1.1.3 Cell Division, Cell Diversityand cellular organisation

Objectives

Cell cycle – stages (Go, S, G1, M); mitosis and meiosis. Budding in yeast; binary fission (bacteria)Significance of mitosis and meiosis; Mitosis – produces clones; growth, asexual reproduction, cancer. Meiosis – gametes; maintain chromosome number; cellproduced not genetically identicalDNA, genes and packaging informationClones and stem cells (embryonic and adult)Differentiation of stem cells – blood cells (RBC, neutrophils); cambium (xylem + phloem)Cell specialisation - epithelial cells (squamous + ciliated); sperm; palisade; root haircell; guard cell; neuronesCells, tissues, organs, organ systems

HW – Past paper question

The Cell Cycle, Mitosis, and Meiosis

New cells can only be made when existing cells divide. All cells have the ability to divide – but some cells lose this ability.

Intestinal epithelial lining - replaced every five days by cell divisionLiver cells - divide only to repair damage, and then stop dividingBone marrow cells - divide repeatedly to produce red and white blood cellsMeristem cells (tips of roots and shoots) – divide to produce new

growthCambium cells (plants) – divide to form vascular tissue (xylem and phloem)

These are relatively unspecialised cells. Specialised cells often go through the cell cycle only once - the nerve cells, once formed cannot divide again.

In eukaryotic cells, there are two types of cell division – mitosis and meiosis. Mitosis is used to produce new cells for growth and repair. Meiosis is used in the formation of gametes only.

In prokaryotes (bacteria), cell division does not involve mitosis or meiosis – bacteria reproduce asexually, by a type of cell division termed by binary fission. Yeasts reproduce asexually by budding.

The cell cycle is the process that all body cells from multicellular organisms use to grow and divide. The cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells.

http://biolibogy.com/cells.html

1 A bud is formed at the surface of the cell2 Interphase – DNA and organelles replicated3 Set of duplicated chromosomes – each set contained within a nucleus4 One daughter nucleus migrates into the bud5 Bud increases in size and eventually separates from parent cell – producing a new, genetically identical yeast cell

Binary Fision (Bacteria) Budding (Yeast)

The cell cycle can be divided into stages:

G1 (“growth phase” 1) - Cells prepare for DNA replicationS (“synthesis”) - DNA replication occursG2 (“growth phase” 2)- Short gap before mitosisM Mitosis (relatively short)

Affected by availability of nutrients

Between each stage the cell “checks” to see if it is OK to proceed to the next stage.

“Proof-reading” enzymes check the copied chromosomes for mistakes (mutations) – the cell may kill itself (undergo “suicide”) if harmful mutations are – a process known as apoptosis.

Bacterial cells complete the cycle every 20 minutes. Muscle cells never complete the cycle – “terminal differentiation”

Uncontrolled and repeated cell division by mitosis results in cancer (tumours)

Eukaryotic cells have a well- defined cell cycle of growth and division (mitosis). The length of the cycle varies (from minutes to hours, or , longer) ending with mitosis.

Each phase of the cycle involves specific activities, and varies in length from one organism to another.

The Cell Cycle

MITOSIS (M)

Process by which a nucleus divides into two – each with an identical set of chromosomes – the nuclei are genetically identical

Four phases – prophase, metaphase, anaphase, and telophase

Followed by cytokinesis – division of the cell into two genetically identical daughter cells

INTERPHASE

Period of cell growth; cell prepares cell for cell division (mitosis); genetic material (DNA) is copied and checked for errors – prevents mutations being passed on

No apparent activity

New organelles and proteins are made

Divided into three phases (G1, S, and G2 phase)CELL

CYCLE

G1

S phase

G2

Mitosis (M)

Two daughter cells – genetically identical

G1 + S + G2 = INTERPHASE No apparent observable activity

Cytokinesis – cell divides into twoDNA content = 20

G1 - First growth phase – longest phaseProtein synthesis – cell “grows”Most organelles producedVolume of cytoplasm increasesCell differentiation (switching on or off of genes)Length depends on internal and external factorsIf cell is not going to divide again it remains in this phaseDNA content = 20 (arbitary)

S - Replication phase DNA replication – this must occur if mitosis is to take placeThe cell enters this phase only if cell division is to followDNA content = 40

G2 - Second growth phase - shortShort gap before mitosis (cell division)Cytoskeleton of cell breaks down and the protein microtubule components begin to reassemble into spindle fibres – required for cell divisionDNA content = 40

The Cell Cycle

Mitosis occurs rapidly during growth and repair to make new cells.

