITOSIS MEIOSIS Diploid body cell The nucleus divides twice Two diploid daughter cells Meiosis is important as it ensures that, when the gametes fuse at fertilisation, the normal diploid number of chromosomes is maintained; meiosis is also an important source of genetic variation Four haploid, genetically different gametes are produced

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