Chromosomal basis of heredity


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Chromosomal basis of heredity

  1. 1. CHROMOSOMAL BASIS OF HEREDITY <ul><li>The genetic material of eukaryotes is distributed among multiple chromosomes. </li></ul><ul><li>In 1902 W. Sutton and T. Boveri independently recognized that the transmission of chromosomes from one generation to the next parallels the pattern of transmission of Mendelian factors (genes) from one generation to the next. </li></ul><ul><li>They proposed the chromosome theory of inheritance which states that the chromosomes are the carrier of genes (i.e. the genes are located on chromosomes). </li></ul>
  2. 2. Evidence considered <ul><li>1. The number of chromosomes is constant from cell to cell within an organism, from organism to organism with any one species and from generation to generation within that species. </li></ul><ul><li>2. Many eukaryotes have two copies of each type of chromosome in their nuclei. Their chromosome complement is said to be diploid (2N). Paired (homologous) chromosomes separate during gamete formation. Thus, gamete contain half as many chromosomes as somatic cells of the same organism i.e. each gamete has only one set of chromosomes and is said to be haploid (N). Diploid individuals are produced by fusion of two gamete nuclei, one from a male parent and another one from the female parent. </li></ul>
  3. 3. <ul><li>Therefore, the behaviour of Mendel’s factors (genes) during the production of gametes in peas precisely parallels the behaviour of chromosomes at meiosis. </li></ul><ul><li>- genes are in pairs, so are chromosomes, </li></ul><ul><li>- the alleles of a gene segregate equally into gametes, so do the members of a pair of homologous chromosomes, </li></ul><ul><li>- different genes act independently, so do different chromosome pairs. </li></ul><ul><li>3. Except during gamete formation, genes must be accurately reproduced and transmitted to daughter cells in such a way that each cell contains a set identical to its parent. Chromosomes, too, are very carefully replicated and separated during cell division, so that normally all descendants of a cell contain accurate copies of its chromosomes. </li></ul>
  4. 4. <ul><li>4. Visible changes in chromosome structure can often be correlated with genetic changes, e.g. the arms of two homologous chromosomes sometimes exchange parts during meiosis. The visible feature of this exchange is a joining between the arms of two chromosomes at point known as a chiasma. The chisma was found to correlate with interchange of allelic genes between homologous chromosomes. </li></ul><ul><li>This parallel behaviour of genes and chromosomes suggests that the genes are located on chromosomes. </li></ul>
  5. 5. Characteristics of chromosomes <ul><li>In diploids, the members of chromosome pair that contain the same genes and that pair at meiosis are called homologous chromosome, each member is called homolog. Chromosomes that contain different genes and that do not pair during meiosis are called nonhomologous chromosomes. </li></ul><ul><li>Chromosomes related to the sex of the organism are called sex chromosomes, e.g. in humans, females have two X chromosomes (XX), males have one X and one Y chromosomes (XY). Chromosomes other than sex chromosomes are called autosomes. </li></ul>
  6. 6. <ul><li>Each chromosome has a centromere somewhere along its length. A centromere is a specialized region that is seen as a constriction under the microscope. Centromeres are responsible for the accurate segregation of the replicated chromosome during mitosis and meiosis. </li></ul><ul><li>The kinetochore is the point where microtubules of the spindle apparatus attach. Replicated chromosomes consist of two molecules of DNA (along with their associated proteins ) known as chromatids . The area where both chromatids are in contact with each other is known as the centromere the kinetochores are on the outer sides of the centromere. </li></ul><ul><li>Telomeres are the region of DNA at the end of the linear eukaryotic chromosome that are required for the replication and stability of the chromosome. </li></ul>
  7. 10. Structure of a eukaryotic chromosome
  8. 11. <ul><li>Chromosomes differ in size and morphology within and between species. </li></ul><ul><li>Eukaryotic chromosomes can be classified into four types depending on the position of the centromere. </li></ul><ul><li>Metacentric chromosome have the centromere at the centre of the chromosome so that it appears to have two approximately equal arms. </li></ul><ul><li>Submetacentric chromosomes have centromere away from the centre of the chromosome so that one arm is longer than the other. </li></ul><ul><li>Acrocentric chromosomes have centromere towards one end of the chromosome so that it appears to have one arm with a stalk and often with a bulb-like structure on it. </li></ul><ul><li>Telocentric chromosomes have the centromere at one end of the chromosome so that it appears to have only one arm. </li></ul>