The rate is controlled according to the situation – e.g., during growth and repair the rate may be fast, but when growth or repair is achieved the rate declines or mitosis stops.

Cancers (tumours and diffuse) are formed through uncontrolled cell division by mitosis

Mitosis

Mitosis occurs wherever an increase in number of cells is needed.

It is important in the growth and repair of multicellular organisms, and in the population growth of unicellular organisms

During mitosis a cell produces two copies of itself.

Each is genetically identical to the other and to the parent cell from which they were formed.

The parent cell copies its chromosomes by replication prior to cell division.

Chromosomes

A duplicated and condensed eukaryotic chromosome with two sister chromatids

Homologous Chromosomes

Chromosomes occur in pairs – there are 23 pairs of chromosomes. Each pair consists of a chromosome from the male (father) and one from the female (mother). The chromosomes that make up each pair of the 22 pairs are the same size and have the same or alternative versions of genes for particular characteristics. Each individual pair of similar chromosomes is called a homologous pair.

•Replacement of cells - outer layer of skin – replacement of epidermal cells; red blood cells; intestinal lining cells

•Growth of tissues by producing new extra cells – e.g. epithelial tissue; plant meristem cells (stem and root growth).

•Division of zygote into a multicellular organism – in which, all cells are genetically identical.

•Formation of clones of T and B lymphocytes and plasma cells in the immune response

•Asexual reproduction - mitosis produces individual organisms which are genetically identical to each other (clones) and the parent cell – e.g. binary fission in bacteria; budding in yeast.

•The production of genetically identical cells in multicellular organisms allows certain cells to retain the ability to develop into any other type if needed as a result of damage.

•Sometimes, abnormal cells divide by mitosis in an uncontrolled way – giving rise to tumours, and if the cells are malignant, cancers.

Mitosis takes place in the following:

Before a cell divides, its chromosomes are copied exactly in INTERPHASE. This process is called replication; ATP is synthesised – provides energy for cell division; organelles are replicated and proteins are made

PROPHASEThe DNA of each chromosome is copied to form two chromatids (“sister” chromosomes); chromosomes condense – becoming shorter and fatter – visible under LM; nuclear envelope breaks down; chromosomes lie freely in cytoplasm; centrioles move to opposite ends of the cell, forming protein (tubulin) fibres across it called a spindle – fibres extend to the equator of the cell

METAPHASEChromosomes line up at the equator; the spindle fibres from each pole become attached to the centromere of the chromosomes

ANAPHASEThe spindle fibres contract; the centromeres are split and the pairs of sister chromatids are separated and dragged to opposite poles assuming a “V” shape – the centromeres lead; a complete set of chromosomes is therefore found at each pole; energy (ATP) is required

TELOPHASEChromatids reach their respective poles and uncoil – become thin and long again – now called chromosomes again – no longer visible under LM; spindle fibres break down; nuclear envelope forms around each group of chromosomes – forming two nuclei; cytokinesis follows – cytoplasm divides and a plasma membrane forms two form two individual cells; cell enters interphase once again

Cytokinesis

Division of the cytoplasm (cytokinesis) into two equal parts follows mitosis

A “waist” forms in the middle of the cell. Eventually, the plasma membrane from one side of the cell joins that of the opposite side of the cell and the two new cells separate. The two daughter cells are genetically identical.

The equator of the cell is constricted by a ring of contractile proteins (actin) in the process of cleavage, to create two cells.

In plant cells, the Golgi apparatus and associated secretory vesicles assemble at the equator. Their contents are deposited to form a plate (the cell plate). Some vesicles remain intact and make connecting channels, termed plasmodesmata, through the new cell wall.

