LO21-无性与有性繁殖.docx

X Hello and welcome! In this onine session,you wil learn about Asexual and Sexual Reproduction. Page 1/40 School of Biological Sciences At the end of the session,you should be able to: Recognise the difference between asexual and sexual reproduction; Recognise the variety of ways asexual repro...

X Hello and welcome! In this onine session,you wil learn about Asexual and Sexual Reproduction. Page 1/40 School of Biological Sciences At the end of the session,you should be able to: Recognise the difference between asexual and sexual reproduction; Recognise the variety of ways asexual reproduction can occur; ldentify the advantage of asexual reproduction; Identify the necessary players in sexual reproduction; Recognise the various patterns of sexual lifecycle; ldentify the advantage of sexual reproduction;and Explain how meiosis contributes to genetic variation. Page 2/40 School of Biological Sciences Some organisms reproduce simply by cell division,either through binary fission or mitosis and do not involve gametes.This is called asexual reproduction. 一 e.g.,Binary fission in prokaryotes;mitosis in protists. Other organisms are able to reproduce both sexually (i.e.through fertilisation of gametes)and asexually. e.g.,Strawberry plants produce flowers (sexual reproduction) as well as send out runners (asexual reproduction) Page 3/40 School of Biological Sciences Binary fission and Mitosis serve the role of asexual reproduction in unicellular organisms,such as prokaryotes,protists and some fungi.This means that the daughter cells generated through binary fission or mitosis are themselves "offspring". In multicellular organisms,mitosis often simply serves to replenish the body with new cells. -e.g.,For growth and repair,rather than for reproduction However,this does not mean that asexual reproduction in multicellular organisms cannot occur. Page 4/40 Some examples:invertebrates such as sponges,cnidarians, flatworms,annelids and echinoderms -Flatworms reproduce asexually by splitting in half. Sponges,annelids,and echinoderms have the ability to regenerate a full mature body from fragments. Sponges Cnidarians Flatworms Annelids Echinoderms 警率萄 NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences Some examples:invertebrates such as sponges,cnidarians, flatworms,annelids and echinoderms —Flatworms reproduce asexually by splitting in half. Sponges,annelids,and echinoderms have the ability to regenerate a full mature body from fragments. Hydras can reproduce asexually as an outgrowth (bud)of the parent. Echinoderms Asexual Reproduction in Hydra parenatal polyp budding of new polyp Page 5/40 警率萄 NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences Some examples:plants which carry out vegetation propagation Budding off a part of a plant and growing it elsewhere to generate a new plant. 一 Parts that can be broken off for this purpose differ from plant to plant. Players in Vegetative Reproduction Underground branches Suckering Shoots Underground stems Runner/Prostrate aerial stems Peanut Rose Ginger Strawberry Page 6/40 School of Biological Sciences In asexual reproduction,an individual inherits all of its chromosomes from a single parent. It is therefore genetically identical to the parent. In a stable environment,asexual reproduction may prove more advantageous than sexual reproduction because it allows individuals to: 一 Reproduce with a lower investment of energy;and Maintain characteristics from the parent which are successful for survival in this environment. Page 7140 Sexual reproduction occurs when a new individual (offspring)is formed by the union of two cells. The offspring are not Diploid zygote carries chromosomes from two parents. Paternal homologue genetically identical to either Haploid sperm of the parents. Meiosis and fertilisation constitute a cycle of sexual reproduction. Genetic variation in the offspring is brought about by meiosis. Haploid egg Diploid zygote In sexual reproduction,during the life-span of the organism,haploid cells or organisms alternate with diploid cells or organisms in a cyclical manner. Protist Spends most of its life cycle as a haploid individual. Animals Spend most of its life cycle as a diploid individual. Plants