MSPC 112 Mitosis & Meiosis.pdf

MITOSIS AND MEIOSIS 2023 MSPC 112 BY FREDERICK KWAKU ADDAI, PH.D., FWACM DEPARTMENT OF ANATOMY UGMS, KORLE-BU fkaddai/2024 MSPC-112 28/05/2024 2  STUDENT’S LEARNING OUTCOMES AT THE END OF THIS LECTURE, YOU SHOULD BE ABLE TO...

MITOSIS AND MEIOSIS 2023 MSPC 112 BY FREDERICK KWAKU ADDAI, PH.D., FWACM DEPARTMENT OF ANATOMY UGMS, KORLE-BU fkaddai/2024 MSPC-112 28/05/2024 2  STUDENT’S LEARNING OUTCOMES AT THE END OF THIS LECTURE, YOU SHOULD BE ABLE TO DO THE FOLLOWING 1. List key phases in Mitosis in 4. List the correct order of phases their chronological order in meiosis 2. Briefly narrate key processes 5. Compare the phases in mitosis that occur at every phase of and meiosis mitosis 28/05/2024 3 fkaddai/2024 MSPC-112  INTRODUCTION - THE CELL DIVISION CYCLE Every living cell that is not dividing exists in a phase called INTERPHASE Interphase lasts for variable duration from a few hours to a year or more When a cell is dividing it enters a phase called MITOSIS The alternation between mitosis and interphase in the life of a cell is called the cell division cycle fkaddai/2024 MSPC-112 28/05/2024 4  STAGES OF THE CELL CYCLE: Mitosis is the most visible manifestation of the cell cycle Represents the M-phase of the cycle that typically lasts from 1-2 hours fkaddai/2024 MSPC-112 28/05/2024 5  RAPIDLY CYCLING CELLS In cells that are rapidly dividing such as embryonic or tumour cells, G1 is almost non-existent During cleavage of zygote and blastomeres in mammalian development, both G1 and G2 are abolished fkaddai/2024 MSPC-112 28/05/2024 6  G0 -PHASE In cells such as hepatocytes, G1 may be very long (one-half to one year) Terminally differentiated cells including striated muscle cells and nerve cells have completely ceased cycling and are said to be in a special G1 state known as G0. fkaddai/2024 MSPC-112 28/05/2024 7  CELL DIVISION (MITOSIS) Broadly consists of two events 1. Nuclear division or karyokinesis, and 2. Cytoplasmic division or cytokinesis. There are four stages; PROPHASE, METAPHASE, ANAPHASE, and TELOPHASE. fkaddai/2024 MSPC-112 28/05/2024 8  THE CHROMOSOME IN A MITOTIC CELL A cell preparing for MITOSIS duplicates its DNA in the S-phase of the Cell cycle Each chromosome becomes a pair of sister chromatids held together at the centromere Each sister chromatid is a carbon copy (replicate) of its pair Although there are still 46 chromosomes, the DNA has doubled from 2N to 4N before mitosis commences 28/05/2024 fkaddai/2024 MSPC-112 9  MITOSIS -PROPHASE There is coiling of nuclear chromatin, making chromosomes visible (under the light microscope) and revealing their individual identities The nuclear envelope remains intact, but two centrioles separate and migrate to opposite poles of the cell, the spindle microtubules appear between them. fkaddai/2024 MSPC-112 28/05/2024 10  MITOSIS – LATE PROPHASE Towards the end of prophase, the nucleolus disappears, and the nuclear membrane breaks down fkaddai/2024 MSPC-112 28/05/2024 11  MITOSIS - METAPHASE In Metaphase, the nuclear envelope and nucleolus disappear, and the chromosomes occupy the equatorial plane of the spindle microtubules fkaddai/2024 MSPC-112 28/05/2024 12  MITOSIS – EARLY ANAPHASE (1)  Each chromosome shows a division into two sister chromatids that are attached to the spindle microtubules at the kinetochore (centromere) fkaddai/2024 MSPC-112 28/05/2024 13  MITOSIS – EARLY ANAPHASE (2)  Sister chromatids migrate towards the opposite poles of the cell, following the direction of the spindle microtubules fkaddai/2024 MSPC-112 28/05/2024 14  MITOSIS – LATE ANAPHASE The poles move farther apart and the microtubules lengthen A separation of chromatids occurs in anaphase as well, where the proteins holding together sister chromatids are inactivated A cleavage furrow appears