TOPIC 3.1C, PART 3 MAMMALIAN GAMETES AND MEIOSIS

TOPIC 3 SPECIFICATION 8 Explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles and genes including independent assortment and crossing over (details of the stages of meiosis are not required). 9 Explain how mammalian gametes are specialised for their functions. 10 Describe the process of fertilisation in mammals and flowering plants (starting with the acrosome reaction in mammals and pollen tube growth in plants and ending with the fusion of the nuclei) and explain the importance of fertilisation in sexual reproduction.

SPECIALISED CELLS - GAMETES All eukaryotic cells share the basic structure and many functions. A zygote divides by mitosis to form a ball of eight cells that are totipotent – able to differentiate into any type of cell. After more divisions, the cells start to take on specialised roles – nerve cells, muscle cells, bone cells, gametes. What are the names of the human gametes?

MAMMALIAN GAMETES Define Haploid: describes a cell which has a single set of unpaired chromosomes in its nucleus. Gametes are haploid and are formed by meiosis. Diploid: describes a cell which has paired sets of homologous chromosomes in its nucleus, one set from the mother, one from the father, formed by mitosis. Somatic or body cells are diploid. Label ovum and sperm on the first pages in the handout packet using figure 3.11, page 107 Ovum: haploid nucleus, protein and lipid droplets, lysosomes, zona pellucida, follicle cells Sperm: haploid nucleus, middle with mitochondria, acrosome, flagellum

OVUM

SPERM CELL

MAMMALIAN GAMETES Students use the book to complete Activity 3.3a showing features and purpose. Include the relative size of sperm and egg cell

OvumSperm FeaturePurposeFeaturePurpose Activity 3.3a Human Gametes 1. larger cell with more cytoplasm 1. Large cytoplasm contains protein and lipids for early development of embryo 2. Jelly like coating- zona pellucida 2. Thickens after first sperm penetrates to prevent entry of other sperm 3. Cortical granules (lysosomes) 3. Release enzymes by exocytosis after first sperm penetrates to thicken zona pellucida 4. Haploid nucleus 4. When fuses with sperm will produce diploid zygote 1. Smaller cell with small cytoplasm 1. Less energy needed for it to swim 2. flagellum2. Allows sperm to swim to ovum 3. Acrosome 3. Enzymes are released by exocytosis- digest through zona pellucida and follicle cells 4. Haploid nucleus 4. When fuses with ovum will produce diploid zygote

FERTILIZATION OF GAMETES Interactive SNAB activity Complete Activity 3.3, Gametes and Fertilisation, in the hand out packet. Fill out worksheet 3.3b, What happens during fertilisation? using pages

QUIZ ON EGG, SPERM AND FERTILISATION What triggers the acrosome reaction during fertilisation in animals? (1) Describe the events which occur after the digestive enzymes are released from the acrosome up to the fusion of the male and female nuclei.(4) Why are gametes haploid?(1) Describe two features of a human sperm cell and explain how they allow it to carry out fertilisation. (4) Describe two features of a human ovum and explain how they allow it to carry out fertilisation(4)

HUMAN LIFE CYCLE AND MEIOSIS Turn to page 7 in the hand out packet, labeled Human Life Cycle. Label section that is haploid (n) 23 and diploid (2n) 46, processes - fertilization, meiosis Where exactly are ova and sperm produced? Label Ovaries and testes Label the diploid zygote Label sperm and ovum Indicate where mitosis and differentiation takes place

HUMAN LIFE CYCLE AND MEIOSIS

Cell Division Function: Occurs in: Number of cell divisions: Produces: Function: Occurs in: Humans: Plants: Number of cell divisions: Produces:

Meiosis overview Whereas there is only one division in mitosis (separation of the ______________), there are two divisions in meiosis. They are called meiosis I and meiosis II. Human life cycle Meiosis I: Meiosis II: For meiosis to occur, DNA replicates and condenses into chromosomes with two chromatids (see fig 3.32 page 115 Lets draw a chromosome! How many chromosomes in humans? What is a homologous pair: a pair of chromosomes, the same length, one inherited from mother, one from father. (same genes at some loci, different alleles) How many homologous pairs? 22 and 1 pair of sex chromosomes

Indicate which cells are- Haploid Diploid

POLES AND THE EQUATOR OF A CELL On each cell identify the equator and the poles

MEIOSIS OVERVIEW Animation- Watch “stages of meiosis” without sound to watch what happens. For the questions below answer meiosis OR meiosis 2 Animation When does the DNA condense to form chromosomes each with two chromatids? When do the homologous pairs pair up? When do they line up on the equator of the cell? When do they separate? When do chromosomes (NOT as pairs) line up on the equator of the cell? When do they separate?

