Mitosis And Meiosis


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Mitosis And Meiosis

  2. 2. Topics • Highlights • Definition • Chromosomes • Cell Division – mitosis _ meiosis _ significance • A note on chronology
  3. 3. TYPES: • Causal (experimental) embryology • Comparative embryology •Descriptive embryology
  4. 4. EMBRYOLOGY /em” brē-ol’é-je/ • The science of the development of the individual during the embryonic stage and, • by extension, in several or even all preceding and subsequent stages of life cycle.
  5. 5. General definitions of embryology •The branch of biology that deals with the formation, early growth, and development of living organisms •Study of the formation and development of an embryo and fetus
  6. 6. Why medical students should study Embryology? • Know how a single cell develops into a newborn, containing numerous tissues and organs. • Help us understand many complicated facts of adult anatomy. • Explain why some children are born with organs that abnormal. • Appreciate the factors responsible for maldevelopment to assist us in preventing or treating such abnormalities.
  7. 7. Some preliminary consideration • Embryo = developing individual during 1st two months. • Fetus = developing individual from 3rd month until birth. • Gametes : are produced by testes and ovaries. • Testis : produces gametes ( spermatozoa ). • Spermatogenesis : process of producing spermatozoa. • Ovary : produces gametes ( ova ). • Oogenesis : process of producing ova. • Fertilization : takes place when one spermatozoon enters an ovum. • Zygote : the fused ovum and sperm.
  8. 8. What is Embryology? Embryology is the study of the development of an individual before birth. • Every individual spends 266 days— 38 weeks ( to be exact ) • One celled structure pass stages to become organism having billions of cells, numerous tissues and organs are formed and come to function in perfect harmony. The most spectacular of these changes occur in the 1st two months ( the unborn baby acquires its main organs and just begins to be recognized as human ). • 1st two months = Embryo. • 3rd month until birth = fetus. • 1st 28 days post birth = Newborn baby .
  9. 9. •Chromosome • A structure in the nucleus containing a linear thread of DNA, which transmits genetic information and associated with RNA and histones; during cell division. • Chromatid = one of the paired chromosome strands, joined at the centromere, which make up a metaphase chromosome, resulting from chromosome reduplication during the DNA synthetic phase of interphase.
  10. 10. Chromatin • The more readily stainable protein of the cell nucleus, forming a network of nuclear fibrils. It is a DNA attached to a protein primarily (histone) structure base and is the carrier of the genes in inheritance. It coils to form the metaphase chromosome. Gene A segment of DNA molecule that contains all the information required for synthesis of a product ( polypeptide chain RNA molecule ). It is biologic unit of hereditary, self reproducing and transmitted from parent to progeny. Each gene has a specific position (locus) on the chromosome.
  11. 11. DNA THE NUCLEIC ACID IN WHICH THE SUGAR DEOXYRIBOSE, constituting the primary genetic material of all cellular organism and the DNA viruses; and occurring predominantly in the nucleus. DNA is duplicated by replication and it serves as atemplate for synthesis of ribonucleic acid (RNA ) (transcription).
  12. 12. Some facts about chromosomes • Number of chromosomes in each cell is fixed for a given species and in man it is (46). • This is referred to as diploid (or double number ). • In spermatozoa and ova the number of chromosomes in only half the diploid number (23), this is called haploid or half number . • There are (46) chromosomes in human cell (44) autosomal (2) are sex chromosomes. • (44) autosomal chromosomes = (22) pairs. • The chromosomes forming a pair being exactly alike.
  13. 13. • Other facts • Characters of parents are transmitted to offspring through codes borne on strands of DNA. • Genes are made up of such strands of DNA. • Genes are located on chromosomes. • A typical cell contains (46) chromosomes = diploid number. • A gamete contains (23) chromosomes = haploid number. • The diploid number of chromosomes is restored as a result of fertilization.
  14. 14. Human chromosome with coiled and uncoiled
  15. 15. Significance of chromosomes • The entire human body develops from the fertilized ovum. • The fertilized ovum contains all the information necessary for formation of the numerous tissues and organs of the body, and for their orderly assembly and function. • Each cell of the body inherits all directions that are necessary for it to carry out its functions through life. • This tremendous volume of information is stored within the chromosomes of the cell.
  16. 16. Significance of chromosomes(continue) •The nature and functions of a cell depend on the proteins synthesized by it •So the genes control the development and functioning of cells by determining what types of proteins will be synthesized within them. Genes play an important role in the development of tissues and organs of the individual. •Characters ( traits) of an individual are determined by genes carried on his (or her) chromosomes. •Half of these characters are inherited from the father and half from the mother.
