Horse breeding and genetics

Transcription

1 Horse breeding and genetics Pack 2 Meiosis Pack Code: EBS15 This pack will help you to: describe the stages of meiosis explain the genetic importance of meiosis

2 About this pack Objectives When you have completed this pack you should able to explain how cells divide through meiosis. The pack will help you to: describe the stages of meiosis explain the genetic importance of meiosis This pack is relevant to the level 3 unit Understand the Principles of Inheritance and Genetic Manipulation, and in particular to: Outcome 2: Understand the principles of Mendelian genetics It is also relevant to the level 3 unit Understand the Principles of Animal Biology, and in particular to: Outcome 1: Know the functions of the main animal cell organelles Links to other packs This is one of a series of learning packs, each tackling an aspect of equine biology. They are: EBS1: The skeleton EBS2: Joints EBS3: Circulation EBS4: Respiration EBS5: The digestive system of the horse EBS6: The reproductive system EBS7: Oestrus and hormones EBS8: Gestation and birth EBS9: How animal cells work EBS10: How animal cells divide EBS11: Connective and epithelial tissue EBS12: Muscle and nervous tissue EBS13: The horse s sensory organs EBS14: Introduction to genetics EBS15: Meiosis EBS16: Inheritance EBS17: Selective breeding EBS18: Breeding technology EBS19: Managing breeding EBS20: The lymphatic system EBS21: The endocrine system EBS22: The nervous system Animal breeding and genetics 2

3 Introduction As we saw in pack EBS14: Introduction to genetics, meiosis is the process by which cells in sex organs divide to create sperm and eggs. During meiosis a single parent cell divides twice to produce four new daughter cells. Meiosis is a remarkable process that helps to ensure the genetic diversity of offspring. The stages of meiosis Meiosis involves a number of stages, as follows: Before cell division Interphase First cell division (Meiosis 1) Phophase 1 Metaphase 1 Anaphase 1 Telophase 1 Second cell division (Meiosis II) Phophase II Metaphase II Anaphase II Telophase II The stages of each cell division are similar to those of mitosis (see the pack EBS10: How animal cells divide). However, there are some important differences as well, especially in Meiosis 1. Animal breeding and genetics 3

4 Interphase Interphase is the stage before the cell starts to divide. Each cell contains the genetic make-up or DNA of the individual, in the form of chromosomes, which themselves contain genes. The individual inherited chromosomes from its mother and father. So each cell contains pairs of chromosomes. While each chromosome in a pair has similar functions the alleles may be different. For example, in humans one chromosome may contain the allele for blue eyes, while the other may have the allele for brown eyes. The pairs of chromosomes are called homologous chromosomes. During interphase, the chromosomes replicate. Each chromosome now consists of two identical or sister copies which stay joined together and are also called dyads. Meiosis I the first cell division During Meiosis I the parent cell divides into two daughter cells, each with a different genetic make up. Prophase I Prophase I is the longest stage of meiosis and is a complex process. DNA condensation occurs and the replicated chromosomes become visible through a microscope. Pairing. Each chromosome seeks out its equivalent or homologous chromosome. They join together as a homologous pair (one from each of the individual s original parents). Crossing over. As prophase I continues the homologous pairs join together at points called chiasmata to exchange genetic material. This crossing over is of vital importance because it helps to ensure the genetic diversity of any offspring. As Prophase I comes to an end the chromosomes start to separate again however their genetic makeup has now changed. During Prophase I other important changes happen to the cell. The nuclear membrane disappears and centrosomes move to each end of the cell. This is sometimes seen as a separate stage called prometaphase Animal breeding and genetics 4

5 Metaphase I During metaphase I homologous chromosomes line up along the centre of the cell. This lining up is random there is a 50:50 chance that either chromosome from a pair can finish up on either side of the centre, and later on in one of the two new cells. This randomness also helps ensure genetic diversity. Centrosomes at each end of the cell send out spindle fibres which attach to one of each pair of chromosomes. Anaphase I During anaphase I the homologous chromosomes separate. One chromosome from each pair starts to move towards one of the centrosomes, while the other moves in the opposite direction. Because of crossing over, they are now genetically different from how they were before Prophase I. Telophase I During telophase I the chromosomes arrive at opposite ends of the cell: There are now two nuclei, and a membrane reappears round each one. The cell divides into two this process is called cytokineisis because at this point the cell s cytoplasm divides completely to create two distinct cells. Animal breeding and genetics 5

6 This is the end of Meiosis I. There are now two, new cells, and each contains significantly different genetic material as a result of the crossing over and random alignment. Activity 1 How do the following two things help create genetic diversity? a) Crossing over b) Random alignment 2 In which stage do each of the following happen? a) Chromosome replication b) Crossing over c) Random alignment d) Creation of two new cells The following photographs of plant cells show the process of Meiosis I. Animal breeding and genetics 6

7 Meiosis II the second cell division During Meiosis II the two new cells in turn divide to create four new cells in total, each with half the number of chromosomes. Prophase II During prophase II further changes take place in each new cell: Two centrosomes start to move into position at opposite ends of the cell. The membrane round the nucleus starts to disappear. Metaphase II During metaphase II the chromosomes move to the centre of the new cell. Unlike metaphase I it is individual chromosomes, not pairs, that line up. However, each chromosome is still a dyad. Anaphase II During anaphase II the chromosomes split. One chromatid from each chromosome starts to move towards one of the centrosomes, while the other chromatid moves in the opposite direction. Telophase II During telophase II, the process comes to an end: There are now two nuclei, and a membrane reappears round each one. Each cell divides into two new cells, each with half the number of chromosomes. Animal breeding and genetics 7

