Cell Division and Reproduction
- The genetic material responsible for hereditary and variation is found in the nucleus
- The nucleus contains long- thread like structures called chromosomes which mainly consist of DNA and histones.
- Individual chromosomes are only visible when a cell is divided, when it isn't only a mass can be seen known as chromatin.
- Human body cells have 46 chromosomes in each nucleus and such a cell is described as diploid
- The egg and sperm cells only have one set of chromosomes which is the haploid number 23.
- The diploid number is the number of chromosomes in a zygote.
- In body cells the chromosomes can be arranged in pairs, the two members of each pair are identical so are described as homologous pairs.
- Every chromosome has a centromere
- Produces 2 identical daughter cells.
- The cell division of growth
- The bulk of the cell cycle is spent in interphase. The organelles replicate and the cell doubles in size.
1) Longest stage of division
2) The chromosomes become visible because the chromatin fibres shorten and thicken
3) Each chromosome consists of 2 chromatids
4) The centrioles move to opposite poles
5) The nucleolus disappears and the nuclear membrane breaks down
- Metaphase - 1) The chromosomes assemble at the equator of the spindle
2) They become attached to the spindle at their centromeres
- Anaphase - 1) The centromeres divide in two and the spindle fibres pull the daughter centromeres apart. The separated chromatids are pulled along behind.
- Telophase - 1) The chromosomes reach the poles of the spindle
2) A nuclear membrane reforms around each of the two groups of chromosomes and the nucleoli disappear.
3) A membrane is formed
- During sexual reproduction, in the formation of the gametes, a reduction division called meiosis takes place
- The daughter cells have a haploid number of chromosomes.
- Meiosis is 2 divisions
- To find meiosis it is necessary to look inside anthers and ovules
- Meiosis is preceded an interphase stage during which the genetic material replicates.
- Prophase 1
1) The chromosomes shorten and thicken so become visible
2) Homologous chromosomes pair together, a four stranded structure is thus formed
3) Non sister chromatids become joined called chiasmata
4) The chromatids may break and recombine crosswise which leads to genetic variation
- Metaphase 1
1) The pairs of chromosomes line up on the equator of the cell
2) The centromeres attach to the spindle
- Anaphase 1
1) The spindle fibres shorten and pull the homologous chromosomes apart.
2) One of each pair of homologous chromosomes is pulled to one end of the cell
- Telophase 1
1) The chromosomes group together at opposite poles
2) Nuclear membranes form around each group
- Meiosis 2
1) This is similar to mitosis and during this phase the chromosomes line up on the equator and the chromatids separate
2) There are now 4 identical daughter cells each with a haploid nucleus, these are called gametes.
- Meiosis is important as it leads to genetic variation. It does this by the production of haploid cells for fertilisation, the random assortment of chromosomes in metaphase and anaphase 1 and the crossing over of chromatids in prophase 1.
- The human sperm consists of 3 main regions
- The head contains a little cytoplasm filled with the nucleus. It is haploid
- At the front of the sperm head is a thin cap called the acrosome which contains lysosomes and digestive enzymes
- The mid-piece contains closely packed mitochondria which provides ATP for locomotion. The flagellum of the tail has contractile fibrils which propels the sperm forwards.
- The egg cells has a much bigger volume then a sperm, egg cell has a haploid nucleus
- It has a dense cytoplasm with mitochondria, rough ER, cortical granules and yolk droplets.
- The cortical granules contain enzymes that alter the structure of the outer egg when fertilised
- The yolk droplets provide food for the embryo.
- On it's journey to the egg the sperm undergoes capacitation. This makes the plasma membrane more permeable and makes the acrosome develop
- The ovum is surrounded by follicle cells which form the corona radiata and a jelly layer called the zona pellucida.
- On contact the acrosome reaction takes place. Enzymes are released from the acrosome which digest the zona pellucida.
- The sperm fuses with the ovum membrane
- The sperm nucleus shoots into the ovum
- There is a change in the electrical potential which weakly blocks further fusion.
- Enzymes released from lysosomes in the ovum thicken the jelly layer, preventing entry of more sperm
- Nuclei fuse
- The male gametes are found inside the pollon grains which are made in the anthers
- The nucleus of the pollon grain now divides into two by mitosis to form a pollon tube nucleus and a generative nucleus.
- Pollen grains are carried from one flower to another during pollination and are deposited on the stigma of the flower
- Inside the female carpel of the flower is an ovary. Here one of more ovules develop. Inside each ovule is an embryonic sac with a female gamete nucleus.
- When a pollon grain has landed on the stigma a pollon tube is made by the pollon tube nucleus.
- The male gametes then pass down this tube
- The generative nucleus in the pollon grain divides to form two male nuclei.
- The pollon tube enters an ovule
- The pollon tube nucleus degenerated and the tip of the tube bursts open.
- One nucleus fuses with the female gamete nucleus forming a zygote.
- The other nucleus fuses with 2 polar nuclei forming an endosperm which is a food source.