Title: The cell cycle and apoptosis 15 th January 2014 Learning question: What happens in interphase? What is apoptosis? Can you arrange these images in sequence? Homework: Read pages and answer textbook questions for Friday 17 th January. Make sure you mark the questions please! Starter: ,1,4,5,2

Learning outcomes (a) describe the cell cycle, with reference to interphase (G 1,S and G 2 ), mitosis and cytokinesis leading to diploid cells; (b) describe the structure of DNA and explain the importance of complementary base pairing and hydrogen bonding; (c) explain how DNA replicates semi- conservatively during the S phase of the cell cycle (HSW1); (d) outline the processes taking place at the G 1 and G 2 points in the cycle; (e) describe the appearance of the components of the nucleus and cell during mitosis with reference to: nuclear envelope, centrioles, spindle fibres, centromere, chromatids and chromosomes; (f) define the term apoptosis; (g) state that cell deletion by apoptosis and cell addition by mitosis are essential for normal growth and repair;

The Cell Cycle Mitosis only accounts for a small part of the total cell cycle. The majority of the cell cycle is interphase and it is divided into the following parts: G1 S G2

G 1 and S G 1 - First growth phase – Proteins are made – Proteins are used to build organelles – More cytoplasm is made S – synthesis – DNA is replicated

G 2 & M G 2 – second growth phase – Cell organelles grow and divide – Cell builds up energy stores M – mitosis phase – Nucleus undergoes mitosis and then cytokinesis (cell division

Control of Cell Cycle To know when one phase has ended and when to start the next, there are a series of checkpoints. In a cell, these checkpoints are proteins and enzymes. Proteins that signals the start of the next phase are call cyclins Enzymes that activate cyclins are called cyclin-dependent kinases (CDKs) Cyclins + CDKs = next phase activation

Important checkpoints G 1 checkpoint – Late G 1 triggers initiation of S phase – Replication of organelles G 2 checkpoint – Late G 2 triggers mitosis M(etaphase) checkpoint – Controls entry to anaphase

Signal Control of Cell Cycle Internal and external signals cue the assembly or disassembly of the cyclin-CDK completes

Signal Control of Cell Cycle What would happen to the cell cycle if a defective mitotic CDK was produced, resulting in a deformed active site? Mitotic cyclin would not be able to bind to the active site The initiation of the M phase would not result Mitosis phase may not occur / occurs at a very slow rate

Signal Control of Cell Cycle Stopping the cell cycle can be helpful – why? Allows DNA repair to take place In some forms of breast cancer, over-stimulation of Ck4 (a CDK) leads to increased cell division The protein p53 in a helpful protein that inhibits cell division. In over 50% of cancers contain a defective p53 gene

Apoptosis Apoptosis is programmed cell death in development – Series of biochemical events leading to an orderly and tidy cell death Hayflick Constant – Cells undergo about 50 mitotic divisions before apoptosis Necrosis – Untidy and damaging cell death occurring after trauma

Sequence of Apoptosis Enzymes breakdown cell cytoplasm Cytoplasm becomes dense Organelles are tightly packed Cell surface membrane changes and blebs form Chromatin condenses, nuclear envelope breaks Cell breaks into vesicles phagocytosis

Apoptosis

Control of Apoptosis Apoptosis is controlled by cell signalling – Cytokines from the immune system – Hormones and growth factors – Nitric oxide Makes inner mitochondrial membrane more permeable to hydrogen ions

Apoptosis and tissue development The rate of cells dying should balance the rate of cells produced by mitosis – Not enough apoptosis leads to the formation of tumours – Too much leads to cell loss and degeneration Cell signalling plays a role in maintaining the correct balance

Apoptosis in Development The formation of the digits (fingers and toes) occurs due to apoptosis during the development of the embryo.

Apoptosis and metamorphosis As tadpoles grow they develop legs, change their body shape and lose their tails The tail is lost by apoptosis

APOPTOSIS L/O: What is it? Why is it important? How is it controlled? What is its role in age-related disease? Apoptotic and necrotic cells, respectively

APOPTOSIS Programmed cell death Orderly cellular self destruction Process: as crucial for survival of multi-cellular organisms as cell division

Quick Questions 1.What word means “programmed cell death”? 2.Give an example of apoptosis in the human body 3.What would happen if the rate of apoptosis exceeded the rate of mitosis? Use whiteboards to show your answers

Tasks 1.Create a flow diagram of the stages that a cell goes through during apoptosis. 2.Make a list of key words for apoptosis then write a description or definition for each. 3.Summarise apoptosis and its role in development. – Be sure to include any examples; The consequences of too much/too little apoptosis; Explain why cells commit apoptosis.