Spend significant portions of its lfe cycle as haploid and diploid individuals. In animals,the cells that will eventually undergo meiosis are reserved and set aside during the animals'growth and development for the purpose of reproduction. They are referred to as germ-line cells and are diploid,like somatic cells. -The germ-line cells will undergo meiosis to produce haploid gametes. —In most animals,this is the only stage during which the animal maintains its haploid state. Once fertilisation occurs,the diploid state predominates. Diploid (2n) Haploid (n) fenue gem tre ceh pkadj n Msisk 5pen tholsd n fwitatun Mtonk Zroote hslod m Page 13/40 Grows into adult male or adult female Female germ-line cells (diploid) 2n Page 13/40 (haploid)n Fertilisation Zygote (diploid)2n e mkiat ai= Copyright The McGraw-Hll Companies,Inc. In a human germ-line or somatic cell,there are 23 homologous pairs of chromosomes.(2x23=46 chromosomes). They are numbered 1 to 22 as shown >autologous chromosomes. The last pair is not numbered,but referred to as the sex chromosome.If an individual possesses XX,it is equivalent to female;if XY,it is equivalent to male. School of Biological Sciences Meiosis brings about genetic variation in two key ways: During prophase l Crossing over Page 18/40 Crossing over Page 18/40 Crossing over Crossing over Firsily,when homologous chromosomes line up together as a pair crossing over occurs between non-sister chromatids of homologous chromosomes. The chromatids break in the same place and sections of chromosomes are swapped. School of Biological Sciences Meiosis brings about genetic variation in two key ways: During prophase l Page 19/40 Page 20/40 XX X× Page 20/40 熟 V XX xk Page 20/40 Page 20/40 Page 20/40 )( ( ts5 1() Page 20/40 Page 20/40 xx 8 Page 20/40 )( ( 455 l() Page 20/40 Page 20/40 Page 20/40 Page 20/40 Page 20/40 Page 20/40 xx ,)( () 4s5 1() Page 20/40 Page 20/40 Page 20/40 School of Biological Sciences Meiosis brings about genetic variation in two key ways: During prophase l XX xx XX XX uu Page 21/40 XX X8 uu Page 21/40 Ⅱ vo g Page 21/40 Ⅱ Vu Page 21/40 vv ,)( ()x (ss 1() Page 21/40 vv Page 21/40 Ⅱ v* Page 21/40 v* ))( ()c tsS 1() Page 21/40 器 V 3 Page 21/40 Ⅱ V* V Page 21/40 器 W ))( ()c ts{ 1() Page 21/40 v Page 21/40 Ⅱ v Page 21/40 Vy j( S /<, Page 21/40 Page 21/40 )( )( ))( )》( ( ( ( 4s5 as5 4s5 435 1 1) 1() 1() Combination of Independent Assortment and Crossing Over >2 Combination of Independent Assortment and Crossing Over Page 21/40 Combination of Independent Assortment and Crossing Over Crossing over >2 Fertilisation -The union of male and female gametes. Page 22/40 Fertilisation -The union of male and female gametes. Chromosomes donated by the parents are combined in the resulting zygote. School of Biological Sciences Page 23/40 Paternal gamete Maternal gamete 3 chromosomesin a haploid set->2 possible combinations in each collection of parental gametes. If one is picked from each parent,2³×2³=(2³}²=64 chromosomally different zygotes are possible. Page 24/40 If crossing over occurs only once: —(4²³}²,or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible. School of Biological Sciences Take Home Messages In this section on Asexual and Sexual Reproduction,you need to take home the following messages: 1.Asexual reproduction is reproduction from a single parent,and the offspring is genetically identical to the parent. 2.Sexual reproduction requires the involvement of two parents, each contributing half of its genetic material to the offspring. As a result,the offspring is not genetically identical to one or the other of the parent. 3.In a stable environment,asexual reproduction may be more advantageous than sexual reproduction,due to low energy expenditure and predictable inheritance of parental traits. Page 30/40 School of Biological Sciences In this section on Asexual and Sexual Reproduction,you need to take home the following messages: 4.Sexual reproduction is advantageous because it generates genetic variation,thus ensuring genetic diversity within a population for greater survivability. 5.Genetic variation in sexual reproduction is generated through a combination of independent assortment and crossing over of chromosomes during meiosis. Page 30/40