  9. 12. <ul><li>Chromosomes length and centromere positions are constant for each chromosome and help in the identification of individual chromosomes. </li></ul><ul><li>Sex chromosomes plus the autosomes constitute a genome, which is the total set of chromosomes in a cell. </li></ul><ul><li>Genomes in which chromosomes occur in pairs are said to be diploid, and the members of a pair are called homologues. </li></ul><ul><li>Sexually reproducing organisms have two sorts of cells: somatic (body) cells and germ (sex) cells. Somatic cells are haploid or diploid depending on the type of the eukaryote, e.g. yeast are haploid, higher eukaryotes are diploid. Germ cells (gametes) are haploid. </li></ul>
  10. 13. Chromosome number in various organisms 78 Chicken 44 Rabbit 62 Donkey 64 Horse 60 Cattle 54 Sheep 60 Goat 38 Pig 78 Dog 38 Cat 46 Human Total number of chromosomes organism
  11. 14. Cell Division <ul><li>All somatic cells reproduce by a process called mitosis. Mitosis is a process of cell division which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. </li></ul><ul><li>The growth of a single-celled zygote into multicellular adult individual involves the process of mitosis. </li></ul><ul><li>Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material such that each sperm and each ovum contains one member of each pair of chromosomes. This is because m eiosis comprises two successive nuclear divisions with only one round of DNA replication. </li></ul>
  12. 15. <ul><li>In both unicellular and multicellular eukaryotes, cellular reproduction is a cyclical process of growth, mitosis (nuclear division) and cell division (cytokinesis). The cycle of growth, mitosis and cell division is called the cell cycle. </li></ul><ul><li>The cell cycle consists of two phases: the mitotic (dividing) phase (M) and the interphase between two divisions. </li></ul><ul><li>Interphase consists of three stages: G 1 (gap 1), S (synthesis) and G 2 (gap 2). </li></ul><ul><li>During G 1 (presynthesis) stage the cell prepares for DNA and chromosome replication, which takes place in S (synthesis) stage. In G 2 (postsynthesis) stage the cell prepares for cell division, which takes place in the M phase. </li></ul>
  13. 16. <ul><li>Cytokinesis is the process where one cell splits off from its sister cell. It usually occurs after cell division. </li></ul><ul><li>Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They acquire ATP and increase in size during the G 1 phase of Interphase. Most cells are observed in Interphase, the longest part of the cell cycle. After acquiring sufficient size and ATP, the cells then undergo DNA Synthesis (replication of the original DNA molecules, making identical copies, one &quot;new molecule&quot; eventually destined for each new cell) which occurs during the S phase. </li></ul><ul><li>Since the formation of new DNA is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G 2 phase. The energy acquired during G 2 is used in cell division (in this case mitosis). </li></ul>
  14. 17. <ul><li>Some cells can exit the cell cycle and enter a quiescent, nondividing state called G 0 . </li></ul><ul><li>The cell cycle </li></ul>
  15. 18. The cell cycle
  16. 19. Mitosis <ul><li>Mitosis is the process of forming (generally) identical daughter cells by replicating and dividing the original chromosomes. Mitosis deals with the segregation of the chromosomes into daughter cells. </li></ul><ul><li>During interphase of the cell division cycle, the individual chromosomes are extended and difficult to see under light microscope. The DNA of each chromosome is replicated and the product of chromosome duplication is two exact copies, called chromatids, which are held together by centromere. </li></ul><ul><li>A chromatid is one of the two visibly distinct, longitudinal subunits of a replicated chromosome. </li></ul>
  17. 20. <ul><li>Mitosis is usually divided into four stages: prophase, metaphase, anaphase and telophase. </li></ul><ul><li>Prophase </li></ul><ul><li>- Chromosomes condenses, the nuclear envelope dissolves, centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form, and the spindle forms. </li></ul><ul><li>Metaphase </li></ul><ul><li>- The chromosomes (which at this point consist of chromatids held together by a centromere) migrate to the equator of the spindle, where the spindles attach to the kinetochore fibers. </li></ul><ul><li>Anaphase </li></ul><ul><li>- Anaphase begins with the separation of the centromeres, and the pulling of chromosomes (we call them chromosomes after the centromeres are separated) to opposite poles of the spindle. </li></ul>
  18. 21. <ul><li>Telophase </li></ul><ul><li>- Telophase is when the chromosomes reach the poles of their respective spindles, the nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus (which had disappeared during Prophase) reform. Where there was one cell there are now two smaller cells each with exactly the same genetic information. These cells may then develop into different adult forms via the processes of development. </li></ul><ul><li>Cytokinesis </li></ul><ul><li>- Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the division of the nucleus, cytokinesis is the splitting of the cytoplasm and allocation of the golgi, plastids and cytoplasm into each new cell. In animal cells cytokinesis occurs by formation of a constriction in the middle of the cell which contracts until two daughter cells are produced. </li></ul>
