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 division•Liver cells - divide only to repair damage, and then stop dividing•Bone marrow cells - divide repeatedly to produce red and white blood cells•Meristem cells (tips of roots and shoots) – divide to produce new

growth•Cambium 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.

• Eukaryotic cells – Mitosis:

• new cells for growth and repair

– Meiosis: • 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.

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)

Cell Cycle• Starts when cell has been produced by cell division• Ends with the cell dividing to produce two identical cells• Can take anything from a few minutes to several hours per cycle

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”

INTERPHASE

CELLCYCLE

G1

S phase

G2

Mitosis (M)

Two daughter cells – genetically identical

Mitosis

•Purpose: increase cell number•Multicellular organisms – growth and repair•Unicellular organisms – population growth

•Parent cell copies (replicates chromosomes prior to division

•Result: Two genetically identical daughter cells produced, identical to the parent cell

•Rate is controlled via cell signalling and internal checks•Uncontrolled and repeated cell division by mitosis results in cancer (tumours and diffuse)

Chromosomes

A duplicated and condensed eukaryotic chromosome with two sister chromatids

Homologous Chromosomes

1. Chromosomes occur in pairs – there are 23 pairs of chromosomes in humans.

2. 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.

3. Each individual pair of similar chromosomes is called a homologous pair.

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

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.

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

Takes place ..

Produces how many cells?

What happens to number of chromosomes?

How do parent and daughter cells differ genetically?

Variation between daughter cells?

Mitosis and Meiosis Compared

  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