Cellular reproduction

1

• Cellular reproduction
• Mitosis
•  Two major processes in Eukaryotes
• Meiosis
• Mitosis- leads to the prod. of cells each with
  the same number of chromosomes as the parent
  cell, happens in somatic cells which are
  continuously reproducing.
•  Meiosis- Reduces the genetic content and the
  number of chromosome by half, this is special
  type of cell division that produces gametes or
  spores.
•  Mitosis
• In some unicellular organisms i.e protozoans some
  fungi and algae, mitosis provides the basis for
  asexual reproduction.
•  
• Multicellular diploid organism single
  cell zygotes development by mitosis
  organism

2

• Mitosis
• Nuclear division first- chromosomes replicate and
  then accurately partitioned - karyokinesis
• This is followed by cytoplasmic division
  (cytokineses)
•  
• The cell cycle
•  Somatic cells undergo a continuous alteration
  between division and non division. The cell cycle
  is divided into 2 broad categories interphase
  (non dividing) and mitosis (dividing)
•  Interphase stage chromosomes unfold and uncoils
  into a diffuse network known as chromatin,
  general state of the cell at non dividing stage
• Interphase is divided into 3 phases
• G1 (pre synthesis)- cell is in very active stage,
  intensive metabolic activity, cell growth and
  differentiation occurs
• S1 (synthesis)- DNA replicates
• G2 (post synthesis)

3

• When interphase ends- mitosis usually begins,
  continuously dividing cells then repeat the cycle
  (G, S, G2, M) over and over.
• Mistosis is usually divided into 4 phases.
• Prophase, Metaphase, Anaphase and Telophase 
• Prophase- centriole divides and move apart,
  (centrioles are responsible for organizing
  cytoplasmic microtubules into spindle fibers)
  centromeres visible, spindle fibers forms,
  nuclear membrane break down, half of mitosis is
  spent in prophase

4

• Metaphase chromosomes align along the metaphase
  plate, spindle fibers attach at the kinetechore
  (the protein structure located at the centromere
  for the attachment of spindle fibers )
• Anaphase- begins with splitting of chromatids and
  move to poles
• Telophase Chromosomes complete migration to
  poles, spindle fibers disappear, nuclear membrane
  reforms

5

• Telophase Cytokinesis

6

• Cell cycle control
• Cell cycle- The events that occur from the
  completion of one mitotic division until the
  beginning of the next division constitute the
  cell cycle.
• A cell has built in control functions as check
  points- controls the proper sequence of events
  in the cell cycle. Any disruption of this
  regulation can lead to uncontrolled cell division
  that characterizes malignancy. Study of
  mutations revealed at least 3 major checkpoints
  exist. Fig.
• G1/S checkpoint- monitors cell size and DNA
  damage
• G2/M checkpoint- monitor if DNA replication or
  repair to any DNA is completed
• M checkpoint- occurs during mitosis proper
  spindle fiber formation and attached to
  kinetechores.

7

• The cells that enter G1 phase either continue to
  proceed through it or enter G0 where it becomes a
  non dividing cell, but still a physiologically
  active.
• Apoptosis programmed cell death during
  development growth and repair.
• Ex mitosis - replaces skin cells lost to
  help injury.
• Apoptosis - programmed cell death during
  
• development growth and repair.
• The human p53 gene product has shown to regulate
  apoptosis. High of human cancers has been found
  to contain mutations in p 53 gene. This gene is
  referred to as tumor suppressor gene

8

• Meiosis
• Meiosis occurs in all sexually reproducing
  organisms and results in a diploid cell producing
  haploid gametes.
• Meiosis occurs in 2 stages
• 1st division reduce the number of chromosomes
  from diploid to haploid (reductional division)
• 2nd division results in separation of the
  sister chromatids ( equatorial division)
• In meiosis pairing of homologues chromosomes
  occur- synapse, each synapsed structure is
  called a bivalent. The bivalent eventually
  gives rise to a tatrad consisting of 4
  chromotids.
• The first meiotic division Meiosis I 3 events
  characterize the initial stage, Prophase I-
  visible chromosomes appear, homologous
  chromosomes undergo synapsis,  crossing over
  (exchange process occurs between sister
  chromatids) . Prophase I can be described in
  several stages

9

• Leptotene stage - chromosomes become visible and
  homology search occurs, chromomeres  resemble as
  beads on a string
• Zygotene stage- synaptonemal complexes begins to
  form, named as bivalent
• Pachytene stage more intimate pairing occurs -
  synapsis occurs, tetrads are visible and within
  each tetrad, non sister chromatids undergo
  genetic exchange known as crossing over, each
  such area is called chiasma (pl chiasmata)
• Diplotene stage- sister chromatids begin to
  separate, chiasmata are visible

10

• Crossover

11

• Diakinesis The chromosomes pull farther apart
  but still keep intact at chiasmata, move towards
  the end of the chromosomes ( terminalization),
  nucleolus and nuclear envelope break down.
• Metaphase I, anaphase I and telophase I occur
  similar to mitosis
• At anaphase I one half of each tetrad (dyad) is
  pulled toward each pole of the dividing cell.
  This separation process is known as
  disjunction
• Meiotic Telophase I is short and cells enter
  second meiotic division
• Prophase II - each dyad is composed of one pair
  of sister chromatids attached by a common
  centromere

12
(No Transcript)
13
(No Transcript)
14

• Significance of Meiosis
•  
• 1) Reduction from 2n n
• 2) Homologues chromosomes pair followed by
  genetic crossing over leads to genetic
  recombination. This event results in extensive
  genetic variation by recombining the maternal
  chromosomes and their paternal chromosomes and
  their random segregation into gametes.
• 3) The meiotic process results in the
  conservation of the number of chromosomes from
  generation to generation.
•  

15

• Meiosis in animals- The events that occur during
  meiotic divisions are similar in all cells, but
• In most animal species there are certain
  differences between the production of a male
  gamete (spermatogenesis) and female gamete
  (oogenesis) fig.
• The primary spermatocyte undergoes the first
  meiotic division and the result is the formation
  of 2 secondary spermatocytes (haploid of
  dyads), each of this then under goes the second
  meiotic division and produces 2 haploid
  spermatids.
• Oogeneis, is similar to spermatogenesis but each
  divison produces an oocyte and a polar body. In
  humans the 1st division begins in the embryonic
  ovary but arrests in prophase I. Meiosis resumes
  just prior to ovulation and the 2nd division is
  completed after ovulation

16

• In multicellular plants, the life cycle
  alternates between the diploid sporophyte stage
  and haploid gametophyte stage. One or the other
  predominates in different plant groups.

17
(No Transcript)
18

• anthers Microspore meiosis
  mitosis
• Mother cell (2n)
  microspores pollen grain
• 
  2 sperm
  nuclei
• 
  tube
  nucleus
• meiosis
  3 of 4 degenerate
• Pistil Megaspore (2n)
  megaspores
• 
  mitosis
• Megagametophyte (one n) embryo
  sac 8 x n
• Double fertilization
• Sperm nucleus oocyte nuclei
  diploid zygote nucelus
  
• Sperm nucleus endosperm nuclei triploid
  endosperm nucleus

19
(No Transcript)