  19. 27. <ul><li>During mitosis replicated chromosomes are positioned near the middle of the cytoplasm and then segregated so that each daughter cell receives a copy of the original DNA (if you start with 46 in the parent cell, you should end up with 46 chromosomes in each daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus ) to &quot;pull&quot; chromosomes into each &quot;cell&quot;. </li></ul><ul><li>Animal cells (except for a group of worms known as nematodes) have a centriole . Plants and most other eukaryotic organisms lack centrioles. Cells that contain centrioles also have a series of smaller microtubules, the aster , that extend from the centrioles to the cell membrane. The aster is thought to serve as a brace for the functioning of the spindle fibers. </li></ul>
  20. 29. Structure and main features of a spindle apparatus
  21. 30. Genetic significance of mitosis <ul><li>Mitosis results in the production of daughter nuclei that contain identical chromosome numbers and that are genetically identical to one another and to the parent nucleus from which they arose. This kind of division produces two genetically identical cells from a single progenitor cell. Thus, mitosis is the way in which the chromosome number is maintained constant during cell division. </li></ul><ul><li>Mitosis is the type of division that allows a multicellular organism to be developed from a single fertilized egg (zygote). </li></ul>
  22. 31. Meiosis <ul><li>Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. </li></ul><ul><li>Meiosis comprises two successive nuclear divisions with only one round of DNA replication. </li></ul><ul><li>Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new &quot;gamete&quot;. Meiosis, therefore, reduces the number of sets of chromosomes by half (involves a reduction in the amount of genetic material), so that when gametic recombination ( fertilization ) occurs the ploidy of the parents will be re-established. </li></ul>
  23. 32. <ul><li>The two nuclear divisions of a normal meiosis are called meiosis I and meiosis II. </li></ul><ul><li>The first meiotic division results in reduction in the number of chromosomes from diploid (2N) to haploid (N). The second division results in separation of the sister chromatids, thus Meiosis II divides the remaining set of chromosomes in a mitosis-like process . </li></ul><ul><li>In most cases the divisions are accompanied by cytokinesis. Thus, the result of the meiosis of a single diploid cell is four haploid cells. </li></ul><ul><li>Meiosis I consists of four stages: prophase I, metaphase I, anaphase I and telophase I. </li></ul>
  24. 33. <ul><li>Prophase I: begins when the chromosomes have already replicated. The chromosome become shorter and thicker , they pair off, crossing-over occurs and the spindle apparatus forms and the nuclear membrane and the nucleolus disappear. </li></ul><ul><li>Prophase I is divided into five stages: </li></ul><ul><li>Leptotene stage (Leptonema): Chromosomes begin to coil. Homologous chromosomes pair. Once pairing has been completed, the event of crossing-over begins. Crossing-over is the reciprocal exchange of chromosome segments at corresponding positions along pairs of homologous chromosomes. </li></ul>
  25. 34. <ul><li>Zygotene stage (Zygonema): A key event in this stage is synapsis - the formation of an intimate association of homologous chromosomes brought about by the formation of a zipper-like structure along the length of the chromatids called the synaptonemal complex. Each synapsed set of homologous chromosomes consists of four chromatids and it is called a bivalent. </li></ul><ul><li>- Pachytene stage (pachynema): the formation of the synaptonemal complex is completed in pachynema, the structure is disassembled at the end of pachynema. </li></ul><ul><li>- Diplotene stage (diplonema): the result of crossing-over becomes visible during diplonema as a cross-shaped structure called a chiasma (plural- chiasmata). </li></ul><ul><li>- Diakinesis: the chiasmata often terminalize, that is, they move down the chromatids to the ends. </li></ul><ul><li>- Crossing-over between homologous chromosomes produces chromosomes with new associations of genes and alleles </li></ul>
  26. 36. <ul><li>When the two homologous pairs are aligned, side-by-side, the pair is called a tetrad . </li></ul><ul><li>A tetrad is composed of two chromosomes - one maternal (M) and one paternal (P). </li></ul><ul><li>A tetrad have two centromeres and four chromatids because it is made from two chromosomes. </li></ul><ul><li>Recall that a dyad was a single chromosome so a tetrad is composed of two dyads . </li></ul>
