Meiosis and Sexual Reproduction. Chapter 10

Transcription

1 Meiosis and Sexual Reproduction Chapter 10

2 Number of Chromosomes As sexual organisms, humans receive a set of chromosomes from each parent Humans have 46 chromosomes, two copies of each chromosome (23 from mom, 23 from dad) A cell that contains one copy of each chromosome is called haploid A cell that contains two copies of each chromosome is called diploid In diploid cells, the two identical chromosomes are called homologous chromosomes

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4 Number of Chromosomes Homologous chromosomes contain the same genes, but not necessarily the same traits Ex: Chromosome #6 contains a gene for eye color, but one chromosome may have a blueeyed trait while the other may have a browneyed trait After S phase, when each chromosome is copied in preparation for division, each cell will then have four copies of each chromosome

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6 Meiosis During sexual reproduction, each parent will donate a gamete to the offspring Gametes are reproductive cells (sperm and egg) When gametes fuse, the cell is called a zygote. Gametes are haploid. They only contain one copy of each chromosome The zygote is diploid Meiosis is the system that produces a haploid gamete for sexual reproduction

7 Meiosis Mitosis: The daughter cells are exact copies of the parent There are only two cells produced The daughter cells are diploid In Meiosis The daughter cells contain a full set of chromosomes, but which traits each one receives are random mixtures of their parent s traits Four daughter cells are produced The daughter cells are haploid

8 Meiosis For gametes, interphase occurs as it does in mitosis Meiosis then goes through two series of steps, each similar to mitosis Meiosis I: the cells are reduced from diploid to haploid Meiosis 2: the chromatids on each individual chromosome are separated Meiosis only occurs in reproductive organs, while mitosis occurs in all other cells

9 Meiosis I Prophase I The nuclear envelope disappears, and spindle reappears The chromosomes also appear The homologous chromosomes undergo a process called crossing over. During prophase I, homologous chromosomes line up next to each other, matching gene for gene They then trade sections of each other s chromosomes at random locations. After crossing over, each chromosome contains parts of it s original strand of DNA and part of it s homologue. Thus, each chromatid is a unique sequence of DNA

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12 Meiosis I Metaphase I Each pair of homologous chromosomes will line up next to each other on the metaphase plate The Law of Independent Assortment: when chromosomes line up in metaphase I, they do not line up in any particular order or on any particular side. Each chromosome has an equal chance of being placed on the same side with any other chromosome in the cell When the cell undergoes the first division, it ensures each cell will have at least one copy of each chromosome

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14 Meiosis I Anaphase I During Anaphase 1, the spindle fibers begin pulling each homologous chromosome toward each centriole *Unlike in mitosis, each chromosome still contains two chromatids at this point*

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16 Meiosis I Telophase I In telophase I, a small plasma membrane is built between the two cells, but cytokinesis usually does not occur The nuclear envelope reforms briefly, then disappears again There are now two separate copies of nonidentical chromosomes in two different cells

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18 Meiosis II (Similar to Mitosis) Prophase II Chromosomes appear and nuclear envelopes disappear again Metaphase II Chromosomes line up at the center of the cell at the metaphase plate Anaphase II Individual chromatids separate and move toward separate centrioles Telophase II Cell parts reform, and cytokinesis occurs Four new daughter cells have been produced

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20 Human reproductive cycles When the zygote is formed, it is a combination of the two sets of DNA in each gamete Males undergo spermatogenesis, which occurs in the testes and produces 4 sperm Females undergo oogenesis, which occurs in the ovaries and produces 1 egg An egg is 20 times larger than a sperm cell. The other three eggs that aren t used are broken up and their parts go to make the larger egg. Most of these will never be used for sexual reproduction, but are produced anyway It is completely random which genes will be in which gamete during the moment of fertilization

21 Why all the mixing? Crossing over, independent assortment, and random fertilization all exist to ensure gametes are random samples of parental DNA Why is this good? 1) Certain genes may be good for one environment, but what if the environment changes? Only the same genes will be passed on to the next organism If the genes are now unsuccessful, its dangerous for the species 2) Shuffling of genes allow for unique combinations of genes to occur

22 Why all the mixing? 3) Allows for different genders With no mixing, how do you decide what gender offspring will be? 4) The organisms with the best traits will most likely survive and, thus, will most likely reproduce With random shuffling, the best genes will also be most likely to pass on over multiple generations 5) Variety of organisms A single set of human chromosomes, if randomly fertilized, allowed to go through independent assortment, and allowed to cross over only once will result in approximately how many genetically different human zygotes?