in the equatorial plane of the cell membrane 28/05/2024 15 fkaddai/2024 MSPC-112  MITOSIS - TELOPHASE Telophase begins after the chromosomal movement stops. There is the reappearance of the nuclei in daughter cells The chromosomes revert to their semi- dispersed state The nucleoli, chromatin, and nuclear envelope reappear The cleavage furrow deepens 28/05/2024 16 fkaddai/2024 MSPC-112  CYTOKINESIS While these nuclear events are taking place, the cleavage furrow that developed in the equatorial plane of the parent cell progresses until it divides the cytoplasm and its organelles in half; thus cytokinesis fkaddai/2024 MSPC-112 28/05/2024 17  MEIOSIS - SYNOPSIS Meiosis is a process of cell division characterized by two consecutive divisions: 1. Meiosis I, and 2. Meiosis II Note that: no DNA synthesis (or S phase) between the two divisions fkaddai/2024 MSPC-112 28/05/2024 18  MEIOSIS - SYNOPSIS 4 cells result, each with half the number of chromosomes of the starting cell –i,.e. 2n → n Fusion of two such cells produces a 2n zygote or diploid zygote fkaddai/2024 MSPC-112 28/05/2024 19  SYMBOLS & TERMINOLOGIES Diploid – set of 23 pairs of (or 46) chromosomes symbolized as 2n Haploid – half the diploid number of chromosomes (or 23) symbolized as n DNA in a diploid cell before S-phase is 2N After the S-phase (duplication of DNA) in preparation for mitosis the DNA is 4N 28/05/2024 20 fkaddai/2024 MSPC-112  MEIOSIS - PROPHASE I When the chromosomes first become visible they are already doubled, each homologue having been duplicated during the preceding S phase fkaddai/2024 MSPC-112 28/05/2024 21  MEIOSIS - PROPHASE I Non-sister chromatids exchange bits Process is called CROSSING OVER or CHIASMATA FORMATION fkaddai/2024 MSPC-112 28/05/2024 22  MEIOSIS - PROPHASE I It is the longest part of meiosis fkaddai/2024 MSPC-112 28/05/2024 23  METAPHASE I Microtubules of the spindle fibers attach to : i. Sister kinetochores of one homologue, pulling both sister chromatids toward one pole of the cell. ii. sister kinetochores of the other homologue pulling those sisters toward the opposite pole. fkaddai/2024 MSPC-112 28/05/2024 24  PROPHASE II Typical for this stage is the presence of a haploid number of chromosomes that condense again The two sister chromatids of a chromosome are still coupled together at the centromere fkaddai/2024 MSPC-112 28/05/2024 25  METAPHASE II The chromosomes move again to the equatorial plane between the poles However, this plane is oriented perpendicularly, with respect to the previous one of meiosis I fkaddai/2024 MSPC-112 28/05/2024 26  ANAPHASE II The centromeres separate and the sister chromatids are pulled apart to opposite poles by the spindle 28/05/2024 27 fkaddai/2024 MSPC-112  TELOPHASE II The former sister chromatids have reached the poles A nuclear envelop is formed around each nucleus, while chromosome uncoil again fkaddai/2024 MSPC-112 28/05/2024 28  GENETIC RECOMBINATION Meiosis not only preserves the genome size of sexually reproducing eukaryotes but also provides three mechanisms to diversify the genomes of the offspring; viz: 1. Crossing Over 2. Random Assortment 3. Fertilization fkaddai/2024 MSPC-112 28/05/2024 29  CROSSING OVER - 1 Chiasmata represent points where nonsister chromatids had swapped sections The process is called crossing over. It is reciprocal; the segments exchanged by each nonsister chromatid are identical (but may carry different alleles) fkaddai/2024 MSPC-112 28/05/2024 30  CROSSING OVER - 2 Each chromatid contains a single molecule of DNA So the essence of crossing over is really to swap portions of adjacent DNA molecules It must be done with great precision so that neither chromatid gains or loses any genes. fkaddai/2024 MSPC-112 28/05/2024 31  THE END HAVE A GOOD EVENING fkaddai/2024 MSPC-112 28/05/2024 32