MEIOSIS + COMPARISON WITH MITOSIS.
LEARNING OBJECTIVES:
•At the end of the lecture, students should be able to:
•Describe Meiosis and its first and second meiotic divisions.
•Know the phases of meiotic divisions.
•Differentiate between mitosis and meiosis.
LECTURE OUTLINE
•MEIOSIS.
•Meiosis is a special type of cell division , occurs only in germ cells.
•Occurs during the process of their development and maturation to form gametes.
•It comprises of two successive cell divisions with only one replication of chromosomes.
•Referred as “ Meiosis I” and “Meiosis II”.
•Two meiotic divisions form four cells
•Gametes that develop have haploid no. of chromosomes
•It has four phases:
prophase
•metaphase
•anaphase and
•telophase.
.
•PROPHASE:
•Prophase of meiosis I is divided into five stages:
•Leptotene.
•Zygotene.
•Pachytene.
•Diplotene.
•Diakinesis
•PROPHASE.
•1. LEPTOTENE(thin threads)
•Chromosomes become visible in nucleus as long , thin strands.
•2. ZYGOTENE:(paired threads)
•Homologous chromosomes come together in close apposition along entire length.
•Pairing of homologous chromosomes is termed as synapsis
•Homologous chromosomes
•are chromosome pairs of the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci.
•One homologous chromosome is inherited from the organism's mother; the other from the organism's father.[1]
•They pair (synapse) during meiosis, or cell division that occurs as part of the creation of gametes
•3. PACHYTENE:(thick threads)
•Chromosomes become thicker and shorter due to coiling.
•Each chromosome pair is called a bivalent.
•4. DIPLOTENE:(two threads)
•Chromosomes begin to separate.
•Each Chromosome consists of two chromatids.
•Each bivalent consists of four chromatids.
•During diplotene it becomes clearly visible that each replicated chromosome consists of two sister-chromatids.
•Each bivalent consist of a bundle of four homolog chromatids.
. The crossings chiasma, between non-sister-chromatids are visible.
• Each bivalent shows in general one or more chiasma, where crossing-overs have occured.
•5. DIAKINESIS:(moving through)
•In this stage, separation of chromosomes continues.
•Chiasma and chromosomes are more obvious during this stage.
•The nucleolus and nuclear envelope disappear by the end of diakinesis.
•METAPHASE I.
•A spindle of microtubules is produced by centrioles.
•the homologous chromosomes align along an equatorial plane that bisects the spindle.
•ANAPHASE I.
•In anaphase I the chromatids holding the chromosomes together loosen.
• The two homologous chromatids of each tetrad are separated into separate poles.
•TELOPHASE I.
•In this stage, the nuclei are reconstituted and cytokines divides the parent cell into two daughter cells.
•Each daughter cell contains haploid (i.e 23) chromosomes.
•But each chromosome is a double structured chromosome consisting of two sister chromatids.
•MEIOSIS II.
•Each of two daughter cells (resulting from meiosis-I) soon enters meiosis II, consists of four stages prophase, metaphase, anaphase and telophase.
•The behaviour of chromosomes is same as in mitosis.
•Division of centromere in each doubled chromosome takes place.
•Sister chromatids separate and move independently to opposite poles.
•Soon cytokinesis divides the cell into two daughter cells
•CONCLUSION:
•At the end of second meiotic division four cells are obtained.
•Nucleus of each of these cells contains haploid chromosomes.
•The second meiotic division is similar to an ordinary mitosis except that the chromosome number of the cell entering the second meiotic division is haploid
•Mitosis v/s Meiosis
•Mitosis v/s Meiosis - Differences in Purpose :
•Both Meiosis and Mitosis are found in complex organisms which reproduce sexually.
• Mitosis may be used for human growth, the replenishment of depleted organs and tissues, healing, and sustenance of the body. Identical versions of cells can be created to form tissues through Mitosis.
•Meiosis is a special process reserved for the creation of the egg and sperm cells.
•meiosis
•Provides constancy of the chromosome number from generation to generation by reducing the chromosome number from diploid to haploid, thereby producing haploid gametes.
•Relocates segments of maternal and paternal chromosomes by crossing over of chromosome segments,
• which "shuffles" the genes and produces a recombination of genetic material
•The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction.
•When the separation is not normal, it is called nondisjunction.
•This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy.
•Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
•Abnormal Gametogenesis
•Disturbances of meiosis during gametogenesis, e.g., nondisjunction , result in the formation of chromosomally abnormal gametes.
• If involved in fertilization, these gametes with numerical chromosome abnormalities cause abnormal development such as occurs in infants with Down syndrome
• Meiosis Mitosis
•Occurrence of Crossing Over: Yes No
•Number of Daughter Cells produced: 4 2
•Creates: Sex cells only: all except sex cells
•Definition: A type of cellular reproduction in which the number of chromosomes are reduced by half through the separation of homologous chromosomes in a diploid cell.
•A process of asexual reproduction in which the cell divides in two producing a replica, with an equal number of chromosomes
•Produces: four haploid daughter cells two diploid daughter cells
•Steps: The steps of meiosis are Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II and Telophase II.
•The steps of mitosis are Interphase, Prophase, Metaphase, Anaphase, Telophase and Cytokinesis hide
•Genetically: different identical
•Cytokenesis: Occurs in Telophase I & Telohpase II Occurs in Telophase
•Number of Divisions: 2 1
•
•Function: sexual reproduction Cellular Reproduction & general growth and repair of the body
•Chromosome Number: Reduced by half Remains the same
•Crossing Over: Mixing of chromosomes Does not occur
•
•Significance of Mitosis vs. Meiosis :
•The importance of mitosis is the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.
•Occurs in :
•Meiosis is found to occur in Human, animals, plants while Mitosis is found in single-cell species as well.