1. 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.
YouTube binary fission and budding

2. Yeasts reproduce asexually by budding.
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.

3. Binary Fision (Bacteria) Budding (Yeast)
1 A bud is formed at the surface of the cell
2 Interphase – DNA and organelles replicated
3 Set of duplicated chromosomes – each set contained within a nucleus
4 One daughter nucleus migrates into the bud
5 Bud increases in size and eventually separates from parent cell – producing a new, genetically identical yeast cell
Recall bacterial structure leaves loops of DNA

4. 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

5. Divided into stages:
G1 (“growth phase” 1) - Cells prepare for DNA replication
S (“synthesis”) - DNA replication occurs
G2 (“growth phase” 2)- Short gap before mitosis
M 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”

6. G1 S phase G2 Mitosis (M) INTERPHASE CELL CYCLE
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
Mitosis: 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
Mitosis (M)
Two daughter cells – genetically identical

8. No apparent observable activity
The Cell Cycle
Cytokinesis – cell divides into two
DNA content = 20
G2 - Second growth phase - short
Short gap before mitosis (cell division)
Cytoskeleton of cell breaks down and the protein microtubule components begin to reassemble into spindle fibres – required for cell division
DNA content = 40
G1 - First growth phase – longest phase
Protein synthesis – cell “grows”
Most organelles produced
Volume of cytoplasm increases
Cell differentiation (switching on or off of genes)
Length depends on internal and external factors
If cell is not going to divide again it remains in this phase
DNA content = 20 (arbitrary)
S - Replication phase DNA replication – this must occur if mitosis is to take place
The cell enters this phase only if cell division is to follow
DNA content = 40
G1 + S + G2 = INTERPHASE
No apparent observable activity

9. Purpose: increase cell number
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)
Show picture of teratoma

10. Chromosomes
A duplicated and condensed eukaryotic chromosome with two sister chromatids
Homologous Chromosomes
Chromosomes occur in pairs – there are 23 pairs of chromosomes in humans.
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.
Chromosome numbers in other animals

11. 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
PROPHASE
The 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
METAPHASE
Chromosomes line up at the equator; the spindle fibres from each pole become attached to the centromere of the chromosomes
ANAPHASE
The 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
TELOPHASE
Chromatids 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

12. 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.

13. Prophase

14. Metaphase

15. Anaphase
Telophase

16. 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.

18. 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 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.
Talk through, draw on board

19. 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.

20. Mitosis and Meiosis Compared
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?

21. 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 cell
Diploid = 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

22. 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

23. 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