8 Activity Some of the following statements are in the correct columns, but the others are muddled up. Reorganise them so they are all correct. Prophase Metaphase Anaphase Telophase Meiosis I Homologous pairs exchange DNA Alignment of individual chromosomes Chromatids separate from individual chromosomes Results in two new cells each with the full number of chromosomes Meiosis II No DNA exchange takes place Random alignment of homologous pairs Homologous pairs of chromosomes separate Results in two new cells each with half the number of chromosomes The following photographs again of plant cells show the process of Meiosis II. Prophase II Metaphase II Anaphase II Telophase II Animal breeding and genetics 8

9 After meiosis The haploid cells that result from meiosis are called gametes. Because there is half the number of chromosomes in the haploid cell, the process is sometimes called reduction division. In mares: Meiosis 1 begins in the foetus so all potential eggs are present at birth though many degenerate before puberty. Meiosis 1 is completed at ovulation, and Meiosis II happens after fertilisation. Gametes produced by females are eggs (ova). The creation of ova is called oogenesis. Only one of the gametes produced by a female will be a viable egg. In stallions: Meiosis begins at puberty and continues through life. Gametes produced by males are sperm. The creation of sperm is called spermatogenesis. All the gametes will be viable sperm. Each gamete has a single set of chromosomes and is therefore a haploid cell. During fertilisation an egg and sperm unite to create a new dipoid cell which will go on to become an embryo. This combination of egg and cell from parents with different DNA leads to even more genetic diversity. The new cell will divide by mitosis into 2, then 4, then 8, 16, 32 cells and so on as the embryo and later the foetus develops. Because meiosis is such a complex process, things can go wrong. The body has mechanisms to monitor the process and to stop it if this happens. However in some cases problems can lead to miscarriage, or to genetic mutations or defects. Animal breeding and genetics 9

10 Glossary Centrosomes Chiasmata Chromatids Crossing over Cytokinesis Diploid Dyad Gamete Haploid Homogolous pairs Nucleus Oogenesis Organelle Spermatogenesis Separate the chromosomes in the nucleus. Two centrioles are contained within a centrosome Points where pairs of chromosomes have swapped sections when crossing over Replicated copies of a chromosome Exchange of DNA during Prophase I The division of a cell s cytoplasm Cells with the full number of chromosomes, each in pairs The result of DNA replication prior to meiosis. A dyad is a chromosome with two halves or chromatids Egg or sperm cell-gametes are haploid cells Cell (gamete) with half the number of chromosomes, and no pairs Pairs of chromosomes within a cell that are inherited from the father and mother Cell organelle that contains the cell s genes and DNA The creation of an ovum or egg Component of a cell The creation of sperm Animal breeding and genetics 10

11 Resources and further reading There are many very useful sources of information on the internet: There are useful web sites on horse genetics at: and There is an excellent talking glossary of genetic terms at: The Genetics Home Reference site is at Answers to activities Activity on page 6 1 During crossing over homologous pairs exchange DNA. Random alignment means there is a 50:50 chance of either chromosome from a pair becoming part of a new cell. 2 Chromosome replication Interphase. Crossing over Prophase I. Random alignment Metaphase I. Creation of two new cells Telophase I. Activity on page 8 Prophase Metaphase Anaphase Telophase Meiosis I Homologous pairs exchange DNA Random alignment of homologous pairs Homologous pairs of chromosomes separate Results in two new cells each with the full number of chromosomes Meiosis II No DNA exchange takes place Alignment of individual chromosomes Chromatids separate from individual chromosomes Results in two new cells each with half the number of chromosomes Animal breeding and genetics 11

12 Knowledge quiz 1 What are the four main stages of Meiosis I? a) b) c) d) 2 What is the crucial difference between Prophase I and Prophase II? 3 Which of these statements describe Metaphase I and which describes Metaphase II? a) Lining up of individual chromosomes b) Random lining up of homologous pairs 4 Meiosis II is a similar process to mitosis. What is the crucial difference? 5 What is the difference between a haploid cell and a diploid cell? 6 Give two differences between meiosis in males and females: a) b) Animal breeding and genetics 12

13 Acknowledgements This learning pack has been produced by the Land Based Colleges National Consortium Ltd. The LBCNC is a consortium of colleges working in the land-based sector which cooperate in the development and production of quality flexible learning materials which encourage independent learning. We would like to acknowledge the contributions made by the following individuals and colleges in the development of this learning pack. Initial guidance and source material: Claudine Sutton, Derby College Debbie Smith, Bridgwater College Critical readers Rachel Unwin, Sparsholt College Cover photograph: Steve Watson, Riseholme College Foal photo: Developed and produced for LBCNC by Learners First Revised in September The Land Based Colleges National Consortium Ltd. All rights reserved. Permission to photocopy or adapt the material in this learning pack is granted to members of the Land Based Colleges National Consortium Ltd. only. For further information please contact the LBCNC project management team at 7 Tyne Road, Bishopston, Bristol BS7 8EE. Tel Animal breeding and genetics 13