Dr. Ramesh U4L3 Meiosis

Transcription

1 Dr. Ramesh U4L3 Meiosis

2 The Cell Cycle and Cell Division: MEIOSIS

3 The Cell Cycle and Cell Division KEY CONCEPT: Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

4 Organisms have two basic strategies for reproducing themselves: Asexual reproduction Sexual reproduction

5 Asexual reproduction The offspring are clones genetically identical to the parent Any genetic variations are due to mutations (changes in DNA sequences due to environmental factors or copying errors)

6 Sexual reproduction Involves fusion of gametes Results in offspring with genetic variation Gametes form by meiosis a process of cell division that reduces genetic material by half

7 DNA in eukaryotic cells is organized into chromosomes. Somatic cells: body cells not specialized for reproduction Each somatic cell contains two sets of chromosomes that occur in homologous pairs. One homolog came from the female parent and one from the male parent and have corresponding genetic information.

8 Gametes have only one set of chromosomes one homolog from each pair. They are haploid; number of chromosomes = n Fertilization: two haploid gametes fuse to form a zygote They are diploid; number of chromosome in zygote = 2n

9 All sexual life cycles involve meiosis: Gametes may develop immediately after meiosis Or each haploid cell may develop into a haploid organism (haploid stage of the life cycle) that eventually produces gametes by mitosis Fertilization results in a zygote and begins the diploid stage of the life cycle.

10 The essence of sexual reproduction is: Random selection of half the diploid chromosome set to form a haploid gamete Followed by fusion of haploid gametes from separate parents to make a diploid cell This results in shuffling of genetic information in a population, and no two individuals have exactly the same genetic makeup.

11 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis consists of two nuclear divisions but DNA is replicated only once. The haploid cells produced by meiosis are genetically different from one another and from the parent cell.

12 Mitosis and Meiosis: A Comparison

13 PROCESS OF MEIOSIS Meiosis I occurs after DNA has been replicated. Meiosis I divides homologous chromosomes in four phases.

14 PROCESS OF MEIOSIS Meiosis II divides sister chromatids in four phases. DNA is not replicated between meiosis I and meiosis II.

15 Meiosis differs from mitosis in significant ways. Meiosis has two cell divisions while mitosis has one. In mitosis, homologous chromosomes never pair up. Meiosis results in haploid cells; mitosis results in diploid cells.

16 Haploid cells develop into mature gametes. Gametogenesis is the production of gametes. Gametogenesis differs between females and males. Sperm become streamlined and motile. Sperm primarily contribute DNA to an embryo. Eggs contribute DNA, cytoplasm, and organelles to an embryo. During meiosis, the egg gets most of the contents; the other cells form polar bodies.

17 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity The function of meiosis is to: Reduce the chromosome number from diploid to haploid Ensure that each haploid cell has a complete set of chromosomes Generate diversity among the products

18 Shuffling of genetic material during meiosis occurs by two processes: Crossing over In prophase I homologous chromosomes (synapsis) and the four chromatids form a tetrad, or bivalent.

19 The homologs seem to repel each other at the centromeres but remain attached at chiasmata.

20 Crossing over results in recombinant chromatids Genetic material is exchanged between nonsister chromatids at the chiasmata. Crossing over results in recombinant chromatids and increases genetic variability of the products.

21 Crossing Over Forms Genetically Diverse Chromosomes

22 Independent assortment At anaphase I, it is a matter of chance which member of a homologous pair goes to which daughter cell. The greater the number of chromosomes, the greater the potential for genetic diversity. In humans, 2 23 (8,388,608) different combinations of maternal and paternal chromosomes can be produced.

23 Meiosis is complex, and errors can occur. Nondisjunction Homologous pair fails to separate at anaphase I Sister chromatids fail to separate at anaphase II Both result in aneuploidy an abnormal number of chromosomes.

24 Nondisjunction Leads to Aneuploidy

25 Most human embryos from aneuploid zygotes do not survive. Many miscarriages are due to this. The most common human aneuploidy is trisomy 16. Trisomy 21 (Down syndrome) is one of the few aneuploidies that allow survival.

26 Polyploidy Sometimes, organisms with triploid (3n), tetraploid (4n), and even higher numbers can form. This can occur through an extra round of DNA replication before meiosis, or lack of spindle formation in meiosis II. Polyploidy occurs naturally in some species and can be desirable in plants.

27 Translocation Crossing over between non-homologous chromosomes in meiosis I Location of genes relative to other DNA sequences is important, and translocations can have profound effects on gene expression.

28 A translocation that occurs in humans between chromosomes 9 and 22 can result in a form of leukemia.