MEIOSIS OVERVIEW In meiosis I: first division, DNA condenses to form visible chromosomes, homologous pairs join and line up on the equator and are then pulled apart, halving the number of chromosomes per cell In meiosis II: second division, chromosomes line up on the equator of the cell and sister chromatids are pulled apart What is the purpose of meiosis? Adds genetic variation Results in haploid cells that are necessary to result in a diploid zygote after fertilization

GENETIC VARIATION What actually causes the genetic variation? Independent /random assortment Crossing over When in meiosis does reduction of the number of chromosomes per cell happen? Meiosis 1 Notes on these processes can be made from later slides on the sheet “ Meiosis leads to genetic variation among offspring of sexual reproduction the three ways:”

INDEPENDENT/RANDOM ASSORTMENT Animation- watch “random orientation of chromosomes during meiosis” and fill in blanks Animation During meiosis I, homologous pairs of chromosomes lie in pairs on the equator of the spindle. It is completely random which way the pairs lie- either the maternal or paternal homologue can orient towards a given pole. Therefore different combinations of maternal and paternal chromosomes will be inherited by the gametes. Complete Independent Assortment- Mendel’s Second Law- while watching animation. Note: # 6 is not necessaryanimation

CROSSING OVER- OCCURS DURING MEIOSIS 1 Homologous chromosomes pair up and the non-sister chromatids make contact The non-sister chromatids break and exchange sections of DNA with each other at points called chiasmata (singular – chiasma). This produces a new combination of genes in each chromosome The reformed chromosomes are called recombinants.

REDUCTION AND FUSION OF GAMETES Meiosis produces haploid gametes- the reduction in chromosome number occurs in meiosis 1. An ovum can fuse with a sperm producing a new combination of genes with a diploid number.

MITOSIS VERSUS MEIOSIS MitosisMeiosis One nuclear division, separating chromatids 2 nuclear divisions, separating homologous chromosome pairs into chromosomes with new combinations of DNA in gametes. 2 identical daughter cells produced Four daughter cells produced Chromosome number is maintained, diploid cells produced Chromosome number is halved (haploid cells produced) No association between homologous chromosomes Homologous chromosomes pair and at points where they make contact, chiasma, sections are exchanged. No crossing over occurs, the homologous chromosome pairs remain the same. Crossing over occurs at chiasma. At metaphase, chromosomes only form a single row at the equator At metaphase, chromosomes form a double row at the eqator. (Independent assortment) Daughter cells are identical to the parent cell. Daughter cells differ genetically from each other and the parent.

PLANT FERTILISATION

PART OF A FLOWER-REVIEW Draw and label the parts of a flower: stamen: anther and filament, carpel, stigma, style, ovary, with ovules, petals, sepal

PLANT FERTILISATION Describe the functions of petals, anther, stigma, ovary. Petals: attract insect pollinators. Anther: Part of the pollen producing reproductive organ of a flower, together with a filament, they make up the stamen. Stigma: The pollen receiving part of the flower. Ovary: the female reproductive organ in flowers, contains the ovules.

What are the plant gametes ? Pollen grains contain male nuclei, ovule contains female nuclei Define pollination: Transfer of pollen from anther to stigma either by insect of wind Define fertilisation: Fusion of male and female gametes Where does fertilisation take place in mammals? Fallopian tubes/ oviduct In plants? In ovule

DOUBLE FERTILISATION 1. Pollination (of the correct species, ensured by sugar solution from nectaries, ridge pattern of pollen, and presence of other chemicals) 2.Pollen has 2 haploid nuclei : a pollen tube nucleus and a generative nucleus. 3.The pollen tube nucleus controls development of a pollen tube, which grows down through the style 4.The generative nucleus divides by mitosis: 2 haploid nuclei are formed: these are the male gamete nuclei

5. The pollen tube grows further down the style to the ovary. 6. The growth of the tube is directed towards an opening in the ovule called the micropyle. Chemicals are released which attract the tube growth. Inside each ovule there is an embryo sac FERTILISATION IN FLOWERING PLANTS

7.The pollen tube nucleus degenerates 8.The 1 st male nucleus fuses with the haploid egg nucleus : diploid zygote is produced 9.The 2 nd male nucleus fuses with the 2 polar nuclei (haploid): triploid endosperm is produced 10.This is called double fertilization 11.The zygote divides by mitosis and develops into embryo 12.The primary endosperm divides by mitosis and develops into endosperm tissue: this nourishes the developing embryo 13.The style and stigma shrivel 14.The ovule becomes the seed and the ovary becomes the fruit

DOUBLE FERTILISATION Complete the Double fertilisation gap fill in the handout packet.

TOPIC 3 SPECIFICATION 8 Explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles and genes including independent assortment and crossing over (details of the stages of meiosis are not required). 9 Explain how mammalian gametes are specialised for their functions. 10 Describe the process of fertilisation in mammals and flowering plants (starting with the acrosome reaction in mammals and pollen tube growth in plants and ending with the fusion of the nuclei) and explain the importance of fertilisation in sexual reproduction.

DID NOT WANT TO DO THIS LAST YEAR Animations (For crossing over- unique features of meiosis) Animations Vocabulary from the videos: Synapsis: pairing of the chromosomes in meiosis Homologue: chromosome pair Reduction division: the first cellular division in meiosis where the number of chromosomes per cell is halved. Crossing over: the exchange of genetic material between homologous chromosomes.