Prophase

Metaphase

Anaphase

Telophase

Meiosis

Meiosis is a type of cell division used in the formation of gametes (spermatozoa and ova) in animals, and spore formation (which precedes gamete formation) in plants - it is termed reduction division.

Meiosis creates genetically different cells and variation within species

The number of chromosomes is halved - from diploid (46) to haploid (23).

Involves two distinct divisions

Meiosis I – homologous pairs of replicated chromosomes are separated

Meiosis II – reduction division (chromatids separated into individual haploid gametes)

Four cells (gametes/spores), genetically different from each other, result from one parent cell – each cell containing half the original number of chromosome

Production of haploid (n) gametes ensures the maintenance of the diploid (2n) number in subsequent generations

Meiosis involves certain “mixing” events, and these, along with the random fusion of gametes in fertilisation to form a diploid zygote, results in genetic variation in the offspring which may be of adaptive advantage.

The DNA of each chromosome is replicated to form two chromatids.

chromosomes arrange themselves into homologous pairs (both coding for the same characteristics), and prepare for cell division. At this point maternal and paternal chromatids can exchange bits of DNA to recombine their genetic material and increase the potential for variation.

Homologous pairs of chromosomes separate and move to the poles of the parent nucleus. For each of the 23 pairs there is a 50-50 chance as to which pole the paternal or maternal pair of chromatids go. With over 8 million possibilities there are many opportunities for variation.

Nucleus now divides to form two daughter nuclei, each with a mixture of paternal and maternal chromosomes but with half the full complement of genetic material (and no pairs at all). This division is called Meiosis 1.

The two daughter nuclei themselves divide to form gametes. This second division - Meiosis 2 - works just like mitosis. The chromosomes (really pairs of chromatids) split apart to form the genetic material of the four new cells. The end result is four sex cells each with a complete but single set of 23 chromosomes.

On fertilisation the nuclei of the sperm and the egg join to form a new nucleus, called the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg.

Meiosis leads to variation among the offspring of sexual reproduction in 3 ways:

Crossing over – crossing over of genes from one chromatid of one chromosome to the chromatid of the other homologous chromosome

Reduction and fusion of gametes from different individuals – gametes have haploid number of chromosomes – allows a gamete from one of these cells to fuse with another cell with a different haploid set, producing a zygote which has the normal diploid number of chromosomes but a new combination of genes

Independent (random) assortment – when the chromosomes line up as pairs at the equator (metaphase I) of the spindle, it is by chance which “way round” each pair lies.

  Mitosis Meiosis

Purpose

To make daughter cells identical to the parent cells - eg during growth and repair

To produce sex cells (gametes)

Takes place ..In all cells apart from gametes

In the reproductive organs (ovaries and testes)

Produces how many cells?

Two daughter cells Four gametes

What happens to number of chromosomes?

Same number as in parent cellDiploid = 46 (in pairs)

Half as many as in parent cell (The original number of chromosomes is restored when two gametes fuse to form a zygote.)Haploid = 23 (single)

How do parent and daughter cells differ genetically?

Not at all - genetic material is copied exactly (replicated)

Contain a mixture of chromosomes from two parent gametes - so cannot be identical

Variation between daughter cells?

No - they are clones of each other

Yes - they are genetically different from each other because chromosomes get shuffled up during division

Mitosis and Meiosis Compared

The drugs vincristine and vinblastine from the periwinkle plant inhibit spindle assembly – used in the treatment of cancers (leukaemia and lymphomas) and gout

Taxol form the Pacific ewe inhibits depolymerisation of spindle fibres (de-assembly), which effectively stops cell division – used to treat ovarian cancers

Stem Cells

Multicellular organisms function as a result of many different cell types that are specialised for their function – e.g:

Neurones (nerve cells) – specialised for the transmission of electrical nerve impulses

Red blood cells (erythrocytes) – specialised for the carriage of oxygen aroung the body

Vascular tissue (xylem and phloem) in plants - cells in the cambium tissue of plants differentiate to form vascular tissue – specialised for transport

All cells in multicellular organisms originate from stem cells. These are unspecialised cells that divide to become new cells, which then differentiate to become specialised into different cell types.

Differentiation is achieved due to the switching on and off of relevant genes