Unit 2 Reproduction Development Cellular Reproduction The cell

Unit 2: Reproduction & Development Cellular Reproduction The cell cycle Cell growth Cell division

Why do cells have to reproduce? • Cells must continuously reproduce to repair damaged tissues & replace lost cells • During the initial development: cell reproduction is important for the growth of the organism & production of different types of cells (differentiation)

How do cells reproduce? • Continual cycle of growth & division: cell cycle • Growth stage is called: interphase • Division stage is called: mitosis

Interphase • Majority of the cell cycle • Growth stage – cell produces new molecules which increase the volume & mass of the cell • DNA replication takes place • 3 stages of interphase: G 1, S, & G 2 • If a cell does not divide, it can stay in G 1 for a rest period

3 stages: 1. Cell growth (phase G 1) 2. DNA replication (S phase – synthesis) 3. Cell prepares itself to divide (phase G 2)

2 Checkpoints during Interphase • At the end of G 1 – cell will not continue to S phase if: ▫ ▫ Cell is not big enough Unfavourable environment DNA is damaged Other problems. . . • At the end of G 2 – cell will not continue to mitosis if: ▫ Cell is not big enough ▫ All of the DNA has not replicated http: //www. cellsalive. com/cell_cycle. htm

So, why do cells divide instead of simply growing larger & larger? • Cell will not be able to take in enough nutrients • Surplus of wastes • Cells die – new ones have to be produced

Mitosis • Part of the cell cycle where the nucleus is divided into 2 • Daughter cells receive the same number of chromosomes & the same genetic material as the parent cell • Therefore: mitosis creates two identical cells • Mitosis takes place in almost all body cells

Mitosis events: 1. Replication of genetic material in the parent cell 2. Separation of the replicated material 3. Formation of two daughter cells

Before we talk about mitosis. . . • • • Chromosome: Chromatin: Sister chromatids: Centromere: Haploid: Diploid:

Stages of mitosis: • • prophase metaphase anaphase telophase ▫ cytokinesis http: //www. cellsalive. com/mitosis. htm

Prophase • During this stage, chromatin (DNA & proteins) condenses, thickens & forms X-shaped chromosomes • Each chromosome was duplicated already during interphase • Nuclear membrane & nucleolus break down & disappear • The centrioles migrate towards the opposite poles • Spindle fibres (microtubules) start to form between the centrioles

Metaphase • Spindle fibres attach to the centromeres • Guide the chromatids to the centre of the cell – equatorial plate • One of the chromatids is attached to a fibre at one pole, the other is attached to the opposite pole

Anaphase • Centromeres split & the chromatids are pulled towards the opposite poles by the shortening of the spindle fibres

Telophase • Chromatids are separated to either pole & are referred to as individual chromosomes now • Nuclear membrane starts to reform around the groups of chromosomes • Nucleolus reforms • Spindle fibres disappear

Cytokinesis • Part of telophase • Division of the cytoplasm into 2 new cells • Animal cells: cell membrane is the last structure to form • Plant cells: cell wall forms & separates the 2 new nuclei

Can cells continue to divide perpetually? • Cells have a “biological clock” ▫ Different types of cells have different limits on the number of times that they are able to divide ▫ “Old” cells have undergone many divisions • Specialization decreases a cell’s ability to reproduce ▫ Skin cells – less specialized – can continue to divide ▫ Muscle cells – more specialized – reproduce less often

Perpetual division • Two types of cells are able to divide “endlessly” ▫ Spermatocytes – produce sperm ▫ Cancer cells – reproduce quickly, form tumours �Quick reproduction prevents the cells from specializing & carrying out normal functions