Unit 2: Multicellular Organisms Sub Topic 2.3 Reproduction On completion of the sub topic I will be able to state that: Gametes are sex cells. Gametes are haploid. Haploid cells contain one set of chromosomes. In plants pollen grains contain the male gametes, which are made in the anthers. In plants the female gametes are called ovules and they are made in the ovaries. In animals male gametes are called sperm, which are produced in the testes. In animals the female gametes are called eggs or ova which are produced in the ovaries. The cells which produce the gametes are diploid. Diploid cells contain two sets of chromosomes. All body cells are diploid, with the exception of red blood cells and gametes. During fertilisation the nuclei of the male and female haploid gametes fuse. Fertilisation results in the production of a zygote, which is diploid. The fertilised egg is known as a zygote. A zygote divides to form an embryo. Page 1 of 11
Gamete (Sex Cell) Production In both animals and flowering plants sex cells are known as gametes. Gametes are haploid (n), this means they have one set of chromosomes. Gametes are produced during a process called meiosis. The cells in the body which have two sets of chromosomes are described as being diploid (2n). DIPLOID GAMETE MOTHER CELL (2n) Haploid gamete (n) Haploid gamete (n) Haploid gamete (n) Haploid gamete (n) Gametes are formed by a special case of cell division where the chromosome number is halved, leaving each newly formed sex cell with a single set of chromosomes. At fertilisation, each gamete provides one full set of chromosomes so that the zygote (fertilised egg) will have two matching sets. Page 2 of 11
Sexual Reproduction in Flowering Plants The anther produces pollen grains which contain the male gametes. The ovary produces ovules which contain the female gamete. petal stigma ovary ovule anther stamen sepal nectary Name of Structure Function Anther Stigma Ovary Ovule Sepal Nectary Petal Page 3 of 11
Pollination Pollination is the transfer of pollen from the anther to the stigma. This can be performed by either wind or insects. When a pollen grain lands on the stigma a sugary substance on the surface causes it to grow a pollen tube. pollen grain pollen tube ovary Fertilisation ovule The pollen tube grows down into the ovary. The nucleus of the male sex cell then leaves the pollen grain and travels down the pollen tube into the ovary to reach the nucleus of the female sex cell. pollen grain male male sex gamete cell nucleus nucleus pollen tube female sex gamete cell nucleus nucleus ovary ovule The fusion of the nucleus of the male and female gamete is called fertilisation. The fertilised egg is a single cell called a zygote. The zygote contains two sets of chromosomes, one from the male gamete and one from the female gamete and is a diploid cell (2n). This describes the fact it has two sets of chromosomes. Page 4 of 11
Sexual Reproduction in Mammals. In mammals, the male gametes (sperm) contain one set of chromosomes so are haploid (n). Sperm cells are produced in the testes. Female gametes (eggs or ova), which are also haploid (n), are produced in the ovaries. Male reproductive system: D Letter Structure Function A Testis B Sperm tube C Penis D Urethra Page 5 of 11
Female reproductive system: Letter Structure Function A Oviduct B Ovary C Uterus D Vagina Page 6 of 11
Fertilisation Fertilisation is the fusion of the nucleus of a sperm cell (haploid) with the nucleus of an egg cell (haploid). human sperm cell (23 chromosomes) human egg cell (23 chromosomes) After fertilisation the zygote (fertilised egg) is diploid (2n). The zygote divides repeatedly by mitosis to form a ball of cells called an embryo. In humans, all body cells (except red blood cells and gametes) are diploid (2n). Each human diploid cell contains 46 chromosomes or two sets of 23. Red blood cells have no nucleus and therefore no chromosomes and gametes have 23 chromosomes which is one set. Page 7 of 11
Examining Gametes Using a microscope, observe some prepared slides of sperm cells, ovaries, pollen grains and stigmas to look at the differences between sex cells in plants and in mammals. Record your observations including labels for any of the cell structures that are visible. Your teacher will help you to identify the cell structures. Specimen name: Magnification: Specimen name: Magnification: Specimen name: Magnification: Specimen name: Magnification: Page 8 of 11
Key Questions Answer the following questions in sentences: 1. Add a line to show the path taken by the growing pollen grain and add an X to show the destination of the male gamete once it has completed its journey. pollen grain pollen tube ovary ovule 2. The following statements refer to the stages that occur after pollination. A B C D E Fertilisation takes place. A pollen tube grows out from a pollen grain. The pollen tube grows down through the stigma. The male gamete moves towards the ovule. The pollen tube grows through the ovary wall. Use the letters of the statements to complete the sequence of stages. B E 3. How many chromosomes are present in the nucleus of a normal body cell of a human being? 4. How many matching sets of chromosomes are present in a normal body cell of a human being? Page 9 of 11
5. How many matching sets of chromosomes are present in a sperm mother cell? 6. Delete one word from each set of brackets to complete the sentences correctly: During gamete formation, (sex/body) cells receive different combinations of the (single/paired) chromosomes present originally in the gamete mother cells. Since the products of gamete formation are (identical/non-identical) this leads to increased (variation/uniformity) among the members of the species following sexual reproduction. 7. The diagram below represents fertilisation in humans. from ovary Cell A (in testis) fertilisation Cell B Cell C Cell D Complete the following table by naming the cells and stating the number of chromosomes present in each: Cell Name of cell Number of chromosomes A gamete mother cell B sperm C 23 D 46 Page 10 of 11
8. Describe what happens during fertilisation. 9. The diagram below shows the chromosome complement of a cell about to divide to form gametes. a) How many sets of chromosomes does this cell contain? b) How many chromosomes does this cell contain? Page 11 of 11