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Meiosis, crossing over and linked genes Dr.Aida Fadhel Biawi 2013.

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Presentation on theme: "Meiosis, crossing over and linked genes Dr.Aida Fadhel Biawi 2013."— Presentation transcript:

1 Meiosis, crossing over and linked genes Dr.Aida Fadhel Biawi 2013

2 The Mitotic Process

3 Figure :Cell division cycle. A dividing cell with 46 chromosomes passes through the G1, DNA synthesis (S), and G2 phases before entering mitosis, which consists, in succession, of prophase (P), metaphase (M), anaphase (A), and telophase (T). The end result of the cell division cycle is the formation of two daughter cells, each with 46 chromosomes.

4 Figure : Essential features of the phases of the cell division cycle of a cell with two pairs of chromosomes.

5 Figure : Metaphase chromosome. The centromere (primary constriction) is marked by the downward diagonal pattern. The kinetochore is the region of attachment of spindle fibers that forms on the outward portion of the centromere. At metaphase the chromosome is duplicated, and each complete chromosome is a chromatid.

6 The Meiotic Process

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8 Figure : Meiotic process. Meiosis I consists of G1, DNA synthesis (S), and G2 phases; prophase I (PI), which includes leptotene (L), zygotene (Z), pachytene (Pa), diplotene (D), and diakinesis (Di); metaphase I (MI), anaphase I (AI), and telophase I (TI). The products of meiosis I carry 23 duplicated chromosomes (23d) from cells that entered meiosis I with 46 chromosomes. Meiosis II consists of a combined G1/G2 phase followed by prophase II (PII), metaphase II (MII), and telophase II (TII). The final products of the meiotic process are cells that contain 23 chromosomes that comprise one of each of the original set of 23 pairs of chromosomes. In human females, each meiotic process produces one functional gamete and three nonfunctional cells called polar bodies. In human males, all meiotic products are functional.

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13 Figure : Essential features of chromosome distribution during the phases of the meiotic process of a cell with two pairs of chromosomes.

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15 Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes. This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic division reduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis. The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs of chromosomes are indicated by size, and parental origin of chromosomes by color.

16 Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

17 Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells.

18 Why do we need meiosis? Meiosis is necessary to halve the number of chromosomes going into the sex cellsMeiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parentsAt fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

19 This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

20 Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

21 Meiosis I : Separates Homologous Chromosomes InterphaseInterphase –Each of the chromosomes replicate –The result is two genetically identical sister chromatids which remain attached at their centromeres

22 Prophase I This is a crucial phase for mitosis.This is a crucial phase for mitosis. During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. Extremely IMPORTANT!!! It is during this phase that crossing over can occur.Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over is the exchange of segments during synapsis.Crossing Over is the exchange of segments during synapsis.

23 Metaphase I The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. –Still in homologous pairs

24 Anaphase I The spindle guides the movement of the chromosomes toward the polesThe spindle guides the movement of the chromosomes toward the poles –Sister chromatids remain attached –Move as a unit towards the same pole The homologous chromosome moves toward the opposite poleThe homologous chromosome moves toward the opposite pole –Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

25 Telophase I This is the end of the first meiotic cell division.This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells.The cytoplasm divides, forming two new daughter cells. Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell divisionEach of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

26 Cytokinesis Occurs simultaneously with telophase IOccurs simultaneously with telophase I –Forms 2 daughter cells Plant cells – cell platePlant cells – cell plate Animal cells – cleavage furrowsAnimal cells – cleavage furrows NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSISNO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

27 Figure :The stages of meiotic cell division: Meiosis I

28 Meiosis II : Separates sister chromatids Proceeds similar to mitosisProceeds similar to mitosis THERE IS NO INTERPHASE II !THERE IS NO INTERPHASE II !

29 Prophase II Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equatorEach of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

30 Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis-like fashionThe chromosomes are positioned on the metaphase plate in a mitosis-like fashion

31 Anaphase II The centromeres of sister chromatids finally separateThe centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite polesThe sister chromatids of each pair move toward opposite poles –Now individual chromosomes

32 Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occursNuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cellsAfter completion of cytokinesis there are four daughter cells –All are haploid (n)

33 Figure 13.7 The stages of meiotic cell division: Meiosis II

34 One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 = ???!

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36 Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes. This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic division reduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis. The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs of chromosomes are indicated by size, and parental origin of chromosomes by color.

37 Figure :Crossing over recombines genes. Crossing over generates genetic diversity by recombining genes and thereby mixing parental traits. The capital and lowercase forms of the same letter represent different variants (alleles) of the same gene. A chromosome actually has hundreds to thousands of genes.

38 Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

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40 - Genes close on a chromosome are packaged into the same gametes and are said to be “linked” - Linkage has this very precise meaning in genetics.. - Linkage refers to the transmission of genes on the same chromosome. Linked genes do not assort independently.

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44 Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

45 Boy or Girl? The Y Chromosome “Decides”

46 Meiosis – division error Chromosome pair

47 Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

48 Thank You


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