  17. 17. Significance of chromosomes CONTINUE • Each chromosome bears on itself a very large number of genes. • Genes are made up of a nucleic acid called DNA and all information is stored in the molecules of this substance. • Genes are involved in synthesis of proteins. • Proteins are the most important constituents of our body, they make up the greater part of each cell and intercellular substance. ‘Enzymes, hormones and antibodies are also proteins.
  18. 18. Chromosome structure • In resting cell, the chromosomes aren’t visible under light microscope. • During cell division the chromatin network becomes condensed into a number of chromosomes. (visible under light microscope) • Chromosome is made up of two ( rod shaped structure) called chromatids. Each chromatid has two arms. • Chromatids unite at an area called centromere ( kinetochore ).
  19. 19. Chromosome structure (continue) Individual chromosome differs from one another in total length, relative length of the two arms and in various other characteristics and these differences enable us to identify each chromosome individually . Classification of chromosomes in this way is called KARYOTYPING. Karyotyping makes it possible for us to detect abnormalities in chromosome number or in the individual chromosome.
  20. 20. MITOSIS /maitosis/ MEIOSIS /maiosis Cell Division Cell Division Cell Division
  21. 21. MITOSIS /maitosis/ • This term is used interchangeably with cell division, • but strictly speaking it refers to nuclear division, • whereas cytokinesis refers to division of cytoplasm.
  22. 22. Cell Division • multiplication is an essential feature for embryonic development and is necessary after birth for growth and for replacement of dead cells. • Chromosomes with the nuclei of cells carry genetic information that controls the DEVELOPMENT and FUNCTIONING of various cells and tissues. • When cell divides the daughter cells must have chromosomes identical in number ( and in genetic content ) to those in the mother cell. • This type of division is called mitosis
  23. 23. Cell Division (continue) • A different kind of cell division is called MEIOSIS occurs during the formation of gametes. • MEIOSIS consist of two successive divisions. • The cells resulting from the first and second MEIOTIC divisions (gametes) differ from other cells of the body in that: 1. The number of chromosomes is reduced to half the normal number. 2. The genetic information in various gametes produced isn’t identical.
  24. 24. Mitosis •Multiplication of cells takes place by cell division. • The usual method of cell division, seen in most tissues. •Daughter cells resulting from mitotic division are similar to the parent cells and have the same number of chromosomes (46). •Many cells of the body have limited span of functional activity, At the end of functional activity, cells undergo division into two daughter cells. •The daughter cells in turn have their own span of activity; followed by another division. •The period between two successive division is called interphase.
  25. 25. Stages of MITOSIS • MITOSIS IS DIVIDED INTO FOUR STAGES 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase The best phase for understanding is TELOPHASE
  26. 26. Interphase • During a specific period of the interphase, the DNA content of the chromosome is duplicated. So that Interphase • Another chromatid is refers to all stages of the formed.(identical to the original cell cycle one) other than mitosis. • Chromosome now is made up of two chromatids. • In Early interphase = chromosomes are in the form of extended threads. • In Late interphase DNA of each chromosome has undergone duplication.
  27. 27. Prophase • The chromatin of the chromosome becomes gradually more and more coiled, • That means the chromosome becomes recognizable as a thread-like then as a rod like appearance. • Toward the and of prophase, the two chromatids become distinct and the chromosome now has its typical structure. • The two centerioles separate and move to opposite poles of the cell. • The two centerioles produce a number of microtubules that pass from one centeriole to the other and form a spindle. • The nuclear membrane breaks down. • The nucleoli disappear.
  28. 28. Metaphase • With the formation of spindle, chromosomes move to a position midway between the two centerioles. ( at the equator of the cell) • Each chromosome becomes attached to microtubules of the spindle by its cenrtomere.
  29. 29. Anaphase • The centromere splits longitudinally into two, • The chromatids now become independent chromosomes. • The cell contains (46) pairs of chromosomes. • One chromosome moves along the spindle to either pole of the cell.
  30. 30. Telophase • Two daughter nuclei are formed by appearance of nuclear membranes. • Chromosomes gradually elongate and become indistinct. • Nucleoli reappear. • Centeriole is duplicated at this stage or in the early interphase. • The division of the nucleus is accompanied by the division of the cytoplasm = (CYTOKINESIS). In this process • THE ORGANELLES are presumably duplicated . • Each daughter cell comes to have a full complement of them.
