Chapter 3.1 Noor Sufi

Two basic types of cells: * Eukaryotic ( Eukaryotic Cells Two basic types of cells: * Eukaryotic ( * Prokaryotic (Bacteria) EUKARYOTIC CELLS: Cells contain membrane bound organellles such as nucleus, mitochondria and chloroplasts. Larger than prokaryotic cells. Not all eukaryotic cells have a cell wall like Prokaryotic cells.

- Their cells do not have a nuclei or other membrane bound organelles. Prokaryotic Cells. - Their cells do not have a nuclei or other membrane bound organelles. Pili – Thin, protein tubes. - Allows bacteria to attach to other cells. Cell Wall: - Helps maintain structure of the cell. Flagellum: - Helps cell to move.

Protein trafficking. Ribosomes on rough ER form vesicles with proteins inside. All these bubbles join together to form flattened stacked cisternae (Golgi apparatus) Proteins are modified within the Golgi apparatus. Glycoprotein is formed by carbohydrates amd protein joining. Glycoprotein vesicles pinches off which contain the modified protein. Vesicles fuse with cell surface membrane and release of contents (exocytosis) Fig 1. The production of proteins and their route through the cell.

Gametes & Fertilisation Ovum: Once a sperm fuses with and penetrates the membrane surrounding the egg, chemicals released by the ovum cause the zona pellucida, to thicken, preventing any further sperm entering the egg. Gametes & Fertilisation Steps of Fertilisation.. Sperm enters the ovum. Chemicals are released from cells surrounding the ovum triggering the acrosome reaction. Acrosome swells, fusing with the cell surface membrane. Digestive enzymes in the acrosome are released. Enzymes digest through the follicle cells and then through the zona pellucida. Sperm fuses with the ovum membrance. Sperm nucleus enters the ovum, Enzymes released from the lysosomes in the ovum thicken the zona pellucida to prevent further entry of sperm. Fig 2. Ovum

Sperm..

Gamete cells Human cells contain 46 chromosomes – 23 homologous pairs. Gametes have half the number of chromosomes found in normal cells which is very necessary. Why is this?! - If the sperm and ovum cell both had the full number of chromosome number, then the zygote would have 92 chromosomes. When this individual reproduced, the zygote would contain 184 chromosomes so with each generation the number of chromosomes would double, which obviously wouldn’t work.

How do gametes form? 2 different types of cell division in living organisms: * Mitosis – Produces new body cells as organism grows and develops. Retains full number of chromosomes, called the diploid number (2n) (46 in humans) Meiois – Produces gametes with only half the number of chromosomes (n) (23 in humans) This form of cell division occurs in the ovaries and testes of animals Meiosis has two important roles in biology: * It results in haploid cells, which are necessary to maintain the diploid number after fertilisation. * It helps create genetic variation among offspring.

Gamete production by meiosis.

Independent assortment… During meiosis only one chromosome from each pair ends in up in each gamete. The independent assortment of the chromosome pairs as they line up during meiosis I is a source of genetic variation. This process is random as either chromosome from each pair could be in any gamete. Crossing over: During the first meiotic division, homolgous Chromosomes come together as pairs and all four chromatids come into contact. At these contact points the chromatids break and Rejoin, exchanging sections of DNA.

Test yourself… What would the chromosomes in the gametes produced from the homologous chromosomes in cells A and B look like after crossing over has occurred?  Cell A  Cell B

Fertilisation in flowering plants… Fertilisation takes place in the embryo sac within the ovule. The pollen grain germinates on the style and a pollen tube grows down through the style towards the ovary, with its growth controlled by the tube nucleus. The pollen grain contains two nuclei; the tube nucleus and the generative On germination of the pollen, the generative nucleus divides to form two haploid gamete nuclei which move down the pollen tube. The tube grows into the embryo sac and the two male gamete nuclei enter the sac. One fuses with the egg cell and forms a diploid zygote and the second fuses with two nuclei in the embryo sac called polar nuclei to form a triploid cell. The diploid zygote divides to form the embryo!

Exam Questions… Describe the sequence of fertilisation in flowering plants. Describe the process of fertilisation in mammals