23 4,951,760,200,000,000,000,000,000,000

24 Mitosis vs Meiosis You should be able to answer the following for each stage: Number of daughter cells Haploid or diploid Identical or non-identical How to identify each stage by name, picture, or description Examples of cells/organisms that undergo mitosis and meiosis.

25 Changes in Chromosome number Chromosomal mutations occur when the gametes formed during meiosis contain unusual numbers of chromosomes After meiosis I, you should have two cells, each with two copies of each chromosome After meiosis II, you should have four cells, each with one copy of each chromosome It s always possible that homologous chromosomes fail to separate or sister chromatids fail to break

26 Polyploidy Polyploidy is a chromosomal mutation where a cell has more than two copies of each chromosome Triploid = 3n Tetraploids = 4n Pentaploids = 5n Polyploidy occurs often in plants, and is one reason for huge evolutionary diversity of flower designs and plant species *A set of gametes may go through meiosis and form polyploids, but only 1 out of these 4 gametes will be fertilized.*

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28 Monosomy and Trisomy If an organism has one less or one more of a specific chromosome, it is referred to as monosomy or trisomy Monosomy and Trisomy is a result of nondisjunction Both members of a homologous chromosome enter the same daughter cell during meiosis I Both sister chromatids fail to separate in meiosis II Down syndrome is the most common human trisomy, when the individual has a third #21 chromosome.

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30 Sex Chromosome number It is possible to inherit an abnormal number of sex-chromosomes as well. XO Turner syndrome, female Short, broad chested and extra folds of neck skin. Do not undergo puberty, menstruation, but are capable of giving birth (thanks to modern infertility treatments) and can lead fairly normal lives XXY Klinefelter syndrome, male The male sexual organs are underdeveloped, but the female organs (breasts, in particular,) are slightly overdeveloped Very large hands, feet and arms XXX Poly-X female Usually, poly-x females lead normal lives. Only slight physical abnormalities are evident XYY Jacob s syndrome, male Taller than average, persistent acne, and communication problems

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33 Chromosome Structure Some chromosomes are more fragile than others, and damage is always possible Most of the time damage is repaired by the cell. If not, it can result in more chromosome mutations Deletion In a deletion, a chromosome breaks, usually at the end of the chromosome Williams syndrome is a loss of the end of #7 chromosome Williams children are academically and physically awkward, but show excellent communication and musical skills Cru de Chats syndrome is a loss of the #5 chromosome end Individuals are mentally retarded with facial abnormalities, and an enlarged larynx results in an infant cry similar to a cat s

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35 Chromosome Structure A translocation is when a chromosome section moves from one chromosome to another Down syndrome (Trisomy 21) is a result of a section of the #21 chromosome breaking and attaching to a #14 chromosome, resulting in three #21 chromosomes Alagille syndrome, when a translocation occurs between #20 and #2 chromosomes The missing section of chromosome #20 results in severe itching and irritation of eyes, skin, and genital areas

36 Chromosome Structure Other types of chromosome break mutations: Duplication A chromosome section breaks and reattaches to it s sister chromatid, resulting in a repeated section Inversion A chromosome section breaks and reattaches, but backwards

37 Extra Credit Mitosis Question This question is worth an extra 5% on your essay exams You may check your answers with me ahead of time for a yes or no response as many times as you like. Turn in your answer to the box by Friday, the 15 th. At the beginning of mitosis, there is one cell. At the end of mitosis there are two cells. These two cells are identical in every way to their parent cell. Absolutely no construction, synthesis, diffusion or osmosis occurs during mitosis. Where does the cell get the materials for building a 2 nd identical cell if it can t build or accept any materials during mitosis?