  27. 37. <ul><li>Metaphase I </li></ul><ul><li>- By the beginning of metaphase I the nuclear membrane has completely broken down. The bivalent become aligned on the equatorial plane of the cell. The spindle apparatus is completely formed and the microtubules are attached to the centromeres of the homologs. </li></ul><ul><li>Anaphase I </li></ul><ul><li>- The chromosomes of each of the homologous pairs disjoin and they migrate toward opposite poles, the area in which new nuclei will form. Each of the separated chromosomes is called a dyad. Note that at this time the segregated sister chromatid pairs remain attached at their respective centromeres. </li></ul>
  28. 38. <ul><li>Telophase I: The dyads complete their migration to opposite poles of the cell and new envelopes form around each haploid grouping. In most species, cytokinesis follows, producing two haploid cells. </li></ul>
  29. 43. Meiosis II <ul><li>The second meiotic division is very similar to a mitotic division. </li></ul><ul><li>Prophase II: N uclear envelopes (if they were formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. </li></ul><ul><li>Metaphase II: chromosomes line upon the equatorial area of the cell and spindle apparatus attach to the opposite sides of the centromeres in the kinetochore region. </li></ul><ul><li>Anaphase II : T he centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell. </li></ul>
  30. 44. <ul><li>Telophase II: A nuclear envelope forms around each set of chromosomes and cytokinesis takes place. After telophase II, the chromosomes become more extended and are invisible under light microscope. </li></ul><ul><li>The end products of the two meiotic divisions are four haploid cells from one original diploid cell. Each of the four progeny cells has one chromosome from each homologous pair of chromosomes. However, these chromosomes are not exact copies of the original chromosomes because of the crossing-over that occurs between chromosomes during prophase I of meiosis I. </li></ul><ul><li>The events that occurs in meiosis are the basis for the segregation and independent assortment of genes according to Mendel’s laws. </li></ul>
  31. 48. Genetic significance of meiosis <ul><li>Meiosis generates haploid cells with half number of chromosomes found in diploid cells. Fusion of the haploid nuclei during fertilization restores the diploid number. Therefore, through a cycle of meiosis and fertilization, the chromosome number is maintained constant in sexually reproducing organisms from generation to generation. </li></ul>
  32. 49. <ul><li>(ii) Meiosis generates variability between individuals in two ways:- </li></ul><ul><li>Through various ways in which maternal and paternal chromosomes are combined in progeny nuclei. In metaphase I of meiosis, each maternally derived and paternally derived chromosome has an equal chance of aligning on one or the other side of the equatorial metaphase plate. As a result, each nucleus generated by meiosis will have different combination of maternal and paternal chromosomes. </li></ul><ul><li>The crossing-over between maternal and paternal chromatid pairs generates more variations in the final combination of chromosomes. Since the sites of crossing-over vary from one meiosis to another, the number of different kinds of progeny nuclei produced by the process is extremely large. </li></ul>
  33. 50. Comparison of Mitosis and Meiosis
  34. 51. Comparison of Mitosis and Meiosis
  35. 53. Summary of the process of meioses I and II and two cytokinesis processes.
  36. 54. Location of meiosis in the life cycle <ul><li>Life cycles are a diagrammatic representation of the events in the organism's development and reproduction. </li></ul><ul><li>Most multicellular animals are diploid for most of their life cycle. The gametes are the only haploid stages of the life cycle. </li></ul><ul><li>Meiosis produces haploid gametes, which produce a diploid zygote when their nuclei fuse in the fertilization process. The zygote then divides mitotically to produce the new diploid organism. </li></ul>
  37. 56. Life cycle of an animal
  38. 57. Gametogenesis <ul><li>Gametogenesis is the process of forming gametes (haploid cells, n) from diploid cells of the germ line. </li></ul><ul><li>Spermatogenesis is the process of forming sperm cells by meiosis in animals in specialized organs known as gonads (in males these are termed testes ). After division the cells undergo differentiation to become sperm cells. </li></ul><ul><li>Oogenesis is the process of forming an ovum (egg) by meiosis in animals in specialized gonads known as ovaries . </li></ul>
  39. 58. <ul><li>Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis , where they mature and are stored. </li></ul><ul><li>The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte . At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause. </li></ul>
  40. 59. <ul><li>Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis produces only one mature gamete (the ovum). The other cells, the polar bodies, do not develop. This all the cytoplasm and organelles go into the egg. </li></ul><ul><li>Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle . </li></ul><ul><li>In humans, all oocytes are formed in the foetus and one oocyte completes meiosis I each month in the adult female, but does not progress further through meiosis unless stimulated to do so by fertilization with a sperm. </li></ul>
  41. 60. Gametogenesis
  42. 62. Oogenesis