Unit 3 CROSSING OVER.pptx

Introduction, Types-somatic and germinal; Mechanism of meiotic crossing over�synapsis, duplication of chromosomes, breakage and union, terminalization;
Cytological basis of crossing over - Stern’s experiment in Drosophila; Creighton
and McClintock’s experiment in Maize; Crossing over in Drosophila, Construction
of genetic maps in Drosophila - two point and three-point crosses; Interference and
coincidence.

1. CROSSING OVER Dr. Jagadisha T V Assistant professor Kristu Jayanti college Bangalore

2. CROSSING-OVER The physical exchange of parts of non-sister chromatids of homologous chromosomes following synapsis at meiosis is known as crossing-over . Results in the recombination of genes. 1,or 2, or more fragments may be interchanged during crossing-over.

3. Definition Crossing over refers to the interchange of parts between non-sister chromatids of homologous chromosomes during meiotic prophase (pachytene). In other words, crossing over results from exchange of genetic material between non-sister chromatids involving breakage and reunion at precise point

4. Frequency of crossing over of a given pair of genes is not constant. Crossing over frequencies are higher in the female sex than in males. Temperature, nutrition, sex, age, etc. influence crossing over. It is a part of meiotic mechanism during gametogenesis. Brings about variation and leads to evolution through natural selection.

5. Types of Crossing Over

6. 1. Somatic Crossing Over: Definition Somatic crossing over refers to the exchange of genetic material between homologous chromosomes in non-reproductive or somatic cells of an organism Somatic cells are all the cells in the body except for the germ cells (sperm and egg cells )

7. Occurrence Multicellular organisms during their development or in specific tissues, such as in plants during growth or repair. It is responsible for generating genetic diversity within somatic cells but does not directly contribute to the genetic diversity of offspring since somatic cells are not involved in reproduction.

8. Somatic crossing over can lead to genetic mosaicism within an individual. Different somatic cells may carry different combinations of alleles due to crossing over events, potentially resulting in varying traits within the same organism. In some cases, somatic crossing over can be linked to the development of certain genetic disorders or cancers if it occurs in critical genes. Consequences:

9. 2. Germinal Crossing Over Definition: Germinal crossing over refers to the exchange of genetic material between homologous chromosomes in the germ cells (sperm and egg cells) of an organism. Germinal cells are responsible for giving rise to the next generation through fertilization

10. Occurrence: Germinal crossing over takes place during meiosis, specifically during prophase I of meiosis I. It is a crucial process that promotes genetic diversity among offspring by creating new combinations of alleles on homologous chromosomes. Consequences : Germinal crossing over is directly related to the genetic diversity of offspring . It ensures that the genetic material inherited by an individual is a unique combination of alleles from both parents. This genetic diversity is important for adaptation to changing environments and the survival of species.

11. According to its occurrence in the somatic or germ cells, crossing over is of two types : 1. Somatic or mitotic crossing over When the process of crossing over occurs in the chromosomes of the body or somatic cells of an organism during the mitotic cell division it is known as somatic or mitotic crossing over . 2. Germinal crossing over Usually , the crossing over occurs in germinal cells during gametogenesis in which the meiotic cell division takes place

12. Somatic or Mitotic Crossing Over

13. Germinal & Meiotic Crossing Over Mechanism of Meiotic Crossing over: Process of crossing over includes following steps: Synapsis Chromosome duplication Crossing over by breakage and union Terminalisation

14. Syn a psis Meiosis starts with one cell that contains a set of 23 chromosomes, giving a total of 46 chromosomes. Just before the start of meiosis, the chromosomes duplicated, so there are two copies of the mom's chromosomes and two copies of the dad's chromosomes. The two copies of each chromosome are referred to as homologous chromosomes .

15. Synapsis

16. Chromosomal Duplication A bivalent is one pair of chromosomes (sister chromatids) in a tetrad . A tetrad is the association of two pairs of homologous chromosomes (4 sister chromatids) physically held together by at least one DNA crossover .

17. Crossing over by Breakage and Union Crossing over of chromatid pair occurs in PROPHASE-I, it is the first phase of meiosis-1. In prophase-1, 3rd phase is of PACHYTENE in which crossing over takes place due to formation of recombinant nodules

18. T erminalization After the exchange of segments, the two chromosomes start moving away from each other as the synaptic force lapses. The separation begins from the centromere and moves towards the ends of the chromosomes. It is this uncoupling of chiasma that is called terminalization. During diakinesis the homologous chromosomes get separated except at their ends.

19. 1. SINGLE CROSSING-OVER Only one chromatid of each chromosome is involved in single crossing over. Single crossing over is of most frequent occurrence.

20. 2. DOUBLE CROSSING-OVER Both the chiasmata may be between the same chromatids or between different chromatids. Two, or three, or all the four chromatids of the homologous pairs of chromosomes are involved in the process of double crossing-over. Double cross-over gametes are produced. This is of less frequent occurrence.

21. 3. MULTIPLE CROSSING-OVER More than two chiasmata are formed. Corresponding to the number of chiasmata formed, it is called triple (3 chiasmata), quadruple (4 chiasmata), and so on. Multiple crossing-over does not occur frequently.

22. SIGNIFICANCE Provides a direct evidence of the linear arrangement of genes in the chromosomes. Chromosome maps can be constructed. Gives rise to new combinations of genes, and hence, variations in offspring.

23. Cytological basis of crossing over – Stern’s experiment in Drosophila ; C. Stern in 1931 by direct cytological evidence, demonstrated that crossing over involves the interchange of parts of homologous chromosomes .

24. The female Drosophila carries XX chromosome and the male Drosophila carries one X chromosome and one Y chromosome. He made the two X chromosomes of the female are different from each other X chromosomes by treating such flies with X-rays. One X chromosome had a part of a Y chromosome attached to one end. The other X chromosome has been broken into two unequal segments.

25. Creighton and McClintock experiment in Maize

26. Creighton and McClintock experiment in Maize In 1931, the geneticists Harriet Creighton and Barbara McClintock provided an elegant demonstration that the recombination of genes linked on a chromosome requires the physical exchange of segments of the chromosome with its homologous partner. During their studies of linkage in corn, they developed a strain of corn that had one chromosome (number 9 of 10 pairs) with two unusual features: A knob at one end of the chromosome and An extra piece at the other. The extra piece of chromosome was the result of a translocation that had occurred at an earlier generation.

27. Here was an organism with a rare chromosomal aberration that made it possible to distinguish two homologs from each other under the microscope . Furthermore, this unusual chromosome carried the dominant allele for colored kernels (C) and the recessive allele for waxy endosperm ( wx ). Its normal-appearing mate carried the recessive allele for colorless kernels (c) and the dominant allele for normal (starchy) endosperm ( Wx ).

28. the parental kinds Cwx  and cWx ) and the recombinant kinds cwx  and CWx

29. G ametes by gametes containing a chromosome of normal appearance and both recessive alleles cwx (a typical testcross) should produce 4 kinds of kernels: 1. colored waxy ( Ccwxwx ) kernels 2. colorless kernels with normal endosperm ( ccWxwx ) 3. colorless waxy ( ccwxwx ) and 4. colored kernels with normal endosperm ( CcWxwx ).