  31. 31. Meiosis special kind of cell division takes place in the testis and ovary for formation of gametes. The gametes resulting from meiosis have the haploid number of chromosomes (23). The various gametes formed don’t have the same genetic content.
  32. 32. Meiosis (continue) • Consists of two successive divisions. • They are called the 1st and 2nd meiotic divisions. • During the interphase preceding the 1st division; DUPLICATION OF DNA as in mitosis. Another chromatid identical to the original one is formed. • Each chromosome is made up of two chromatids.
  33. 33. Meiosis • Prophase Leptotene Zygotene Pachytene Diplotene • Metaphase • Anaphase • Telophase
  34. 34. First Meiotic Division  The Prophase is prolonged and is usually divided into a number of stages as follow: 1. Leptotene 2. Zygotene 3. Pachytene 4. Diplotene
  35. 35. Leptotene • Chromosomes become visible ( as in mitosis). • Chromatids aren’t distinguished.
  36. 36. Zygotene • Pairing =two chromosomes lie parallel to each other. • Synapsis = conjugation = pairing BIVALENT
  37. 37. Pachytene • Chromatids become distinct. • The bivalent has (4) chromatids is called a TETRAD. • There are (2) central and (2) peripheral chromatids one from each chromosome. • The (2) central chromatids become coiled over each other at a number of points = CROSSING OVER. • Chromatids become adherent at crossing points, these points are called chiasmata.
  38. 38. Diplotene • Exchange of genetic material between chromatids. • The two chromosomes of • bivalent move apart • This result in crossing over “BREAK” at the points of crossing over. • Loose pieces become attached to the opposite chromatid.
  39. 39. Metaphase • As in mitosis the (46) chromosomes become attached to the spindle at the equator. • The two chromosomes of pair being close to each other.
  40. 40. Anaphase • ( differs from that in mitosis). • There is no splitting of the centromeres. • One entire chromosome of each pair moves to each pole of the spindle. • Daughter cells have (23) chromosomes (each made up of two chromatids.
  41. 41. Telophase • Two daughter nuclei are formed. • The division of the nucleus is followed by division of the cytoplasm Telophase- the final stage of meiosis Telophase 1
  42. 42. Second Meiotic Division • The first meiotic division is followed by short interphase. • There is no duplication of DNA. (Such duplication is unnecessary as chromosomes of cells possess two chromatids each). • The 2nd meiotic division is similar to mitosis. • The daughter cells aren’t identical in genetic content because of the crossing over has occurred during the 1st division .
  43. 43. Significance of Meiosis WHY ARE NO TWO PERSONS ALIKE? EXCEPT IDENTICAL TWINS NO TWO PERSONS ARE ALIKE! 1. There is a reduction of the number of chromosomes from diploid to haploid. At time of fertilization the diploid number is restored. This provides consistency of chromosome number from generation to generation. 2. Chromosomes from mother and father are distributed between the daughter entirely. 3. Crossing over results in thorough SHUFFLING of genetic material (ova and spermatozoa all have a distinctive genetic content). 4. A THIRD STEP OF SHUFFLING TAKES PLACE AT FERTILIZATION.
  44. 44. Teratogen • The development of defects in an embryo • Abnormalities in development can result due to exposing the embryo to certain agents (chemical or physical). • The list of teratogens keeps increasing. • Some particular organs are most sensitive to teratogens when they are passing through critical phase in their development. • This period of great susceptibility to teratogens differs from organ to organ. • In early stages of development , the age reckoned in days. • Later when the events are less dramatic, age can be expressed in weeks or months. However , the exact age of embryo isn’t always known.
  45. 45. Teratogen (continue) • An estimate can be made by observing the size of the embryo (expressed as C.R. length). Some other feature like the number of somites. • There are numerous references to the timing of embryonic events (most commonly in terms of C.R. length). • The disadvantage of doing so is that it adds yet one more complication to understanding of an already intricate subject.
  46. 46. CRL • Drown-rump length = the length of any embryo, fetus, or infant from the crown of the head to the breech; used in estimating the age of the embryos from the fourth to the eighth week . • It is the equivalent of sitting vertex height in older individuals.
  47. 47. Somite /so’mīt/ • One of the paired, block like masses of mesoderm, arranged segmentally alongside the neural tube of the embryo, forming the vertebral column and segmental musculature; called also mesodermic, mesoblastic, primitative, primordial, or protovertebral segment.
  48. 48. This shows prophase, prometaphase, metaphase, anaphase and telophase
  49. 49. Aster short fibers produced by cells during mitosis and meiosis. The end Thank you next lecture = spermatogenesis and oogenesis